Intraoperative Hypocapnia in PROVHILO and PROBESE

Associations of Intraoperative Hypocapnia With Patient Demographics, Ventilation Characteristics and Outcomes--Statistical Analysis Plan for an Individual Patient Data Analysis of PROVHILO and PROBESE

To gain a better understanding of the epidemiology of intraoperative hypocapnia, in particular the associations of intraoperative hypocapnia with patient demographics, ventilator characteristics, and perioperative complications we will perform an individual patient-level meta-analysis of two recent randomized clinical trials of intraoperative ventilation, the 'PROtective Ventilation using High versus LOw PEEP trial' (PROVHILO), and the 'Protective intraoperative ventilation with higher versus lower levels of positive end-expiratory pressure in obese patients trial' (PROBESE).

Lung-protective intraoperative ventilation (LPV) has the potential to improve the outcome of surgery patients through a reduction in postoperative pulmonary complications. Use of intraoperative ventilation strategies that use a low tidal volume could result in intraoperative hypercapnia. However, hypocapnia remains surprisingly common during intraoperative ventilation, possibly meaning that anesthesiologists continue to use high, if not too high respiratory rates or tidal volumes.

Previous studies suggested associations between intraoperative derangement of end-tidal carbon dioxide (etCO2) and postoperative outcomes. Indeed, two studies in highly selected patient groups showed associations of intraoperative hypocapnia with prolonged length of hospital stay, in patients undergoing pancreaticoduodenectomy, and in patients undergoing hysterectomy.

To gain a better understanding of the epidemiology of intraoperative hypocapnia, in particular the associations of intraoperative hypocapnia with patient demographics, ventilator characteristics, and perioperative complications we will perform an individual patient-level meta-analysis of two recent randomized clinical trials of intraoperative ventilation; PROVHILO and PROBESE.

Inclusion Criteria:

• Planned for major (abdominal) surgery.
• At risk for postoperative pulmonary complications.

Exclusion Criteria:

• Planned thoracic surgery or neurosurgery.
• Unscheduled surgery (i.e., urgent, or emergent surgeries) were excluded because these patients may have had metabolic abnormalities at the moment of surgery, i.e., metabolic acidosis, for which the anesthesiologist may have adjusted the intraoperative ventilator settings. This may have led to a 'compensatory' low etCO2.
• Patients with etCO2 recordings are missing from the study databases.