Changes in Morphology of the Paced QRS Complex Related to Atrial Contraction

A patient with 2:1 AV block underwent temporary ventricuJar pacing. AU the paced stimuli resuited in ventricular capture, but a marked variability in morphology of the paced QRS complexes occurred. Two different types of paced QRS complex (labeled A and B) were recognized. Type B complexes were manifest only when the pacing stimulus was preceded hy a sinus P wave within a time interval ranging from 0.15 to 0.52 sec. The P wave-induced changes in morphology of the paced QRS complexes were interpreted as due to displacement of the pacing ventricular lead caused by atrial systole.     

Heterogeneity in electrical activity of the canine ileal circular muscle: interaction of two pacemakers

Abstract In cross-sectioned slabs from the muscularis externa or in the isolated circular muscle devoid of longitudinal muscle and myenteric plexus (MyP) of ileum, a slow wave, sigmoidal or triangular in shape, was recorded from microelectrode impalements near the deep muscular plexus (DMP) region in the whole-thickness preparation. From the MyP region of whole-thickness preparations, a slow wave which oscillated at a similar frequency (9–10 cycles min^?1) was characterized by a fast upstroke and a square shape. Slow waves of μmixed pattern were recorded in the outer circular muscle (OCM) while triangular slow waves were present near the submucous plexus (SMP). In this preparation but not in isolated circular muscle, the inhibitory junction potentials (IJPs) produced by supramaximal electrical field stimulation triggered slow waves. The amplitudes (15–25 mV) of spontaneous and triggered slow waves (TSWs) were greatest in the MyP region, significantly so compared to those of DMP and SMP regions and to those in all regions of isolated circular muscle. Frequencies of slow waves recorded from the MyP and DMP were slightly but significantly higher than those recorded from either the OCM or the SMP or from all regions of isolated circular muscle. A 10–15 mV gradient in resting membrane potential (more hyperpolarized near MyP) existed across the intact (but not the isolated) circular muscle layer. Both types of slow waves, TSWs and IJPs were unaffected by muscarinic, adrenergic or tachykinergic blockade. We suggest that a MyP pacemaker network generated a plateau-type slow wave while a DMP one induced a triangular slow wave. Each source can function independently but the MyP network may dominate and entrain DMP slow waves.     

Left Origin of the Atrial Esophageai Signal as Recorded in the Pacing Site

Clear atrial depolarizations from inside the esophagus have always been recorded in electrocardiology, but their precise origin is still under discussion. Though atriol signals are recorded along most of the esophagus, pacing of the atria is possible only in a short tract, probably where the esophagus is in contact with the posterior left atrium wall. In order to ascertain which portion of atria gives rise to the esophageai atrial signal recorded in the atrial pacing segment, we examined 37 patients with normal P waves on the standard ECG by inserting esophageai and endocavitary catheters. The interval between the earliest start of the P wave and the bipolar atrial deflection, was measured both through the esophagus (PA-Eso) and the Hisian region (PA-His) (the latest depolarization of interatrial septum). The former was longer than the latter (P < 0.001) in 36 of 37 patients, showing that the esophagus recorded atrial signal, at the site of effective pacing, originates outside the interatrial septum. As the atrial depolarization recorded through the esophagus is significantly delayed compared with the Hisian region recording, a pure left origin of the esophageai signal can be hypothesized. This is supported by the well-known delayed depolarization, during sinus rhythm, of the left atrium posterior wall compared with the right atrium and interatrial septum. Measuring the interval between the standard ECC P wave and atrial depolarization recorded through esophagus in the site of effective pacing, provides a reliable noninvasive estimate of interatrial time conduction.     

Irregular Sinus Parasystole Due to Intermittency and Modulation of Parasystolic Activity

Intermittent Sinus Parasystole. A case of intermittent sinus parasystole in which the parasystolic focus is protected from the dominant sinus rhythm only during the second half of its intrinsic cycle is reported. In addition, a modulating (i.e., electrotonic) effect is often clearly exerted from the dominant rhythm upon the focus during the protected period. Coexistence of both modulation and intermittency in sinus parasystole, as well as a modulating effect limited to the second part of the parasystolic cycle, have not been previously reported.