Measuring dead-space in acute lung injury.

Several recent studies have advanced our understanding of dead-space ventilation in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS). They have demonstrated the utility of measuring physiologic dead-space-to-tidal volume ratio (VD/VT) and related variables in assessing outcomes as well as therapeutic interventions. These studies have included the evaluation of mortality risk, pulmonary perfusion, as well as the effectiveness of drug therapy, prone positioning, positive end-expiratory pressure (PEEP) titration, and inspiratory pattern in improving gas exchange. In patients with ALI/ARDS managed with lung-protective ventilation a significant relationship between elevated VD/VT and increased mortality continues to be reported in both early and intermediate phases of ALI/ARDS. Some clinical evidence now supports the suggestion that elevated VD/VT in part reflects the severity of pulmonary vascular endothelial damage. Monitoring VD/VT also appears useful in assessing alveolar recruitment when titrating PEEP and may be a particularly expedient method for assessing the effectiveness of prone positioning. It also has revealed how subtle manipulations of inspiratory time and pattern can improve CO(2) excretion. Much of this has been accomplished using volumetric capnography. This allows for more sophisticated measurements of pulmonary gas exchange function including: alveolar VD/VT, the volume of CO(2) excretion and the slope of the alveolar plateau which reflects ventilation: perfusion heterogeneity. Many of these measurements now can be made non-invasively which should only increase the research and clinical utility of volumetric capnography in studying and managing patients with ALI/ARDS.