Intra-operative mechanical ventilation strategies in cardiac surgical patients

Cardiac surgery causes substantial respiratory morbidity, leading to prolonged intensive care and hospital length of stay. Although ‘lung protective’ ventilation bundles (low tidal volume, high positive end-expiratory pressure and alveolar recruitment manoeuvres) have evidence to show effectiveness in intensive care unit patients with pre-existing lung injury and non-cardiac surgical patients with ‘non-injured’ lungs, the situation with its intra-operative use is controversial and as a result it is often not employed in the context of cardiac surgery. ¹ Traditionally, ventilator settings in cardiac surgery patients included large tidal volumes (10–15 ml/kg actual body weight) in order to minimise atelectasis, and minimal levels of positive end-expiratory pressure (PEEP) to reduce haemodynamic consequences. ² We hypothesised that no consensus exists among UK cardiothoracic centres with regard to optimal intra-operative mechanical ventilation practices.

An Association for Cardiothoracic Anaesthesia and Critical Care (ACTACC) research committee-approved, online survey (Survey Monkey; San Mateo, CA, USA) was disseminated to the ACTACC linkmen of 32 cardiothoracic UK centres. The survey was available from 8 January to 6 February 2017 (30 days) and consisted of 10 questions relating to intra-operative ventilation strategy and in particular, tidal volume (V_T), levels of PEEP, ventilation during cardiopulmonary bypass (CPB) and use of alveolar recruitment manoeuvres.

From the 32 centres queried, a total of 20 (62.5%) centres participated, and all 20 surveys were included in data analysis. The final sample of respondents comprised consultants in cardiothoracic anaesthesia. Six out of 20 (30%) centres reported having a peri-operative ventilatory bundle protocol within their departments. Volume control ventilation was the most commonly reported ventilation mode used. V_T of 6 ml/kg predicted body weight (PBW) is being used in 58% of the participating centres and V_T < 8 ml/kg PBW in 79% (median V_T 6 ml/kg, (95% CI 6.6 ± 0.39)). The majority of the respondents (75%) cited using a PEEP level of ≤5 cmH₂O (median 4 cmH₂O; 95% CI: 4.6 ± 0.47) and cessation of mechanical ventilation during CPB was reported in 94% of sites. Forty-five percent of sites stated that the target driving pressure (a surrogate of lung strain) was <15 cmH₂O and 50% perform alveolar recruitment manoeuvres (ARM) after sternal closure.

Our results illustrate that although the majority of UK cardiothoracic centres use ‘low tidal volume’ ventilation (6–8 ml/kg PBW) and low levels of PEEP, there is variation in clinical practice across sites and that decisions are frequently surgeon or anaesthetist dependent. This may be explained by lack of institutional protocols and highlights the need for adequately powered multicentre randomised trials examining the effect of intra-operative single interventions (such as high PEEP or ARM) on clinical outcomes, in cardiac surgical patients receiving low tidal volume ventilation. These trials will inform the development of evidence-based clinical guidelines for least damaging mechanical ventilation during cardiac surgery.