pacing is a form of dual-chambered pacing in which the atria and the ventricles are paced. in ddd
pacing the atrium and the ventricle are sensed and paced or inhibited, depending on the native cardiac activity sensed (fig. 4
). other forms of dual-chambered pacing are available, such as dvi and vdd, but ddd
is the most common. the principle advantage of dual-chambered pacing is that it preserves av synchrony. because of this advantage, dual-chambered pacing is increasingly common.
[color=#006666]fig. 4 [color=#006666]ddd pacing. (a) atrial pacing with native ventricular qrs activity without ventricular pacing because of intact av conduction. (b) there is atrial sensing of native p waves (a) that triggers subsequent ventricular pacing. (c) atrial and ventricular pacing occur. from chan tc, et al., eds. ecg in emergency medicine and acute care. philadelphia: mosby, 2004; with permission.
pacing, if the pacemaker
does not sense any native atrial activity after a preset interval, it generates an atrial stimulus (fig. 4a
). an atrial stimulus, whether native or paced, initiates a period known as the av interval. during the av interval the atrial channel of the pacemaker
is inactive, or refractory. at the end of the present av interval, if no native ventricular activity is sensed by the ventricular channel, the pacemaker
generates a ventricular stimulus (fig. 4b
). following the av interval, the atrial channel remains refractory during a short, post-ventricular atrial refractory period (pvarp) so as to prevent sensing the ventricular stimulus or resulting retrograde p waves as native atrial activity. the total atrial refractory period (tarp) is the sum of the av interval and the pvarp. in a simple ddd pacemaker
, the tarp determines the upper rate of the pacemaker
(upper rate [beats per minute] = 60/tarp).
as one might imagine, if a patient who has a ddd pacemaker
were to develop supraventricular tachycardia, the pacemaker
might pace the ventricles at the rapid rate based on the atrial stimulus (up to the preprogrammed upper rate limit). to prevent this, most ddd pacemakers
now use mode switching algorithms, whereby if a patient develops an atrial tachydysrhythmia, the pacemaker
switches to a pacing mode in which there is no atrial tracking, such as vvi. on cessation of the dysrhythmia, the pacemaker
reverts to ddd
mode, thus restoring av synchrony without being complicit in the transmission of paroxysmal atrial tachydysrhythmias.
:typing i hope it helps