The purpose of temporary pacemakers is to sense intrinsic electrical activity and pace the myocardium from the inside (ie, transvenous wires) or outside (epicardial wires). Following cardiac surgery, surgeons often place bipolar leads on the right ventricle (RV) for ventricular sensing/pacing +/- right atrial (RA) leads for synchronized atrioventricular pacing. A current is delivered between the two bipolar leads and “captured” by the myocardium resulting in depolarization.
There are three major settings for pacers:
- Rate: the number of impulses/minute
- Output (mA): the current needed for the myocardium to “capture” the paced signal. In other words, how energy much are you jolting the heart with. Atrial output and ventricular output range from 0.1 – 20 mA and 0.1 – 25 mA, respectively.
- Sensitivity (mV): the point at which the pacing device interprets a native myocardial signal as a pulse. The greater the millivolts, the less sensitive the device is to native cardiac activity which can lead to undersensing and overpacing potentially causing the pacer to fire on a native heart beat (R-on-T phenomenon). At higher sensitivities (lower millivolts), any – little – signal (including breathing, patient movement, etc.) will be interpreted as a native beat (oversensing).
To “test” a pacer, I select a mode (typically DDD for A-V pacing wires and VVI for only V pacing). I’ll start with the default output and sensitivity settings and increase the pacer rate to something above the patient’s native heart rate. Then I’ll watch the continuous EKG to ensure that the signal is indeed captured at the preset rate while dialing down the output.
If a pacer loses its ability to capture, one must consider possible changes in sensitivity thresholds, electrolyte changes, damaged lead(s), battery depletion, or faulty cables. If I seem to have competition between the pacer and the patient’s native heart rate, I’ll adjust the sensitivity to avoid the aforementioned scenarios of over/undersensing.
Drop me a comment below with questions! Also, be sure to check out my more thorough post on pacemaker nomenclature and whether or not a magnet is needed.