Comparison 1. Comparison 1: PEEP versus ZEEP.

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1 Mortality	6	268	Risk Ratio (M‐H, Fixed, 95% CI)	0.97 [0.20, 4.59]
2 Oxygen efficiency ‐ PaO2/FiO2	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only
2.1 Day 1 postop	2	80	Mean Difference (IV, Fixed, 95% CI)	22.98 [4.40, 41.55]
2.2 Day 3 postop	2	80	Mean Difference (IV, Fixed, 95% CI)	12.59 [‐6.78, 31.96]
3 Pneumonia	3	120	Risk Ratio (M‐H, Fixed, 95% CI)	0.40 [0.11, 1.39]
4 Atelectasis	2	88	Std. Mean Difference (IV, Fixed, 95% CI)	‐1.29 [‐1.78, ‐0.79]