Active shortening of the ascending epicardial segment continues for 92+/-33 ms during the rapid fall of ventricular pressure, indicating 'isovolumetric relaxation' involves active contraction.
Active myocyte shortening continues during the rapid fall of ventricular pressure, challenging the classic concept of 'isovolumetric relaxation' and suggesting it is an active process of 'systolic ventricular filling'.
OBJECTIVE: To study the 'isovolumetric relaxation' phase of rapid ventricular filling by analysis of the shortening of cardiac muscle in the endocardial and epicardial segments of the left ventricle in the dual helical model of the ventricular band, described by Torrent-Guasp. METHODS: In 10 pigs (27-82 kg), temporal shortening by sonomicrometer crystals was recorded while recording ECG, and measuring intraventricular pressure and dP/dt with Millar pressure transducers. RESULTS: The following sequence was observed; shortening began in descending or endocardial segment, and 82+/-23 ms later it was initiated in the epicardial or ascending segment of the band. The descending segment stops shortening during the rapid filling phase of fast descent of ventricular pressure, but the ascending segment shortening continues for 92+/-33 ms, so that active shortening continues during the period of isovolumetric relaxation. During the rapid filling phase, dopamine decreased the interval between completion of endocardial and termination of epicardial contraction from 92+/-20 to 33+/-8 ms. Conversely propranolol delayed the start of epicardial shortening from 82+/-23 to 121+/-20 ms, and prolonged the duration of endocardial contraction, causing a closer (21+/-5 ms vs 92+/-20 ms) interval between termination of contraction of endocardial and epicardial fibers. The resultant slope of the rapid descent of the left ventricular pressure curve became prolonged. CONCLUSIONS: These time sequences show that ongoing unopposed ascending segment shortening occurs during the phase of rapid fall of ventricular pressure. These active shortening phases respond to positive and negative inotropic stimulation, and indicate the classic concept of 'isovolumetric relaxation', IVR, must be reconsidered, and the new term 'isovolumetric contraction', IVC, or systolic ventricular filing may be used.
Buckberg et al. (2006) studied Ventricular filling / isovolumetric relaxation (n=10). Dopamine and propranolol was evaluated on Temporal shortening of cardiac muscle in endocardial and epicardial segments. Active shortening of the ascending epicardial segment continues for 92+/-33 ms during the rapid fall of ventricular pressure, indicating 'isovolumetric relaxation' involves active contraction.