Post-operative Pulmonary Complications

"Despite a large number of candidate definitions, the group was unable to reach a consensus on the best definition of postoperative pulmonary complications" (BJA, 2018)

• This statement about sums up the difficulty in defining POPC, though I've used the consensus definitions from the paper below
• NB - They do not align with the European Perioperative Clinical Outcome definitions (2015), which include bronchospasm, pleural effusion and pneumothorax

• Post-operative pulmonary complications are a composite of respiratory diagnoses sharing common pathophysiological mechanisms, including pulmonary collapse and airway contamination
• POPC include:
• Atelectasis - as demonstrated by CXR or CT
• Pneumonia - as per the US CDC definition (see below)
• Pulmonary aspiration - as demonstrated by clear history and radiographic findings
• ARDS - as per the Berlin criteria
• Non-ARDS respiratory failure characterised by either:
• Re-intubation and mechanical ventilation within 30 days of surgery
• >24hrs mechanical ventilation post-operatively


• These definitions exclude PE, pleural effusion, pneumothorax, bronchospasm and cardiogenic pulmonary oedema owing to separate pathophysiological processes

1. Chest radiograph with new/progressive and persistent infiltrates, consolidation or cavitation


2. And one of:
• Pyrexia >38°C
• WCC <4 or >12 x 10⁹/L
• Altered mental state of no other apparent cause in a patient >70yrs


3. And two of:
• New onset purulent sputum/increased respiratory secretions/change in sputum character
• New onset or worsening cough, dyspnoea or tachypnoea
• Bronchial breath sounds or rales
• Worsening gas exchange e.g. hypoxaemia, increased F_iO₂, increased ventilatory requirements

• POPC are relatively common, though inconsistent definitions has led to a variously quoted, remarkable and unhelpfully broad incidence range (3.4% - 51.2%)
• The incidence of any POPC for all-comers may in the 5 -10% range, but can rise significantly in select patient groups, particularly those at higher risk
• The prospective ISOS cohort study (2016) demonstrated the post-operative incidence of pneumonia (1.6%) and ARDS (0.3%) globally

• It would perhaps be easier to describe patients not at higher risk of POPC, such is the breadth of risk factors:

• Asthma
• Poor spirometry; not effective at independently predicting an individuals risk for POPC
• Pre-operative hypercarbia on ABG; not independently associated with POPC
• Abnormal chest X-ray alone
  • If the patient is felt to clinically warrant a CXR then it is associated with higher risk, presumably due to coalescing pathological process such as pneumonia or IECOPD

• The ARISCAT scoring system stratifies patients into low, intermediate and high-risk of POPC
• It scores for: age, sats ≤95%, Hb<100g/L, LRTI in past month, intrathoracic/upper abdominal surgery, emergency surgery and duration of surgery >2hrs
• Compared to the low-risk category, patients risk-stratified as being at high risk had:
• An absolute risk difference for POPC of +12% (AVATAR, 2021)
• A relative risk increase for POPC of 3.16 (LAS VEGAS, 2017 )

• Naturally there will be an impact from patient factors implicated in POPC, be it:
• Advancing age
• Existing respiratory disease such as smoking, COPD or OSA
• Existing non-respiratory disease such as obesity or cardiac disease
• The presence of acute lung pathology e.g. chest infection

• Anaesthetic agents and a host of other drugs used in the perioperative period disrupt the central regulation of breathing
• Most agents impair ventilatory responses to hypoxia or hypercarbia, even at low levels
• This leads to altered neural drive to the respiratory muscles, including the diaphragm


• Other effects of GA (even without NMBA) include:
• Cephalad displacement of the diaphragm, reducing FRC 15 - 20%
• Altered V/Q matching, with areas tending towards higher or lower ratios (i.e. more lung units closer to dead space or shunt)
• Atelectasis from hyperoxia; pre-oxygenation with 100% oxygen leads to up to 5.6% atelectasis on CT
• Mucociliary disruption and retained secretions owing to ETT and GA


