Although DDD pacing allows physiologic pacing, its effectiveness is limited by chronotropic incompetence as demonstrated by the following two ECGs.
Atrial fibrillation, with right bundle branch block and a very slow regular ventricular response, i.e., complete heart block. Ventricular pacing would provide a satisfactory resting heart rate, but no rate response to physiologic demand.
Sinus bradycardia, sino-atrial block, sinus arrest and junctional escape beats, i.e., severe sick sinus syndrome by ECG criteria. Atrial pacing would provide a satisfactory resting heart rate, but no rate response to physiologic demand.
Poor rate response with sick sinus syndrome and a pacemaker programmed DDD.
Holter monitor rate profile with a low rate of 60 bpm (red line). The maximum rate achieved over the 24 hours was about 80 bpm due to the native rhythm.
The poor rate response to exertion can be overcome with rate adaptive pacing. This requires a sensor which accurately emulates changes in cardiac output in response to physiological and emotional stimuli. In order to be incorporated into a pacing system, the sensor has to be small, solid state, low cost, low power consumption, reliable, non-toxic, non-thrombogenic, non-carcinogenic. The sensor should lie within the hermetically sealed pulse generator rather than on the lead. Between 1976 and 1992, thirteen different rate adaptive designs were developed and implanted. However, only those incorporated into a standard pacing system were commercially successful; activity/accelerometer, minute ventilation and closed loop stimulation. More recently, central venous temperature has been incorporated into a leadless pacing system.
The most popular sensor by far is the relatively non-physiologic accelerometer, available from all manufacturers. Today all pacemakers have an incorporated sensor (R). Once programmed ON, the letter R is added to the pacing mode.
With rate adaptive pacing, there is a pacing rate increase with exercise testing. This can be seen with VVIR pacing for underlying atrial fibrillation.
or DDDR pacing with sick sinus syndrome.
Are there any characteristic features on the ECG associated with sensors?
Several rate adaptive pacing systems have used impedance measurements to document physiologic changes in order to alter the pacing rate. The systems use low voltage pulses, usually between one of the poles on the lead and the pulse generator can. Because these are effectively unipolar, they can create characteristic interference patterns on the surface ECG. In turn, these patterns are modified by the filters used in the ECG machine.
Two of the rate adaptive sensors used today use impedance measurements, minute ventilation and closed loop stimulation.
Minute ventilation, the product of respiratory rate and tidal volume is a highly physiologic variable, which closely reflects the metabolic demands of exercise, including cardiac output and heart rate. Transthoracic impedance varies with respiration, falling with inspiration and rising with expiration. The system can use a conventional bipolar pacing lead without the need for a special sensor.
A: Low energy, subthreshold pulses about every 50 ms. The pulse generator recognizes the changes in impedance (green arrow) as changes in tidal volume and the oscillations as respiratory rate, the product of which is minute ventilation.
B: The ECG demonstrates the pulses as continual regular interference (red highlight).
The 12-lead ECG can be very dramatic and is usually diagnosed as extra-corporeal interference with the ECG repeated several times.
Note the absence of artefact in lead I (red highlight). The arm leads lie at a distance from the current and do not detect the pulses. The ECG machine also recognizes the pulses as interference (yellow highlight). The interference is present with both paced and sensed rhythms.
Because of ECG filtering, a variety of appearances can be seen.
Hyperventilation will result in pacing at the programmed upper sensor rate, which may result in the diagnosis of ventricular tachycardia.
The patient suffered from severe anxiety. Following sedation, the rate fell to 90 bpm. The underlying rhythm is atrial fibrillation and there are no interference pulses.
The pulse generator may also detect extra-corporeal pulses resulting in inappropriate sensor upper rate pacing.
Patient in emergency room, awake and alert. On attachment of ECG monitoring electrodes, the heart rate rose to sensor upper rate. With this monitor, a small current is passed through the chest electrodes and the skin impedance measured. A marked rise occurs if a monitoring lead is detached from the skin. Despite the stimulus artefacts (V5) on a simultaneous 12-lead ECG (red highlight), ventricular tachycardia was diagnosed, and cardioversion unsuccessfully attempted many times.
Closed loop stimulation (CLS). The increase in catecholamine stimulation during exercise and emotional stress will increase ventricular contractility, which can be indirectly measured using intracardiac impedance changes between the cathode of a standard pacing lead and the pulse generator can.
About 16 subthreshold biphasic pulses are used each 46 milliseconds and in contrast to the minute ventilation sensor, the pulses commence 50 milliseconds into the sensed or paced QRS (red highlight) and cease after 300 milliseconds (red and yellow highlight), creating a diagnostic artefact appearance on the ECG.
12-lead ECG demonstrating the typical interference pattern from a CLS rate adaptive pacing system (Biotronik, Germany). There are no artefacts for the first 50 ms of the QRS (red highlight) and artefact for the next 250 ms (yellow highlight). The sensor is useful for neuro-cardiogenic syncope and thus most examples do not show paced rhythm.
An example showing a paced rhythm.
VVIR with underlying atrial fibrillation.
Once again, ECG filters can modify the artefact appearance, with recognition depending on the artefact being limited to a window within the QRS/T wave (red highlight).
It is important to identify and exclude atrial and ventricular stimulus artefacts (blue highlight).
Pattern recognition is important to prevent having to repeat the ECG!
In 49+ years as a practicing cardiologist, Dr Harry Mond has published 260+ published manuscripts & books. A co-founder of CardioScan, he remains Medical Director and oversees 500K+ heart studies each year.
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