An unusual case of upper rate behaviour

Abstract

because of high-degree atrioventricular (AV) block due to Lyme Figure: Tracing A: ECG at exercise stress test with sudden onset of a 2:1 block and occasional appearance of late coupled beats with a different QRS morphology (most likely delayed afterdepolarisations) at a heart rate of 135 bpm. Tracing B: Pacing in VVI mode (maximum output of 7.5 volts and maximum pulse width of 1.5 milliseconds) with loss of 1:1 ventricular capture and onset of first 3:2 followed by 2:1 ventricular capture at heart rate of 140 bpm.