INTRODUCTION: Spontaneous breathing with a conventional T-piece (TT) connected to the tracheal tube orotraqueal has been frequently used in clinical setting to weaning of mechanical ventilation (MV), when compared with pressure support ventilation (PSV). However, the acute effects of spontaneous breathing with TT versus PSV on autonomic function assessed through heart rate variability (HRV) have not been fully elucidated. OBJECTIVE: The purpose of this study was to examine the acute effects of spontaneous breathing in TT vs PSV in critically ill patients. METHOD: Twenty-one patients who had received MV for >/= 48 h and who met the study inclusion criteria for weaning were assessed. Eligible patients were randomized to TT and PSV. Cardiorespiratory responses (respiratory rate -f, tidal volume-V(T) , mean blood pressure (MBP) and diastolic blood pressure (DBP), end tidal dioxide carbone (P(ET) CO(2) ), peripheral oxygen saturation (SpO(2) ) and HRV indices in frequency domain (low-LF, high frequency (HF) and LF/HF ratio were evaluated. RESULTS: TT increased f (20 +/- 5 vs 25 +/- 4 breaths/min, P<0.05), MBP (90 +/- 14 vs 94 +/- 18 mmHg, P<0.05), HR (90 +/- 17 vs 96 +/- 12 beats/min, P<0.05), P(ET) CO(2) (33 +/- 8 vs 48 +/- 10 mmHg, P<0.05) and reduced SpO(2) (98 +/- 1.6 vs 96 +/- 1.6%, P<0.05). In addition, LF increased (47 +/- 18 vs 38 +/- 12 nu, P<0.05) and HF reduced (29 +/- 13 vs 32 +/- 16 nu, P<0.05), resulting in higher LF/HF ratio (1.62 +/- 2 vs 1.18 +/- 1, P<0.05) during TT. Conversely, V(T) increased with PSV (0.58 +/- 0.16 vs 0.50 +/- 0.15 L, P<0.05) compared with TT. CONCLUSION: Acute effects of TT mode may be closely linked to cardiorespiratory mismatches and cardiac autonomic imbalance in critically ill patients.