Systemic inflammation impairs respiratory chemoreflexes and plasticity

Denne studien beskriver hvordan systemisk betennelse påvirker pustefunksjonen og gjør at det blir vanskeligere å endre pustemønser, f.eks. å gjøre pusteøvelser, eller å tilpasse pusten til aktivitetsnivå. Spesielt den kjemiske og motoriske delen av pustefysiologien blir dårligere. Noe som viser seg i laver CO2 sensitivitet (kjemisk) og svakere pustemuskler (Motorisk).

Nevner spesielt at det er mikroglia celler i CNS som påvirkes av betennelse, og som kan oppretthodle betennelse siden de sender ut cytokiner, m.m. Astrosytter kan også bidra mye siden de aktiverer NFkB. Den gode nyheten her er at økt CO2 nedregulerer NFkB. TLR-4 (Toll-like receptor) aktiveres av patogener og problemer i cellene, og aktiverer NFkB, og nedreguleres av økt CO2.


Many lung and central nervous system disorders require robust and appropriate physiological responses to assure adequate breathing. Factors undermining the efficacy of ventilatory control will diminish the ability to compensate for pathology, threatening life itself. Although most of these same disorders are associated with systemic and/or neuroinflammation, and inflammation affects neural function, we are only beginning to understand interactions between inflammation and any aspect of ventilatory control (e.g. sensory receptors, rhythm generation, chemoreflexes, plasticity). Here we review available evidence, and present limited new data suggesting that systemic (or neural) inflammation impairs two key elements of ventilatory control: chemoreflexes and respiratory motor (vs. sensory) plasticity. Achieving an understanding of mechanisms whereby inflammation undermines ventilatory control is fundamental since inflammation may diminish the capacity for natural, compensatory responses during pathological states, and the ability to harness respiratory plasticity as a therapeutic strategy in the treatment of devastating breathing disorders, such as during cervical spinal injury or motor neuron disease.

Most lung and CNS disorders are associated with systemic and/or neural inflammation, including chronic lung diseases (Stockley, 2009), traumatic, ischemic and degenerative neural disorders (Teeling and Perry, 2009) and obstructive sleep apnea.

Systemic inflammation affects sensory receptors that modulate breathing, but can also trigger inflammatory responses in the central nervous system (CNS) through complex mechanisms. The primary CNS cells affected during systemic inflammation are microglia, the resident immune cells of the CNS, and astrocytes (Lehnardt, 2010).

Even when in their “resting state,” microglia are highly active, surveying their environment (Raivich, 2005,Parkhurst and Gan, 2010). When confronted with pathological conditions, such as neuronal injury/degeneration or bacterial/viral/fungal infection, they become “activated,” shifting from a stellate, ramified phenotype to an amoeboid shape (Kreutzberg, 1996). Activated microglia can be phagocytic, or they can release toxic and protective factors, including cytokines, prostaglandins, nitric oxide or neurotrophic factors (e.g. BDNF) (Kreutzberg, 1996Graeber, 2010). Despite the importance of microglia in immune function, they are diffuse in the CNS (~70-90% of CNS cells are glia; microglia are ~5-10% of those cells).

Astrocytes, on the other hand, contribute to the overall inflammatory response since they release cytokines, triggering nuclear factor-kappa B (NFκB) signaling elsewhere in the CNS. Further, they express many TLRs, including TLR-4, capable of eliciting an inflammatory response (Li and Stark, 2002Farina et al., 2007,Johann et al., 2008). Given their relative abundance, astrocytes may play a key role in CNS inflammatory responses.

TLR-4 receptors are cytokine family receptors that activate transcription factors, such as NFκB (Lu et al., 2008). NFκB regulates the expression of many inflammatory genes, including: IL-1β, -6 and -18, TNFα, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) (Ricciardolo et al., 2004Nam, 2006). Endogenous molecules known to activate TLR-4 receptors include (but are not limited to) heat shock proteins (specifically HSP60, Ohashi et al., 2000Lehnardt et al., 2008), fibrinogen, surfactant protein-A, fibronectin extra domain A, heparin sulfate, soluble hyaluronan, β-defensin 2 and HMGB1 (Chen et al., 2007).

The role of inflammation (and specifically microglia) in chronic pain has been studied extensively (reviewed in Woolf and Salter, 2000Trang et al., 2006Mika, 2008Abbadie et al., 2009Baumbauer et al., 2009). A remarkable story has emerged, demonstrating the interplay between neurons, microglia, inflammation and plasticity in this spinal sensory system. In short, inflammation induces both peripheral and central sensitization, leading to allodynia (hypersensitivity to otherwise non-painful stimuli) and hyperalgesia (exaggerated or prolonged responses to a noxious stimulus) (Mika, 2008).

An important aspect of ventilatory control susceptible to inflammatory modulation is the chemoreflex control of breathing. Chemoreflexes are critical for maintaining homeostasis of arterial blood gases viaclassical negative feedback (Mitchell et al., 2009), or acting as “teachers” that induce plasticity in the respiratory control system (Mitchell and Johnson, 2003). Major chemoreflexes include the hypoxic (Powell et al., 1998) and hypercapnic ventilatory responses (Nattie, 2001), arising predominantly from the peripheral arterial and central chemoreceptors (Lahiri and Forster, 2003).

To date, no studies have reported the impact of systemic inflammation on hypercapnic responses. However, increased CO2 suppresses NFκB activation, possibly suppressing inflammatory gene expression (Taylor and Cummins, 2011). In fact, hypercapnia has been used to treat ischemia/reperfusion injury to decrease inflammation and reduce lung tissue damage (Laffey et al., 2000O’Croinin et al., 2005Curley et al., 2010Li et al., 2010).

Further work concerning the influence of systemic inflammation on hypercapnic ventilatory responses is warranted, particularly since impaired CO2 chemoreflexes would allow greater hypercapnia and minimize the ongoing inflammation; in this sense, impaired hypercapnic ventilatory responses during inflammation may (in part) be adaptive.

Differential blood flow responses to CO2 in human internal and external carotid and vertebral arteries

Denne viser hvordan CO2-responsen er litt forskjellige i forskjellige blodkar. Den er sterkere i blodkar inni hjernen enn i blodkar i kraniet, ansiktet og ryggraden. Blodkar i ryggraden har større respons enn blodkar i ansiktet, men mindre respons enn blodkar i hjernen.

Because of methodological limitations, almost all previous studies have evaluated the response of mean blood flow velocity (Vmean) in the middle cerebral artery (MCA) to changes in CO2 as a measure of CO2 reactivity across the whole brain (Aaslid et al. 1989Ainslie & Duffin, 2009Ainslie & Ogoh, 2009).


