I recently had a test with six blood pressure cuffs; 2 each on arms, ankles and big toes. During the test I could feel the pulses in my arms and legs, and noticed a distinct ~0.2 second delay between the pulse in my arms and ankles.
I used my pulse rate at the time and the fraction of the pulse period between arm and leg to get the 0.2 seconds, and I unscientifically give myself a +/- 20% error bar on that number.
I was at first quite surprised, because the speed of sound in water is roughly 1500 meters per second, and since water and presumably blood is mostly incompressible I couldn't figure out what could cause such a long delay.
Then I realized that arteries must be somewhat elastic, and that the propagation of the pulse must involve the mass/inertia of the blood and the restoring force of the artery, and so this was a biophysics problem of some kind.
It turns out that the results of the test include a report of the propagation velocity which was of the order of 1100 centimeters/second. I'm not sure how that connects with my estimate of 0.2 seconds as it implies a distance of 2.2 meters and there's no way that my elbow is 2.2 meters farther from my heart than my ankle (I'm ~1.7 meters tall).
Questions: How to understand the biophysics and math behind a 0.2 second delay between the pulse in my arm and my ankle?
- Is there an equation that relates the delay between arm and ankle pulse to the elasticity of the arteries?
- How is the velocity calculated from the measure delay in this case?