Influence of breathing frequency on the pattern of respiratory sinus arrhythmia and blood pressure: old questions revisited (2010) – describes how the breathing frequency controls HRV, or RSA (Respiratory Sinusoid Arythmia) as it is called in this article. It mentiones that 6 bpm (breath per minute), or 0.1Hz, gives a 1,8 times higher HRV than 12 bpm. And it confirms that it is the Vagus nerve that controls HRV since betablockers (reducing blood pressure) does not affect HRV. http://ajpheart.physiology.org/content/298/5/H1588

Cardiovascular and Respiratory Effect of Yogic Slow Breathing in the Yoga Beginner: What Is the Best Approach? (2013) – describes how 5-6 bpm gives the best activation of the Vagus nerve, and that its better to use the same length on both in- and outbreath. http://www.hindawi.com/journals/ecam/2013/743504/

Slow Breathing Increases Arterial Baroreflex Sensitivity in Patients With Chronic Heart Failure (2002) – mentiones that 6 bpm gives the best response on the HRV and Vagus nerve, in addition to a significant lowering of the blood pressure. This study showed results after only 4 minutes of the breathing exercise. http://circ.ahajournals.org/content/105/2/143.full

Slow Breathing Improves Arterial Baroreflex Sensitivity and Decreases Blood Pressure in Essential Hypertension (2005) – mentiones how 6 bpm increases HRV and the vagus nerves effect on the heart, and that it is the best way to breathe to lower bloodpressure. http://hyper.ahajournals.org/content/46/4/714.full

Heartbeat Synchronizes With Respiratory Rhythm Only Under Specific Circumstances (2004) – shows that HRV is highest at 5-6 bpm. http://journal.publications.chestnet.org/article.aspx?articleid=1082860

Autonomic system modification in zen practitioners (2013) – says a lot about how breathing frequency affects HRV and other factors. And especially interesting is how it changes the normal breathing in the long run. They confirm that 6 bpm gives the highest HRV. http://www.ncbi.nlm.nih.gov/pubmed/24469560

Breathing at a rate of 5.5 breaths per minute with equal inhalation-to-exhalation ratio increases heart rate variability. (2014) – this one mentions that 5.5bpm gives the best HRV and that inbreath and outbeath should be the same length to get the best results. http://www.ncbi.nlm.nih.gov/pubmed/24380741

Modulatory effects of respiration (2001) – shows that HRV is highest at 5-6 bpm and that is sinks when the breathing frequency gets higher or lower. http://www.sciencedirect.com/science/article/pii/S1566070201002673

Matter Over Mind: A Randomised-Controlled Trial of Single-Session Biofeedback Training on Performance Anxiety and Heart Rate Variability in Musicians (2012) – shows that you get the same results if you breath 6bpm or use biofeedback. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3464298

Heart Rate Variability Biofeedback Increases Baroreflex Gain and Peak Expiratory Flow (2003) – mentions that the HRV training with breathing exercises have long term effects on heart diseases and thet it involves neurplasticity. Confirms that 6bpm increases the vagal tone. Thay also show that you get reultst with only the breathing exercise, but that the vagus nerve responds better with biofeedback. http://www.ncbi.nlm.nih.gov/pubmed/14508023