Cardiac cycle

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

Biomechanics of the aortic valve in the continuous flow VAD-assisted heart.

Abstract: The biomechanics of the aortic valve are altered in patients with ventricular assist devices (VADs). During high VAD flow and low cardiac function, transvalvular pressure is high, and the aortic valve remains closed throughout the cardiac cycle. This condition has been linked to the development of aortic valve fusion and incompetence during VAD use. Thus, physicians try to maintain pulsatile flow to assure periodic valve opening. The aim of this study was to determine the extent of aortic valve opening and alterations in valve leaflet strain before and during VAD support using a specially designed mock loop. The results showed that diastole is prolonged during VAD use. In addition, there is a reduction in valve opening area, producing a VAD-related functional stenosis. The average leaflet strain increased during VAD support, primarily due to an increase in the minimum strain, during systole, rather than the maximum strain during diastole. The findings support our hypothesis that altered biomechanics in the VAD-assisted heart results in increased strain in the aortic valve leaflets, which can stimulate soft tissue remodeling. The implication for clinical use is that valve opening during parallel VAD flow is reduced compared with normal flow conditions. Consequently, current clinical practice for VAD patients may not be achieving sufficient valve opening to prevent changes such as fusion and incompetence.

Change in heart period: A function of sensorimotor event timing within the cardiac cycle

Abstract: In a fixed-foreperiod reaction time experiment with 66 male college students, the heart rate for the same cardiac cycle within which the imperative stimulus occurred was slowed. The slowing was greater for imperative stimuli presented early in the cardiac cycle than late. The “monotonic” (linear) trend was highly significant, p< 10-8. There was also a significant “bitonic” (quadratic) trend, p <.01. Three subgroups of subjects, split on the basis of heart rate level, each showed significant trends with significant differences in the trend-components among groups. Analyses of ready signal presentations and of a control point in the intertrial interval revealed no comparable effects. The results were replicated in a second experiment in which 20 male college students self-initiated tachistoscopic exposures. In this experiment, the next cardiac cycle, subsequent to the cardiac cycle in which the self-initiated response occurred, was also shown to exhibit systematic modification with changes in the temporal placement of the response within the preceding cycle. The phenomena described are strikingly parallel with results obtained in animals upon direct stimulation of the vagus and carotid sinus nerves, and in man upon stimulation of the carotid sinus with neck suction. The results constitute the first demonstration in intact man of differential modification of heart period with temporal variations of the placement of “significant” sensorimotor events within the cardiac cycle.

Regulation of contractile force during ventricular arrhythmias

Abstract: Causes of fluctuations in ventricular pressure during arrhythmias were studied in anesthetized dogs. Excitability and contractility were measured throughout the cardiac cycle and their relationship to the problem was assessed. The strength of premature contractions was found to vary inversely with prematurity. Weakness of premature beats could not be ascribed to subnormal excitability nor to insufficient ventricular filling. Contractility was virtually zero at the end of the total refractory period, increased progressively thereafter and was fully restored only at the very end of the cardiac cycle. Sensitivity of contractile force to increments in ventricular filling paralleled the return of contractility. The strength of the postcompensatory contraction was always enhanced, this potentiation increasing as a linear function of the extrasystole's prematurity. Postextrasystolic potentiation was attributed to the extrasystole itself rather than to the pause which follows it or to increases in ventricular filling and distention. It was therefore concluded that pressure fluctuations during ventricular arrhythmias are not due to subnormal excitability and would occur independently of variations in ventricular filling.

The evaluation of ventricular function using gated myocardial imaging with Tc-99m MIBI.

Abstract: Left ventricular function is an important prognostic indicator in patients with coronary artery disease. We have assessed a method of providing this information as an adjunct to myocardial perfusion imaging using Tc-99m MIBI (2-methoxy-2-methyl-isopropyl-1-isonitrile). Two separate studies, at rest and during exercise, were performed following an injection of 400-600 M Bq of Tc-99m MIBI in 62 patients. Cardiac gating permitted excellent myocardial edge definition during the cardiac cycle. Radionuclide fractional shortening (RFS) was calculated from the anteroposterior (AP) and the septum to lateral wall (SL) axes in diastole and systole. Results were compared with echocardiographic fractional shortening (EFS) and the ejection fraction (EF) obtained from the gated equilibrium blood pool using Tc-99m-labelled red blood cells. The RFS in the AP axis correlated closely with echocardiographic FS (r = 0.89, P less than 0.001). The RFS in both axes was averaged to provide a global RFS. Global RFS correlated closely with LV radionuclide EF (r = 0.83, P less than 0.001). Inter- and intra-observer reproducibility studies have shown a variability for the procedure of less than 10%. In conclusion, gated perfusion imaging with Tc-99m MIBI, provides useful functional information as an adjunct to perfusion imaging.