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270 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

FIGURE 11.33 Atrial pacing at 70/min with first-degree AV block. Note the long P–R interval, at almost 0.4 sec; particular caution is warranted in increasing
the rate, as, although AV conduction is 1 : 1, it is already very slow. See the next 2 figures for worsening of AV block as the atrial rate is increased.

PR longer dropped PR longer dropped PR longer dropped
FIGURE 11.34 Second-degree AV block type I with 3 : 2 conduction. The same patient as above, with worsening AV block after increasing the atrial pacing
rate to 80/min. Note: the 1 : 1 conduction has been lost and there are dropped beats. After each of the dropped beats the P–R is 0.30 sec, which extends
to 0.46 sec on the next beat, before dropping of the 3rd beat of each cycle.

FIGURE 11.35 The same patient again, now with the atrial pacing at 86/min. At the faster atrial rate, AV conduction has worsened further. There is now
a 2 : 1 block yielding a ventricular rate of 43/min.

have made it the predominant mode in both permanent ventricular conduction produces a contractile pattern
and temporary epicardial pacing. which is superior to the contraction from ventricular
pacing. This may result in better haemodynamics than
Pacing stimuli are delivered to the atria and ventricles at when the ventricles are paced. There has been increasing
a selected rate. After delivery of the atrial stimulus there interest in permitting native AV conduction because of
is a delay of usually 0.16–0.24 seconds (equivalent to a these above reasons, and also on the basis of recent data
P–R interval) before delivery of the ventricular pacing from the DAVID trial which revealed that chronic ven-
stimulus (Figure 11.36). If the patient is able to conduct tricular pacing induces negative ventricular remodelling
the atrial depolarisation to the ventricles themselves and worsening of heart failure. Prolonging AV delays to
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before the ventricular pacing is due, then the pacemaker permit native conduction has become commonplace, but
senses the resultant QRS and inhibits ventricular pacing.
carries some slight arrhythmic risk (Figure 11.37).
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DDD Pacing: The ‘Universal’ Pacing Mode
Practice tip The introduction of the DDD mode of pacing added an
important new dimension to dual-chamber pacing, that
If AV block is encountered during atrial pacing and is causing
significant bradycardia, consideration should be given to is, the ability to synchronise ventricular pacing to spon-
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reducing the atrial pacing rate to see whether the severity of taneous atrial activity in patients with AV block. In addi-
the AV block can be reduced. tion to the normal bradycardia and AV block protection,
the DDD mode features a ‘triggered’ function. If the pace-
maker detects a P wave but a QRS does not follow within
the preset AV interval (AV block), the pacemaker will be
A dual-chamber pacemaker may demonstrate AV pacing triggered to provide ventricular pacing at the end of the
at the set rate and the set AV delay as described above, or programmed AV interval. This means that the ventricular
may operate as simply atrial pacing if normal AV node rate can be brought back under control of the sinus node,
conduction occurs before the programmed AV delay has even though there is AV block. Consequently, in a DDD
elapsed. Deliberately prolonging the programmed AV pacemaker it is common to see ventricular pacing at a
delay provides greater opportunity for patients to conduct range of different rates as it responds to sinus activity.
to the ventricles by themselves. In some patients intrinsic This triggered behaviour of the DDD device is sometimes