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Physiology

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Cardiovascular System

Question
12 out of 12
 

Oxygen demand of heart:(AIPG JUN 2009)



A Increases proportionately with heart rate

B Depends upon duration of systole

C Is negligible when heart is at rest

D Is in constant relation with amount of work done.

Ans. A Increases proportionately with heart rate.

I. The O2 consumption by the heart is determined primarily by the intramyocardial tension, the contractile state of the myocardium, and the heart rate.

II. It Has no relation with duration of systole. Ventricular work per beat correlates with O2 consumption.

III. The work is the product of stroke volume and mean arterial pressure in the pulmonary artery (for the right ventricle) or the aorta (for the left ventricle).

IV. Since aortic pressure is seven times greater than pulmonary artery pressure, the stroke work of the left ventricle is approximately seven times the stroke work of the right ventricle.

V. Pressure work produces a greater increase in O2 consumption than volume work. In other words, an increase in afterload causes a greater increase in cardiac O2 consumption than an increase in preload does.

VI. This is why angina pectoris due to deficient delivery of O2 to the myocardium is more common in aortic stenosis than in aortic insufficiency.

VII. In aortic stenosis, intraventricular pressure must be increased to force blood through the stenotic valve, whereas in aortic insufficiency, regurgitation of blood produces an increase in stroke volume with little change in aortic impedance. So the relation between work done and O2 demand is not linear.

VIII. We can see from the table that oxygen demand in 267 ml/min even at rest.

Changes in Cardiac Function with Exercise.

Work (kg-m/min)

O2 Usage (mL/min)

Pulse Rate (per min)

Cardiac Output (L/min)

Stroke Volume (mL)

A-V O2 Difference (mL/dL)

Rest

267

64

6.4

100

4.3

288

910

104

13.1

126

7.0

540

1430

122

15.2

125

9.4

900

2143

161

17.8

110

12.3

1260

3007

173

20.9

120

14.5

Cardiovascular System Flashcard List

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