Pulmonary effects of body position, PEEP, and surfactant depletion in dogs

Abstract

The influence of position (sphinx, lateral, supine), surfactant depletion, and different positive end-expiratory pressure (PEEP) on functional residual capacity (FRC), series dead space (VdS) and compliance of the respiratory system (Crs) were evaluated in five dogs. Ventilation homogeneity as measured by an index (multiple breath alveolar mixing efficiency), oxygenation, and cardiovascular hemodynamics were additionally examined. The dogs were anesthetized with halothane, paralyzed, and mechanically ventilated. FRC and VdS were found to be notably large in dogs, 45 +/- 8 ml/kg and 6 +/- 1 ml/kg, respectively. FRC and ventilation homogeneity were improved in the sphinx position (prone position with upright head). Surfactant depletion by lung lavage with 37 degrees C saline caused an immediate and stable decrease in FRC, Crs, and oxygenation (P less than 0.05, respectively) for about 5 h without marked effects on the circulatory system. FRC and VdS increased with increasing PEEP. At the highest PEEP, 10 cmH2O (1 kPa), Crs decreased (P less than 0.05) and ventilation became more uneven, indicating alveolar overdistension.

Effects of positive end-expiratory pressure on dead space and its partitions in acute lung injury

“…Fletcher used the original principle of Fowler [25] that probably gives higher values because correction for the alveolar slope, that decreases the airway dead space calculated [20], was not performed. High airway pressure induced by PEEP increased VD aw as previously observed [27].…”

Prone position improves gas exchange – but how?

“…It has even been suggested that an increased FRC prone was the mechanism for the improved gas exchange in this posture. In several previous studies, both experimental and clinical, the prone position has been shown to result in greater FRC (43, 44). There are also several other investigations, experimental as well as clinical, that find an unchanged FRC in the prone as compared with the supine position (36, 40, 45).…”

Effects of positive end-expiratory pressure increments can be predicted by computer simulation based on a physiological profile in acute respiratory failure

“…The observed decrease in V . CO2 30 s after large increments of PEEP may be explained by increased airway dead space [6,18]. Alternative explanations are PEEP related effects on hemodynamics: Compression of capillaries in open lung regions may cause diversion of blood flow toward collapsed areas [19,20].…”