Explanation for the choice of comparators

In normal weight patients, a PEEP setting of 5 mbar is considered as clinical standard. With an increasing number of overweight patients, a weight-adapted PEEP adjustment may be necessary. To be able to clinically apply optimal and standardized ventilation parameters for individual, weight-adjusted ventilation, an easy-to-apply calculation method is needed. Thus, we consider a comparison between a control group with standard therapy (PEEP 5 mbar) and an intervention group (PEEP BMI/3 mbar) to be useful. The same inclusion and exclusion criteria will be used to recruit the intervention and control group.

Intervention description

Only patients who provide consent will be included in the study. After obtaining written informed consent, total height, weight, and waist to hip ratio will be measured. On the day of surgery, a preoperative lung ultrasound examination will be carried out immediately before the patient is admitted to the operating room and the lung aeration score will be calculated as described in [12]. After induction of general anesthesia, patients will be assigned to a group by opening a sealed envelope containing the allocation. All patients will be ventilated in a volume-controlled mode (VCV) with constant inspiratory flow at a tidal volume of 7 ml per kg predicted body weight (PBW), with an I:E ratio of 1:2 and an inspiratory pause of 30%. PBW will be calculated using the NIH-NHLBI ARDS Network formula. ♂: PBW (kg) = 50 + 0.91 (height (cm) − 152.4) and ♀: PBW (kg) = 45.5 + 0.91 (height (cm) − 152.4) [19].

In the intervention group, PEEP will be adjusted to a value of BMI/3 mbar for the duration of general anesthesia. In the control group, PEEP will be set at a fixed value of 5 mbar for the duration of general anesthesia. Sustained-inflation recruitment maneuvers will be performed after disconnection of ventilator circuit and in case of a drop in oxygen saturation below 90% despite ventilation with more than 60% of oxygen, with an airway pressure of 20 mbar above PEEP for 10 s in the intervention group and with an airway pressure of 30 mbar for 10 s in the control group. The lung ultrasound data will be stored anonymously and evaluation of the ultrasound images for calculation of lung aeration score will take place without knowledge of the group allocation (Fig. 1).

Fig. 1

SPIRIT figure: Schedule of enrolment, allocation, interventions, and assessments

Criteria for discontinuing or modifying allocated interventions

If instances of hypotension or hemodynamic instability necessitating intervention occur, or if the patient experiences acute cardiac or pulmonary decompensation, the intervention will be discontinued. Treating physicians (who are not part of study team) can suspend the study at any time. The reason will be documented in the case report form. Furthermore, participants may withdraw from the study without citing a reason at any time. There is no provision for modifying the assigned intervention.

Strategies to improve adherence to interventions

Adherence to the study protocol is ensured by continuous monitoring of the patient throughout the intervention by a member of the research team. This investigator will not only perform the pre- and postoperative sonographic examinations but will also ensure that the intervention is intraoperatively performed correctly by the clinician.

Relevant concomitant care permitted or prohibited during the trial

All procedures, interventions, and medications required for optimal patient treatment are allowed during the study. Any medications administered or additional interventions required during the trial will be documented on the case report form. All procedures, interventions, and medications administered in this study adhere to established internal standard procedures and clinical protocols. Potential side effects primarily include hemodynamic or respiratory complications. However, to mitigate these risks, each patient will be under the direct supervision of a board-certified anesthesiologist with extensive experience in managing such complications during surgical procedures. Consequently, we assess the likelihood of experiencing adverse events as low. Detailed information regarding possible adverse events is provided in Item 22. Additionally, as stated in Item 30, patient insurance coverage is available to address any adverse events should they occur.

Provisions for post-trial care

A patient insurance with an insurance number of maximum 5,000,000 Euro (maximum 500,000 Euro per participant) exists with HDI Global SE (99.9%) and HDI-Haftpflichtverband der Deutschen Industrie V.a.G. (0.1%) if study-associated complications should occur. The monitoring period ends after discharge from the recovery room. Further follow-up is not required.

Outcomes

The primary endpoint of this trial is the difference in driving pressure (ΔP) between the intervention and control group during general anesthesia in endotracheal intubation and volume-controlled ventilation (VCV). Secondary endpoints include:

Mechanical Power of ventilation (elastic-dynamic, tidal and total).

Compliance of the respiratory system.

Intraoperative fluid requirements.

Average intraoperative vasopressor requirements (µg/kg/min).

Number of intraoperative hypotension events (MAD < 65 mmHg for > 1 min).

Time-weighted average of hypotension.

Number of alveolar recruitment maneuvers (to be performed if SpO2 < 90% with FiO2 > 60%).

SpO2 after arrival in recovery room (without oxygen insufflation).

Number of patients with indication for oxygen insufflation (to maintain SpO2 ≥ 90%).

Change in lung aeration score between before surgery and after arrival in recovery room(determined by lung ultrasound).

Overall postoperative pulmonary complications.

For further clarification see, Table 1 (“Outcome Definition”).

Table 1 Outcome definitionParticipant timeline

Within the week before surgery, patients will visit the pre-medication outpatient clinic. Participants will be screened and, if eligible, will be asked to provide written informed consent to participate in the study. Only patients who provide consent will be included in the study. After obtaining consent, total height, weight, and waist to hip ratio will be measured. On the day of surgery, a preoperative lung ultrasound examination will be carried out immediately before the patient is admitted to the operating room. After induction of general anesthesia, patients will be randomized by opening a sealed envelope. The protocol is then carried out accordingly for the control group or for the intervention group. After extubation, patients will be transferred to the recovery room. A postoperative lung ultrasound examination will be carried out. Additional oxygen will be applied if oxygen saturation drops below 90% breathing room air. Patients will be discharged from the recovery room according to routine clinical criteria (oxygen saturation > 90% or similar to preoperative value breathing room air, hemodynamic stability, patient awake and oriented, sufficient pain control). Data collection ends after patient is discharged; no further follow-up is needed (Fig. 2).

Fig. 2

Patient timeline showing the different steps of the study process

Sample size

Sample size calculation was performed with G*power 3.1 Software [13]. Assuming a mean (± SD) ΔP of 10 (± 3) mbar at PEEP = 5 mbar and a mean ΔP of 8 (± 2) mbar at PEEP = BMI/3 mbar, α = 0.05, 1 − β = 0.8, G*Power yielded a required sample size of n = 54 (27 per group) patients. The anticipated effect size is based on the results previously published by Pereira et al. [10]. To compensate for refusal, technical difficulties, or possible dropouts, we will increase the sample size by 10%, resulting in a total sample size of 60 patients.

Recruitment

Recruitment in the pre-medication outpatient clinic will be continuously conducted by a member of the research team until the target randomized sample size of 60 participants is achieved. Based on current clinical case numbers, this will take approximately 6 months.