Clinical update no. 132 14 May, 2009

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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
WorkArea/DownloadAsset.aspx?id=42248 also
device does not sense the intrinsic QRS activity
presented at AAEM 2009, downloadable
and generates a paced beat anyway.
from www.emedhome.com
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
block with

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
system
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
detailed.
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
subclavian artery.

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
underlying bradycardia.
Failure to capture: pacing spike with no QRS
complex.
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
component failure

if there are spikes at a rate slower than the
programmed rate, then there is battery
failure
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.