Apparent bradycardia-dependent sinoatrial block associated with respiration

In our previous patients, apparent bradycardia-dependent block has been sho wn in the atrioventricular (AV) junction and in the accessory pathway. It w as suggested that these previous cases were not of true bradycardia-depende nt block; namely, that, as a result of periodic increases in vagal tone ass ociated with respiration, conductivity in the Av junction or in the accesso ry pathway was depressed to a greater degree than automaticity in the sinus node. In the present article, 3 patients with frequent sinoatrial (SA) blo ck were reported. In 1 patient, sinus escape-capture bigeminy caused by SA block was found. In these present patients, when the sinus cycle lengthened , SA block occurred. The purpose of the present article is to show that the patients have apparent bradycardia-dependent SA block, namely, not true br adycardia dependent SA block. In all patients, the respiration curve was re corded simultaneously with the electrocardiogram. In all patients, during i nspiration, the sinus cycle gradually shortened; on the other hand, during expiration, the sinus cycle gradually lengthened, and then a sinus impulse was blocked in the SA junction. These findings suggested that increased vag al tone during expiration depressed conductivity in the SA junction to a gr eater degree than automaticity in the sinus node.