A recent article examines the principles underpinning intraoperative mechanical ventilation (MV) and synthesises current evidence to guide optimal ventilator settings during general anaesthesia. With more than 230 million patients worldwide undergoing surgery with MV each year, postoperative pulmonary complications (PPCs) remain common and are strongly associated with increased morbidity and mortality. Optimising intraoperative ventilation is a major priority in perioperative medicine.
Although operating room (OR) ventilation strategies historically differ from ICU practice, growing evidence shows that ventilator settings during surgery influence postoperative outcomes. Historically, high tidal volumes (Vt ≥ 10 ml/kg) were routinely used. This practice originated in the 1960s when larger volumes were shown to improve oxygenation in the absence of positive end-expiratory pressure (PEEP). However, this approach has since been challenged. The IMPROVE trial in 2013 demonstrated that a lung-protective strategy combining lower Vt with the application of PEEP reduced PPCs and extrapulmonary complications by approximately 60% following major abdominal surgery. These findings prompted widespread adoption of lower tidal volumes and PEEP, but subsequent studies produced mixed results when examining each parameter independently.
Trials comparing high versus low PEEP (such as PROVILHO and PROBESE) found no significant differences in postoperative complications. Similarly, studies comparing low and higher tidal volumes at equivalent PEEP levels also failed to show clear benefit. Comparable findings were observed in ICU patients without acute respiratory distress syndrome. These contradictory outcomes raised the question of why combining lower Vt and PEEP was beneficial when modifying either alone did not improve results.
The concept of driving pressure (DP), defined as plateau pressure minus PEEP, has emerged as a key determinant of ventilator-induced lung injury and mortality. DP reflects tidal volume scaled to the functional size and compliance of the lung. Patients with identical tidal volumes may experience different lung stress depending on their respiratory system compliance. Reductions in Vt confer benefit if they reduce DP. Evidence indicates that the advantages of lower tidal volumes are most pronounced in patients with reduced compliance. Thus, the effectiveness of lung-protective ventilation appears to depend less on any single setting and more on the combined impact on driving pressure.
Despite widespread adoption of a standard PEEP of 5 cmH₂O during general anaesthesia, evidence suggests this level is frequently inadequate. Many patients exhibit ongoing alveolar recruitment and derecruitment at this setting. Individualised PEEP titration has consistently improved oxygenation, particularly during one-lung ventilation, and may reduce PPCs in this context. However, trials investigating routine PEEP individualisation in standard two-lung ventilation have shown inconsistent benefits. Studies such as iPROVE, IMPROVE-2, and DESIGNATION did not demonstrate overall reductions in complications or mortality. These results suggest that benefits may vary among patients and that a single universal PEEP target is overly simplistic.
Beyond tidal volume and PEEP, respiratory rate (RR) is increasingly recognised as an important contributor to lung injury. Higher intraoperative RR and minute ventilation, often leading to hypocapnia, have been associated with greater risk of PPCs. Hypocapnia may worsen lung injury through mechanisms such as ergotrauma. This has led to interest in the broader concept of mechanical power, which integrates tidal volume, driving pressure, respiratory rate, and other factors to quantify the total energy delivered to the lungs. Higher mechanical power correlates with worse outcomes, even in patients without ARDS. Compensatory increases in RR may negate the benefits of reducing tidal volume by increasing overall ventilation intensity. Observational data suggest that reducing Vt while increasing RR during transitions from OR to ICU may increase mortality.
Although mechanical power offers a promising framework, uncertainties remain about how best to calculate and apply it clinically, particularly regarding the contribution of PEEP and adjustments for lung size. Prospective perioperative studies are still lacking. Until clearer evidence emerges, the authors advocate focusing on lowering driving pressure as a practical surrogate for reducing injurious ventilation. Mechanical power may help clinicians decide whether to prioritise lowering tidal volume or respiratory rate, especially in patients with preserved compliance where Vt reductions may have limited effect.
Overall, the authors recommend moving away from rigid, one-size-fits-all ventilator settings and towards individualised intraoperative strategies. Clinicians should aim to minimise driving pressure, adjust PEEP to maintain recruitment without overdistension, and avoid excessive respiratory rates or oxygen concentrations. A tidal volume around 8 ml/kg predicted body weight is often acceptable, with plateau pressures and DP monitored regularly. If driving pressure exceeds a relatively safe threshold (commonly 11–14 cmH₂O), tidal volume should be reduced while avoiding compensatory increases in RR. Tailoring ventilation to each patient’s physiology, rather than relying on fixed parameters, is presented as the most rational approach to reducing postoperative pulmonary complications.
Source: Intensive Care Medicine
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