Regional ventilation, pulmonary perfusion and gas exchange in supine and prone positions

The rationale for treating patients with acute lung injury in the prone position springs from a number of observations. Clinical reports have documented improved oxygenation when patients are turned prone. Regional vascular conductance is higher in dorsal lung regions regardless of posture. Lung perfusion in dogs is similarly distributed in prone and supine positions. Ventilation to perfusion matching is better in the prone than supine position. These observations provided the incentive for the present series of studies. All studies focus on determinants of gas exchange in supine and prone posture. In paper I, 13 patients with severe acute lung insufficiency were treated in the prone position. They suffered from multiple organ failure due to septicemia, trauma, aspiration, intoxication and bum injury. In paper 11, gas exchange was studied in 8 pigs before and after an experimentally induced abdominal distension, while in the prone and supine posture. In paper 111, lung perfusion distribution was studied in 10 healthy volunteers using single photon emission computed tomography (SPECT), during normal and CPAP (10 cmH20) breathing. In paper IV, ventilation (V), lung perfusion (Q) and V/Q ratios were studied in both supine and prone positions during controlled mechanical ventilation (CMV) using an in vitro microsphere technique. In the final paper, using SPECT, V, Q and V/Q ratios were studied in 16 volunteers, during normal and CPAP (10 cmH20) breathing while prone and supine. The most important findings in this thesis are that prone position improves oxygenation in the majority of patients with acute lung insufficiency and that lung perfusion is more uniform in that posture. V/Q ratios are more uniform when prone in spontaneously breathing humans with healthy lungs. During experimental conditions, using CMV in healthy lungs, V/Q is more uniform prone than supine. During abdominal distension, the prone position results in a more markedly improved gas exchange. The use of CPAP in healthy humans shows an unfavourable V/Q in the prone as compared to the supine position