ICA, VA and BA CO2 reactivity was significantly higher during hypercapnia than during hypocapnia (ICA, P < 0.01; VA, P < 0.05; BA, P < 0.05), but ECA and MCA were not significantly different.

The major finding from the present study was that cerebral CO2 reactivity was significantly lower in the VA and its distal artery (BA) than in the ICA and its distal artery (MCA). These findings indicate that vertebro-basilar circulation has lower CO2 reactivity than internal carotid circulation. Our second major finding was that ECA blood flow was unresponsive to hypocapnia and hypercapnia, suggesting that CO2 reactivity of the external carotid circulation is markedly diminished compared to that of the cerebral circulation. These findings suggest that different CO2 reactivity may explain differences in CBF responses to physiological conditions (i.e. dynamic exercise and orthostatic stress) across areas in the brain and/or head.

Hypercapnic cerebral CO2 reactivity in global CBF was greater than the hypocapnic reactivity (Ide et al. 2003) (Table 3). The mechanisms underlying this greater reactivity to hypercapnia compared with hypocapnia may be related to a greater influence of vasodilator mediators on intracranial vascular tone compared with vasoconstrictive mediators (Toda & Okamura, 1998Ainslie & Duffin, 2009). In humans, Peebles et al.(2008) recently reported that, during hypercapnia, there is a large release of nitric oxide (NO) from the brain, whereas this response was absent during hypocapnia.

The difference in CO2 reactivity between vertebro-basilar territories (VA and BA) and the cerebral cortex (ICA and MCA) may be due to diverse characteristics of vasculature, e.g. regional microvascular density (Sato et al. 1984), basal vascular tone (Ackerman, 1973Haubrich et al. 2004Reinhard et al. 2008), autonomic innervation (Edvinsson et al. 1976Hamel et al. 1988) and regional heterogeneity in ion channels or production of NO (Iadecola & Zhang, 1994Gotoh et al. 2001).

Interestingly, the response of the ECA to changes in CO2 may be similar to other peripheral arteries. It has long been appreciated that the vasodilatory effect of hypercapnia is much more profound in cerebral than in peripheral vasculature, particularly leg (Lennox & Gibbs, 1932Ainslie et al. 2005) and brachial arteries (Miyazaki, 1973). These findings suggest that control of CO2 is particularly important in the cerebral circulation. The high resting metabolic requirements of the brain, compared with that of other vasculature, might be one reason why this circulatory arrangement is desirable (Ainslie et al. 2005). Specifically, high CO2 reactivity may be a way for the brain to match metabolism with flow (Ainslie et al. 2005).

Lower CO2reactivity in the vertebro-basilar system may be important for maintaining central respiratory function because Graphic in central chemoreceptors is regulated by Graphic and blood flow to maintain breathing stability.

In summary, our study shows that cerebral CO2 reactivity in the vertebro-basilar circulation is lower than that in the internal carotid circulation, while CO2 reactivity in the external carotid circulation is much lower compared with two other cerebral arteries. These findings indicate a difference in cerebral CO2 reactivity between different circulatory areas in the brain and head, which may explain different CBF responses to physiological stress. Lower CO2 reactivity in the vertebro-basilar system may be beneficial for preserving blood flow to the medulla oblongata to maintain vital systemic functions, while higher CO2 reactivity in the internal carotid system may imply a larger tolerance for varied blood flow in the cerebral cortex.

Morning attenuation in cerebrovascular CO2 reactivity in healthy humans is associated with a lowered cerebral oxygenation and an augmented ventilatory response to CO2

Denne beskriver hvordan blodkarenes respons på CO2 er dårligere om morgenen, og det er derfor det skjer flere slag og slikt om morgenen. Den nevner mange interessante prinsipper. Bl.a. at lavere vasomotor respons (på CO2) gir mindre oksygen til hjernen. Og at i opptil 20 sekunder etter en 20 sekunder holdning av pust (etter utpust) øker fortsatt oksygenmengden og blodgjennomstrømningen i hjernen. Nevner også at siden blodkarene i hjernen reagerer dårligere på CO2 om morgenen blir det lett at pusten over- eller underkompenserer, så pustemønsteret blir uregelmessig om morgenen. Spesielt om man har underliggende faremomenter som hjerte/karsykdommer.


Furthermore, our results suggest that morning cerebral tissue oxygenation might be reduced as a result of a decreased cerebrovascular responsiveness to CO2 or other factors, leading to a higher level of desaturation.

Our data indicate that the cerebrovascular reactivity to CO2 in healthy subjects is significantly reduced in the morning and is strongly associated with an augmented ventilatory response to CO2. It is likely that this reduction in MCAV CO2 reactivity, by reducing blood flow through medullary respiratory control centers, increases both the arterial-brain tissue PCO2 difference and the H+ concentration presented to the central chemoreceptor(s) (1144). In effect, it appears the brain tissue is more susceptible in the morning to changes in arterial PCO2, which could increase the likelihood of ventilatory overshoots and undershoots.

However, as was the case with the hypercapnic challenge, subjects holding their breath in the morning experienced a significantly blunted increase in MCAV compared with evening, likely a result of a reduced cerebrovascular responsiveness to CO2.

In conclusion, our results suggest that early morning reductions in cerebrovascular CO2 reactivity strongly influence the magnitude of the ventilatory response to CO2. This may have significant implications for breathing stability, increasing the chances of periodic breathing in the morning in patients with additional risk factors. The early morning reduction in cerebral oxygenation with hypercapnic challenge, mild hypoxemia, or during apnea may be a contributing factor in the high prevalence of early morning stroke. Whether differences in the responses of CBF, oxygenation, or V̇E to CO2challenge are associated with other risk factors for stroke, such as gender or age, remains to be elucidated.


Dette er en veldig viktig artikkel for å forstå diafragmas rolle i både pust og bevegelse, og ifh smertetilstander i ryggraden. Nevner en lovende teknikk for å styrke diafragma og støttemuskulatur hvor man blåser opp en ballong og strammer kjernemuskulaturen. Nevner Zone of Apposition (ZOA) som beskriver diafragmas bevegelsesmuligheter. Ved lav ZOA har diafrgma lite bevegelse. Vi ønsker å øke ZOA. Denne øvelsen er konstruert basert på fysioterapeutisk prinsipper, men i Verkstedet Breathing System har vi øvelser som er gir samme resultater på diafragma, men bygget på lang og erfaringsbasert tradisjon fra tibetansk buddhisme.

Nevner også hvordan mage-pust minker bevegelsen i diafragma.

