Clinical update no. 132 14 May, 2009
Clinical update no. 132
14 May, 2009
Problems with a pacemaker and ICDs
there is oversensing – the device believes it
has detected a QRS signal, and so does not
generate a paced beat.
If there are paced beats despite intrinsic QRS
From ACEP 2008 http://www.acep.org/
activity then there is a lack of sensing – the
device does not sense the intrinsic QRS activity
presented at AAEM 2009, downloadable
and generates a paced beat anyway.
Pacemakers are indicated for various chronic
conditions essentially related to bradycardia and
Oversensing – no spikes when there should be
Undersensing – spikes when there shouldn’t be
syncope, or to manage CCF. Acutely in the
If there is a pacing spike but no QRS generated,
context of ischaemia and AMI, a temporary
then there is failure to capture.
pacing wire is indicated for transient advanced AV
bundle branch block (BBB) or
persistent advanced 2nd degree AV block or
The wires run down the subclavian vein into the
right atrium and ventricle, crossing the midline
and coursing anteriorly – see below.
greater with block in the His-Purkinje
The type of pacemaker can be identified from a
code that can be seen on a CXR, although
patients often carry a card with the type of device
Problems mostly relate to these actions.
If no QRS is sensed, then the device will pace,
and capture is recognised by a paced QRS beat.
Complications from lead misplacement can be
seen on CXR – the following shows the lead in the
If there is no pacing despite a period of
left ventricle after incorrect placement in the
bradycardia, then either
the battery is flat and the device is not
working at all, or
A magnet will turn off the sensing function, and
the device will generate pacing at a programmed
rate until the magnet is removed.
The second half of the trace is after application of
a magnet. The magnet can allow identification of
battery or device failure, and whether there is
output from the device
failure to pace
failure to capture
If a patient is bradycardic or if symptoms suggest
Failure to sense: pacing spikes are generated
without regard to the underlying QRS complexes.
Failure to pace: no pacing activity despite
Failure to capture: pacing spike with no QRS
bradycardia that has resolved, then applying a
magnet can identify problems as follows:
if there is pacing at the programmed rate,
then there has likely been oversensing, with
Pacemaker re-entrant tachycardia precipitated
inappropriate inhibition of spikes
by a PAC or PVC – can be terminated by placing a
if there are no spikes then there is
if there are spikes at a rate slower than the
programmed rate, then there is battery
A pacemaker can generate a re-entrant
tachycardia (analogous to re-entrant SVT). A
magnet will interrupt the re-entrant circuit and
terminate the SVT (analogous to adenosine).
For an AICD giving recurrent shocks, a magnet
will prevent further shocks.
magnet over the pacemaker.
An acute infarct can be diagnosed despite a paced
rhythm by application of Sgarbossa’s criteria.
There is the rule of appropriate discordance. If
there is concordance of QRS and ST segments
(arrows), then an acute MI is suggested.