Blood Pressure Measurement

The heart pumps blood with fairly short strokes and then rests approximately 3/4 second between strokes. Blood pressure measurement procedure is to first close the arteries and then slowly allow them to open. This is done by applying external pressure to an extremity like the upper arm as shown in Canvas 0. The external pressure that just closes the artery, even when the heart-induced pressure is maximum, is called the systolic pressure-this pressure is the same as the minimum pressure needed for a tourniquet. The pressures are measured by pumping air into a cuff that surrounds the cross section of the extremity. As the pressure in the cuff is increased it will reach a value where the pressure monitor will show short, slight surges. These are due to the arteries expanding and contracting during the heart's pump strokes. The pressure in the cuff is increased until the pressure monitor no longer shows these small surges in pressure. After that, the pressure in the cuff is slowly bled off while the pressure is monitored for the small surges. The pressure reading during the bleed off, where the small surges are again first detected is the systolic pressure. This is where the heart's peak pressure is just enough to start to open the artery. These pressure surges are monitored as the bleed off continues and the arteries open farther. When the surges are no longer detected, that pressure is called the diastolic pressure. The reason the surges stop is that the cuff pressure is to longer sufficient to even partially close the arteries at peak heart pressure. The diastolic pressure is greater than zero because, between heart pulses, the blood never has time to flow into the smaller arteries and capillaries during the filling phase of the heart. The flow into these smaller vessels continues driven by the elastic contraction of the larger arteries (see link 1) 1. The walls of the artery are made up of several layers and are actually pretty thick compared to the lumen which is the inner diameter which carries the blood. The walls get even thicker and the artery has less capacity to carry blood when they build up a layer called plaque, often due to high blood sugar content.Reference 2

Pulse Pressure and the Effect of Exercise

The difference between the systolic and distolic pressure is called the pulse pressure. The pulse pressure is proportional to the heart's stroke volume. When the person is at rest and not emotionally agitated, the product of the stroke volume times the heart rate (beats per minute) should be constant because that is the volume per minute of oxygenated blood that the person needs to keep him alive. When the person performs mechanical work (exercises), this flow rate (volume per minute) has to increase to support the muscle cells. Since the heart volume can't increase much, the heart beats per minute (bpm) has to increase. Also, since the arteries and the capillaries still offer the same resistance to blood flow, the pulse pressure has to increase to support the increased blood volume per minute so that higher bpm causes increased pulse pressure. Reference 3 There is an equation that shows the relation between pulse pressure and stroke volume: Reference 4

`"Pulse Pressure"=(Stroke Volume)/(Arterial Compliance)`

The artery compliance is a measure of how easily the large arteries stretch (comply, usually radially) so as to accommodate a larger volume of blood when the heart places pressure on them. The typical values for younger adults are artery compliance of 2 milliliters per mm Hg while stroke volume is typically 80 milliliters which results in a pulse pressure of 40 mm Hg which is also the average. As discussed in Reference 3, during exercise, the stroke volume increases slightly while the arterial compliance also increases which might increase pulse pressure a bit but the main increase in the pulse pressure is due to the large increase in heart rate:

"During exercise, the cardiac output increases more than the total resistance decreases, so the mean arterial pressure usually increases by a small amount. Pulse pressure, in contrast, markedly increases because of an increase in both stroke volume and the speed at which the stroke volume is ejected. The cardiac output increase is due to a large increase in heart rate and a small increase in stroke volume."

Canvas 0:Cross Sectional View of the
extremity where Blood Pressure is measured

Explanation of the Graphics

Canvas 0 shows a cross section of the extremity (in this case the arm) where the measurement cuff is centered. The cuff, arm, artery wall and the artery are clearly labeled. When the button "Start Inflation" is pressed, as one would expect, the cuff becomes thicker radially by the inner radius getting smaller and the outer radius getting larger. The radius of the artery gets smaller and smaller until the systolic cuff pressure is reached, where even at the maximum heart muscle contraction, its radius stays zero. A cuff pressure meter scaled in millimeters of mercury (mm Hg) is shown just to the right.
The pressure meter shows gradual pressure increase when the button "Start Inflation" is pressed. It also shows fluctuations during the increase up to a pressure that represents the systolic pressure. After the systolic pressure is reached, the meter fluctuations stop. They stop because the artery remains closed even when the heart's highest pressure is incident on the artery.
When the "Start Deflation" button is pressed, the cuff pressure is reduced and the meter fluctuations re-occur down to what is the diastolic pressure. At this pressure the artery is fully open to blood flow.

The next graphic is a horizontal top view of the extremity (e.g. arm) where blood pressure is being measured. In this view it is convenient to show the Blood flow. This flow is depicted by white chevrons pointing to the right. These chevrons move when two conditions are met:

1. The heart muscle is contracting

2. The cuff pressure is lower than the systolic pressure.

Just as in Canvas 0, when the button "Start Inflation" is pressed. the cuff inflates and, at the systolic pressure, moves inward enough to choke off the artery. As might be expected, the blood flow speed decreases when the cuff pressure increases and becomes zero at the systolic pressure.

Canvas 1: Arm Showing Cuff, Artery and Blood Flow:Top View

Explanation of the Plots Vs Time

The remainder of the graphics (you might have to scroll down) are moving plots of the heart's electrical signals, and the heart muscle and blood flow responses to those electrical signals. To see these plots, the Animation has to be running. Animation is started by pressing either "Start" button.

First is the very familiar EKG that is obtained by measuring the voltage differences between probes strategically placed on the chest. The EKG is some measure of the electrical signal that the heart will see. The EKG has 5 components, P, Q, R, S, T. The R peak is by far the largest. Medical doctors critically examine the size and placement of the EKG components. Sometimes, the frequency of the EKG is sporadic and that can be a serious condition called fibrillation.

Next is a plot of the electrical signal that the heart itself sees. Reference 5.   Typically this signal is much wider time wise than the signal that the skeletal muscles use. The reason for the wider width is that the heart muscle has to remain contracted for roughly 1/4 of its period in order for it to refill.

Next is a plot of the heart muscle's response to the previous plot. As might be expected, there is a slight lag to the response and the muscle pulse width is approximately the same as the width of the heart's electrical signal.

Last is a plot of the blood flow (e.g. milliliters per second) rate versus time. It is scaled to the value when the artery is fully open and the heart muscle contraction is maximum. Of course it goes to zero and stays there when the cuff pressure is above the systolic pressure.