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

Abstract

Monitoring the partial pressure of end-tidal carbon dioxide ( ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ ) 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 ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ and ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ 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. ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ 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 ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ determination. RESULTS: ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ 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 ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ -to- ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ gradient [ ${\text{P}}_{\left(\text{et}\right){\text{CO}}_{\text{2}}}$ ] 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: ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ and ${\text{P}}_{\left(\text{et}\right){\text{CO}}_{\text{2}}}$ varied with breathing pattern; however, the relationship remained constant for each breath type during the weaning process. We recommend that the breath-specific or maximum ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ value used to calculate initial ${\text{P}}_{\left(\text{et}\right){\text{CO}}_{\text{2}}}$ be used as the frame of reference for estimating ${\text{P}}_{{\text{etCO}}_{\text{2}}}$ during weaning from mechanical ventilation.