This study explored the breathing patterns and arterial blood gases before and during cuprophane (CU) bicarbonate and polysulfone (PS) bicarbonate dialysis in six chronic dialysis patients with mild chronic obstructive pulmonary disease (COPD). The studies were performed in random order during two consecutive dialyses. Breathing patterns were monitored by respiratory impedance plethysmography. Apneic episodes, defined as a decrease in tidal volume of 75% lasting 10 sec, were present before and during hemodialysis. In these patients with COPD a high number of apneic episodes (17 +/- 6 [SE]) were observed during CU bicarbonate hemodialysis. Most of these episodes were central rather than obstructive in character. There were fewer events when the same patients were dialyzed with PS membranes (10 +/- 5; p = 0.05). The decrement in PO2 (baseline to 60 min) was 17 +/- 7 during CU and 4 +/- 5 mmHg during PS dialysis (p = 0.10). Minute ventilation decreased in four of six patients on CU bicarbonate and increased in all six patients on PS bicarbonate. It was concluded that bicarbonate hemodialysis does not completely prevent hypoxemia or apnea during dialysis in patients with COPD. Apneic episodes and hypoxemia appear to be less severe during PS bicarbonate than during CU bicarbonate hemodialysis.