Suboptimal breathing patterns and impairments of posture and trunk stability are often associated with musculoskeletal complaints such as low back pain. A therapeutic exercise that promotes optimal posture (diaphragm and lumbar spine position), and neuromuscular control of the deep abdominals, diaphragm, and pelvic floor (lumbar-pelvic stabilization) is desirable for utilization with patients who demonstrate suboptimal respiration and posture. This clinical suggestion presents a therapeutic exercise called the 90/90 bridge with ball and balloon. This exercise was designed to optimize breathing and enhance both posture and stability in order to improve function and/or decrease pain. Research and theory related to the technique are also discussed.

Many muscles used for postural control/stabilization and for respiration are the same, for example: the diaphragm, transversus abdominis, and muscles comprising the pelvic floor.16 Maintaining optimal posture/stability and respiration is important and is even more challenging during exercise. Exercise increases respiratory demand (e.g. running) and limb movements (e.g. arms moving while standing still) increase postural demands for stabilization.3,7

Many factors are potentially involved with suboptimal respiration and suboptimal (faulty) posture and may be associated with musculoskeletal complaints such as low back pain, and/or sacroiliac joint pain.8 (Table 1)

Suboptimal Respiration and Posture
Decreased/suboptimal Zone of Apposition of diaphragm
Decreased exercise tolerance
Decreased intra-abdominal pressure
Shortness of Breath/Dyspnea
Decreased respiratory efficiency
Decreased expansion of lower rib cage/chest
Decreased appositional diaphragm force
Decreased length of diaphragm (short)
Decreased transdiaphragm pressure
Increased use of accessory muscles of respiration
Poor neuromuscular control of core muscles
Increased lumbar lordosis
Increased anterior pelvic tilt
Increased hamstring length
Increased abdominal length
Rib elevation/external rotation
Sternum elevation
Increased activity of paraspinals
Increased lumbar-pelvic instability
Low back pain
Sacroiliac Joint pain
Thoracic Outlet Syndrome

One of the most critical factors, often overlooked by physical therapists, is maintaining an optimal zone of apposition of the diaphragm.3,911 The zone of apposition (ZOA) is the area of the diaphragm encompassing the cylindrical portion (the part of the muscle shaped like a dome/umbrella) which corresponds to the portion directly apposed to the inner aspect of the lower rib cage.12 The ZOA is important because it is controlled by the abdominal muscles and directs diaphragmatic tension. When the ZOA is decreased or suboptimal, there are several potential negative consequences. (Table 1) Two examples include:

  1. Inefficient respiration (less air in and out) because the transdiaphragmatic pressure is reduced.11 The smaller the ZOA, there will be less inspiratory action of the diaphragm on the rib cage.11
  2. Diminished activation of the transversus abdominis which is important for both respiration and lumbar stabilization.11,13

The incidence of LBP has been documented to be as high as 30% in the athletic population, and in many cases pain may persist for years.15 Low back pain is frequently correlated with faulty posture such as an excessive lumbar lordosis.1618 Excessive lumbar lordosis may be associated with over lengthened and weak abdominal musculature.1820 Poor neuromuscular control of core muscles (transversus abdominis, internal oblique, pelvic floor and diaphragm) has been described in individuals with SIJ pain21 and in individuals with lumbar segmental instability, potentially adversely affecting respiration.22

Richardson et al.27 describe coordination of the Transversus abdominis and the diaphragm in respiration during tasks in which stability is maintained by tonic activity of these muscles. During inspiration, the diaphragm contracts concentrically, whereas the transversus abdominis contracts eccentrically. The muscles function in reverse during exhalation with the diaphragm contracting eccentrically while the transversus abdominis contracts concentrically. Hodges et al. noted that during respiratory disease the coordinating function between the transversus abdominis and diaphragm was reduced.6 Thus, it is also possible that faulty posture such as over lengthened abdominals and excessive lordosis could reduce the coordination of the diaphragm and transversus abdominis during respiration and stabilization activities.

O’sullivan et al.21 studied subjects with LBP attributed to the sacroiliac joints and compared them to control subjects without pain. O’sullivan et al. compared respiratory rate and diaphragm and pelvic floor movement using real time ultrasound during a task that required load transfer through the lumbo-pelvic region (the active straight leg raise test). Subjects with pain had an increase in respiratory rate, descent of their pelvic floor and a decrease in diaphragm excursion as compared to the control subjects, who had normal respiratory rates, less pelvic floor descent, and optimal diaphragm excursion. While O’sullivan et al. concluded that an intervention program focused on integrating control of deep abdominal muscles with normal pelvic floor and diaphragm function may be effective in managing patients with LBP,21 they did not describe strategies or exercises to achieve this goal.21

While the role of the Transversus abdominis in lumbar stability is well documented, less well known is the role of the diaphragm in lumbar stability. While the primary function of the diaphragm is respiration, it also plays a role in spinal stability.3,28

The right hemidiaphragm attaches distally to the anterior portions of the first through third lumbar vertebrae (L1-3) and the left hemidiaphragm attaches distally on the first and second lumbar vertebrae (L1-2).29 This section of the diaphragm is referred to as the crura. Of interest is the asymmetrical attachment of the diaphragm with the left hemidiaphragm attaching to L1-2 and the right portion attaching to L1-3.

During the inhalation phase of ventilation, the dome of the diaphragm moves caudally like a piston creating a negative pressure in the thorax that forces air into the lungs. This action is normally accompanied by a rotation of the ribs outward (external rotation) largely in part due to the ZOA.12 (Figure 1) Apposition is a term that means multiple layers adjacent to each other.33 The normal force of pull on the sternal and costal portions of the diaphragm would produce an internal rotation of the ribs. The ZOA creates an external rotation of these ribs primarily because the pressure in the thoracic cavity prevents an inward motion. The crural portion of the diaphragm assists the caudal motion of the dome. It also pulls the anterior lumbar spine upward (cephalad and anterior). Additionally, the abdominal muscles and pelvic floor musculature are less active to allow visceral displacement due to the dome of the diaphragm dropping. With exhalation, this process is reversed. Abdominal muscle activity compresses the viscera in the abdominal cavity, the diaphragm is forced cephalad and the ribs internally rotate. As exhalation becomes forced as during exercise, abdominal activity (rectus abdominus, internal obliques, external obliques, and transversus abdominis) will be increased.3436

When the ZOA is optimized, the respiratory and postural roles of the diaphragm have maximal efficiency.37 In suboptimal positions (i.e. decreased ZOA), the diaphragm has a decreased ability to draw air into the thorax because of less caudal movement upon contraction and less effective tangential tension of the diaphragm on the ribs and therefore lower transdiaphragmatic pressure.38 This decreased ZOA is accompanied by decreased expansion of the rib cage, postural alterations, and a compensatory increase in abdominal expansion.12 (Figure 2)

