True about myocardial O2 demand is (AIIMS 2011 MAY)
|A||Directly proportional to duration of systole|
|B||Inversely proportional to heart rate|
|C||Negligible in quiescent heart|
|D||Has a constant relation to the external work done by heart|
I. The O2 consumption by the heart is determined primarily by the intramyocardial tension, the contractile state of the myocardium, the heart rate duration of systole.
II. We also use tension-time indexdeveloped as a measure of the work done of the left ventricle and is a product of the mean systolic pressure, times the duration of systole, times the heart rate.
III. Quiescent heart is an alive but non beating heart example conduction block, hypothermia, vagal inhibition of heart. So, it does consume oxygen as it is still alive although very less in amounts.
IV. Ventricular work per beat correlates with O2 consumption. 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).
V. 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.
VI. 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.
VII. This is why angina pectoris due to deficient delivery of O2 to the myocardium is more common in aortic stenosis than in aortic insufficiency. 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.