Functional Phenotyping: A New Role for Electrical Impedance Tomography

Postoperative pulmonary complications (PPCs) are common, costly, and increase patient mortality. Changes to the respiratory system occur immediately on induction of general anaesthesia: respiratory drive and muscle function are altered, lung volumes reduced, and atelectasis develops in > 75% of patients receiving a neuromuscular blocking drug. The respiratory system may take 6 weeks to return to its preoperative state after general anaesthesia for major surgery. Risk factors for PPC development are numerous, and clinicians should be aware of non-modifiable and modifiable factors in order to recognize those at risk and optimize their care. Many validated risk prediction models are described. These have been useful for improving our understanding of PPC development, but there remains inadequate consensus for them to be useful clinically. Preventative measures include preoperative optimization of co-morbidities, smoking cessation, and correction of anaemia, in addition to intraoperative protective ventilation strategies and appropriate management of neuromuscular blocking drugs. Protective ventilation includes low tidal volumes, which must be calculated according to the patient's ideal body weight. Further evidence for the most beneficial level of PEEP is required, and on-going randomized trials will hopefully provide more information. When PEEP is used, it may be useful to precede this with a recruitment manoeuvre if atelectasis is suspected. For high-risk patients, surgical time should be minimized. After surgery, nasogastric tubes should be avoided and analgesia optimized. A postoperative mobilization, chest physiotherapy, and oral hygiene bundle reduces PPCs.

Background Assessing alveolar recruitment at different positive end-expiratory pressure (PEEP) levels is a major clinical and research interest because protective ventilation implies opening the lung without inducing overdistention. The pressure-volume (P-V) curve is a validated method of assessing recruitment but reflects global characteristics, and changes at the regional level may remain undetected. The aim of the present study was to compare, in intubated patients with acute hypoxemic respiratory failure (AHRF) and acute respiratory distress syndrome (ARDS), lung recruitment measured by P-V curve analysis, with dynamic changes in poorly ventilated units of the dorsal lung (dependent silent spaces [DSSs]) assessed by electrical impedance tomography (EIT). We hypothesized that DSSs might represent a dynamic bedside measure of recruitment. Methods We carried out a prospective interventional study of 14 patients with AHRF and ARDS admitted to the intensive care unit undergoing mechanical ventilation. Each patient underwent an incremental/decremental PEEP trial that included five consecutive phases: PEEP 5 and 10 cmH2O, recruitment maneuver + PEEP 15 cmH2O, then PEEP 10 and 5 cmH2O again. We measured, at the end of each phase, recruitment from previous PEEP using the P-V curve method, and changes in DSS were continuously monitored by EIT. Results PEEP changes induced alveolar recruitment as assessed by the P-V curve method and changes in the amount of DSS (p < 0.001). Recruited volume measured by the P-V curves significantly correlated with the change in DSS (r s = 0.734, p < 0.001). Regional compliance of the dependent lung increased significantly with rising PEEP (median PEEP 5 cmH2O = 11.9 [IQR 10.4–16.7] ml/cmH2O, PEEP 15 cmH2O = 19.1 [14.2–21.3] ml/cmH2O; p < 0.001), whereas regional compliance of the nondependent lung decreased from PEEP 5 cmH2O to PEEP 15 cmH2O (PEEP 5 cmH2O = 25.3 [21.3–30.4] ml/cmH2O, PEEP 15 cmH2O = 20.0 [16.6–22.8] ml/cmH2O; p <0.001). By increasing the PEEP level, the center of ventilation moved toward the dependent lung, returning to the nondependent lung during the decremental PEEP steps. Conclusions The variation of DSSs dynamically measured by EIT correlates well with lung recruitment measured using the P-V curve technique. EIT might provide useful information to titrate personalized PEEP. Trial registration ClinicalTrials.gov, NCT02907840. Registered on 20 September 2016. Electronic supplementary material The online version of this article (10.1186/s13054-017-1931-7) contains supplementary material, which is available to authorized users.