EFFECTS OF EXPOSURE TO HYPOXIA ON THE SIGNAL-AVERAGED ELECTROCARDIOGRAM IN HEALTHY-SUBJECTS

The effects of hypoxia on the signal-averaged ECG (SAECG) were investi gated in 26 healthy active subjects with no suggestion of cardiac dise ase. The SAECG was recorded in each resting subject in normoxic and hy poxic normobaric conditions (inspired O-2 fraction 20.7 vs 10.0%) whic h lowered resting arterial O-2 saturation from 98.6 +/- 0.6% to 77.7 /- 8%. Recordings from four subjects (three men) met the definition of abnormal late potentials at baseline; in all these subjects but one, who exhibited an improved but still abnormal QRS duration, these param eters returned to normal in hypoxic conditions. The duration of the fi ltered QRS was significantly reduced (from 107.6 +/- 13.2 to 101.6 +/- 11.3 ms, P<0.01), the duration of the low amplitude signals in the te rminal portion of the QRS <40 mu V (LAS) significantly decreased (from 26.5 +/- 9.5 to 22.7 +/- 7.9 ms, P<0.05) and the root mean square vol tage in the last 40 ms (Term-RMS) increased non-significantly (from 55 .8 +/- 40.2 to 69.1 +/- 38.3 mu V, P=0.058). Hypoxia determined a high er (P<0.05) heart rate increase in subjects with abnormal records than in normal subjects. These data could be related to a sympathic discha rge They suggest that: (1) variation in heart rate could affect the SA ECG; (2) exposure to hypoxia improves SAECG parameters in healthy subj ects, possibly related to sympathetic discharge; (3) abnormal records collected during sinus bradycardia could represent a type of false-pos itive expression of late potentials in young active adults.