OBJECTIVE To evaluate the effects of mechanical ventilation on pulmonary function during short duration of general anesthesia with tracheal intubation, and assess the safety of controlled spontaneous respiration during general anesthesia. METHODS Fifty-three adult patients (aged 18-55 years, ASA physical status I-II) scheduled for elective unilateral tympanoplasty were randomly assigned into mechanical ventilation group (group M, n=28) and spontaneous respiration group (group S, n=25). Anesthesia induction was performed in group M with intravenous propofol (2 mg/kg), fentanyl (3 microg<kg) and vecuronium (0.1 mg<kg), while with propofol (2 mg/kg), fentanyl (3 microg/kg) and sufficient superficial anesthesia on upper airway mucous membrane in group S. After tracheal intubation, mechanical ventilation began with VT 8 ml<kg and RR 10-12 bpm in group M, and spontaneous respiration was maintained in group S. Anesthesia was maintained by 0.7%-0.8% isoflurane and 60%-70% N(2)O at the end respiratory concentration to control MAC between 1.2-1.3. During the surgery, BIS values were controlled between 40-60, and propofol was administered when necessary. Vecuronium (1-2 mg) was given intermittently to maintain muscle relaxation and neostigmine (1 mg) with atropine 0.5 mg was administered intravenously before extubation in group M. No relaxant was used in group S. The parameters including heart rate (HR), mean blood pressure (MAP), pulse oxygen saturation (SpO(2)), and thoracic fluid content (TFC) were recorded before the induction and at 1, 5, 10, 20, 40, 60, 90, 120, and 150 min after intubation. Arterial blood was drawn immediately and 150 min after intubation for blood gases analysis and Alveolar-arterial oxygen gradient (P(A-a)DO(2)), and the respiratory index (RI) and dead volume/tidal volume (VD/VT) were calculated. The incidences of moving, bucking, swallowing, and status of awareness during surgery procedures were also recorded. RESULTS A total of 43 patients (group M, n=23; group S, n=20) were included in the study with 10 dropouts due to failed attempt to obtain arterial blood samples (8 patients) or severe bucking during intubation (2 patients). No significant differences were found in HR and MAP between the two groups (P>0.05). The pH and SpO(2) [ (97.9-/+1.00)% at the lowest] and PaO(2) in group S were significantly lower and the PaCO(2) was higher than those in group M (P<0.05). In group S, the pH values were 7.274-/+0.025 and 7.331-/+0.039, PaCO(2) values were 60-/+6 and 53-/+5 mmHg, and PETCO(2) values were 53-/+ 6 and 48-/+7 mmHg, and the PaO(2) values were 143-/+37 and 165-/+49 mmHg immediately and 150 min after the intubation, respectively. These values were considered safe under the concept of permissive hypercapnia. No significant differences were found in the P(A-a)DO(2), RI, VD/VT and TFC between or within the two groups (P>0.05), nor were moving, bucking, swallowing and awareness recorded during the surgical procedures. CONCLUSIONS In essentially normal lungs, short-term mechanical ventilation during general anesthesia with tracheal intubation does not damage the lung functions, and spontaneous respiration can offer sufficient oxygen supply without causing harmful carbon dioxide retention.