Physiology, Pulse Pressure

Excerpt

Pulse pressure is the difference between systolic and diastolic blood pressures.

1. Pulse Pressure = Systolic Blood Pressure – Diastolic Blood Pressure

The systolic blood pressure is defined as the maximum pressure experienced in the aorta when the heart contracts and ejects blood into the aorta from the left ventricle (approximately 120 mmHg). The diastolic blood pressure is the minimum pressure experienced in the aorta when the heart is relaxing before ejecting blood into the aorta from the left ventricle (approximately 80 mmHg). Normal pulse pressure is, therefore, approximately 40 mmHg.

A change in pulse pressure (delta Pp) is proportional to volume change (delta-V) but inversely proportional to arterial compliance (C):

1. Delta Pp = Delta V/C

Because the change in volume is due to the stroke volume of blood ejected from the left ventricle (SV), we can approximate pulse pressure as:

1. Pp = SV/C

A normal young adult at rest has a stroke volume of approximately 80 mL. Arterial compliance is approximately 2 mL/mm Hg, which confirms that normal pulse pressure is approximately 40 mm Hg.

Arterial compliance is equal to the change in volume (Delta V) over a given change in pressure (Delta P):

1. C = Delta V/Delta P

Because the aorta is the most compliant portion of the human arterial system, the pulse pressure is the lowest. Compliance progressively decreases until it reaches a minimum in the femoral and saphenous arteries, and then it begins to increase again. This concept requires an understanding of the effect of pressure wave reflection on the amplification of aortic pressure and thus pulse pressure. The phenomenon mainly occurs in the lower body, especially the lower extremities where pressure waves reflect back due to vessel branching, and the vessels are less compliant (stiffer) When a reflected wave is in phase with a forward wave, it generates a wave with higher amplitude. An analogy here is waves bouncing off a seawall and interacting with an incoming wave. If they are in phase, the wave height is greater.

A pulse pressure that is less than 25% of the systolic pressure is inappropriately low or narrowed, whereas a pulse pressure of greater than 100 is high or widened.