“Breathe the pressure, come play my game, I’ll test ya… Inhale, inhale – you’re the victim” – The Prodigy
- Can intraoperative mechanical ventilation mode affect rate of postoperative pulmonary complications?
What is known already:
- Anaesthesia and mechanical ventilation can have detrimental effects on postoperative pulmonary function.
- Postoperative pulmonary complications (PPCs) are one of the most common post-operative complications and are associated with increased morbidity and mortality, increased length of hospital stay and healthcare costs, and can adversely affect long-term clinical outcomes. 
- PPCs are inconsistently defined in the literature but generally regarded as pulmonary abnormalities that result a new requirement for oxygen or respiratory support and adversely affect clinical outcome (respiratory failure, pneumonia, atelectasis).
- Occur in up to 12.5% of elective postoperative patients and in 5.8% of patients after major abdominal surgery. 
- PPCs are associated with intraoperative PEEP, ventilator driving pressure and FiO2. An intra-operative lung protective mechanical ventilation (MV) strategy is associated with reduced risk of PPCs.
- A protective MV strategy is broadly defined as a median PEEP ≥5cmH2O, tidal volume (Vt) <10ml/kg predicted bodyweight, plateau pressure <30cmH2 In patients with normal lungs vs acute lung injury PEEP 5cmH2O and plateau pressure <16cmH2O have been shown to be associated with reduced PPCs.
- Driving pressure (inspiratory plateau pressure – end-expiratory pressure) may be a more important determinant of lung damage than Vt.
- No MV mode (volume or pressure) has been shown to be superior to the other in studies in patients with ARDS, but during laparoscopic surgery PCV may be associated with Improved compliance, better oxygenation, reduced stress response.
- The same tidal volume setting delivered by pressure-controlled ventilation (rather than volume-controlled ventillation) will result in lower peak airway pressure, faster Vt delivery, different gas distribution & reduced risk of barotrauma. However, changes in compliance can affect the tidal volume delivered by pressure-controlled ventilation.
- Volume-control ventilation (VCV) rather than pressure-control ventilation (PCV) might modify the rate of PPCs.
- Adults undergoing MV via an endotracheal tube for surgery at three US centres between Jan 2007 – Dec 2015.
- Excluded patients (n=72,661) with pre-existing pulmonary complications in the 7 days before surgery, those requiring prolonged MV (not extubated immediately after surgery), ASA >5 and those with missing data.
- Analysed cases = 109,360
- Retrospective multi-centre cohort study
- Analysed perioperative data from hospital administrative database.
- Categorised patients by mode of ventilation used most during an operation.
- Outcome measure: PPCs within 7 days post-op (re-intubation, pulmonary oedema, respiratory failure, pneumonia) as defined by ICD-9/10 and CPT clinical codes.
- Ethical approval from MGH Institutional Review Board.
- Declaration of funding and conflict of interests.
- Propensity score matching
- Evaluated potential bias from missing data by using multiple imputation and various sensitivity analyses
- Seventeen percent of patients received PCV and 83% received VCV
- PPCs were more common after pressure-controlled ventillation in both unmatched and matched analyses. The Odds Ration is (95% CI) 1.29 (1.21-1.37), unaffected by adjustment for anaesthesia provider or by imputation for missing data.
- PCV resulted in more variable tidal volumes and driving pressures and more frequent delivery of extreme values of each.
- Association of PCV with PPCs most marked with low PEEP.
- Affect of MV mode disappeared when Vt >12ml/kg.
- Subgroup analyses: 21.2% of patients underwent abdominal surgery, 14.8% of whom had PCV. Rate of PPCs higher after PCV than VCV.
- Variable tidal volumes and driving pressures with PCV may result in increased cyclical collapse and opening of lung segments leading to increased strain with subsequent inflammation. Exacerbated by low or no PEEP.
- Appropriate PEEP and minimisation of driving pressure are key to reducing PPCs.
- The study supports the use of volume-controlled ventillation during surgery, particularly for patients more likely to suffer PPCs.
- Large study population.
- Strong methodology and statistical analysis with thorough attempts to control for confounding, including change in practice over time.
- Generalisable results.
- Physiological plausibility.
- Observational study – potential for unmeasured unknown confounders (ethnicity, social deprivation, smoking, analgesia, early mobilisation, incentive spirometry, lung expansion techniques, chest physiotherapy, minimally invasive surgical techniques).
- Administrative data not collected for clinical / research use.
- Composite outcome – PCV may not have same affect on individual outcomes.
- How many patients recruited from each hospital? Didn’t control for hospital.
- Potential for clinical coding errors and misclassification of outcomes.
- No consideration of assist modes / PC-VG nor switching between PCV and VCV.
- No consideration of ventilation with supraglottic airways.
- Patients may be switched to PCV if difficult to ventilate.
- Patients discharged within the 7 day follow-up period could present with PPCs to a hospital not include in the study.
- USA is a very different healthcare where access to healthcare / insurance coverage could affect management of chronic illness and susceptibility to PPCs.
- Highlights importance of tailoring MV mode to individual patient and surgery, with appropriate PEEP and minimisation of driving pressure.
- Use of appropriate alarms, ventilator limits or PC-VG modes where respiratory compliance frequently changing.
- Sufficient equipoise for RCT?
Summary by Dr Dermot McGuckin
 Davies O, Husain T, Stephens RCM. Postoperative pulmonary complications following non-cardiothoracic surgery. BJA Education 2017; 17: 295–300.
 Yang CK, Teng A, Lee DY, Rose K. Pulmonary complications after major abdominal surgery: National Surgical Quality Improvement Program Analysis. J Surg Res 2015; 198: 441–9.