One such adaptive breathing strategy would be to relax the abdominal musculature more than necessary on inspiration to allow for thoraco-abdominal expansion. This situation leads to decreased abdominal responsibility while breathing and can contribute to instability. This would reflect more upper chest breathing and less efficient diaphragm activity. If the body maintains this position and breathing strategy for an extended period of time, the diaphragm may adaptively shorten and the lungs may become hyperinflated.37,39,40 Hyperinflation may also contribute to over use of accessory muscles of respiration such as scalenes, sternocleidomastoid (SCM), pectorals, upper trapezius and paraspinals in an attempt to expand the upper rib cage.4144 Again, without an optimal dome shape/position of the diaphragm or an optimal ZOA the body compensates to get air in with accessory muscles since the more linear/flat/short diaphragm is less efficient for breathing.32

Instructions for Performance of the 90/90 Bridge with Ball and Balloon: 1. Lie on your back with your feet flat on a wall and knees and hips bent at a 90-degree angle. 2. Place a 4-6 inch ball between your knees. 3. Place your right arm above your head and a balloon in your left hand. 4. Inhale through your nose and as you exhale through your mouth, perform a pelvic tilt so that your tailbone is raised slightly off the mat. Keep low back flat on the mat. Do not press your feet into the wall, instead pull down with your heels. 5. You should feel the back of your thighs and inner thighs engage, keeping pressure on the ball. Maintain this position for the remainder of the exercise. 6. Now inhale through your nose and slowly blow out into the balloon. 7. Pause three seconds with your tongue positioned on the roof of your mouth to prevent airflow out of the balloon. 8. Without pinching the neck of the balloon and keeping your tongue on the roof of your mouth, inhale again through your nose. 9. Slowly blow out as you stabilize the balloon with your left hand. 10. Do not strain your neck or cheeks as you blow. 11. After the fourth breath in, pinch the balloon neck and remove it from your mouth. Let the air out of the balloon.12. Relax and repeat the sequence 4 more times. Copyright © Postural Restoration Institute™ 2009, used with permission

The patient/athlete is asked to hold the balloon with one hand and inhale through his/her nose with the tongue on the roof of the mouth (normal rest position) and then exhale through his/her mouth into the balloon. The inhalation, to about 75% of maximum, is typically 3-4 seconds in duration, and the complete exhalation is usually 5-8 seconds long followed by a 2-3 second pause. This slowed breathing is thought to further relax the neuromuscular system/parasympathetic nervous system and generally decrease resting muscle tone. Ideally the patient/athlete will be able to inhale again without pinching off the balloon with their teeth, lips, or fingertips. This requires maintenance of intra-abdominal pressure to allow inhalation through the nose without the air coming back out of the balloon and into the mouth.

When the exercise is performed by the patient/athlete with hamstring and gluteus maximus (glut max) activation (hip extensors) the pelvis moves into a relative posterior pelvic tilt and the ribs into relative depression and internal rotation. This pelvic and rib position helps to optimize abdominal length (decreases) and diaphragm length/ZOA (increases).

Clinical experience with the BBE includes utilization of the exercise for both female and male patients (more females than males), ages 5-89 with a wide variety of diagnoses including: low back pain, trochanteric bursitis, SIJ pain, asthma, COPD, acetabular labral tear, anterior knee pain, thoracic outlet syndrome (TOS) and sciatica.

Carbon dioxide and the critically ill—too little of a good thing?

Omfattende studie av alle de gode egenskapene ved hyperkapni – høyt CO2 nivå. Nevner mange interessante ting, bl.a. at CO2 indusert acidose gir mye mindre fire radikaler enn om pH senkes av andre faktorer. Bekrefter også at oksygen blir sittende fast på blodcellene ved hypokapni, og at melkesyreproduksjonen begrensens når acidosen er pga CO2 men ikke når den er av andre faktorer.

Spesielt med denne artikkelen er at den beskriver forskjellene på en hyperkapni acidose og acidose av andre faktorer. Hyperkapnisk acidose har beskyttende egenskaper.

Permissive hypercapnia (acceptance of raised concentrations of carbon dioxide in mechanically ventilated patients) may be associated with increased survival as a result of less ventilator-associated lung injury.
Accumulating clinical and basic scientific evidence points to an active role for carbon dioxide in organ injury, in which raised concentrations of carbon dioxide are protective, and low concentrations are injurious.
Although hypercapnic acidosis may indicate tissue dysoxia and predict adverse outcome, it is not necessarily harmful per se. In fact, it may be beneficial. There is increasing evidence that respiratory (and metabolic) acidosis can exert protective effects on tissue injury, and furthermore, that hypocapnia may be deleterious.
If hypoventilation is allowed in an effort to limit lung stretch, carbon dioxide tension increases. Such “permissive hypercapnia” may be associated with increased survival in acute respiratory distress syndrome (ARDS);2 this association is supported by outcome data from a 10-year study.3
Furthermore, hypocapnia shifts the oxyhaemoglobin dissociation curve leftwards, restricting oxygen off-loading at the tissue level; local oxygen delivery may be further impaired by hypocapnia-induced vasoconstriction.
Brain homogenates develop far fewer free radicals and less lipid peroxidation when pH is lowered by carbon dioxide than when it is lowered by hydrochloric acid.19
Finally, greater inhibition of tissue lactate production occurs when lowered pH is due to carbon dioxide than when it is due to hydrochloric acid.20
An association between hypoventilation, hypercapnia, and improved outcome has been established in human beings.2521 In lambs, ischaemic myocardium recovers better in the presence of hypercapnic acidosis than metabolic acidosis.22 Hypercapnic acidosis has also been shown to protect ferret hearts against ischaemia,23 rat brain against ischaemic stroke,16 and rabbit lung against ischaemia-reperfusion injury.24 Hypercapnia attenuates oxygen-induced retinal vascularisation,25 and improves retinal cellular oxygenation in rats.26 “pH-stat” management of blood gases during cardiopulmonary bypass, involving administration of large amounts of additional carbon dioxide for maintenance of temperature-corrected PaCO2, results in better neurological and cardiac outcome.27
Hypercapnia results in a complex interaction between altered cardiac output, hypoxic pulmonary vasoconstriction, and intrapulmonary shunt, with a net increase in PaO2 (figure).28 Because hypercapnia increases cardiac output, oxygen delivery is increased throughout the body.28 Regional, including mesenteric, blood flow is also increased,29 thereby increasing oxygen delivery to organs. Because hypercapnia (and acidosis) shifts the haemoglobin-oxygen dissociation curve rightwards, and may increase packed-cell volume,30 oxygen delivery to tissues is further increased. Acidosis may reduce cellular respiration and oxygen consumption,31 which may further benefit an imbalance between supply and demand, in addition to greater oxygen delivery. One hypothesis32 is that acidosis protects against continued production of further organic acids (by a negative feedback loop) in tissues, providing a mechanism of cellular metabolic shutdown at times of nutrient shortage—eg, ischaemia.
Acidosis attenuates the following inflammatory processes (figure): leucocyte superoxide formation,33 neuronal apoptosis,34phospholipase A2 activity,35 expression of cell adhesion molecules,36 and neutrophil Na+/H+ exchange.37 In addition, xanthine oxidase (which has a key role in reperfusion injury) is inhibited by hypercapnic acidosis.24 Furthermore, hypercapnia upregulates pulmonary nitric oxide38 and neuronal cyclic nucleotide production,39 both of which are protective in organ injury. Oxygen-derived free radicals are central to the pathogenesis of many types of acute lung injury, and in tissue homogenates, hypercapnia attenuates production of free radicals and decreases lipid peroxidation.19 Thus, during inflammatory responses, hypercapnia or acidosis may tilt the balance towards cell salvage at the tissue level.
However, we know from several case series that human beings, and animals, can tolerate exceptionally high concentrations of carbon dioxide, and when adequately ventilated, can recover rapidly and completely. Therefore, high concentrations (if tolerated) may not necessarily cause harm.
From the published studies reviewed, and from the pathological mechanisms assessed, we postulate that changes in carbon dioxide concentration might affect acute inflammation,33—36 tissue ischaemia,16 ischaemia-reperfusion,2024 and other metabolic,1221,32 or developmental14 processes.
We argue that the recent shift in thinking about hypercapnia must now be extended to therapeutic use of carbon dioxide. Our understanding of the biology of disorders in which hypocapnia is a cardinal element would require fundamental reappraisal if hypocapnia is shown to be independently harmful.
In summary, in critically ill patients, future therapeutic goals involving PaCO2 might be expressed as:“keep the PaCO2 high; if necessary, make it high; and above all, prevent it from being low”.

