High Mechanical Efficiency of Left Ventricular Arrhythmic Contractions during Atrial Fibrillation

Abstract

Abstract:We analyzed the frequency distribution of the left ventricular (LV) mechanical efficiency of individual arrhythmic beats during electrically induced atrial fibrillation (AF) in normal canine hearts. This efficiency is the fraction of the external mechanical work (EW) in the total mechanical energy measured by the systolic pressure-volume area (PVA). The mean, median, and mode of this efficiency (EW/PVA) were as high as 78%, 80%, and 81%, respectively, on average in six hearts. These high efficiencies were comparable to that of the regular beats in these hearts. The frequency distribution of the EW/PVA during AF tended to skew to the higher side in all the hearts. Since the EW/ PVA is directly related to both the ventriculo-arterial (or afterload) coupling ratio (E a /E max ; E a = effective arterial elastance, E max = end-systolic ventricular elastance) and the ejection fraction on a per-beat basis, we also analyzed their frequency distributions. We found them to skew enough to account for the rightward skewed frequency distribution of the EW/PVA during AF with the unexpectedly high mean EW/PVA. These results indicate that the LV arrhythmia during AF per se does not directly suppress the mean level of LV mechanical efficiency in normal canine hearts.Key words: heart, arrhythmia, external work, pressure-volume area.We have been investigating the statistical characteristics of major mechanoenergetic variables of the left ventricle (LV) beating arrhythmically during atrial fibrillation (AF) in normal canine in situ hearts [1][2][3][4][5][6]. In these studies, we have found the positive correlation between E max and the preceding beat interval [5]; the apparently nonnormal or non-Gaussian frequency distributions of the beat interval, heart rate, and effective arterial elastance (E a ) [1,2]; and the normal or Gaussian distributions of the end-systolic pressure-volume (P-V) ratio or maximal elastance (E max ) as a load-independent contractility index and the systolic P-V area (PVA) as a measure of the LV total mechanical energy [3,7]. As for the E max , E a , and PVA [7][8][9][10][11][12], see the LV P-V diagram schematically shown in Fig. 1A.We have also proposed a new method of estimating LV oxygen consumption of individual arrhythmic beats during AF from a set of E max and PVA of the respective arrhythmic beats [4]. Through these studies, we have elucidated much of the statistical characteristics of the LV mechanoenergetics during AF. We then suspected a possibility that the energy conversion efficiency from PVA to the external mechanical work (EW) of arrhythmic beats in normal hearts could remain high even during AF. This possibility seems to be neglected in discussions on cardiac failure of diseased hearts during AF [13,14].We therefore analyzed the statistical characteristics of the LV mechanical efficiency (EW/PVA) from PVA to EW during electrically induced AF in normal canine hearts. Contrary to the general impression [13,14], we found that even the arrhythmic LV during AF could keep on average a rea...

Mode of frequency distribution of external work efficiency of arrhythmic beats during atrial fibrillation remains normal in canine heart

“…EW), EDV, and ESP also fluctuated simultaneously. As for these fluctuations under AF, we have already analyzed them in detail [4,5,8,[12][13][14][15]. Figure 2 plots LV EW (open circles) and its efficiency (solid circles) against the predicted LV VO 2 of arrhythmic beats for 30 s in one canine heart.…”