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.
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