Cardiovascular and Respiratory Effect of Yogic Slow Breathing in the Yoga Beginner: What Is the Best Approach?

Svært spennende studie ang pustens påvirkning på vagusnerven, som bekrefter Breathing System sin Autonome pust, 5 sek inn og 5 sek ut, altså 6 pust i minuttet.

Nevner hvordan en usymmetrisk pust, f.eks. 3 inn og 7 ut, ikke påvirker vagusnerven i særlig stor grad. Og at ujjayi påvirker vagusnerven dårligere enn uanstrengt sakte pust. Ujjiayi pust har andre positivie effekter.

Nevner også at CO2 synker fra 36 til 30 mmHg når man puster 5/5 i forhold til når man ikke gjør pusteteknikk (spontan pust), men synker til 26 mmHg når man puster 15 pust i minuttet. Selv med 7s utpust synker CO2 ned til 31 mmHg. Dette er motsatt av hva studien på CO2 hos angstpasienter viser, hvor CO2 øker selv når pustefrekvensen senkes fra 15 til 12, og øker mer jo saktere pustefrekvensen er.

Nevner også noe svært interessant om at små endinger i oksygenmetning kan gi store endringer oksygentrykket pga bohr-effekt kurven som flater veldig ut ved 98% slik at en 0.5% økning i oksygenmetning kan likevel gir 30% økning i oksygentrykket.

The slow breathing with equal inspiration and expiration seems the best technique for improving baroreflex sensitivity in yoga-naive subjects. The effects of ujjayi seems dependent on increased intrathoracic pressure that requires greater effort than normal slow breathing.

Respiratory research documents that reduced breathing rate, hovering around 5-6 breaths per minute in the average adult, can increase vagal activation leading to reduction in sympathetic activation, increased cardiac-vagal baroreflex sensitivity (BRS), and increased parasympathetic activation all of which correlated with mental and physical health [14]. BRS is a measure of the heart’s capacity to efficiently alter and regulate blood pressure in accordance with the requirements of a given situation. A high degree of BRS is thus a good marker of cardiac health [5].

The slow breathing-induced increase in BRS could be due to the increased tidal volume that stimulates the Hering-Breuer reflex, an inhibitory reflex triggered by stretch receptors in the lungs that feed to the vagus [6]. In addition, the slow breathing increases the oxygen absorption that follows greater tidal volume , as a result of reduction in the effects of anatomical and physiological dead space [78]. This might in turn produce another positive effect, that is, a reduction in the need of breathing. Indeed, a reduction in chemoreflex sensitivity and, via their reciprocal relationships, an increase in BRS, have been documented with slow breathing [913].

 pustmønster CO2
In comparison to spontaneous breathing, fast breathing led to a reduction in BRS, whilst all slow breathing (with or without ujjayi breathing) increased BRS. This increase was seen in both the symmetrical (5 second inspiration and expiration) and asymmetrical (3 second inspiration and 7 second expiration) slow breathing conditions. Engaging ujjayi breathing on the exhalation had the effect of reducing the increase in BRS of slow breathing alone, and this was further reduced with ujjayi on the inspiration and expiration (which was not significantly higher than baseline). These differences were even more pronounced with respect to controlled breathing at 15 breath/minute, which also showed highly significant differences with respect to spontaneous breathing, but in the opposite direction.
When slow breathing was done in conjunction with ujjayi breathing, oxygen saturation further increased, though only slightly. Overall, however, this was a highly significant change given that baseline oxygen saturation was already high approximately 98.3% (Table 3).
However, with 15 breath/minute controlled breathing the increase in oxygen saturation occurred with a large relative increase in Ve and a marked drop in end-tidal carbon dioxide. Conversely, with slow breathing, the increase in oxygen saturation occurred with only a moderate increase in Ve and drop in carbon dioxide.
The greatest improvement was found in slow breathing without ujjayi, while breathing controlled at a rate of 15/min caused a drop in BRS. In all forms of slow breathing there was a statistically significant increase in oxygen saturation from the mean baseline of 98.3%, confirming the relationship between high levels of oxygen absorption and BRS.
In this study, we show that slow breathing and increased oxygen absorption lead to enhanced BRS. This might result from several possible factors, all interrelated. In theory, the increase in arterial oxygen partial pressure increases blood pressure, which in turn could stimulate the baroreceptors and improve the BRS gain. This was recently observed in healthy [28] and diabetic subjects [25]. The seemingly small extent of the increase in oxygen saturation should not be overlooked. In fact, the haemoglobin dissociation curves states that at higher saturation values small changes reflex large changes in the partial pressure of oxygen.
Because the oxygen tension (and not oxygen saturation) is the chemoreflex input signal, this explains why in a previous study the administration of oxygen in normoxia induced a significant increase in BRS and parasympathetic activity despite a small increase in oxygen saturation [25].
We did not find any significant difference between asymmetrical and symmetrical breathing during slow breathing. We suggest that most of these results could be due to the prolonged expiratory time (in fact the 3-second inspiratory time of the asymmetrical breathing was very close to the spontaneous breathing). In the yoga tradition several degrees of asymmetries were adopted. While some of these could have specific effects (and could be matter for further investigations), our results suggest that an expiratory time of at least 5 seconds was sufficient to elicit most of the results observed.
Based on our findings, slow breathing with similar inspiration and expiration times appears the most effective and simple way to heighten the BRS and improve oxygenation in normoxia. Ujjayi breath demonstrates limited added benefit over slow breathing done at 6/min in normoxia; however, the effects could be more pronounced in hypoxia, and this could be matter for future investigations.


