Post-extrasystolic potentiation in a human fetus detected during measurement of systolic time intervals in labour

Abstract

Measurement of the pre-ejection period (PEP), a non-invasive index of cardiac function, was made in a human fetus with a disturbance of cardiac rhythm manifested by: (i) intermittent mechanically ineffective extrasystoles (ventricular premature beats) and (ii) pauses. The mean interval between the sinus beats before and following extrasystoles was virtually the same as that before and following pauses. The PEP was shortened at the first sinus beat following both extrasystoles and pauses, but to a greater extent following extrasystoles. The PEP was still shortened at the second sinus beat following extrasystoles, but prolonged at the same cardiac cycle following pauses. These data demonstrate that fetal cardiac post-extrasystolic potentiation was taking place and that this was due in part to inotropic potentiation of the post-extrasystolic beats. Inotropic potentiation of post-extrasystolic beats is due to a fundamental property of cardiac muscle relating frequency of stimulation to strength of contraction (the so-called ‘force-interval’ relationship). Our data present the first evidence, to our knowledge, that the inotropic state of the human fetal heart in vivo is modulated by this mechanism.