Reply to the Editor

We thank Dr Macedo and colleagues for their interest in our study on low-frequency ventilation during cardiopulmonary bypass (CPB) that was recently published in the Journal.1 They share our interest in lung injury during cardiac surgery and how to prevent it. We believe this is relevant to current practice with an ever-increasing population of high-risk patients.

The rationale beyond our interest in lung injury is based on the simple observation that in current clinical practice, once CPB is instituted, the patient's lungs are disconnected from the ventilator and left open to the air, fully collapsed and poorly perfused for the duration of CPB. Our report showed significant ischemic damage and atelectasis in the collapsed lungs during and after CPB, and these were improved by using low-frequency ventilation during CPB.1 Although the underlying mechanisms are still unclear, there have been reports suggesting that ventilation during CPB may reduce damage in the lungs.2 We have taken this forward and are now about to start a large clinical trial to assess low-frequency ventilation in clinical practice.

The issue of pulmonary perfusion during cardiac surgery is an interesting one. Current practice is based on the assumption that bronchial blood supply is sufficient for lung protection during CPB.3 Yet, during cardiac surgery the bronchial circulation (only 1%–2% of the cardiac output) is jeopardized by CPB and surgery-related factors, including lung collapse, internal thoracic artery harvesting, aortic manipulation, hypothermia, and loss of pulsatile flow. Schelensak and colleagues4 showed that there is significant decrease in bronchial arterial flow during the initial phase of total CPB and that flow remains low until the end of CPB, returning to normal by 60 minutes of reperfusion.

This highlights the importance that pulmonary circulation may have for alveolar oxygenation, which is also supported by the observation that during lung transplantation surgery the bronchial arteries rarely are reconnected without obvious lung ischemia.

Pulmonary artery perfusion is an obvious potential intervention to prevent lung ischemia. Along with Dr Macedo and colleagues, we are currently focusing on this in ongoing experimental research.

Others have suggested that ventilation during CPB can reduce lung injury.4, 5 In our view, the safety, efficacy, and surgical practicality of pulmonary perfusion during CPB remains to be proven beyond any doubt before establishing it in clinical practice. Also, there is still a lot of research to undertake to ascertain the mechanistic insights, the dominance of ventilation versus perfusion or vice versa as a protective intervention, and the best route or mode of delivery of pulmonary perfusion.

With regard to the possible resistance by surgeons to adopt these interventions, we think the availability of compelling evidence in support of these interventions to prevent lung injury will eliminate any practical concerns.

Dr Macedo and colleagues refer to a trial that will assess the effect of lung perfusion/ventilation during CPB. We look forward to their results, which will hopefully add further knowledge to face the continued ch allenge of lung injury during cardiac surgery.