Inotropic effect of post-stimulation potentiation in man: an echocardiographic study

Mechanical alternans in human idiopathic dilated cardiomyopathy is caused with impaired force-frequency relationship and enhanced poststimulation potentiation

Mechanical alternans (MA) is frequently observed in patients with heart failure, and is a predictor of cardiac events. However, there have been controversies regarding the conditions and mechanisms of MA. To clarify heart rate-dependent contractile properties related to MA, we performed incremental right atrial pacing in 17 idiopathic dilated cardiomyopathy (DCM) patients and in six control patients. The maximal increase in left ventricular dP/dt during pacing-induced tachycardia was assessed as the force gain in the force-frequency relationship (FG-FFR), and the maximal increase in left ventricular dP/dt of the first post-pacing beats was examined as the force gain in poststimulation potentiation (FG-PSP). As a result, MA was induced in 9 DCM patients (DCM MA(+)) but not in the other 8 DCM patients (DCM MA(-)), and not in any of the control patients. DCM MA(+) had significantly lower FG-FFR (34.7 ± 40.9 vs 159.4 ± 103.9 mmHg/s, P = 0.0091) and higher FG-PSP (500.0 ± 96.8 vs 321.9 ± 94.9 mmHg/s, P = 0.0017), and accordingly a wider gap between FG-PSP and FG-FFR (465.3 ± 119.4 vs 162.5 ± 123.6 mmHg/s, P = 0.0001) than DCM MA(-) patients. These characteristics of DCM MA(+) showed clear contrasts to those of the control patients. In conclusion, MA is caused with an impaired force-frequency relationship despite significant poststimulation potentiation, suggesting that MA reflects ineffective utilization of the potentiated intrinsic force during tachycardia.

Postextrasystolic potentiation. Do we really know what it means and how to use it?

Postextrasystolic potentiation (PESP), the increase in contractility that follows an extrasystole, is an interesting phenomenon that has been known for almost 100 years. The literature on this effect is reviewed. It is found that there is significant evidence that the phenomenon is independent of muscle loading and represents a distinct property of the myocardium. Examination of the literature pertaining to the cause of the effect suggests that calcium shifts within the sarcoplasmic reticulum are responsible, although there are some conflicts with this conclusion. Regarding the utility of PESP as a diagnostic test of latent viability of ischemic myocardium, the literature review reveals contradictions and conflicts with several methodological problems of the experiments. Finally, concerning the utility of continuous PESP (paired-pacing) to augment ventricular function in the failing ventricle, the studies again are inconclusive and methodologically suspect. Conditions for the proper analysis of the PESP response are reported, and suggestions for future studies are introduced.

Assessment of potentially salvageable myocardium during acute myocardial infarction: use of postextrasystolic potentiation

Twenty-three patients with evolving acute myocardial infarction (AMI) undergoing catheterization for thrombolytic therapy had interventional contrast ventriculography using programmed atrial stimulation. Postextrasystolic (PES) potentiation was present in 67% of infarct-related segments up to 9 hours after the onset of AMI. The presence of segmental potentiation was not related to time from onset of pain to ventriculography, initial ejection fraction, presence of collaterals, left ventricular end-diastolic pressure or the PES delay. In 18 patients reperfusion was successful using intracoronary streptokinase an average of 6.2 hours after the onset of AMI; in these patients repeat contrast ventriculography was performed an average of 11 days after AMI. Improved chronic segmental ventricular function was predicted by the presence of collaterals to the infarct-related artery at the time of acute catheterization (p = 0.02), but was best predicted by analysis of acute PES potentiation (p less than 0.0001). The predictive value of PES analysis was highest in segments without collaterals. Thus, atrial stimulation is safe during AMI and analysis of segmental ventricular function shows potentially viable myocardium up to 9 hours after the onset of AMI. In addition, analysis of PES segmental function can predict chronic function if reperfusion is successful, especially in segments without collaterals. PES ventriculographic analysis may allow prospective determination of which patients during AMI are most likely to benefit from acute thrombolytic therapy.

