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Q1: What are the two significant electrocardiographic findings in this clinical context?
Intermittent sinus rhythm and intermittent ventricular paced beats with retrograde ventriculoatrial conduction (QRS complexes 1, 2, 8, and 9 in the rhythm strip).
Q2: What is the diagnosis?
Q3: What is the definitive treatment?
Dual chamber pacemaker with mode switching.
During the initial presentation, the 24 hour Holter revealed atrial fibrillation with pauses of 6 seconds. Hence a VVIR pacemaker was inserted. However subsequent 24 hour Holter revealed intermittent sinus beats as well as paced beats with ventriculoatrial retrograde conduction; these are two of the key factors that could cause the pacemaker syndrome. The initial 24 hour tape picked up only atrial fibrillation and intermittent sinus rhythm could have developed subsequently. Ventriculoatrial retrograde conduction explained in detail later is a particular problem when patients are paced for atrial fibrillation with prolonged pauses rather than for complete heart block.
Before proceeding with the discussion of pacemaker syndrome, brief review of pacemaker terminology is not inappropriate. Pacemakers are described by using three to five letters. The first letter describes which heart chamber or chambers paced: A for atrium, V for ventricle, or D for dual (both the atrium and the ventricle). The second letter refers to the chamber in which the pacemaker is able to sense intrinsic cardiac events: A for atrium, V for ventricle, or D for dual. The third letter indicates the pacemaker’s mode of response to intrinsic cardiac events: I for inhibit, T for trigger, and D for dual. Inhibition means the pacemaker will not pace after it senses intrinsic depolarisation. Triggered applies to dual chamber pacemakers and single chamber ventricular pacemakers that can sense the atrium (for example, VDT). When an intrinsic atrial depolarisation is sensed, ventricular pacing is triggered. D as a third letter means that the pacemaker is capable of both being inhibited and triggered by intrinsic cardiac events. R as a fourth letter indicates rate modulation, which allows the pacemaker to pace above its programmed upper limit in response to exercise. Antitachycardia functions may be represented in the fifth letter.
Pacemaker syndrome was first described in 1969 by Mitsui et al and he also linked it to the atrioventricular dysynchrony caused by single chamber ventricular pacing.1 However it can also be associated with dual chamber pacing, usually as a result of suboptimal atrioventricular synchrony or rapid atrial or ventricular pacing.2
The prevalence of pacemaker syndrome may be underestimated for two reasons. Firstly, the syndrome is mistaken for worsening heart failure or coronary heart disease. Secondly, often the pacemaker recipient who experienced syncope before pacemaker implantation may not report the less troublesome signs and symptoms of pacemaker syndrome to their healthcare providers.5
The risk factors for developing pacemaker syndrome are listed in the box 1.
Box 1: Main risk factors for development of pacemaker syndrome
Single chamber ventricular pacemaker (VVI/VVIR).
Retrograde ventriculoatrial conduction.
Associated left ventricular disease.
Atrioventricular dysynchronous pacing such as VVI/VVIR modes results in loss of atrial contribution to ventricular filling and the cardiac output may go down up to 35%, especially in diseased ventricles.
Another factor that contributes to pacemaker syndrome is retrograde conduction (from ventricle to atria). It occurs in some patients who receive single chamber ventricular pacemakers. Because the lead of a ventricular pacemaker is positioned in the apex of the right ventricle, paced depolarisation of the ventricles proceeds in a cell-to-cell fashion from the right ventricular apex to the entire right and left ventricles. It can then continue to the atria, causing atrial depolarisation. This process is termed retrograde depolarisation of the atria. As 90% of retrograde conduction occurs through the atrioventricular node, the potential for retrograde conduction is preserved unless the atrioventricular node is diseased.6 It causes atrial contraction against closed atrioventricular valves thus increasing atrial and pulmonary venous pressures. This activates systemic vasodepressor reflexes, which are mediated by vagal stimulation, sympathetic inhibition, or release of atrial natriuretic peptide thereby resulting in hypotension.
Though pacemaker syndrome may very rarely occur with atrial pacing as well as dual chamber pacing, it is very much less common in these modes when compared with VVIR mode. With atrium paced, atrium sensed, inhibitory pacing (AAI), atrioventricular dyssynchrony may result from very long PR intervals, particularly at faster paced beats, during rate responsive (AAIR) pacing. Atrioventricular dysynchrony can also occur in dual chamber pacing with non-atrial tracking pacing modes, long programmed atrioventricular delays, loss of atrial capture as well as during pacemaker-mediated tachycardia. Prolonged intra-atrial conduction times can lead to simultaneous atrial and ventricular activation thereby resulting in the pacemaker syndrome in dual chamber paced, dual chamber sensed, dual response to sensing (triggered and inhibitory) (DDD) pacing.
The key signs and symptoms of pacemaker syndrome are listed in the box 2.
Box 2: Key signs and symptoms of pacemaker syndrome
The diagnosis requires clinical correlation with 24 hour Holter monitoring/event recorder.
In patients with paroxysmal atrial fibrillation as in this patient, the treatment of choice is the upgrading of pacemaker to a dual chamber device with mode switching.
Pacemakers with mode switching function can sense the conversion from a sinus rhythm to atrial fibrillation or atrial flutter and automatically switch from DDD mode to VVI pacing to prevent rapid ventricular pacing. When the patient flips to sinus rhythm, mode switch reprograms the pacemaker to DDD thereby maintaining atrioventricular synchrony. Normally, mode switch will not cause pacemaker syndrome; however, inappropriate mode switching in response to muscle artifact rather than rhythm change can result in pacemaker syndrome.
Allowing sinus rhythm preference by decreasing the lower pacemaker rate in VVI pacing can also be useful. However it often requires upgrading to dual chamber pacemaker. In one study, 26% of patients who received single chamber ventricular pacemakers experienced pacemaker syndrome, which required upgrading to dual chamber devices.7 Pacemaker upgrade is a more complex procedure than initial system implantation. Though it is usually possible to upgrade the pacemaker, it takes longer and there is a higher incidence of complications, particularly atrial lead displacement when compared with initial system implantation.
Pacemaker syndrome mainly occurs in VVI pacing due to loss of atrioventricular synchrony.
Low cardiac output symptoms in the presence of VVI pacing should prompt investigations.
The optimum mode of pacing in patients with paroxysmal atrial fibrillation is DDD pacing with mode switch.
Pacemaker syndrome due to paroxysmal atrial fibrillation.
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