The DOO operating mode is reversed compared to the ODO mode: no sensing is possible and pacing is at fixed intervals. Asynchronous modes at fixed rate were the only modes available on the first pacemakers. The DOO mode induces asynchronous sequential atrioventricular pacing, without inhibition by intrinsic events. As seen on this tracing, when the patient is not device-dependent, parasystole occurs with competition between spontaneous activities and paced activities. This mode allows to verify the effectiveness of the pacing and to avoid inhibition in case of exposure to an external interference (electric scalpel in a dependent patient, for example). Pacing is effective and captures the atrium or ventricle only when it occurs outside the absolute physiological refractory period following a spontaneous atrium or ventricle.
This tracing shows the risk of this type of mode. Several ventricular pacings 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. The risk of ventricular fibrillation is limited although increases in the presence of myocardial ischemia or metabolic disorder. Similarly, asynchronous atrial pacing in an atrial vulnerable period can induce atrial fibrillation.
Asynchronous modes are now obsolete and are only used in 2 specific circumstances: 1) in a magnetic mode or magnet mode; indeed, the application of a magnet causing AOO, VOO or DOO pacing depending on the programmed mode, 2) the DOO mode can be programmed temporarily in dependent patients with an MRI-compatible pacemaker who need to undergo an MRI.