Bench-to-bedside review: Carbon dioxide

Om CO2 i helbredelse av vev. Viktig oversikt som nevner CO2 sin bane og effekt gjennom hele organismen – fra DNA til celle til vev til blod. Bekrefter ALT jeg har funnet om CO2 og pusteknikkene. Nevner også potensiell farer, som kun skjer ved akutt hypercapnia. Nevner også en meget spennende konsept om å buffre CO2 acidose med bikarbonat (Natron). I kliniske tilfeller på sykehus kan det ha negative effekter, men hos normale mennesker vil det virke som en effektiv buffer.

Hypercapnia may play a beneficial role in the pathogenesis of inflammation and tissue injury, but may hinder the host response to sepsis and reduce repair. In contrast, hypocapnia may be a pathogenic entity in the setting of critical illness.

For practical purposes, PaCO2 reflects the rate of CO2 elimination.

The commonest reason for hypercapnia in ventilated patients is a reduced tidal volume (VT); this situation is termed permissive hypercapnia.

High VT causes, or potentiates, lung injury [4]. Smaller VT often leads to elevated PaCO2, termed permissive hypercapnia, and is associated with better survival [5,6]. These low-VT strategies are not confined to patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS); they were first reported successful in severe asthma [7], and attest to the overall safety of hypercapnia. Indeed, hypercapnia in the presence of higher VT may independently improve survival [8].

Hypocapnia is common in several diseases (Table ​(Table1;1; for example, early asthma, high-altitude pulmonary edema, lung injury), is a common acid-base disturbance and a criterion for systemic inflammatory response syndrome [9], and is a prognostic marker of adverse outcome in diabetic ketoacidosis [10]. Hypocapnia – often prolonged – remains common in the management of adult [11] and pediatric [12] acute brain injury.

Table 1

Causes of hypocapnia

Hypoxemia Altitude, pulmonary disease
Pulmonary disorders Pneumonia, interstitial pneumonitis, fibrosis, edema, pulmonary emboli, vascular disease, bronchial asthma, pneumothorax
Cardiovascular system disorders Congestive heart failure, hypotension
Metabolic disorders Acidosis (diabetic, renal, lactic), hepatic failure
Central nervous system disorders Psychogenic/anxiety hyperventilation, central nervous system infection, central nervous system tumors
Drug induced Salicylates, methylxanthines, β-adrenergic agonists, progesterone
Miscellaneous Fever, sepsis, pain, pregnancy

CO2 is carried in the blood as HCO3-, in combination with hemoglobin and plasma proteins, and in solution. Inside the cell, CO2 interacts with H2O to produce carbonic acid (H2CO3), which is in equilibrium with H+ and HCO3-, a reaction catalyzed by carbonic anhydrase. CO2 transport into cells is complex, and passive diffusion, specific transporters and rhesus proteins may all be involved.

CO2 is sensed in central and peripheral neurons. Changes in CO2 and H+ are sensed in chemosensitive neurons in the carotid body and in the hindbrain [13,14]. Whether CO2 or the pH are preferentially sensed is unclear, but the ventilatory response to hypercapnic acidosis (HCA) exceeds that of an equivalent degree of metabolic acidosis [15], suggesting specific CO2 sensing.

An in vitro study has demonstrated that elevated CO2 levels suppress expression of TNF and other cytokines by pulmonary artery endothelial cells via suppression of NF-κB activation [18].

Furthermore, hypercapnia inhibits pulmonary epithelial wound repair also via an NF-κB mechanism [19].

The physiologic effects of CO2 are diverse and incompletely understood, with direct effects often counterbalanced by indirect effects.

Hypocapnia can worsen ventilation-perfusion matching and gas exchange in the lung via a number of mechanisms, including bronchoconstriction [21], reduction in collateral ventilation [22], reduction in parenchymal compliance [23], and attenuation of hypoxic pulmonary vasoconstriction and increased intrapulmonary shunting [24].

CO2 stimulates ventilation (see above). Peripheral chemoreceptors respond more rapidly than the central neurons, but central chemosensors make a larger contribution to stimulating ventilation. CO2increases cerebral blood flow (CBF) by 1 to 2 ml/100 g/minute per 1 mmHg in PaCO2[25], an effect mediated by pH rather than by the partial pressure of CO2.

Hypercapnia elevates both the partial pressure of O2 in the blood and CBF, and reducing PaCO2 to 20 to 25 mmHg decreases CBF by 40 to 50% [26]. The effect of CO2 on CBF is far larger than its effect on the cerebral blood volume. During sustained hypocapnia, CBF recovers to within 10% baseline by 4 hours; and because lowered HCO3-returns the pH towards normal, abrupt normalization of CO2 results in (net) alkalemia and risks rebound hyperemia.

Hypocapnia increases both neuronal excitability and excitatory (glutamatergic) synaptic transmission, and suppresses GABA-A-mediated inhibition, resulting in increased O2 consumption and uncoupling of metabolism to CBF [27].

Hypercapnia directly inhibits cardiac and vascular muscle contractility, effects that are counterbalanced by sympathoadrenal increases in heart rate and contractility, increasing the cardiac output overall [28].

Indeed, a large body of evidence now attests to the ability of hypercapnia to increase peripheral tissue oxygenation, independently of its effects on cardiac output [30,31].

