Mobitz Type II Second‐Degree Atrioventricular Block during Dobutamine Stress Echocardiography. True or False?

Abstract

Dobutamine is a sympathomimetic drug and a structural analog of isoproterenol. Its primary mechanism is the direct stimulation of the b 1 receptors of the sympathetic nervous system. When used for stress echocardiography, it rarely causes transient second-degree atrioventricular (AV) block (0.6-1.1%). 1-3 The development of transient complete AV block is even rarer. 4-6 In this respect, Varga et al. 4 found only 2 cases of transient complete AV block in a study of 35 103 cases of dobutamine stress echocardiography in an investigation where the incidence of seconddegree AV block was not examined.Transient second-or third-degree AV block induced by dobutamine during stress echocardiography may be produced by several mechanisms such as myocardial ischemia, the Bezold-Jarisch reflex, and latent abnormalities in the His-Purkinje system. The latter are important because an erroneous diagnosis of infranodal block or Mobitz type II AV block may lead to inappropriate pacemaker implantation. So far, Mobitz type II AV block has been reported in only one study (2% of patients) which is widely cited in review articles about dobutamine stress echocardiography. 7 The question then arises as to whether the occurrence about Mobitz type II AV block is true or false.Can Dobutamine Cause Mobitz Type II Second-Degree AV Block?: Hung et al. 7 found 12 patients who developed transient second-degree AV block (AVB) in 302 patients who underwent dobutamine stress echocardiography. The incidence of AV block was 4.0%, indicating a higher incidence than that reported in the safety studies. The duration of transient AV block ranged from 0.75 to 3.93 minutes. Mobitz type I AV block was noted in 6 patients and Mobitz type II block in the other 6 patients. One to one AV conduction resumed after intravenous injection of atropine in the 6 patients with Mobitz type I AV block, but not in the 6 patients with Mobitz type II AV block. The incidence of Mobitz type II AV block was 2% and its occurrence had not been previously reported. Nine of these 12 patients were subsequently shown to have coronary artery disease (CAD) and three had no significant coronary artery stenosis. Mobitz type II AV block was observed only in patients with CAD, whereas Mobitz I block occurred in 3 patients with and three patients without CAD (P < 0.05). Eight of the 9 patients with CAD underwent successful coronary angioplasty with or without stenting and repeat dobutamine stress echocardiography revealed no recurrence of inducible AV block except in 1 patient so that Hung et al. 7 postulated that AV block was related to myocardial ischemia though they admitted that neurally mediated vagal reflex might have also been responsible for the result. Mechanisms of AV Block Induced by Dobutamine:Vagal-mediated effects by the Bezold-Jarisch vasodepressor reflex from excessive inotropic stimulation of left ventricular mechanoreceptors may have been a contributing factor in the study of Hung et al., 7 an assumption supported by positive head-up tilt testing in all 3 patients with Mobi...