Relation between myocardial injury and postextrasystolic potentiation of regional function measured by two-dimensional echocardiography

An experimental study was designed to validate postextrasystolic potentiation assessment of myocardial viability or functional reserve of cardiac segments after acute coronary occlusion. Segmental systolic fractional area changes and wall thickening in pacing-induced postextrasystolic beats were mapped in 12 closed chest dogs by two-dimensional echocardiography during a control period and from 20 minutes to 3 hours after occlusion of the left anterior descending coronary artery. The extent of myocardial ischemic and necrotic zones was evaluated in left ventricular slices and subsegements corresponding to echographic cross sections. During two-dimensional echocardiography, left ventricular segments that were found to be neither ischemic nor necrotic always exhibited a significant augmentation of both fractional area change and wall thickening during the postextrasystolic beat that followed an induced premature contraction with a 42.4% coupling interval. In segments without necrosis but with varying degrees of ischemia, significant postextrasystolic potentiation was also demonstrated, even after 3 hours of occlusion. In contrast, segments that developed more than 80% necrosis failed to potentiate systolic fractional area change after 2 hours, and systolic wall thickening, even after 20 minutes of coronary occlusion. Statistical evaluation revealed a characteristic threshold at 41 to 60% necrosis, beyond which no potentiation of function could be elicited 3 hours after occlusion. Extrapolation from the experimental data suggests that when two-dimensional echographic studies in myocardial ischemia indicate postextrasystolic augmentation of segmental left ventricular function, the latter segments may be assumed to contain only small infarcts or to consist of reversibly ischemic and normal myocardium. Conversely, segments that fail to exhibit postextrasystolic potentiation can be assumed to be more than 60% necrotic.

Effect of beam directional alterations on left ventricular shortening fraction

In this study, the dependence on beam direction of the echocardiographically derived fractional shortening of the left ventricular minor axis was evaluated. A directionally oriented, steerable cursor was used to select M-mode lines from a 2-dimensional image at the chordal level. Seven M-mode traces were derived from each image, 1 along the true ventricular meridian and 3 at known deviations on either side of the meridian. M-mode traces, thus derived, all appeared acceptable for measuring wall or cavity dimensions. Results indicate a progressive increase in shortening fraction (p less than 0.05) for deviations more than 25% of the distance between the meridian and the lateral or medial papillary muscle heads. Decreases in left ventricular dimensions in diastole and systole with progressive deviation from the meridian accounted for the decrease in shortening fraction. Spatial orientation is required for reproducible shortening fraction measurements.

Beat-to-beat left ventricular performance assessed from the equilibrium cardiac blood pool using a computerized nuclear probe

The feasibility, accuracy and reproducibility of continuous beat-to-beat evaluation of left ventricular performance with a computerized nonimaging scintillation probe was assessed in 71 patients. This portable instrument has enough sensitivity to generate a real-time relative left ventricular volume curve using the labeled equilibrium blood pool without electrocardiographic gating. The probe was positioned at the left ventricular and background regions of interest using a systematic series of computerized algorithms and operator routines that were developed and standardized during the initial phase of this study. In each patient, left ventricular ejection fraction was calculated manually from the strip-chart recording in 10 consecutive sinus beats. Beat-to-beat left ventricular ejection fraction determined by the probe correlated well with first-pass studies obtained using a computerized multicrystal scintillation camera (r = 0.92). There was no systematic over- or underestimation, and the correlation was evident over a wide range of first-pass values (15-81%). There was excellent agreement between initial and repeat analyses (n = 58, r = 0.97) and between initial and repeat studies (n = 48, r = 0.94). The absolute variability of beat-to-beat ejection fraction measurements determined from all 710 beats was +/- 5.9% (expressed in ejection fraction units as +/- 2 SD). This technique should provide a reliable means of addressing pathophysiologic questions that require sampling of data directly on a beat-to-beat basis.