A typical pacemaker has 2 electrodes – one in the right atrium and one in the right ventricle. If the myocardium produces its own electrical discharge (intrinsic beat), the pacer will turn itself off and not produce a pacer spike; this is called sensing. If a pacer possesses a sensor, this is called a demand pacemaker, i.e. it will only discharge if a long pause occurs.
The electrodes touch the endocardium and stimulate the myocardium much like the terminal Purkinje cells do. If the myocardium depolarizes following the pacer spike, this is called capturing.
Hence we frequently describe the pacer as:
- 100% sensing – turns off when indicated and produces a spike when needed
- 100% capturing – every spike is followed by a P-wave or QRS complex
Pacemakers are typically coded according to the pacing chamber, sensing chamber and the response to sensed event. Two common pacers are as follows:
VVI – Paces the ventricle, senses the ventricle activity, inhibited by intrinsic activity
DDD – Paces the atrium and ventricle, senses the atrium and ventricle activity, inhibited or triggered according to need
- D = Dual Chamber
- V = Ventricle
(T + I) or (Atrium + Ventricle) Where…
- I = Inhibited
- T = Triggered
Clinical Comment: Please remember that the electrical activity of pacer spike to P-wave to pacer spike to QRS complex is not synonymous with a pulse. This might represent Pulseless Electrical Activity (PEA). You must check to see if a pulse is present with each beat.
To continue our discussion of the pacer rhythm, let us look at the example rhythm strip below. The pacing electrode is typically placed in the apex of the right ventricle. The right ventricle is depolarized first in time sequence followed by the depolarization of the left ventricle. Consequently, the ECG tracing will appear as a LBBB pattern. The pacer spike is the leading electrical activity of the QRS complex, but please be aware that sometimes the pacer spike is so tiny that it is virtually imperceptible.
When searching for the STEMI of a pacemaker rhythm patient, my advice is simple:
Analyze the tracing as if it were simply a LBBB pattern (ignore the pacemaker spike since it is simply there to remind you it is an electronic pacer rhythm). Analyze the LBBB looking to see if it satisfies the Modified Sgarbossa Criteria:
If present – It is a STEMI
If not present – It is not a STEMI
Now click “MARK COMPLETE” or “NEXT”.