I was handed this ECG and asked if it was an example of a rate adaptive pacing, which uses changes in transthoracic impedance to increase the pacemaker rate in response to physiologic demand.

Firstly, note that this is not a paced rhythm. However, these pacing algorithms create regular artefact on the ECG tracing, whether they are pacing or not in the ventricle. In the above illustration, the artefact initially appears regular at about every 50 msec. It is seen best in the limb leads, but with the rhythm strip of lead II, they “deteriorate” later in the strip.I then reprinted the ECG as rhythm strips.

Quite clearly the this is not regular in all leads and is therefore extracorporeal artefact.In 1987, the Australian pacemaker company Telectronics, created a rate adaptive sensor, requiring a standard bipolar lead. The rate response was achieved by passing a very small subthreshold unipolar current of 1mA for 15 microseconds every 46 ms from the anode to the pulse generator can across the thorax.

Sensing of this current was between the cathode and the pulse generator can. With each respiratory cycle, the impedance of the current changed reflecting changes in respiratory rate and tidal volume i.e. minute ventilation.The ECG shows the small very regular spikes each 46 msec.Boston Scientific still uses minute ventilation as a dual sensor with activity.Here are two 12-lead ECG examples, showing the sensor artefact in patients who are ventricular paced.

The size of the s[pikes is very dependent on the ECG filters and not infrequently very small in leads I, V5 and V6. This must be the way the ECG leads are orientated.Another impedance sensor is “Closed Loop Stimulation or CLS” by Biotronik. This is not transthoracic, but rather biphasic intraventricular stimulation and the energy is delivered into the paced or sensed QRS, 50 msec after the commencement of depolarization and continues for 250 msec.

The ECG features are very characteristic and cannot be mistaken for the minute ventilation sensor. Note that in this example, there is no ventricular pacing.Harry Mond