The external work (EW) efficiency of individual arrhythmic beats of the left ventricle (LV) cannot directly be obtained since LV O(2) consumption (VO(2)) of each beat cannot directly be measured under beat-to-beat varying contractile and loading conditions. We, however, have recently reported that VO(2) of each arrhythmic beat can reasonably be estimated by VO(2) = aPVA + bE(max) + c even under varying PVA and E(max). Here, PVA is the LV pressure-volume (P-V) area as a measure of the LV total mechanical energy, E(max) is the LV end-systolic elastance as an index of the LV contractility, a is a constant O(2) cost of PVA, b is a constant O(2) cost of E(max), and c is the basal metabolic VO(2) of the beat, all on a per-beat basis. Using the above formula in this study, we calculated VO(2) of the individual arrhythmic beats from their measured PVA and E(max) during electrically induced atrial fibrillation (AF) in normal canine hearts. We then calculated their LV EW efficiency by dividing their measured EW with the estimated VO(2). We found that the thus calculated EW efficiency of the arrhythmic beats had a rightward skewed distribution with a mode of 15% and a maximum of 18% around a mean of 13% on average in six hearts. This mode remained comparable to the efficiency (15%) at regular tachycardia though 22% lower than mean arrhythmic tachycardia.