Certain characteristics of myocardial contractility of isovolumic dog heart at randomly variable heart rhythm

Abstract

The relationship "heart rate - left ventricular pressure" was investigated in the isolated canine heart perfused with constant pressure at different preloads. Rhythmical stimulation was performed with constant stimulus interval duration and with stimulus intervals randomly changed near the average value (150-200 stimuli in series). Correlation and dispersion function analysis show that rhythm dispersion had a negative inotropic effect which was independent of the preload of the ventricle in the range of 120-180 beat/min, but this dependence occurred with low rats of stimulation. This method is proposed for the assessment of contractility under conditions of heart rate variations (physiological and pathological arrhythmias).

Autocorrelation Patterns in the Left-Ventricular Pressure Course of Isolated Working Hearts at Sinus Rhythm

“…It is known (and quite understood) that the first or a few beats after shortening the BI develop slower and lower contraction force [1,21]; enough time for a Bowditch phenomenon to settle was awaited before starting the LVP recordings. Later on, neither the change in heart rate nor in its variability cause contractile effects in the present frequency range [27,28].…”

“…The response curve is adjusted (shifted) by other regulatory pathways and attributed to Ca 2+ sensitivity of the myocardial filament which increases with precontraction sarcomere length (length dependent activation) [20,22,23,24]. As standing to reason, this effect has been thoroughly investigated by sudden preload changes [25,26] and in the presence of spontaneous [16] or artificially induced ventricular arrhythmia [27,28]. Physiological importance of Frank-Starling's law is acknowledged in attuning right and left cardiac output [21]; apart of that, it is considered somewhat to compensate for lacking neuro-humoral regulation (elderly [19]), transplant recipients [5]) or arrhythmia [16,28].…”

“…The persistence of the emerged alternating pattern confirms, in terms of regulation theory [29], that heterometric autoregulation reacts lag-free and does not propagate regulatory information to consecutive beats. A Bowditch effect (development of higher inotropy at pacing, due to higher intramyocytal Ca 2+ turnover [19,21,27]), if any present at this slight overpacing, may further stabilise this alternating beat-to-beat imbalance, because there is no hemodynamic need for an elevated excitation-contraction coupling, which, in turn, only provides more elbowroom to heterometric beat-to-beat regulation. It is known (and quite understood) that the first or a few beats after shortening the BI develop slower and lower contraction force [1,21]; enough time for a Bowditch phenomenon to settle was awaited before starting the LVP recordings.…”

“…Extensive research has been done especially for the ("chrono-")inotropic effects of systematic or random changes in the rhythm of myocardial excitation [1,15,16,27,28]. Its most widespread result says that any remarkable variability in the heart rhythm (at given mean frequency) has a negative inotropic effect.…”