Abstract
<pre class="western">Background
 Observations of short term uniformity or microrhythmical undulation in ventricular pressure courses at sinus rhythm are lacking but necessary to describe cardiac status. The present investigation aims for (a) detecting repetitive similarity patterns in ventricular pressure at isolated sinus rhythm, (b) tagging hemodynamic parameters which contribute most to dissimilarity, and 
 (c) for a stochastical characterisation of the random component in mutual similarity.
 
 Methods
 Left-ventricular pressure curves from isolated working small animal hearts, at sinus rhythm and electrical stimulation, are analysed by autocorrelation.
 
 Results
 Ventricular pressure courses consistently reach their peak coefficient of autocorrelation either at one-beat lag (monorhythm) or at two-beat lag (duorhythm). Replacing sinus rhythm with strictly even electrical right-atrial stimulation provokes more duorhythms to occur (Langer paradox). Duorhythms become scarcer at hypothermia and high cardiac output. Repetition of very similarly shaped beats accords with an exponential law. Variability of twelve hemodynamic parameters, regarding microrhythm, is given as a Table.
 
 Conclusions
 (a) Incidence of alternating patterns (duorhythms) suggests the presence of effective heterometric autoregulation (Frank-Starling law). Consequently, a pacing test may assess contractile reserve in certain conditions. Higher multi-beat patterns occur by chance at isolated sinus rhythm. (b) Interbeat variability of relevant hemodynamic parameters complies with the initial conclusion. Statistical pooling of data from consecutive beats seems permissible; pooling alterant beats separately will do better. (c) Random fluctuation is a constituent part of medium-term ventricular pressure course. Mutually very similar beats occur stochastically by a Poisson process with fade-out. Deviations accord with the presence of mono- or duorhythms.</pre>
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