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.