Same patient as in tracing 1.
Tracing 2a: programming in DOO mode 50 beats/minute;
- atrioventricular pacing with effective and complete capture (AP-VP);
- absence of sensing of the atrium and ventricle with ineffective asynchronous atrioventricular pacing since occurring in the atrial and ventricular refractory periods;
- new asynchronous atrioventricular pacing; the ventricular stimulus occurs at the peak of the T wave in a vulnerable ventricular period without capture;
Tracing 2b: programming in VOO mode 50 beats/minute;
- asynchronous ventricular pacing in the vulnerable period of the preceding unsensed spontaneous ventricular complex;
Tracing 2c: programming in VOO mode 60 beats/minute;
- ventricular capture at a rate slightly higher than the sinus rate;
- ventricular pacing in a vulnerable period of an unsensed premature ventricular contraction.
Asynchronous modes at fixed rates were the only available modes on the first-generation pacemaker models. The D00 mode induces asynchronous atrioventricular sequential pacing, without inhibition by intrinsic events. As seen on this tracing, when the patient is not pacemaker-dependent, a parasystole occurs with competition between spontaneous activities and paced activities. This mode allows verifying the effectiveness of the pacing and avoids inhibition in case of exposure to external interference (for example, an electric scalpel in a pacemaker-dependent patient). Pacing is effective and captures atrial or ventricular activity when it occurs outside the absolute physiological refractory period following a spontaneous atrial or ventricular event.
These tracings illustrate the risks associated with this type of mode. Several ventricular pacing pulses occur at the peak of the T wave of an unsensed spontaneous QRS. This is the vulnerable period with risk of induction of a ventricular rhythm disorder. While the risk of ventricular fibrillation is limited, it does increase considerably in the presence of myocardial ischemia or metabolic disorder (hypokalemia and long QT). Similarly, asynchronous atrial pacing in the atrial vulnerable period may induce atrial fibrillation. Increasing the minimal pacing rate promotes ventricular capture and reduces the risk of pacing during a vulnerable period. However, tracing 3 shows that the risk of pacing in a vulnerable phase of a premature ventricular contraction nonetheless persists.
Asynchronous modes are now obsolete and are only used in 2 specific circumstances: 1) in magnetic or magnet mode, in which the application of a magnet leads to a A00, V00 or D00 pacing according to the programmed mode; 2) the D00 mode can be programmed temporarily in pacemaker-dependent patients with an MRI-compatible device who are scheduled to undergo an MRI.