We examined the effect of lung hyperinflation and suction on PaO2, heart rate, and rhythm in patients after coronary artery bypass graft surgery (N = 26). Three lung hyperinflation breaths, at one of five randomly ordered volumes (tidal volume, 12 cc/kg, 14 cc/kg, 16 cc/kg, or 18 cc/kg of lean body weight) were delivered, by a ventilator (fraction of inspired oxygen 1.0), followed by 10 seconds of continuous suction. Lung hyperinflation and suctioning were repeated three times. Repeated-measures analysis of variance revealed a statistically significant increase (p = 0.000) in PaO2 immediately after the third suction pass, which was volume dependent (p = 0.009). A statistically significant (p less than 0.001) increase in heart rate from baseline occurred over the three lung hyperinflation-suctioning sequences that was not volume dependent. The mean increase in heart rate was 6.8 beats/min. The majority of rhythm changes for lung hyperinflation and suctioning were from normal sinus rhythm to sinus tachycardia. Suction was associated with a greater incidence of rhythm (53.9%) and arrhythmia (80.8%) changes. The most frequent arrhythmia was premature atrial contraction.