How do pulse pressure and mean arterial pressure differ, and how are they related?
Mean arterial pressure (MAP)
= 1/3*(systolic blood pressure) + 2/3*(diastolic blood pressure)
= cardiac output [heart rate*stroke volume] * systemic vascular resistance
= ¼ diastolic blood pressure + 0.4*(pulse pressure).
MAP is an important way to measure blood pressure; MAP is what we use to titrate pressors and measure perfusion. However, consider the case of person #1 with a BP 130/80 and person #2 with a BP 160/60. Both of them have a MAP of 100 mm Hg by the first calculation, but clearly, the blood pressures are different–what is different is the originating pressure from the left ventricle, and pulse pressure correlates better with this. This chapter explains the factors that contribute to pulse pressure elegantly, but the simple equation is:
Pulse pressure (PP) = systolic blood pressure – diastolic blood pressure
What is a “high” PP and what is a “low” PP, and what do they signify?
There is no specific cut-off that I could find. However, if you think about 120/80 as a “normal” blood pressure, that indicates that a “normal” PP=40. The study cited below uses a cutoff of <30 mm Hg as a “low” PP. Therefore, it’s reasonable to think that a “high” PP is >50 and a “low” PP is <30.
Traditionally, we have been warned against high pulse pressure. Why? High PP is associated with increased cardiovascular death, as well as CAD, MI, and heart failure, because it signifies either/and a high systolic blood pressure and low diastolic blood pressure. In addition to heart failure, other conditions associated with a high PP include severe anemia, sepsis, thyrotoxicosis, aortic dissection, aortic regurgitation, neurological conditions (i.e. hemorrhage) or AVMs (things that might cause a high output heart failure state).
However, low PP can also be dangerous. If you think about it, someone with a systolic and diastolic blood pressure that are almost the same is in a “low flow” state; their blood pressure demonstrates they can’t push blood forward effectively. This study is one report about an association between low PP and cardiovascular death in patients with heart failure, specifically, advanced heart failure (NYHA Class III-IV). In addition to advanced heart failure, other conditions associated with a low PP include cardiogenic shock, tamponade, and severe aortic stenosis.
Bonus question: what is “true MAP” and how is it calculated? “True MAP” is direct measurement of pulsatile flow…think about that…it’s measured through tonometry, usually of the brachial artery, which involves placement of an arterial line.