Should the patient with trifascicular disease be routinely paced? If not, why not?

Normal activation of the ventricles below the bundle of His occurs by way of three “fascicles” – the right bundle branch and the anterosuperior and posteroinferior divisions of the left bundle branch. Conduction block in two of the three fascicles is bifascicular block. Additional prolongation of the PR interval results in “trifascicular block” implying abnormal conduction through or above the remaining fascicle. The concern is that conduction will fail in the remaining fascicle, i.e. complete heart block will develop with a slow and unreliable ventricular escape rhythm. Potential consequences include syncope and death.

There have been no randomised trials of pacing vs no pacing in patients with chronic bi- or trifascicular block. Clinicians must therefore be guided by knowledge of the natural history of the condition without pacing, and expert consensus guidelines. The largest prospective study of patients with bi- and trifascicular block followed 554 asymptomatic patients for a mean of 42 months. The five year mortality from an event that may conceivably have been a bradyarrhythmia was just 6%, a figure that must inevitably include some non-bradyarrhythmic deaths. The five year incidence of complete heart block was also low at 5%. A prolonged PR interval was associated with a higher incidence of potentially bradyarrhythmic deaths but not with the development of complete heart block. An important finding of this study was a five year all cause mortality of 35% reflecting the high incidence of underlying coronary heart disease and congestive cardiac failure.

The available evidence would suggest that asymptomatic patients with trifascicular block should not be paced routinely. A history of syncope should prompt thorough investigation for both brady- and tachyarrhythmic causes. If intermittent second or third degree block is documented permanent pacing is indicated. If tachyarrhythmias are implicated then therapy is likely to include antiarrhythmic drugs, which may exacerbate AV block and prophylactic permanent pacing would seem wise. Bi- and trifascicular block are associated with a high incidence of underlying coronary heart disease and heart failure. Attention should therefore be directed towards the detection of these conditions and the use of therapies known to improve their prognosis.