CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 377

V Sense

V Sense

A Sense

A

R

V

AV Tout

V Pace

A Pace

A Tout

R Tout

Figure 8.7 Dual-chamber pacemakers synchronize the activation of the ventricles to atrial activity. This simplified DDD pacemaker state
machine can pace both atria ([A Pace]) and ventricles ([V Pace]). It has the following possible states: [A], atrial and ventricular alert, dur-
ing which the pacemaker may sense signals from either or both chambers; [V], ventricular alert, during which sensing of intrinsic events is
enabled only for the ventricle; and [R], refractory state. Possible events that cause transition between states are: [A Sense], sensing of an
intrinsic atrial event; [V Sense], sensing of an intrinsic ventricular event; [A Time Out], maximum time that the state machine waits for an
atrial sense event after exiting from the refractory period; [AV Time Out], maximum time that the state machine waits for a ventricular sense
event after an atrial event; and [R Time Out], refractory period.

Possible events that cause transition between states are:

• [A Sense] Sensing of an intrinsic atrial event

• [V Sense] Sensing of an intrinsic ventricular event

• [A Time Out] Maximum time that the state machine waits for an atrial sense event

after exiting from the refractory period

• [AV Time Out] Maximum time that the state machine waits for a ventricular sense

event after an atrial event

• [R Time Out] Refractory period

Possible actions taken by the state machine are:

• [A Pace] Pacing pulse delivered to the atrium

• [V Pace] Pacing pulse delivered to the ventricle

The states of this pacemaker state machine are:

• [A] Atrial and ventricular alert, during which the pacemaker may sense signals

from either or both chambers

• [V] Ventricular alert, during which sensing of intrinsic events is enabled only for

the ventricle

• [R] Refractory state

If intrinsic events are not detected from the heart, the state machine's period is given by
the following sequence:

1. Pacing pulse delivered to the atrium
2. AV delay ([AV Time Out])
3. Pacing pulse delivered to ventricle