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1 Department of Cardiothoracic Surgery and the Department of Biophysics and Physiology, Albert Einstein College of Medicine, Bronx, New York 10461; 2 Department of Cardiovascular and Thoracic Surgery, Research Institute of the Palo Alto Medical Foundation, Stanford University School of Medicine, Palo Alto, California 94306
Left ventricular relaxation is dependent on afterload conditions during systole. An abrupt increase in afterload while the ventricle is actively contracting prolongs the duration of systole. An increase in afterload during ventricular relaxation shortens the duration of systole. Therefore, we hypothesized that the point during systole when an abrupt increase in afterload had no effect on the duration of systole represented the onset of ventricular relaxation. To determine when this point occurs, we performed aortic occlusions progressively throughout the duration of systole in six dogs. We determined the change in systolic time (tsys) after an intervention normalized to tsys of a control beat (tsys,i/tsys,c) as a function of systolic occlusion time as a percentage of total systolic time (tocc/tsys,c), where tsys is the duration from time of left ventricular end-diastolic pressure to the time of minimum first derivative of left ventricular pressure. Our results show the onset of left ventricular relaxation during normal ejection occurs at 34 ± 3% of systolic time and ~16% after the onset of ejection. Thus the beginning of relaxation occurs soon after the beginning of ejection, suggesting that relaxation is modulated by variable loading conditions during ejection, significantly before what has been conventionally been assumed to be the beginning of ventricular relaxation.
afterload; left ventricular relaxation
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