Effects of exposure to hypoxia on the signal-averaged electrocardiogram in healthy subjects

The effects of hypoxia on the signal-averaged ECG (SAECG) were investigated in 26 healthy active subjects with no suggestion of cardiac disease. The SAECG was recorded in each resting subject in normoxic and hypoxic normobaric conditions (inspired O2 fraction 20.7 vs 10.0%) which lowered resting arterial O2 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 parameters returned to normal in hypoxic conditions. The duration of the filtered QRS was significantly reduced (from 107.6 ±13.2 to 101.6 ± 11.3 ms, P<001), the duration of the low amplitude signals in the terminal portion of the QRS <40 μ V (LAS) significantly decreased (from 26.5 ± 9.5 to 22.7 ± 7.9 ms, P<005) and the root mean square voltage in the last 40 ms (Term-RMS) increased non-significantly (from 55.8±40.2 to 69.1±38.3 μV, P=0.058). Hypoxia determined a higher (P<0.05) heart rate increase in subjects with abnormal records than in normal subjects. These data could be related to a sympathic discharge. They suggest that: (1) variation in heart rate could affect the SAECG; (2) exposure to hypoxia improves SAECG parameters in healthy subjects, possibly related to sympathetic discharge; (3) abnormal records collected during sinus bradycardia could represent a type of false-positive expression of late potentials in young active adult