Nonrandom ventricular rhythm in horses with atrial fibrillation and its significance for patients.

RR interval sequences during spontaneous atrial fibrillation in eight horses were analyzed as in previous studies in patients and dogs using histograms and serial auto-correlograms. In patients and dogs with spontaneous atrial fibrillation, ventricular rhythms were always random. In the horses, the histograms were skewed with median RR intervals of approximately 1,000 ms. A striking finding in these animals was the presence of long RR intervals up to 5,000 ms in duration. The shortest RR intervals lasted 400 to 600 ms. In contrast to findings in dogs and patients, the serial autocorrelograms showed periodicity that was reenforced by digitalis (n = 3), but eliminated by quinidine (n = 2) and atropine (n = 2). Quinidine and atropine eliminated the longer RR intervals, whereas digitalis increased the number of long RR intervals. In one horse, it was possible to measure intraarterial pressure, and large fluctuations in pressure were observed as the RR intervals varied from over 3,000 to less than 500 ms. It is postulated that these changes in blood pressure are associated with baroreceptor responses that may alter the electrophysiologic behavior of the atria and atrioventricular node. These changes cause the nonrandom patterns of ventricular rhythm in the horse. Because such very long RR intervals do not occur in human beings or dogs during atrial fibrillation, the random ventricular rhythm in these groups is maintained even during digitalis treatment.

Role of atrial fibrillation and atrioventricular conduction (including Wolff-Parkinson-White syndrome) in sudden death.

A short refractory period of the accessory pathway is considered a major threat for sudden death in patients with Wolff-Parkinson-White syndrome and atrial fibrillation. RR interval and QRS signal analysis together with signal analysis of a bipolar high right atrial electrogram were obtained in six patients with Wolff-Parkinson-White syndrome and either induced or spontaneous atrial fibrillation. A record of a sufficiently long episode of atrioventricular (AV) conduction by way of the bypass tract that could be used for satisfactory RR interval sequence and QRS analysis was obtained from only one patient. The results were compared with those of a representative patient with atrial fibrillation and normal AV nodal-His conduction. In a patient with Wolff-Parkinson-White syndrome, atrial fibrillation and AV conduction by way of the bypass tract may exhibit high ventricular rates (median RR intervals of about 300 ms) and long/short RR interval ratios of just over 1 (RR intervals not exceeding 400 ms). The right atrial electrogram showed a noiselike excitation pattern. This study suggests that rather than a short refractory period of the bypass tract, it is lack of concealed conduction, responsible for the presence of long RR intervals, that allows the ventricles to reach very high ventricular rates and at times to fibrillate. The normal AV nodal-His system seems to protect the heart against high ventricular rates and ventricular fibrillation during atrial fibrillation by its relatively long refractory period and capacity to induce long RR intervals by means of concealed conduction.

Low urinary oestrogen excretion during pregnancy, due to an impairment of fetal adrenocorticotrophic hormone secretion.

Four cases of newborn infants whose mothers had a low urinary oestrogen excretion during pregnancy are reported. Placental sulphatase deficiency in placental insufficiency were excluded by a dehydroepiandrosterone sulphate loading test. Postnatally, they developed a clinical picture characterized by an inappropriate secretion of cortisol which, by the results of an adrenocorticotrophic hormone stimulation test, appeared to be due to an impairment of adrenocorticotrophic hormone secretion. The prenatal and postnatal steroid metabolism is discussed.

Postextrasystolic relaxation in the dog heart.

Left ventricular relaxation was studied in 8 dogs using parameters derived from the left ventricular pressure: the fastest pressure fall and the time constant of pressure decline. Effects of extrasystolic rhythm interventions were examined on the relaxation parameters of the post-relative to the preextrasystolic beat. Postextrasystolic potentiation of these parameters could not be demonstrated. Possible influences of physiologic variables as peak left ventricular pressure, endiastolic aortic and enddiastolic left ventricular pressure on relaxation mechanism were evaluated. The nonselective description of myocardial relaxation by pressure derived parameters is discussed.