To the Editor:—
We were interested to read the article by Slinger et al. regarding the effect of positive end-expiratory pressure (PEEP) on arterial oxygenation during one-lung ventilation (OLV). 1When application of 5 cm H2O PEEP to the ventilated lung causes total PEEP (plateau end-expiratory pressure) to increase from a low level toward the maximum curvature point (so-called “lower inflection point”) on the static inspiratory pressure-volume (PV) curve during OLV, arterial oxygenation is improved. However, we have two comments regarding the conclusions made.
First, the “lower inflection point” on the inspiratory PV curve does not accurately indicate PEEP required to prevent end-expiratory lung collapse in patients with acute respiratory distress syndrome (ARDS), because PEEP is an expiratory phenomenon. 2Holzapfel et al . reported that the inflection point on the expiratory PV curve was greater than the “lower inflection point” on the inspiratory PV curve, and PEEP set at the inflection point on the expiratory PV curve abruptly decreased pulmonary shunt in the early stage of ARDS. 3
Second, the conclusions of the authors suggest that increases in total PEEP would improve hypoxemia during OLV in many patients with good elastic recoil. However, we believe that increased end-expiratory volume of the ventilated lung would not improve hypoxemia during OLV. We quantified the magnitude of auto-PEEP during OLV, and found that Pao2on Fio2of 1.0 during OLV was extremely variable and levels of auto-PEEP did not correlate with Pao2during OLV. 4In addition, Katz et al. demonstrated that application of 10 cm H2O PEEP to the ventilated lung did not significantly affect pulmonary shunt during OLV. 5These clinical findings suggest that end-expiratory volume of the ventilated lung may not represent a major determinant of arterial oxygenation during OLV. Pulmonary blood flow has been considered to redistribute from the nondependent to the dependent lung in the lateral decubitus position. However, an animal study has recently demonstrated that pulmonary blood flow does not redistribute with repositioning from the supine to the left lateral decubitus position during two-lung ventilation. 6Furthermore, Glenny et al . most recently demonstrated that the structure of the pulmonary vascular tree, and not gravity, is the primary determinant of the distribution of pulmonary blood flow in pigs. 7Whether complete lung collapse produces significant reduction in pulmonary blood flow to the nondependent lung has yet to be fully elucidated. However, these recent experimental studies suggest that excessive pulmonary blood flow to the nonventilated lung, as opposed to decreased end-expiratory volume of the ventilated lung, might play a key role in the development of hypoxemia during OLV.