The beneficial effects of HCA in such models are increasingly well understood, and include attenuation of lung neutrophil recruitment, pulmonary and systemic cytokine concentrations, cell apoptosis, and O2-derived and nitrogen-derived free radical injury.

Concern has been raised regarding the potential for the anti-inflammatory effects of HCA to impair the host response to infection. In early pulmonary infection, this potential impairment does not appear to occur, with HCA reducing the severity of acute-severe Escherichia coli pneumonia-induced ALI [41]. In the setting of more established E. coli pneumonia, HCA is also protective [42].

Hypocapnia increases microvascular permeability and impairs alveolar fluid reabsorption in the isolated rat lung, due to an associated decrease in Na/K-ATPase activity [47].

HCA protects the heart following ischemia-reperfusion injury.

Hypercapnia attenuates hypoxic-ischemic brain injury in the immature rat [52] and protects the porcine brain from reoxygenation injury by attenuation of free radical action. Hypercapnia increases the size of the region at risk of infarction in experimental acute focal ischemia; in hypoxic-ischemic injury in the immature rat brain, hypocapnia worsens the histologic magnitude of stroke [52] and is associated with a decrease in CBF to the hypoxia-injured brain as well as disturbance of glucose utilization and phosphate reserves.

Indeed, hypocapnia may be directly neurotoxic, through increased incorporation of choline into membrane phospholipids [56].

Rapid induction of hypercapnia in the critically ill patient may have adverse effects. Acute hypercapnia impairs myocardial function.

In patients managed with protective ventilation strategies, buffering of the acidosis induced by hypercapnia remains a common – albeit controversial – clinical practice.
While bicarbonate may correct the arterial pH, it may worsen an intracellular acidosis because the CO2 produced when bicarbonate reacts with metabolic acids diffuses readily across cell membranes, whereas bicarbonate cannot.

Hypocapnia is an underappreciated phenomenon in the critically ill patient, and is potentially deleterious, particularly when severe or prolonged. Hypocapnia should be avoided except in specific clinical situations; when induced, hypercapnic acidosis should be for specific indications while definitive measures are undertaken.

Human skeletal muscle intracellular oxygenation: the impact of ambient oxygen availability

Viktig studie som beskriver alt om hvordan oksygen-nivået synker fra innpust gjennom blodkar og ut til celler, og relasjonen til trening hvor cellene ikke mottar oksygenet pga lav CO2 som følge av hyperventillering.

Intracellular oxygen (O2) availability and the impact of ambient hypoxia have far reaching ramifications in terms of cell signalling and homeostasis; however, in vivo cellular oxygenation has been an elusive variable to assess.

These data are the first to document resting intracellular oxygenation in human skeletal muscle, highlighting the relatively high PiO2 values that contrast markedly with those previously recorded during exercise (∼2–5 mmHg). Additionally, the impact of ambient hypoxia on PiO2 and the relationship between changes in SaO2 and PiO2 stress the importance of the O2 cascade from air to cell that ultimately effects O2 availability and O2 sensing at the cellular level.

Changes in intracellular oxygen availability have far reaching consequences likely involved in such diverse processes as angiogenesis (Richardson et al. 1999c; Wagner, 2001) and hypoxic pulmonary vasoconstriction (Wang et al. 2005; Wolin et al. 2005).

Therefore, although it is known that musclePiO2 falls to very low values of 2–5 mmHg during exercising (Mole et al. 1999;Richardson et al. 2001), the starting point for skeletal muscle oxygenation or resting PiO2is, as of yet, unknown.

Hypoxia is both an important stimulus and a constant threat to the human body and its vital organs throughout life. Environmental changes such as exposure to high altitude reduce ambient O2 availability, while lung, vascular, and sleep disorders can result in hypoxia under normoxic conditions. It is known that hypoxia mediates adaptive changes in metabolism, O2 sensing and gene expression. However, although much research has examined the consequences of experimental hypoxic conditions, data documenting hypoxically mediated changes in cellular oxygenation in humans are sparse, if not non-existent.

Specifically, it was determined that in normoxia Mb was 9 ± 1% deoxygenated and this increased to 13 ± 3% in hypoxia. In our view, any degree of Mb deoxygenation supports the role of Mb as a facilitator of O2 diffusion, and thus the observation that Mb is somewhat desaturated in normoxia and furthermore that Mb desaturation increases in hypoxia is consistent with Mb playing a significant role in O2 transport from blood to cell.

Theoretically, because of the O2 cascade from air to tissue, graded reductions in FIO2 should ultimately alter in vivo O2 availability all the way to the myocyte (Richardson et al. 1995b).

In fact, this hypoxic ventilatory response (HVR) varies widely between individuals, and has been used to distinguish between those who will thrive and those who will perish at high altitude (Bartsch et al. 2001).

50% of the variance in PiO2 could be explained by the change in arterial PO2 (Fig. 4). Hence, the fall in skeletal muscle PiO2was attenuated in those subjects with a brisk HVR, making teleological sense and providing perhaps the first evidence, through arterial O2 saturation, of the importance of human HVR in terms of cellular O2 homeostasis.

Despite an apparently strong HVR in some subjects, the ambient hypoxia of 10% O2significantly reduced the average intracellular Mb saturation by ∼44% and calculated PiO2by ∼33%. Although the complete ramifications of such a change within resting muscle cells are unknown (cell signalling and growth factor responses) it is clear that such a perturbation, although relatively large, still leaves the cells far above the suggested ‘critical PO2’ (between 0.1 and 0.5 mmHg) below which muscle metabolism is compromised (Chance & Quistorff, 1978; Wilson et al. 1979; Richmond et al. 1999).

Taken together these data reinforce the concept that O2 availability and metabolism are more tightly coupled during exercise when PiO2 falls to low levels than at rest when there is a relative abundance of O2.

Specifically, in the current study a reduction in the ambient O2 to 10% resulted in an ∼11 mmHg change in PiO2 at rest (from 34 to 23 mmHg), whereas in previous investigations during exercise (using 12% O2) we have repeatedly seen closer to a ∼1 mmHg reduction (from 3 to 2 mmHg) (Richardson et al.1995b, 1999b, 2002).

This phenomenon may occur as a result of the mitochondrial transition from a somewhat quiescent state during rest to an active more governing role, in terms of determining PiO2, during exercise. Therefore, these data support the theory that during a hypoxic challenge resting PiO2 is most likely the simple consequence of ambient hypoxia upon passive diffusion, while during exercise the large increase in metabolic rate and subsequent O2consumption reduce PiO2 and facilitate O2 transport to a greater extent, somewhat staving off the effect of ambient hypoxia.

