An experiment: three years ago, I decided to see how low I could get my respiratory rate in one hour. I just counted breaths, so it was obviously just a quick and dirty estimate and the actual rate in the last few minutes would have been much lower than in the beginning.
I don't remember the number, just that the average was between 1 and 2 breaths per minute for the hour.
I used what was basically a modified pranayama technique. I lay in savasana (corpse pose), took slow deep breaths, and gently and comfortably extended the retention after the exhale. Focused on the heartbeat in between breaths, really trying to catch the exact moment when the sensations of the pulsatile wave in the abdominal aorta passed away.
The rationale: increasing the length of time after exhalation increases vagal tone, which decreases the respiratory rate. Parasympathetic nervous system activation also decreases the heart rate, but I didn't have a monitor, sadly.
I will be repeating the experiment in a few weeks if anyone cares to join me. Young, cardiovascularly fit males should in theory have the greatest advantage, as they will have the highest vagal tone. My hypothesis is that my average will increase due to decreased vagal tone because I've been slacking on cardio lately. Or will it decrease because I've got three more years of meditation and pranayama practice? Or stay the same? DIY science will tell us!
How low can you go?