Pacing Modes of Operation in a Pacemaker

The pacing modes of the pacemaker are classified as Competitive and Non – competitive modes.

Competitive Mode of the Pacemaker

The competitive mode of pulse generators produces fixed rate pulses that are independent of natural cardiac activity. Instead, the fixed rate pulses are generated alongside with heart’s natural pulses. It competes with them to control the heartbeat. This fixed rate pacemakers are implanted for the patient whose SA node has failed from its operation. The disadvantage of this type is less battery life.

Non – Competitive Mode of the Pacemaker

This type of pacing mode either generates ventricular programmed or atrial programmed pulses. Among these, the ventricular-programmed pacemaker will operate in either demand mode (R wave stopped) or standby mode (R wave triggers). Atrial programmed pacemaker works in synchronization with P wave of ECG waveform.
The pacing modes of pacemakers are classified into following types.
human heart

Ventricular Asynchronous Pacemaker (Fixed Rate Pacemaker)

This type of pacemaker is installed in atrium or ventricles for the patients who suffer from total AV block and atrial arrhythmia. The generated impulses and normal heartbeat competes with each other. When looking into the electrical circuit of the pacemaker it has square wave generator and monostable multivibrators.


  1. In this type of pacemaker heartbeat cannot be altered.
  2. When the impulse in atrium and ventricles beats at different rate ventricular fibrillation occurs.

Ventricular Synchronous Pacemaker (Standby Pacemaker)

This type of pacemaker is used for patients who suffer from a short period of AV block. The electrode is placed in the right ventricle and it senses R wave. R wave triggers the pacemaker. The detected signal is given to amplifier and filter circuit. this pacemaker does not compete with normal heart beat signal. Filters remove the unwanted noise signal, hence P wave is removed. The obtained signal is given to the refractory period and timing circuit. If it detects heartbeat below a particular level, fixed rate pacemaker block is turned on. The impulses from fixed rate pacemaker is amplified and given through electrode. On the other case, if heartbeat is normal fixed rate pacemaker block is not turned on. The pacemaker delivers impulses only when R wave is detected and remains at certain level. This type of pacemaker avoids ventricular fibrillation.

Ventricular Inhibited Pacemaker (Demand Pacemaker)

The timing circuit consists of RC network, reference voltage source and a comparator. Comparator is used to determine the pacing rate of generator. The obtained output is given to the second RC network. The duration of stimulating pulse is determined with pulse width circuit. After this, the signal goes to the third RC network, which is rate-limiting circuit. It disables comparator and helps to limit pacing rate. The final output obtained is given as impulse for stimulating heart. Voltage monitor circuit controls the rate and used as energy compensation circuit. Rate slow down circuit stops amount of current that goes to the basic timing. This helps to slow down the pulse rate during depletion of cell. Sensing circuit identifies R wave and resets the oscillator timing of capacitor. When R wave is absent, the oscillator in the circuit produces pulses at preset rate. The inhibited pacemaker makes heart to produce pulse that can beat at its own rhythm whenever possible. When R wave is missing for a short time period, pacemaker will turn ON to produce stimulus for heart. Hence, it is called Demand Pacemaker.

Atrial Synchronous Pacemaker

This pacemaker is used for patients with stable block and it provides temporary pacing. The electrode placed in the atrium senses the P wave. The sensed signal is given to amplifier circuit, which amplifies the P waveform. The signal is given to the delay circuit to give a delay of 0.12 second. The output from the delay circuit is given to the refractory and preset multivibrator block. When amplitude of P wave is not normal, fixed rate pacemaker is ON. Fixed pacemaker will be OFF if amplitude is normal.

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