Cardiac cycle

About: Cardiac cycle is a research topic. Over the lifetime, 3290 publications have been published within this topic receiving 96159 citations.

Influence of heart rate on left ventricular performance in conscious dogs.

Abstract: The effects of atrial pacing on the left ventricular end-systolic pressure-volume relation, a relatively load-insensitive index of left ventricular performance, were studied in 8 chronically instrumented, conscious dogs. Six of the dogs were studied while autonomically intact, and 2 were studied after autonomic blockade with 2 mg/kg i.v. propranolol and 0.2 mg/kg i.v. atropine. Left ventricular pressure was measured with a micromanometer and left ventricular volume was determined from 3 ultrasonic orthogonal dimensions. Pressure was varied by caval occlusions at control heart rate and after atrial pacing at 100, 120, 140, 160, 180, and 200 bpm. The end-systolic pressure-volume relation was linear in every case (r = 0.97 +/- 0.03, SD). In the autonomically intact dogs, Emax, the slope of the end-systolic pressure-volume relation, was directly and monotonically related to heart rate in every dog, increasing to 238 +/- 99% of control at peak pacing rate (p less than 0.05). V0, the zero pressure intercept of the relation was also directly related to heart rate in every dog and increased 8.6 +/- 5.5 ml from control to peak pacing rate (p less than 0.05). Autonomic blockade did not attenuate these effects. This rightward shift of the end-systolic pressure-volume relation results in a reduced stroke volume from any end-diastolic volume, modulating the hemodynamic benefits of enhanced contractility. T, the time constant of isovolumic pressure fall during ventricular relaxation, was determined from beats with matched end-systolic pressures. T was related to heart rate, falling by 20 +/- 10.3% over the range of rates studied in the autonomically intact dogs and by 23.1 +/- 6.2% in the autonomically blocked dogs. Thus, the ventricle relaxes more rapidly at higher heart rates. We conclude that the frequency of contraction is concluded as an important determinant of overall pump function throughout the cardiac cycle in conscious dogs.

Hemodynamic effects of cardiac cycle-specific increases in intrathoracic pressure

Abstract: Changes in intrathoracic pressure (ITP) can influence cardiac performance by affecting ventricular loading conditions. Because both systemic venous return and factors determining left ventricular (LV) ejection may vary over the cardiac cycle, phasic increases in ITP may differentially affect preload or afterload if delivered at specific points within the cardiac cycle. We studied the hemodynamic effects of cardiac cycle-specific increases in ITP (pulses) delivered by a high-frequency jet ventilator in an acute closed-chested canine model (n = 11), using electromagnetic flow probes to measure biventricular stroke volume. Measurements were taken during a control condition after the induction of acute ventricular failure (AVF) by propranolol hydrochloride and volume infusion. ITP was independently varied without changing lung volume by the inflation of thoracoabdominal binders. Although synchronous pulses had minimal hemodynamic effects in unbound controls, binding pulses timed to occur in early diastole resulted in decreases in LV filling pressure and left ventricular stroke volume (SVlv) (P less than 0.05). In the AVF condition, pulses increased LV performance, evidenced by increases in SVlv (P less than 0.01), despite decreases in LV filling pressure (P less than 0.05). This effect is maximized by binding and by timing the pulses to occur in systole. We conclude that cardiac cycle-specific increases in ITP can significantly affect cardiac performance. These effects appear to be related to the ability of such timed pulses to selectively affect LV preload and afterload.

Pulmonary blood flow after total cavopulmonary shunt.

Abstract: The pattern of pulmonary blood flow was studied in three patients after a total cavopulmonary shunt procedure. Doppler studies showed a phasic pattern of flow which varied with the respiratory cycle. Pulmonary blood flow was increased with normal inspiration, and was much augmented by the Mueller manoeuvre. This suggests that flow was occurring when a negative intrathoracic pressure was generated. During a brief Valsalva manoeuvre blood flowed away from the lungs. With a sustained Valsalva manoeuvre there was no spontaneous forward flow; instead there was low velocity pulsatile pulmonary blood flow that coincided with ventricular systole. Left ventricular cavity dimensions decreased, reflecting a considerably reduced pulmonary blood flow. Pulmonary blood flow after the total cavopulmonary shunt operation is critically dependent on changes in intrathoracic pressure. This has important implications in terms of the immediate postoperative management of these patients.

Abnormal mitral valve coaptation in hypertrophic obstructive cardiomyopathy: Proposed role in systolic anterior motion of mitral valve

Abstract: Two dimensional echocardiographic evaluation utilizing the apical four chamber and apical long axis views was made in 10 normal subjects and 35 patients with hypertrophic cardiomyopathy, including 16 without and 19 with systolic anterior motion of the mitral valve. Mitral valve leaflets were imaged throughout the cardiac cycle. The normal subjects and the patients with hypertrophic cardiomyopathy without systolic anterior motion demonstrated mitral valve coaptation that seemed to involve the distal tip of the anterior leaflet. In contrast, patients with hypertrophic cardiomyopathy with systolic anterior motion demonstrated abnormal coaptation, so that the posterior leaflet coapted with a mid portion of the anterior leaflet, leaving a distal “residual” anterior leaflet in the left ventricle during systole. A sharp angulation of this distal leaflet in mid systole toward the interventricular septum is thought to represent systolic anterior motion. It is speculated that abnormal mitral leaflet coaptation is necessary for development of leaflet systolic anterior motion, which Is correlated with dynamic left ventricular outflow gradients in hypertrophic obstructive cardiomyopathy.