[Clinical and electrophysiological aspects of median intra-His bundle block with normal electrocardiogram at rest].

The clinical and electrophysiological features and the natural history of median intra-His block with a normal resting electrocardiogram were studied: 11 patients had a fixed split H1-H2 potential with a spontaneous or induced block between H1 and H2. The patients (5 men and 6 women) were aged 17 to 70 years (average 53 years). Associated pathology included 2 cases of aortic stenosis (1 severe), 1 case of ischaemic heart disease (effort angina), 1 case of mitral valve prolapse and 2 cases of hypertension. The presenting symptoms were syncope (4 cases), dizziness (2 cases), effort angina (1 case) and tiredness (3 cases); 1 patient was asymptomatic. Holter monitoring (24 hours) was performed in 8 patients and s-owed paroxysmal conduction defects in 6 cases; 4 Mobitz II 2nd degree AV block, 1 3rd degree AV block with narrow QRS complexes and 1 case of blocked atrial extrasystoles at coupling intervals longer than 480 ms and sinus cycle lengths of over 800 ms. Exercise testing by bicycle ergometry (4 patients) was normal in 1 case and revealed Mobitz II 2nd degree AV block in 3 cases. Baseline electrophysiological studies showed an A-H1 interval ranging from 60 to 100 ms (average 78 ms), a H1-H2 interval of 20 to 40 ms (average 31 ms) and a H2-V interval of 30 to 50 ms (average 32 ms). Block between H1 and H2 was observed: "spontaneously" during electrophysiological investigation in 6 cases, after IV atropine in 1 case, during overdrive atrial pacing at rates slower than 150/min in 7 cases, after atrial extrastimulus with a functional intra-His refractory period of over 420 ms in 7 cases, after ajmaline in 3 of the 4 cases in which this test was performed. A cardiac pacemaker was implanted in 10 patients in whom the initial symptoms have all regressed; the remaining patient considered to be "epileptic" had another syncopal attack under therapy and was finally paced. This series demonstrates that the diagnosis of median intra-His block depends on precise electrophysiological criteria and should be looked for even when the presenting symptoms are atypical; some of our patients complained only of tiredness. The value of Holter monitoring and careful endocavitary investigation is emphasised. Median intra-His block should be distinguished from longitudinal and functional His bundle dissociation.