• The surgeons are to blame too; surgical intervention can also damage respiratory muscles and restrict ventilation
• Mechanisms include:
• Direct functional disruption e.g. via incision of muscles, pneumoperitoneum
• Voluntary limitation of muscle use due to post-operative pain
• Reflex inhibition of motor signals to the phrenic, vagus and somatic nerves following traction on the viscera e.g. gallbladder, oesophagus


• This leads to a combination of chest wall (& thus underlying lung) distortion and poorly coordinated respiratory muscle effort
• There is consequential:
• Reduced VC and ventilatory efficiency, with alveolar de-recruitment
• Reduced FRC due to collapse ± consolidation of alveoli
• Impaired pulmonary gas exchange and hypoxaemia


• The net effects are atelectasis, pneumonia and acute lung injury from inflammatory responses to tissue injury i.e. POPC

• Issues continue in recovery, owing to an interaction between:
• Residual effects of anaesthetic drugs depressing consciousness (airway obstruction) and respiratory drive
• Residual effects of NMBA
• Ongoing effects from opioid analgesia, including impaired ventilatory response to hypoxia and hypercarbia
• Pain-related impairments in ventilation and coughing


• Atelectasis, and associated reduced FRC, remain an issue for a prolonged period post-operatively;
• Up to 60% of patients will still have radiographic atelectasis at 24 - 72hrs
• FRC may reach its nadir day 2 post-operatively and only return to normal values by day 7
• The A-a gradient may remain elevated for days post-operatively

• Optimise existing cardiorespiratory disease e.g.:
• Smoking cessation
• Pre-operative physiotherapy e.g. inspiratory muscle training
• Prehabilitation
• Continue regular treatments
• Ensure compliant with CPAP if OSA


• Risk prediction models e.g. ARISCAT may help identify patients at high risk of complications and allow stratified pre-operative intervention and more informed consent

• Encourage use of a minimally-invasive technique
• Selective (rather than routine) use of NG tubes, i.e. only for symptom relief or surgical indications

• Avoid GA if possible
• If a GA is required, a spontaneously ventilating patient on a supraglottic device is preferable to endotracheal intubation (BJA, 2021)
• Interestingly:
• Use of an ETT is associated with higher risk of POPC, even if NMBA aren't used
• The difference in POPC risk between SAD and ETT is abolished by giving patients ventilated via a SAD an NMBA

• This table is the lovechild of international consensus recommendations (BJA, 2019) and the Q&A following an RCoA Webinar (2021) on POPC

• Interestingly, the AVATAR study found no ventilatory variables to be independently associated with risk of POPC in adults undergoing robotically-assisted abdominal surgery

• Why the moderate PEEP in the recommendations? Given the key role of atelectasis in the pathophysiology of POPC, one would expect gratuitous PEEP to be beneficial. This doesn't appear to be the case...


• A meta-analysis (BJA, 2022) of studies on higher (>10cmH2O) vs. lower (<4cmH2O) PEEP and rate of day 7 POPCs showed no statistically significant difference between groups
• Both groups were ventilated using 7ml/kg IBW and the rate of POPCs in both was approximately 30%
• The higher PEEP group did have fewer interventions for desaturation but at the expense of significantly more frequent intra-operative hypotension
• Higher (12cmH2O) PEEP not associated with reduced risk of POPC than low (4cmH2O) PEEP in obese patients according to the PROBESE trial (JAMA, 2019)
• A low VT, moderate PEEP strategy reduced incidence of POPC compared to standard mechanical ventilation, according to a 2020 network meta-analysis (BJA)
• A secondary analysis (2021) of the iPROVE and iPROVE-O2 trials found that an open lung approach (stepwise ventilator recruitment manoeuvre to 40cmH2O followed by PEEP titrated to optimum compliance) was associated with a reduced incidence of POPC at 7 and 30 days post-operatively
• A high (>12cmH2O) vs. low (2cmH2O) PEEP strategy did not reduce risk of POPC in the PROVILHO trial (Lancet, 2014)

