The force-interval relationship of the left ventricle: a quantitative description in patients with ischemic heart disease

Effect of pacing site on systolic mechanical restitution curves in the in vivo canine model

INTRODUCTION

Pacing site is known to influence the contractile state of the ventricle. Non-physiologic pacing sites such as the right ventricular apex (RVA) or left ventricular freewall (LVFW) have been shown to decrease the contractile state of normal myocardium, due to abnormal electrical propagation. The impact of pacing at these sites may alter mechanical restitution (MR), a fundamental cardiac property involving the electro-mechanical regulation of contraction. This, in turn, may affect cardiac function. The present study was conducted to determine if pacing site alters the time constant of MR: tau.

METHODS AND RESULTS

Anesthetized canines (n = 6) were acutely paced at four sites: right atrium (RA), RVA, right ventricular septum (RVS), and LVFW. MR data was captured by the S1-S2 pacing protocol and used to create MR curves, generating a restitution time constant, tau, at each site. No significant difference in tau was found between pacing sites. A linear regression analysis of MR curves revealed that there was no significant difference in slope between pacing sites.

CONCLUSION

Although pacing site has been found to influence the contractile state of the ventricle, this is the first known study to demonstrate no change in tau in an in vivo preparation. This suggests that alteration of electro-mechanical coupling described by MR is not sufficiently robust to provide insight into pacing site and cardiac function in healthy hearts.

Attenuation of the Negative Inotropic Effects of Metoprolol at Short Cycle Lengths in Humans

OBJECTIVES

This study sought to compare the influence of changes in systolic interval on the negative inotropic effects of metoprolol, sotalol, and verapamil in patients with ischemic heart disease.

BACKGROUND

The long-term symptomatic and prognostic effects of antiarrhythmic agents are not easily predicted on the basis of acute hemodynamic actions at rest, but may be unmasked during tachycardia. However, this has not been studied extensively in vivo.

METHODS

The force-interval relationship of the intact human left ventricle was examined before and 10 min after intravenous bolus administration of the negatively inotropic agents metoprolol, sotalol, or verapamil in patients undergoing diagnostic cardiac catheterization.

RESULTS

All three drugs similarly reduced maximal rate of increase of left ventricular pressures (LV+dP/dt(max)) by approximately 10%, but diversely modified the force-interval relationship. The parameter c (the reduction in LV+dP/dt(max) of a fixed premature stimulus on mechanical restitution) was significantly reduced by metoprolol (by 9+/- 4%, p < 0.05), was increased by verapamil (by 6 +/- 2%, p < 0.05), and was not significantly changed by sotalol. Similarly, metoprolol had a minimal effect on the extent of frequency potentiation, whereas sotalol and verapamil attenuated frequency potentiation (the relative response to 10 s of rapid pacing was 1.19 +/- 0.58-fold, 0.07 +/- 0.35-fold, and 0.03 +/- 0.17-fold of the baseline response after 10 min of metoprolol, sotalol, or verapamil, respectively).

CONCLUSIONS

These results show that the negative inotropic effects of metoprolol are attenuated and those of verapamil are accentuated at short cycle lengths; sotalol is intermediate between the two. These properties may contribute to the relative safety of these agents in patients prone to hemodynamic deterioration during sustained tachycardia.

The positive inotropic effects of milrinone but not of digoxin are attenuated at short cycle lengths

The effects of inotropically active agents on the left ventricular force-interval relation are a potential determinant of their clinical utility and safety. However, little information is available concerning the effects of noncatecholamine positive inotropic agents on this relation. Therefore this study compared the short-term effects of digoxin and milrinone on resting hemodynamics, frequency potentiation (FP), and mechanical restitution (MR) in patients undergoing nonemergency cardiac catheterization. Both digoxin and milrinone produced similar increases in LV + dP/dt at rest (12.2 +/- 1.3%, p < 0.000001 and 11.4 +/- 3.2%, p < 0.01, respectively). The positive inotropic effects of digoxin were marginally attenuated during FP (by 8.5 +/- 4.2% and 4.6 +/- 2.9% at 10 and 60 s, respectively, both p = NS compared with baseline). Similarly, on MRC analysis, the parameter c (a measure of sensitivity of contractile performance to reductions in cycle length) increased by 3.6 +/- 3.7% (p = NS). Whereas the positive inotropic effects of milrinone were not significantly attenuated during FP, they were abolished and possibly reversed at short cycle lengths on MR curve construction (6.8 +/- 5.9% negative inotropic effect at 60% of resting cycle length; p = NS; p < 0.05 vs. resting cycle length). In conclusion, in patients with well-preserved left ventricular systolic function, the positive inotropic effects of milrinone but not of digoxin are markedly dependent on heart rate. These properties may influence both relative safety and efficacy of both agents.