Relation of the static compliance curve and positive end-expiratory pressure to oxygenation during one-lung ventilation

Background: Positive end-expiratory pressure (PEEP) is commonly applied to the ventilated lung to try to improve oxygenation during one-lung ventilati on but is an unreliable therapy and occasionally causes arterial oxygen par tial pressure (Pao,) to decrease further. The current study examined whethe r the effects of PEEP on oxygenation depend on the static compliance curve of the lung to which it is applied. Methods: Forty-two adults undergoing thoracic surgery were studied during s table, open-chest, one-lung ventilation. Arterial blood gasses were measure d during two-lung ventilation and one-lung ventilation before, during, and after the application of 5 cm H2O PEEP to the ventilated lung. The plateau end-expiratory pressure and static compliance curve of the ventilated lung were measured with and without applied PEEP, and the lower inflection point was determined from the compliance curve. Results: Mean (+/- SD) Pao, values, with a fraction of inspired oxygen of 1 .0, were not different during one-lung ventilation before (192 +/- 91 mmHg) , during (190 +/- 90), or after (205 +/- 79) the addition of 5 cm H2O PEEP. The mean plateau end-expiratory pressure increased from 4.2 to 6.8 cm H2O with the application of 5 cm H2O PEEP and decreased to 4.5 cm H2O when 5 cm H2O PEEP was removed. Six patients showed a clinically useful (> 20%) incr ease in Pao(2) with 5 cm H2O PEEP, and nine patients had a greater than 20% decrease in Pao2. The change in Pao(2) with the application of 5 CM H2O PE EP correlated in an inverse fashion with the change in the gradient between the end-expiratory pressure and the pressure at the lower inflection point (r = 0.76). The subgroup of patients with a Pao, during two-lung ventilati on that was less than the mean (365 mmHg) and an end-expiratory pressure du ring one-lung ventilation without applied PEEP less than the mean were more likely to have an increase in Pao, when 5 cm H2O PEEP was applied. Conclusions: The effects of the application of external 5 cm H2O PEEP on ox ygenation during one-lung ventilation correspond to individual changes in t he relation between the plateau end-expiratory pressure and the inflection point of the static compliance curve. When the application of PEEP causes t he end-expiratory pressure to Increase from a low level toward the Inflecti on point, oxygenation is likely to improve. Conversely, if the addition of PEEP causes an increased inflation of the ventilated lung that raises the e quilibrium end-expiratory pressure beyond the inflection point, oxygenation Is likely to deteriorate.