cardio_sol_corr - Space Biomedical Engineering Spring 2006...

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Space Biomedical Engineering, Spring 2006 SOLUTIONS to Assignment #5 Cardiovascular Homework 1. Compare the heart rate and blood pressure at time = + 100 sec to the baseline values at time = 10 sec. What is the physiologic basis for the observed change in arterial BP? Description: Before and after the stand test, the heart rate remains fixed at 68 beats/min. The systemic arterial pressure (ABP) is 117/72 mmHg (average = 94.5 mmHg) while supine, but it decreases sharply upon standing and has dropped to 60/37 mmHg (average = 48.5 mmHg) by the time 100 seconds have passed. Analysis: When a human moves from a supine to standing position, there is a major translocation of the body’s blood; it shifts towards the feet and pools in the veins of the leg. This shift into the leg veins is effectively an increase in the venous zero pressure filling volume (ZPFV). Since veins are elastic, increasing the ZPFV causes an increase of blood volume in the veins before any stretch in the wall of the veins (and any pressure) appears. Assuming that the volume of blood in the body doesn’t chance, there is less blood in the other parts of the cardiovascular system, including in the systemic arteries. Therefore, the pressure drops in the systemic arteries. With the “operational control systems” switched off, the baroreflex cannot do its job of signaling the heart to increase its cardiac output. Therefore, the heart rate does not increase upon standing in this “no- control” case. 2. The curves in Figure 3 show the patient’s status at time = 10 sec. Using the axes provided, sketch CO/VR curves and a LV P-V loop to represent the hemodynamics at time = + 100 sec. (Hint: First estimate the new CO assuming constant peripheral resistance.) The post-standing LV P-V loop is determined by the post-standing systolic pressure (60 mmHg), the post-standing diastolic pressure (38 mmHg), and the post-standing stroke 1 of 5
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volume (SV is proportional to pulse pressure, which is systolic pressure minus diastolic pressure Æ since the post-standing pulse pressure is about half the pre-standing pulse pressure, the post-standing SV is about half of the pre-standing SV. SV is the distance between the two vertical lines in the cardiac cycle, so the pre-standing SV is about 75 cc. Thus, the post-standing SV is about 38 cc.)
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cardio_sol_corr - Space Biomedical Engineering Spring 2006...

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