Sequential changes in left ventricular compliance during acute coronary occlusion in the isovolumic working canine heart

Regional ischemia increases sensitivity of left ventricular relaxation to volume in pigs

Regional ischemia impairs early diastolic filling due, in part, to changes in left ventricular relaxation. This study uses open-chest pigs instrumented with high-fidelity pressure transducers to investigate the effect of regional ischemia on the active component of relaxation independent of the passive effects of filling and the effect of left ventricular filling and stretch on the rate, duration, and extent of relaxation. During regional ischemia, active relaxation was impaired in the nonfilling ventricle, with a slower rate of relaxation. Stretching the myocardium as the ventricle fills slows the rate of relaxation more during regional ischemia than during normal perfusion, reflecting an increased sensitivity to stretch due to filling and an increased dependence of relaxation on volume. The duration of relaxation depends on the effect of regional ischemia on the end-diastolic pressure-volume relation. Stronger baseline contractile function results in an upward shift in the end-diastolic pressure-volume relation during regional ischemia and no net effect on the duration of relaxation. If this curve is shifted upward, the duration of relaxation shortens. All these effects combine to reduce the atrioventricular pressure gradient and left ventricular filling during regional ischemia.

Significance of right ventricular filling for left ventricular enddiastolic pressure-volume relationship under acute hypoxia in the dog

In 14 closed-chest dogs, the significance of right ventricular filling for left ventricular enddiastolic pressure-volume relationship was investigated under acute hypoxia by means of single plane cineventriculography and simultaneous intraventricular pressure recording. Both after 5 min asphyxia (respirator switched off) (n = 5) and after 3 min hypoxia (ventilation with pure N2) (n = 9), there was a significant leftward shift (p less than 0.005) of the left ventricular enddiastolic pressure-volume curve as compared to the control curves under normoxia. To simulate the elevated filling of the right ventricle under acute hypoxia, rapid intraventricular infusion was applied under normoxic conditions to raise right ventricular enddiastolic pressure to the same values as that measured under hypoxia. The extent of the ensuing leftward shift of the left ventricular enddiastolic pressure-volume curve was on average 60% of the shift under hypoxia in both sets of experiments. Neither the slope of the relationship between volume stiffness and enddiastolic pressure, nor the relationship between tangent elastic modulus and left ventricular wall stress, was affected by hypoxia or asphyxia. Thus, the shift of the left ventricular enddiastolic pressure-volume curve in the early stage of hypoxia is predominantly due to the influence of increased right ventricular filling. Since the increased volume of the atria under acute hypoxia limits left ventricular distensibility additionally, the changes in left ventricular enddiastolic pressure-volume relationships observed in the early stage of hypoxia are mainly, or even entirely, the result of interaction of the various heart compartments, and not a reflection of alterations in myocardial tissue elasticity.

Changes in diastolic properties of the regional myocardium during pacing-induced ischemia in human subjects

Mechanisms related to alterations in the diastolic properties of the left ventricle during angina were studied in seven patients with coronary artery disease. Single plane left ventriculograms were obtained using a high fidelity micromanometer-tipped catheter in both the resting state and immediately after rapid cardiac pacing. In all patients, typical anginal pain developed with pacing stress. After atrial pacing, the left ventricular end-diastolic pressure increased from 10 +/- 3 to 21 +/- 7 mm Hg (+/- standard deviation) (p less than 0.005) regardless of the changes in the end-diastolic volume. The ejection fraction was reduced from 59 +/- 10 to 48 +/- 13% (p less than 0.05). The diastolic pressure-volume curves shifted upward in post-pacing beats in four patients, while in three the curves shifted more to the right. The regional myocardial function was expressed in quantitative terms by a radial coordinate system with the origin at the center of gravity of the end-diastolic silhouette. Two representative radial grids for normal and ischemic segments were selected. In the normal segment, the end-diastolic length was augmented by 15% (p less than 0.005) and was associated with a 24% increase in stroke excursion with pacing stress (p less than 0.05). The increase in diastolic pressure was accompanied by comparable increases in end-diastolic length, and the diastolic pressure-length relation moved up to the higher portion of the single curve. In the ischemic segment, the end-diastolic length remained unchanged in the post-pacing beat, but segment shortening was significantly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Left ventricular diastolic relaxation and pressure-diameter relations during ischaemic left ventricular failure in the dog

The significance of severe ischaemic left ventricular (LV) failure on the LV isovolumic relaxation process and diastolic chamber stiffness has been investigated in nine open-chest pentobarbital-anaesthetized dogs. LV failure was induced by bolus injections of 50 micron microspheres into left coronary vascular bed until LV minor axis diameter had increased about 25% and end-diastolic pressure about 20 mmHg. Such ischaemic LV failure did not shift the relation between diastolic LV pressure and minor axis diameter compared with pressure-diameter curves obtained before induction of failure. Neither inotropic nor chronotropic stimulation evoked such shifts. Assuming exponential pressure decline, LV relaxation was significantly slower during failure, but proceeded in all experimental conditions at rates which indicated complete relaxation in late diastole. Analysis of the pressure decline during LV relaxation demonstrated that this process proceeded faster than assumed by an exponential function both before and during LV failure.

Experimental evaluation of secondary blood cardioplegia

Reperfusion damage after ischemia may be evidenced by myocardial cell edema, intracellular calcium accumulation, and limited utilization of oxygen. The need for cardioplegic arrest during initial reperfusion to allow oxygen to be used for reversing ischemic damage rather than for electromechanical activity has been propounded by some researchers. Reports of greater postischemic compliance and performance, low postischemic edema, and greater oxygen uptake at a perfusion pressure of 50 mm Hg or lower have been cited. The present study was conducted on 24 pigs having 2-hr cardioplegic arrest, which of 12 underwent normal reperfusion and 12 experienced secondary cardioplegia followed by normal reperfusion. The results showed that in spite of improved high-energy phosphate preservation, the secondary cardioplegia group had higher myocardial edema, less coronary flow, and poorer contractility and compliance at the end of 1 hr of reperfusion. Because of these findings and contradictory results reported by other groups, caution is urged in the clinical extrapolation of the results of such studies pending further investigations.

Diastolic tension of rat cardiac muscle during deficiency of oxygen and glucose. Stress-strain relationships and reversibility

In the isometrically contracting rat trabecular muscle strip preparation, contracture tension was induced either by 10 min of hypoxia (low contracture tension) or by 30 min of joint oxygen and glucose withdrawal (severe contracture tension) at optimum length. Using stepwise 0.1 mm releases, length-tension relationships were recorded under control and contracture conditions in both types of contracture. Stress-strain relationships were evaluated as well as the linear function of the tangent elastic modulus delta sigma/delta epsilon versus stress sigma. The slope of the delta sigma/delta epsilon = f (sigma) function was not significantly changed in low contracture compared to control conditions (n = 8; p less than 0.001). When reversibility tests were performed by reoxygenation after 10 min hypoxia and reperfusion with oxygen and glucose after 30 min of glucose and oxygen withdrawal, low contracture tension was completely abolished after 15 min, whereas 28% of severe contracture was maintained even after 60 min of reperfusion. Additional morphological studies revealed uniform sarcomere lengths in low contracture, and two populations of sarcomere lengths in the severe contracture. On the basis of the present and earlier results, a model of contracture tension generation is proposed in which cross-bridges between actin and myosin cycle at a very low rate and are attached in a force-generating position for a long time.