The hysteria surrounding Apple Watch 4’s single lead electrocardiogram (ECG) feature has simmered down over the last few months, but it was brought to my attention that pacemaker activity can also be detected by the application. Unlike 12-lead ECGs which provide significant amounts of data to localize pathologies to certain regions of the heart, 1-lead ECGs provide only basic heart monitoring information (heart rate, arrhythmias like atrial fibrillation, etc.) However, because pacemakers operate at the level of myocardial electrical conduction, it’s no surprise that we can see the characteristic “spikes” on the ECG app!
Now for a real life example – I had full consent from the individual to use their ECG strips, pacemaker information and history.
Let’s say a patient has a dual chamber pacemaker (leads in the right atrium and right ventricle) placed for a slow junctional rhythm in AAI mode at 60 beats per minute (bpm). In this mode, the right atrium is sensed. If an electrical impulse is sensed at least 60 times per minute (ie, from SA node depolarization), then the pacemaker does nothing. If no signal is sensed, then the atrium is paced at 60 bpm. This mode relies on normal conduction through the atrioventricular (AV) node to depolarize the ventricles thereby maintaining AV synchrony.
The top image represents normal sinus rhythm; however, in the same patient, the bottom image shows atrial pacing (at the pre-programmed rate of 60 bpm) with clearly visible atrial spikes.
Now what’s even cooler is this Medtronic pacemaker has a managed ventricular pacing (MVP) feature. This is essentially a ventricular pacing backup if normal AV conduction is lost. The device will switch from an AAI mode to a DDD mode, periodically assess if normal AV conduction returns, and then switch back to AAI pacing at that point. You can even see a short run of atrioventricular pacing (both atrial and ventricular spikes) here on the Apple Watch ECG app! How cool!