Inter- and intraday variability in major electrocardiogram intervals and amplitudes in healthy men and women.

The ECG may vary during the day (intra-day), and between days (interday), for the same subject. Variability in ECG characteristic measurements between different investigators is well documented and is often large. During days 1-6 of each placebo period of a two-way crossover Phase I study, digital ECGs were recorded at about 8 and 12 AM in 16 healthy volunteers (8 men, 8 women).