Blood pressure debunked
By Mike McEvoy
A common myth is that indirect monitoring equipment like a sphygmomanometer (blood pressure cuff) actually measures blood pressure. It doesn’t. Indirect monitors measure flow, not pressure.
Direct monitors such as a catheter placed in an artery (arterial line) measure pressure. Normally, pressure and flow are related, but in hypovolemic and cardiogenic shock states, flow decreases and pressure increases from vasoconstriction.
It stands to reason then, that if we want accurate blood pressure measurements in these shock states, we need to use direct (invasive) monitoring. Hence, our modern day indication for an arterial line has become any patient who appears to be in shock and does not rapidly respond to therapy.
Ejected under tremendous force, blood typically travels from the heart to the capillary beds of body tissues in about ten 10 seconds. Under stressful conditions and during heavy exercise, blood can make the same trip in as little as two seconds. With each contraction of the ventricles, elastic arteries stretch to accommodate the ejected blood.
Recoil of these elastic arteries continually propels blood forward into smaller and smaller vessels, finally reaching the capillaries. Blood return to the heart, through the venous circuit, is powered partially by movement of the leg muscles and partly by negative pressure in the chest produced by breathing.
Physical laws of fluid dynamics tell us that systolic BP increases as blood moves away from the heart, and then falls at the capillary level along with diastolic and mean blood pressure.
These laws of physics are important since BP cuff readings taken in forearms, thighs, and calves will produce higher systolic values than readings on the same patient made closer to the heart. The same is true with arterial line placement: the further away from the heart the catheter is placed, the higher the systolic pressure will be. If you’ve ever used a finger cuff to measure blood pressure, you likely have observed higher- than- expected systolic pressures.