When cardiac changes in the face of a constant SVR, mean arterial pressure is influenced according to the formula Pa = CO X SVR. The influence of a change in cardiac output on mean arterial pressure is independent of the cause of the change— heart rate or stroke volume (remember that CO = SV X HR). In contrast, the effect of a change in cardiac output on pulse pressure greatly depends on whether stroke volume or heart rate changes. Below we consider the effects of changes in heart rate, stroke volume, cardiac output, SVR, and arterial compliance on pulse pressure and mean arterial pressure.
Effect of Changes in Heart Rate and Stroke Volume With No Change in Cardiac Output. If an increase in heart rate is balanced by a proportional and opposite change in stroke volume, mean arterial pressure does not change because cardiac output remains constant. However, the decrease in stroke volume that occurs in this situation results in a diminished pulse pressure,- the diastolic pressure increases, while the systolic pressure decreases around an unchanged mean arterial pressure. An increase in stroke volume with no change in cardiac output likewise causes no change in mean arterial pressure. The increased stroke volume, however, produces a rise in pulse pressure, systolic pressure increases and diastolic pressure decreases.
Another way to think about these events is depicted in Figure 15.3A. The first two pressure waves have a diastolic pressure of 80 mm Hg, systolic pressure of 120 mm Hg, and mean arterial pressure of 93 mm Hg. Heart rate is 72 beats/min. After the second beat, the heart rate is slowed to 60 beats/min, but stroke volume is increased sufficiently to maintain the same cardiac output. The longer time interval between beats allows the diastolic pressure to fall to a new (lower) value of 70 mm Hg. The next systole, however, produces an increase in pulse pressure because of the ejection of a greater stroke volume, so systolic pressure rises to 130 mm Hg. The pressure then falls to the new (lower) di-astolic pressure, and the cycle is repeated. Mean arterial pressure does not change because cardiac output and SVR are constant. The increased pulse pressure is distributed evenly around the same mean arterial pressure.
If an increase in heart rate is balanced by a decrease in stroke volume so that there is no change in cardiac output, the result is no change in mean arterial pressure but a decrease in pulse pressure. Systolic pressure decreases and di-astolic pressure increases.
Effect of Changes in Cardiac Output Balanced by Changes in Systemic Vascular Resistance. Mean arterial pressure may remain constant despite a change in cardiac output because of an alteration in SVR. A good exam-
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...