End-Tidal CO ₂ Monitoring Utility during Weaning from Mechanical Ventilation

Monitoring the partial pressure of end-tidal carbon dioxide ( P etCO 2 ) has been advocated as a way to estimate arterial carbon dioxide partial pressure (Paco) during mechanical ventilation. However, its reliability during titrated weaning from mechanical ventilation remains unsubstantiated, and we therefore studied this matter. METHODS: We evaluated the relationship between P etCO 2 and P etCO 2 in 11 postoperative patients undergoing weaning facilitated by synchronized intermittent mandatory ventilation (SIMV). The SIMV rate was reduced in increments of two breaths/min. Data were collected at the start of weaning and 30 min after each drop in SIMV rate. P etCO 2 was measured with an infrared analyzer for 1 min and manually recorded as being associated with either SIMV or spontaneous breaths, and an arterial blood sample was obtained at the same time for P etCO 2 determination. RESULTS: P etCO 2 was significantly higher with spontaneous breaths (35.1 ± 4.4 torr [4.68 ± 0.59 kPa]) than with SIMV breaths (31.2 ± 2.4 torr [4.16 ± 0.32 kPa]) (p < 0.001). The P etCO 2 -to- P etCO 2 gradient [ P ( et ) CO 2 ] was significantly lower with spontaneous breaths (6.0 ± 2.4 torr [0.80 ± 0.32 kPa]) than with SIMV breaths (9.2 ± 2.7 torr [1.23 ± 0.36 kPa]) (p < 0.001). CONCLUSIONS: P etCO 2 and