Inspiratory muscle training reduces blood lactate concentration during volitional hyperpnoea

Med pustetrening blir melkesyrekonsentrasjonen lavere under trening. 2 studier her, første fra 2008 og den andre fra 2012.

Den første nevner at melkesyre synker med opptil 59% (25+34%)

Although reduced blood lactate concentrations ([lac(-)](B)) have been observed during whole-body exercise following inspiratory muscle training (IMT), it remains unknown whether the inspiratory muscles are the source of at least part of this reduction.

After 6 weeks, increases in [lac(-)](B) during volitional hyperpnoea were unchanged in the control group. Conversely, following IMT the increase in [lac(-)](B) during volitional hyperpnoea was reduced by 17 +/- 37% and 25 +/- 34% following 8 and 10 min, respectively (P < 0.05).

These findings suggest that the inspiratory muscles were the source of at least part of this reduction, and provide a possible explanation for some of the IMT-mediated reductions in [lac(-)](B), often observed during whole-body exercise.

Inspiratory muscle training abolishes the blood lactate increase associated with volitional hyperpnoea superimposed on exercise and accelerates lactate and oxygen uptake kinetics at the onset of exercise.

Den andre viser til en 15% laver melkesyrekonsentrasjon og at årsaken er pustemusklenes evne til å fjerne det.

Following the intervention, maximal inspiratory mouth pressure increased 19% in the IMT group only (P < 0.01). Following IMT only, the increase in [lac(-)](B) during volitional hyperpnoea was abolished (P < 0.05). In addition, the blood lactate (-28%) and phase II oxygen uptake (-31%) kinetics time constants at the onset of exercise and the MLSS [lac(-)](B) (-15%) were reduced (P < 0.05). We attribute these changes to an IMT-mediated increase in the oxidative and/or lactate transport capacity of the inspiratory muscles.

Inspiratory muscle training lowers the oxygen cost of voluntary hyperpnea

Nevner at innpustmuskel trening gir mindre oksygenbehov under trening og dermed mer utholdenhet. Innpustmuskler bruker opp mye av oksygenet kroppen trenger under trening så med svak pustefunksjon blir man fort sliten. Under maksimal trening krever pustemusklene 15% av oksygenet, men med pustetrening synker det til 8%. Den nevner at diafragma og pustemuskler blir sterkere og større. Den henviser også til studier som nevner at det gir mindre melkesyre. Noe av effekten kommer også av at man får en større reserve i lungene ved å øke inn- og utpust styrken.

IMT significantly reduced the O2 cost of voluntary hyperpnea, which suggests that a reduction in the O2 requirement of the respiratory muscles following a period of IMT may facilitate increased O2 availability to the active muscles during exercise. These data suggest that IMT may reduce the O2cost of ventilation during exercise, providing an insight into mechanism(s) underpinning the reported improvements in whole body endurance performance; however, this awaits further investigation.

THE OXYGEN COST of breathing or energy requirement of the respiratory muscles are shown to increase relative to the level of ventilation (V̇E) and the work of breathing (Wb) (1, 8). During moderate-intensity exercise the respiratory musculature requires ∼3–6% of total oxygen consumption (V̇O2T), increasing to ∼10–15% at maximal exercise (1, 3).

Inspiratory muscle training (IMT) is an intervention that has been associated with improvements in whole body exercise performance (24, 31, 34), enhanced pulmonary oxygen uptake kinetics (5), reduced blood lactate concentrations (6, 24), diaphragmatic fatigue, and cardiovascular responsiveness (37).

The oxygen cost of voluntary hyperpnea (V̇O2RM) and V̇O2RM expressed as a percentage of total oxygen consumption (V̇O2T) graphed against V̇E at low (50% V̇O2 max), moderate (75% V̇O2 max), and high (100% V̇O2 max) exercise intensities for both IMT (A) and CON (B) groups, pre- and post-training (means ± SE).
•, Pre-IMT;
○, post-IMT;
▴, pre-CON;
Δ, post-CON.

To our knowledge this study is the first to investigate the influence of IMT on the oxygen cost of voluntary hyperpnea. The main findings of the present study are that the relationship between increasing ventilatory workloads and the O2 cost of voluntary hyperpnea is curvilinear in trained cyclists and that 6 wk of pressure threshold IMT significantly reduced the O2 cost of V̇E at high ventilatory workloads. Importantly, the finding that V̇O2RM is reduced at a V̇E above 50% V̇O2 max suggests that IMT may reduce the energy requirements of the respiratory musculature in maintaining a given V̇E.

The increase in energy expenditure as V̇E increases can be attributed to a variety of sources of respiratory muscle work, including the elastic recoil of the chest and lung wall, airway resistance (4,15), increased EELV (9), and high muscle shortening velocities (19, 23). It has been suggested that as tidal breathing approaches the maximal limits for inspiratory muscle pressure development and expiratory flow rates, energy expenditure may increase to overcome the additional respiratory muscle work (3). Conversely, if one or more of the additional sources of respiratory muscle work are reduced as a result of IMT, it is reasonable to suggest that the increase in the O2 cost maybe attenuated.

In the present study, following 6 wk of IMT, V̇O2RM was significantly reduced from pretraining values at submaximal and maximal levels of ventilation. The O2 cost of voluntary hyperpnea expressed as a percentage of V̇O2T was reduced by 1.5% at a V̇Ecorresponding to 75% V̇O2max following IMT. The greatest reduction in the O2 cost of voluntary hyperpnea was observed at V̇O2 max, where V̇O2RM was significantly reduced from 11% of V̇O2T to 8% V̇O2T following IMT.

Increased ventilatory demand was previously shown to elicit a sympathetically mediated metaboreflex (33), which increases heart rate and mean arterial pressure (MAP), reducing blood flow to the limb locomotor muscles during exercise (16) and potentially reducing whole body endurance performance (18). Furthermore, Witt et al. (37) showed that IMT attenuates this increase in HR and MAP, presumably by reducing or delaying the sympathetically mediated reflex.

The 22% increase in respiratory muscle strength shown in the present study is similar in magnitude to those previously reported using pressure-threshold IMT (11, 22, 30, 32, 37). Respiratory muscle structure has also been shown to change following IMT, with an increase in diaphragm thickness (11, 12) and hypertrophy of type II muscle fibers of the external intercostal muscles (27) being reported.

Aaron et al. (3) demonstrated that individuals who reached their reserve for expiratory flow and inspiratory muscle pressure development required 13–15% of V̇O2T compared with ∼10% of V̇O2T for non-flow-limited individuals. Thus, an increase in maximal expiratory flow rates or inspiratory pressure development would increase the ventilatory reserve, thereby increasing the maximal limits for ventilation.