General anaesthesia for horses is a remarkably high risk procedure. So high risk that most clued up anaesthetists will routine place an arterial catheter for direct measurement of arterial blood pressure, usually using an oscilloscope type display of the wave form, even for "short" procedures. Back in my training days I had the pleasure of many such events and, in those where there was not too much to worry us, my extremely well informed supervisor and I would discuss the applied physiology of keeping 500kg of unspeakably valuable equine alive, asleep and immobile. I remember just one brief comment about the double nature of the systolic pulse peak. Polly's comment was that it was something to do with a pressure wave reflection from the hind limbs (we usually had the catheter in the facial artery, up at the front end, as far away from the surgeons as we could get. If you've never been there...
Anyway. The heart contracts, there is a rise of pressure in the peripheral arteries to the systolic peak followed by a decay down to the diastolic plateau before the next beat. Except it's not smooth. A few milliseconds after the systolic peak, part way down the fall to diastole, there is a second peak. Who ordered that?
It turns out that this second peak really is due to a reflected pressure wave. The systolic pressure wave travels directly from the heart to the facial or the radial artery, wherever you have managed to get an arterial line in. The same pressure wave travels off down the aorta until it hits the resistance of the hind legs. Some of the pressure wave carries on, some is reflected back. The reflected portion goes back up the aorta in the wrong direction before spreading out through the arteries at the front end of the patient, those few milliseconds later.
If all horses where exactly the same size the time between the systolic pulse and the second peak would always be the same if the speed of travel of the pressure wave was always the same.
It's not. The speed of the wave is influenced by the stiffness of the aorta. The stiffer the aorta, the faster the wave travels and the shorter the time to that second peak. Also, the stiffer the aorta in a human the more likely that person is to die of cardiovascular disease. The two things are associated.
So, in a human, if you correct for height, the time difference between first and second peaks gives you an index of aortic stiffness. This usually worsens with age so you can then give a crude cardiovascular "age" to a person.
There are several methods for getting aortic pulse wave velocity and, luckily, most of them don't involve you having a catheter placed in an artery!
The easiest and most expensive way is to buy one of these machines, which is a commercialisation of the system described here. This is from the paper and the caption is self explanatory:
By an absolute quirk of fate I was put in touch with someone who owned one of these commercial machines. I was invited to play with it and came out with a cardiovascular age of 32 years. I think I was 51 or 52 at the time, already several years in to LC/high saturated fat eating. The girl who owned the machine was in to "healthy" eating plus arginine supplementation for her own CV issues. The CardioCheck gave her a CV age of about 70 years and she was having frequent palpitations. I was polite enough not to ask her chronological age but it was a sight younger than mine!
I lent her Life Without Bread. The palpitations stopped immediately on LC but it took six months to get her CV age down to her chronological age. I guess it's well below that now.
I don't have the equivalent of $3,999 in pounds sterling for a CardioCheck machine. But I do have access to a doppler ultrasound used for the determination of blood pressure in cats and dogs.
You can HEAR the double peak in the doppler arterial flow. You can also sit your computer next to the loud speaker of the doppler, record the sound output, smooth it electronically and actually measure the time between the peak systolic flow and the smaller second peak. Factor in your height and you have an aortic stiffness index.
I'm consistently equivalent to the low 30's in CV age. I've played with other people of my age who also eat LC and they too have a "young" CV system. Normal diet people come out all over the place, some scarily high.
Well, it's time to look at Bradley and Hunter's latest sugary offering.
EDIT added in response to comments
OK, the formula is simply height in metres divided by time between peaks in seconds. I can't find the graph anywhere on my hard drive so this jpeg is the closest I can come to putting values in to ball park areas.
One 50+ female LC friend has a time between peaks of 320ms and a height of about 1.65m giving an SI of 5.12. That puts her CV age somewhere in her 20s...
I can't convert the Audacity file to anything visual to show you. The estimations are pretty crude but if you check various pulses you get a reasonable idea of a typical waveform.
Relax before you try, be sitting, no crossed legs!