Fig. 1

a shows the left ventricular (LV) pressure (P)–volume (V) loops of 14 arbitrarily chosen continuous arrhythmic contractions under electrically maintained atrial fibrillation in a canine heart. b shows one of the P–V loops to explain abbreviations. In a, three P–V loops show their E_max values as the slopes of the respective end-systolic P–V lines drawn from the common dead volume on the volume axis. In b, the area within the quasi-rectangular P–V loop, mostly shaded, is the external work (EW), and the quasi-triangular area, also mostly shaded, under the E_max line on the origin side of the EW area is the mechanical potential energy (PE). The systolic pressure–volume area (PVA) is the sum of these two areas, namely, PVA = PE + EW. The stroke volume (SV) is the width of the P–V loop; the end-systolic pressure (ESP) is the pressure at the left upper corner of the P–V loop; the end-diastolic volume (EDV) is the volume at the right lower corner of the P–V loop. V₀ is the dead volume at which end-systolic pressure is zero