Science commentary: PacemakersBMJ 1998; 316 doi: http://dx.doi.org/10.1136/bmj.316.7143.1492a (Published 16 May 1998) Cite this as: BMJ 1998;316:1492
Cardiac pacing began in the 1950s with the aim of preventing symptomatic bradycardia. Single chamber pacemakers, in which one lead is introduced into the ventricle, achieve this but they cannot increase the heart rate when people take exercise. They also fail to ensure that the atria and ventricles contract in synchrony.
Modern pacemakers now include dual chamber and “rate responsive” devices. With dual chamber pacemakers, leads enter the right atrium and ventricle, allowing both atrium and ventricle to be paced and sensed. If the sinus node is working dual chamber pacing enables atrial activity to be tracked by the ventricle. If the sinus node is diseased extrinsic sensors can supply additional information to decide an appropriate pacing rate. This means that when patient demand goes up, the pacemaker responds accordingly.
For example, some pacemakers contain a piezo-electric crystal to detect motion. When the crystal vibrates—in response to body activity—it produces a tiny voltage which feeds into the circuit, triggering an increase in pacing rate. Other sensors detect the release of catecholamines due to exercise or emotion. Catecholamines shorten the QT interval on the electrocardiograph, which can be monitored easily by the device. Other sensors can detect respiratory rate and acceleration.
Many devices also include safety features to avoid inappropriate rate rises. Some rely on a number of sensors as a cross checking mechanism. These can detect the difference between false positive movements caused, for example, by external motion around a person sitting in a train and true movements caused by walking or running.
Other pacemakers include mode switching devices which are designed to detect atrial arrhythmias. These can distinguish the onset of paroxysmal atrial fibrillation from the sudden onset of exercise and will reset the ventricular rate to a non-tracking mode of 70 beats per minute during the period of the arrhythmia.
The average age of patients receiving pacemakers in Britain is 73. Younger patients are likely to get more sophisticated devices. Several multicentre trials have been set up to establish the best policy, including the UKPACE trial which will assess the clinical impact and the cost utility (the value of quality of life benefits) of dual chamber pacing in elderly patients with heart block.