Mean Arterial Pressure and Pulse Pressure





When blood pressure is measured clinically, the systolic and diastolic values are recorded. Neither of these should be above normal, and for the most part these are the values that are followed clinically. But two additional ways of characterizing the blood pressure are important to consider.

Mean Arterial Pressure

Physiologically, the pressure that is primarily regulated is the mean arterial pressure. This is the average level of the blood pressure over several heart beats. This could be determined by measuring the blood pressure at many closely spaced intervals and averaging the numbers. But this is not necessary, since it can be estimated satisfactorily by adding one-third of the pulse pressure (see below) to the diastolic pressure.

What factors determine the mean arterial pressure? To the right you see the figure that we use to analyze the regulation of mean arterial pressure. It shows a balloon with two spouts. Like the arteries, it has an elastic wall. The pressure in the balloon is determined by the amount of water in the balloon at any time. This is determined by how fast water enters the balloon, which corresponds to the cardiac output, and the diameter of the outflow spout, which corresponds to the total peripheral resistance. The latter is determined by the diameters of the smallest vessels of the arterial system, the arterioles. Vasoconstriction narrows the lumen of an arteriole. Vasodilation expands the lumen of an arteriole.


Carotid Baroreceptor Reflex

When the mean arterial pressure is perturbed from the regulated level, the cardiac output and the total peripheral resistance are adjusted to bring the mean arterial pressure back to the specified level. Over seconds to minutes, this regulation is due to the carotid baroreceptor reflex. The sensors for the reflex are in the carotid sinus (and also aortic arch and ventricles). The primary integrating center is in the medulla, with the regulatory outflow carried via autonomic nerves.


QUESTION: Does vasoconstriction of arterioles increase or decrease the total peripheral resistance?

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QUESTION: Does an increase in the total peripheral resistance increase or decrease the mean arterial pressure?

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Pulse Pressure

The pulse pressure is calculated by subtracting the diastolic pressure from the systolic pressure. In other words, it is the change in pressure from the diastolic level to the systolic level. It is determined by two factors, the stroke volume and the compliance of the arterial system. The stroke volume, of course, is the amount of the blood injected into arteries by each heart beat. The compliance is determined by the elasticity of the arterial system. Flexible arteries that expand easily have a high compliance. Stiff arteries have a low compliance.

QUESTION: Arteries tend to stiffen with age. Does their compliance increase or decrease?

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QUESTION: Thus, would you expect the elderly often to have a high pulse pressure or a low pulse pressure?

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Pulse Pressure in a More Complex Structure

The simple balloon model of the arterial system above is usually adequate for analyzing the mean arterial pressure. But it is worth making our model a little more realistic for analyzing the pulse pressure. The arterial system, of course, is a complex structure. It progressively branches into smaller and smaller vessels.

In the figure below, we have the arterial system shown as a long, tapering elastic tube. This is more like the actual arterial system, although, of course, this model doesn't have any branching.

Click the button to inject one stroke volume into the long tube. The pressures recording at successive locations are shown below.

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While the mean arterial pressure should be about the same in all regions, notice how the pulse pressure changes. The last pressure is in a small artery, which has the lowest compliance. For example, compare the two arteries used for the ankle/brachial index. The posterior tibial artery in the ankle is smaller with a lower compliance. Therefore, you expect a higher pulse pressure. A higher pulse pressure implies a higher systolic pressure, assuming the mean arterial pressure is about the same. This is one reason why the normal ankle/brachial index is 1.1 to 1.3. (Another reason is that the posterior tibial artery is just before where the artery branches into many small vessels in the foot. As discussed below, this causes a reflected wave, which in the posterior tibial artery adds to the systolic pressure.)

Consider a patient with claudication due to a constriction in the femoral artery as a result of atherosclerosis. Flow through this region is reduced, and distal to the contriction the mean arterial pressure and other pressures are reduced. This would give an ankle/brachial index below 0.9, which is abnormal. And it is not uncommon in patients where atherosclerosis develops in the leg arteries for the index to be much lower than 0.9.

QUESTION: Compared to the brachial artery, is the systolic pressure normally higher or lower in the posterior tibial artery?

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QUESTION: What is the typical symptom of claudication?

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QUESTION: Compared to the brachial artery, is the compliance normally higher or lower in the posterior tibial artery?

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QUESTION: Compared to the brachial artery, would you expect the diastolic pressure normally to be higher or lower in the posterior tibial artery?

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Effect of Branching of Arteries

Finally, let's go a further step and look at how adding branching to the arterial system affects the pulse pressure. When a discontinuity is added to an elastic system, a propagating wave tends to have a reflection. The branching of the aorta into the common iliac arteries is an especially important place for this.

In a healthy, young aorta the reflection tends to arrive during diastole, which increases the diastolic pressure somewhat. Click the button to inject a stroke volume into the aorta in the figure to the left. Observe the actual pressure in the aorta at the bottom, which is the sum of the initial wave and the reflected wave. The reflected wave increases the diastolic pressure.

Now consider a stiffer, less compliant aorta, which is represented in the figure to the right below. This would be typical in an older person. Both waves travel faster in this stiffer system. Observe that the initial wave creates a somewhat higher pulse pressure because the artery is stiffer. Next, note that the reflected wave occurs earlier, since it travels faster. Thus, now it mainly occurs during systole and not diastole. Now observe the addition of the initial wave and the reflected wave. The reflected wave here increases the systolic pressure, but not the diastolic pressure, as in the more compliant artery shown to the left. Now, the addition of the initial wave and the reflected wave gives a higher systolic pressure and a lower diastolic pressure. This situation is why older people tend to develop a higher pulse pressure. In general, both a higher pulse pressure and a high mean arterial pressure are risk factors for cardiovascular disease.

YOUNG, HEALTHY PERSON

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ELDERLY PERSON, WITH LOW COMPLIANCE

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One disadvantage of the lower diastolic pressure during aging is that it affects coronary blood flow. The small coronary vessels are squeezed shut during systole, so that coronary blood flow occurs only during diastole. Thus coronary blood flow is driven by the diastolic pressure. If this is lower, there is a reduction in the supply of oxygen to the heart and thus a reduced maximum exercise level. This is not a huge issue, but is one reason the maximum exercise tolerance on the average tends to decrease with age.

QUESTION: Does the pulse wave tend to travel faster in a younger or older subject?

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QUESTION: Does coronary blood flow occur during systole or diastole?

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QUESTION: Consider an elderly person with a normal mean arterial pressure, but with a significantly reduced aortic compliance. Based on what we are discussing, why would the reduced compliance tend to cause a reduced maximum exercise tolerance?

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