Effects of systemic hypoxia (3% O2, 5% CO2, Bal N2) on cardiac rhythm (Lead I) in the absence (-ADA) and presence (+ADA) of adenosine deaminase (5 U/Kg/min, i.c.). Top row, one minute of hypoxia, middle row 2 minutes of hypoxia, and bottom row, 3 minutes of hypoxia.