Essex, Hiram E.

Eight dogs with complete atrioventricular heart block showed a definitive relation between the P-Q interval and the systolic left ventricular pressure if the P-Q interval increased or decreased slowly at a regular rate. The ventricular pressure reached a ‘plateau-like’ maximum at a P-Q interval between 0.1 and 0.2 second, and decreased only slightly with somewhat longer P-Q intervals. The lack of abrupt decrease in pressure suggests that the mitral valve closes passively during early atrial diastole and that this prevents rapid regurgitation of blood into the left atrium.

As a result of observations on 50 isolated perfused hearts of guinea pigs it was found that the vagus nerves were active in 48 instances and the average duration of response to stimulation was 108 minutes. In one heart the vagi remained responsive to stimulation for 6 hours. It has again been shown that injections of appropriate doses of l-epinephrine are capable of restoring the unresponsive or exhausted vagal nerves to renewed chronotropic effectiveness.

Potential changes attending excitation of the atrioventricular node, the common bundle, the bundle branches, and initial ramifications of the conducting tissue in the subendocardium of the bovine heart beating in situ have been recorded from unipolar exploring electrodes. Correlations between the form of nodal deflection and the location in the node of the electrode tip have been established. From right bundle branch and subendocardial action potentials, data were derived indicating a rate of transmission through the right branch of 2.02 M/sec. Certain observations on the effects of acute injury and severe hypoxia are recorded. The character of nodal deflections attending retrograde excitation is reviewed. In studies on canine hearts beating in situ , electrograms have been made from the regions in which the atrioventricular node, the common bundle, and bundle branches are believed to lie. Deflections comparable to those recorded in bovine experiments were encountered and, by analogy with those bovine studies, certain morphologic correlations postulated. In general, canine nodal deflections are smaller than their bovine counterparts and lack the variety of form ascribed to potential changes in the several portions of the bovine node. In canine studies, deflections of atrial, atrionodal, and nodal type coexisted in recordings from a single needle electrode suggesting a degree of proximity between the sources of these potential changes not encountered in bovine experiments.

The spread of the excitatory process in the ventricular septum of the dog's heart has been studied in the isolated organ perfused by the Langendorff technic. It has been possible to remove large portions of the free wall of the right and left ventricles and explore the septal surfaces under direct vision. The mean excitatory pathway in the muscular mass of the dog's ventricular septum has been demonstrated to be from apex to base. The left side of the septum is activated before the right side, and the apical part of the right side is activated before its base.

A comparison was made of the circulatory effects of l -epinephrine and l -norepinephrine when various doses were given either intravenously or intra-arterially into anesthetized animals. No significant difference was found between the effects of identical doses of the two drugs on the arterial blood pressure, heart rate and blood flow. The intensity and duration of effect of either drug varied roughly with the dose given.

Solutions of Dextran Ph, Plasmoid, Periston and isotonic saline were compared with serum albumin and whole blood to determine the efficacy of the former as blood substitutes. Rats, guinea pigs and dogs were used as experimental animals. Dextran Ph, Plasmoid and Periston were found to be less satisfactory than whole blood, since 50 to 80 per cent of the rats and guinea pigs survived following their use compared with 100 per cent survival when whole blood was given. There was, however, 100 per cent mortality of the animals when a blood substitute was not given.

On isolated perfused canine hearts, studies were made of the electrocardiographic changes which attend injury to the endocardium of the free wall of the left ventricle and of changes which develop following extension of the injury onto the left ventricular aspect of the septal endocardium. In an epicardial lead, the depression of RS-T segment produced by a subendocardial injury of the free wall of the left ventricle disappeared almost completely when that same type of injury was extended onto the septum.