Tusman 1999.

Methods	Parallel randomized controlled trial High risk of bias
Participants	30 participants randomly assigned Included: age > 60, ASA II/III undergoing surgery > 2 hours in supine position, not expected to directly affect thorax or diaphragm position Exclude: patients undergoing thoracic, upper abdominal, spinal and laparoscopic surgery Age: years (mean 69 ± 8.6, 70.7 ± 7.3, 72.3 ± 7.7) Sex distribution: M/F (3/7, 6/4, 3/7) Country: Argentina
Interventions	20 participants received PEEP, commenced after induction: 10 received 5 cm H2O and 10 received accelerating PEEP, up to15 cm H2O, followed by 5 cm H2O 10 participants received ZEEP
Outcomes	Arterial blood gas parameters Mortality during admission
Notes	Single centre The study authors stated that all participants were followed up until discharge and that no complications occurred in any participant. We thought it reasonable to assume from these statements that no mortality occurred in either group during the admission Could be included in the following subgroup analyses: PEEP 5 cm H2O, age > 60
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Quote: "...patients... were randomized prospectively to one of three groups of 10 by opening sealed envelopes..." Comment: inadequate sequence generation
Allocation concealment (selection bias)	High risk	Quote: as above Comment: Allocation concealment would have been compromised towards the end, given the randomization technique
Blinding of participants and personnel (performance bias) Mortality	Low risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Blinding of participants and personnel (performance bias) Oxigen efficiency ‐ PaO2/FiO2	Low risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Blinding of participants and personnel (performance bias) Atelectasis	Low risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Blinding of outcome assessment (detection bias) Mortality	Unclear risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Blinding of outcome assessment (detection bias) Oxigen efficiency ‐ PaO2/FiO2	Unclear risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Blinding of outcome assessment (detection bias) Atelectasis	Unclear risk	Random assignment occurred after participants were anaesthetized. No comment was made about blinding of the outcome assessor. BUT only outcome used in our review was mortality
Incomplete outcome data (attrition bias) All outcomes	Low risk	All randomly assigned participants analysed
Selective reporting (reporting bias)	High risk	Parameters of lung mechanics not described in the methods section