Traditionally, BP measurements report two basic pressures (Systolic pressure and Diastolic pressure). This article explains the reason why diastolic pressure should be actually called as ‘lowest or baseline systolic pressure.’
Whenever BP is measured, external pressure (cuff-induced) combines with baseline arterial pressure to challenge the heart to pump the blood beyond the artificially vasoconstricted region. For example, when brachial artery is artificially vasoconstricted to greater extent, the heart can’t pump blood beyond that pressurized portion till the pressure is relieved to a level, so that heart can exhibit its maximum strength to work against this elevated resistance. At one point, when the examiner drops the pressure in the cuff, heart sounds start to appear and this point can be called ‘highest systolic pressure.’ This point indicates the maximum resistance against which that individual’s heart can work.
Next, when gradual drop in the cuff pressure takes place, at a point, the heart sounds start to disappear and this point can be called as ‘lowest systolic pressure.’ The place where the heart sounds disappear allows the artery to return back to their original dimension and offer its own resistance only. This point indicates the baseline pressure (no cuff-induced pressure) against which the heart has to work.
Let us look at two definitions for diastole of the heart.
Definition 1: Cardiac diastole is the period of time when the heart relaxes after contraction in preparation for refilling with circulating blood.
Definition 2: Diastole is that phase of the cardiac cycle when the heart is not contracting to propel blood out to the body, brain and lungs but instead is relaxed and filling with incoming blood.
These two definitions are to be correlated with the two formulae.
Formula 1: Blood pressure = Cardiac output x Peripheral vascular resistance
Formula 2: Blood flow = Pressure / Resistance
We have to combine the definitions and formulae to understand the following;
Diastole is the phase where cardiac output becomes zero. So, during diastole, the blood pressure caused by blood flow also becomes zero.
BP (caused by blood flow during diastole) = 0 x Peripheral vascular resistance
= 0 mm Hg
In fact, the pressure in the arteries doesn’t fall to zero during diastole (because some hydrostatic pressure still exists during diastole) but the pressure caused by flow of blood pumped by heart becomes zero.
Since both the extremes of the blood pressure are measured during systolic phase (blood pumping phase) of heart, the term diastolic pressure doesn’t match the actual physiologic scenario of circulation and can lead to misconception. Finally, the traditionally expressed ‘diastolic pressure’ is actually the baseline pressure against which cardiac systole happens. Baseline systolic pressure is the apt term to be used to indicate the lowest pressure detected during BP measurement. For example, if your BP tests for a client (at rest) reveals 120 mm Hg/ 70 mm Hg, then;
(a) 120 mm Hg is the highest resistance (highest systolic pressure) against which his heart can pump the blood.
(b) 70 mm Hg is the lowest resistance (lowest systolic pressure or baseline systolic pressure) against which his heart is working or has to work.
However consensus must be developed for the replacement of ‘diastolic pressure’ as ‘baseline systolic pressure.’ Physicians and health care professionals are actually very much concerned about the baseline BP or baseline resistance against which the heart functions and help stabilizing the abnormalities if any.