Simultaneous recordings of contractile tension and transmembrane potentials from canine ventricular tissue yielded a consistent correlation of action potential (AP) alteration with contractile change associated with abrupt rate change. The AP terminating a relative prolongation of the cycle-length manifested shortening of phase 2 with lengthening of phase 3 and was associated with potentiation of contractile force. Conversely, the AP terminating a relative abbreviation of cycle-length displayed a broader phase 2 with a more precipitous phase 3, while the associated contraction was less forceful than the control. In each circumstance, the relative magnitude of cycle-length change correlated with the extent of both AP change and contractile alteration. Changes in AP configuration may reflect changes in transmembrane flux of K + during repolarization consistent with the findings of prior workers who have related K* efflux to increased contractility. Mechanical altemans, in addition, was frequently observed in association with abrupt rate change and was consistently associated with an electrical altemans manifested by action potentials with alternately wide and narrow plateaus (phase 2). As above, the more forceful contractions were associated with action potentials which displayed a narrower "phase 2. Mechanical altemans initiated by abrupt fate change may represent an adaptive phenomenon prior to the establishment of a stable contractile state, as reflected by a stable AP configuration. ADDITIONAL KEY WORDScardiac repolarization and contractility cardiac contractility cardiac action potentials post-extrasystolic potentiation compensatory pause and repolarization mechanical and electrical altemans rest potentiation cycle-length, repolarization and contractility• Augmentation of cardiac contraction associated with abrupt rate change has been noted repeatedly in a variety of mammals (1) as well as the human (2) since first described by Langendorff in 1875. The mechanism by which such a phenomenon occurs has, nevertheless, remained obscure and of little more than academic concern until recently. With
A B S T R A C T A series of experiments were performed upon intact anesthetized dogs to determine the relevance of a variety of hemodynamic variables to the irregular ventricular performance associated with atrial fibrillation. During experimentally induced atrial fibrillation central aortic pulse pressure was measured in relation to the duration of the preceding diastolic interval, the relative degree of cycle-length change, the magnitude of the preceding aortic end-diastolic pressure, the rate of ventricular tension development (at a fixed diastolic tension), and to ventricular end-diastolic pressure. While all of the latter variables bore a significant relation to the chosen parameters of ventricular function, the most linear correlation lay with the rate of ventricular tension development. It has been suggested, as a consequence, that the irregular ventricular performance observed during atrial fibrillation under these experimental conditions, may be more directly related to variation in the inotropic state of the ventricular myocardium than to an expression of the Frank-Starling concept, resulting from variable ventricular filling. The lability of the inotropic or contractile state has in turn been attributed to abrupt cycle-length change effecting inotropic alteration analogous to postextrasystolic potentiation of contractility and, at rapid rates, effecting an alternation of the contractile state.
Complete bundle-branch block was found in 57 of 1,560 (3.7%) electrocardiograms routinely recorded on members of a retirement community. Of this number, 19 were identified as left bundle-branch block (LBBB), while 38 fulfilled the criteria for right bundlebranch block (RBBB). Individuals with LBBB were found to have cardiomegaly more frequently than did individuals with normal electrocardiograms. Individuals with RBBB, regarded as a homogeneous group, also demonstrated a greater prevalence of cardiomegaly than did the control group, but the difference from normals was less striking than in the case of LBBB. Neither LBBB nor RBBB was significantly associated with hypertension, as defined in the text. When the common, uncomplicated RBBB was regarded as a separate entity, the prevalences of hypertension and cardiomegaly were indistinguishable from those of a control group with normal electrocardiograms. The RBBB variant, regarded as a possible manifestation of left ventricular disease secondary to hypertrophy or myocardial infarction, was found to be frequently associated with hypertension, cardiomegaly, and the need for digitalis therapy. RBBB with left axis deviation, regarded as a complication of the common RBBB due to left ventricular hypertrophy or myocardial infarction, was similarly associated with a high prevalence of cardiovascular disease. None of the 38 cases of RBBB fulfilled the criteria for classic RBBB. It is suggested, therefore, that the chronic uncomplicated RBBB in the adult may indeed bear a less ominous portent than has previously been ascribed to it. This is suggested only in relation to the parameters evaluated, however; prospective evaluation is clearly necessary to assess the validity of such an inference.
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