• The use of any NMBA was associated with an increased incidence of POPC (aOR 1.86) in the POPULAR trial (Lancet, 2019)
• Perhaps counter-intuitively this risk wasn't reduced by use of neuromuscular monitoring, reversal agents (either sugammadex or neostigmine) or reversal to a quantitative ToF ratio of >0.9
• Post-Hoc analysis of the the study demonstrated that targeting a TOFR >0.95 may actually decrease incidence of POPC vs. a TOFR of 0.9 (BJA, 2019)


• If one is obliged to use a NMBA, is there a 'best' option?
• Suxamethonium (aOR 1.29) is more likely to cause POPC than non-depolarising agents (aOR 1.19), especially for short (<2hrs) surgeries (BJA, 2020)
• Use of suxamethonium and non-depolarising agents on the same patient increases the risk even further
• Shorter-acting agents aren't necessarily better, though pancuronium was associated with a higher rate of POPC (16.9%) than atracurium or vecuronium (4.2%)

• Despite the risks of inadequate reversal, up to 15% of patients are still extubated with TOFR <0.9


• We should be using quantitative TOF; it is demonstrably superior to tactile monitoring at predicting hypoxia (<90%) or need for airway manoeuvres in the PACU
• Avoiding such hypoxia is important; earlier desaturation (<90%) after surgery is associated with more severe pulmonary complications inc. pneumonia, ARDS or re-intubation (OR 1.68)
• Residual block is associated with post-operative complications; patients with a critical respiratory event in recovery (0.9%) had a mean TOFR of 0.62 vs. 0.98 for those who did not (from the afore-referenced POPULAR study)


• Acknowledging that reversal of neuromuscular blockade is a good thing, is there a superior agent? Surely it's sugammadex?


• Sugammadex is demonstrably more efficacious than neostigmine at ensuring there's no residual block in recovery
• Sugammadex is not definitively superior to neostigmine at preventing POPC and pneumonia
• However, when compared to neostigmine, sugammadex is associated with a:
• Reduced risk of POPC inc. pneumonia in older patients (effects seen in those >70yrs, >75yrs and >80yrs) (BJA, 2021)
• Lower incidence of total POPC, up to a 1/3^rd reduction according to the much-criticised STRONGER and STRONGER-STIL studies (BJA, 2022)
• Reduced likelihood of desaturation and need for oxygen supplementation
• Less pulmonary morbidity and respiratory failure (BJA, 2022)

• An FiO2 >0.8 during emergence increases atelectasis formation
• Aim sats >94% i.e. not necessary to give all patients oxygen in recovery

• Adequate multi-modal, opioid-sparing analgesia
• Use of post-operative physiotherapy
• Early mobilisation
• Lung expansion techniques e.g. incentive spirometry

• Early application of HFNO does not reduce incidence of POPC according to the OPERA trial (2016)
• There's insufficient evidence to confirm benefits/harms of empirical CPAP on POPC in those undergoing major abdominal surgery as per the Cochrane collaborative (2014)
• Prophylactic post-operative CPAP alongside an individualised intra-operative ventilation strategy aimed at minimising lung de-recruitment did not statistically reduce the incidence of POPC in the iPROVE study (Lancet, 2019)
• CPAP did not reduce incidence of pneumonia, need for re-intubation or death in patients >50yrs undergoing major elective abdominal surgery (PRSIM trial, Lancet 2021)
• Overall neither CPAP, NIV nor HFNO appears to reduce the incidence of pneumonia or POPC in adults (BJA, 2021)


• It would seem routine use of post-operative non-invasive ventilatory support is not beneficial, indeed the afore-linked international consensus statement suggests:

The postoperative prophylactic use of NIPPV or CPAP should be considered for patients who use these modalities to maintain adequate ventilation before operation