Arterial Pressures Are Measured in Terms of Systolic, Diastolic, and Mean Levels
The pressures in the pulmonary artery and aorta are not constant but rather are pulsatile, as shown in Figure 21-1 and repeated in Figure 22-8. With each cardiac ejection, the pulmonary artery and aorta become distended with blood, which causes the pressures within these vessels to increase to peak values, called systolic pressures.
Between cardiac ejections (i.e., during ventricular diastole), blood continues to flow out of the pulmonary artery and aorta into the pulmonary and systemic circulations, respectively. As the volume of blood in these large arteries decreases, the arteries become less distended, so arterial pressure decreases. Pressure continues to decrease until the next cardiac ejection begins. The minimal pressure reached before each new ejection is called the diastolic pressure. Figure 22-8 provides typical values for systolic and diastolic pressures.The amplitude of the pressure pulsations in an artery is called the pulse pressure, specifically:
Aortic pulse pressure =
(Aortic systolic pressure - Aortic diastolic pressure)
and
Pulmonary artery pulse pressure =
(Pulmonary artery systolic pressure - Pulmonary artery diastolic pressure)
Typical values for pulse pressure are given in Figure 22-8. Note how much lower the systolic, diastolic, and pulse pressures are in the pulmonary artery than in the aorta. These differences illustrate why the pulmonary circulation is called the “low-pressure circulation” and the systemic circulation is called the “high-pressure circulation.”
It is important to distinguish among systolic pressure, diastolic pressure, and pulse pressure and to distinguish all of them from mean pressure. Mean aortic pressure is the average pressure in the aorta over the course of one or more complete cardiac cycles. Likewise, mean pulmonary artery pressure is the average pressure in that vessel.
Obviously, the mean pressure in an artery is somewhere between the systolic (maximal) and diastolic (minimal) pressure levels. However, because the pressure wave forms in arteries are not symmetric, the mean pressure is generally not exactly midway between the systolic and diastolic pressures.A popular rule is that mean pressure is about one third of the way up from diastolic toward systolic pressure; that is:
Mean arterial pressure ≈ Diastolic pressure + 1ZaPuIse pressure
Figure 22-8 reveals that this is not a valid approximation for the determination of mean pressure in the aorta. However, the approximation is a good one for pressures measured in the femoral artery or in most other major arteries distal to the aorta. The reason that the rule applies in the distal arteries but not in the aorta is that the waveform of the arterial pressure pulsations changes as the pulses move out, away from the heart. The pressure pulses become narrower and more sharply peaked. This pronounced asymmetry of the pressure pulses causes the mean level in distal arteries to be closer to the diastolic pressure than to the systolic pressure (see Figure 22-8).
For complex reasons, the pulse pressure typically increases as blood flows from the aorta into the distal arteries. However, the mean pressure necessarily decreases in accordance with the principle of the conservation of energy. As stated earlier, mean arterial pressure is a measure of the potential energy in the bloodstream, and this potential energy is used up (converted into heat by friction) as blood flows from the aorta through the systemic circulation. The aorta and large arteries offer only a small resistance to blood flow; mean arterial pressure decreases only 1 to 3 mm Hg between the aorta and the femoral artery (see Figure 22-8). Most of the resistance to blood flow is found in the arterioles and capillaries. Therefore the largest decrements in mean pressure occur in these segments of the systemic circulation (see Figure 22-1).
Mean pressures are the pressures that must be used when calculating vascular resistance from the following equation:
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Unfortunately, the only way to measure mean vascular pressures is by inserting a needle or catheter into the vessel of interest. The first direct measurement of mean arterial blood pressure was carried out by Stephen Hales, an English clergyman. In about 1730, Hales inserted a tube (catheter) into the femoral artery of a conscious horse and found that blood rose in the tube to a height of more than 8 feet. An 8-foot column of blood represents a pressure of more than 180 mm Hg, almost twice the mean arterial pressure expected in a normal resting animal. The high pressure undoubtedly reflected the physical and emotional distress of the horse, which was restrained upside down during the episode. Currently, arterial catheterization (with anesthetic agent to reduce pain) is routine in human medicine (e.g., in cardiac catheterization laboratories) and is becoming more common in veterinary medicine. However, the lesson that physical or emotional distress can dramatically increase blood pressure is as relevant today as it was in Hales* time.
In human medicine, systolic and diastolic arterial pressures can be measured quite accurately with a blood pressure cuff and stethoscope. Blood pressure cuffs are less frequently used on veterinary species, but the pulse is often palpated by placing the fingertips over a major artery, such as the femoral artery. Palpation of an artery allows the clinician to sense the pulse pressure on the basis of the magnitude of the pulsations felt in the artery. A low pulse pressure is referred to as a “thready, or weak, pulse. A high pulse pressure may be called a “bounding,” or strong, pulse.
FIGURE 22-9 Various conditions that increase arterial pulse pressure are compared with regard to their effects on systolic pressure, diastolic pressure, and mean pressure (see text).