Lung volume assessments in normal and surfactant depleted lungs: agreement between bedside techniques and CT imaging

BACKGROUND: Bedside assessment of lung volume in clinical practice is crucial to adapt ventilation strategy. We compared bedside measures of lung volume by helium multiple-breath washout technique (EELV(MBW,He)) and effective lung volume based on capnodynamics (ELV) to those assessed from spiral chest CT scans (EELV(CT)) under different PEEP levels in control and surfactant-depleted lungs. METHODS: Lung volume was assessed in anaesthetized mechanically ventilated rabbits successively by measuring i) ELV by analyzing CO(2) elimination traces during the application of periods of 5 consecutive alterations in inspiratory/expiratory ratio (1:2 to 1.5:1), ii) measuring EELV(MBW,He) by using helium as a tracer gas, and iii) EELV(CT) from CT scan images by computing the normalized lung density. All measurements were performed at PEEP of 0, 3 and 9 cmH(2)O in random order under control condition and following surfactant depletion by whole lung lavage. RESULTS: Variables obtained with all techniques followed sensitively the lung volume changes with PEEP. Excellent correlation and close agreement was observed between EELV(MBW,He) and EELV(CT) (r = 0.93, p < 0.0001). ELV overestimated EELV(MBW,He) and EELV(CT) in normal lungs, whereas this difference was not evidenced following surfactant depletion. These findings resulted in somewhat diminished but still significant correlations between ELV and EELV(CT) (r = 0.58, p < 0.001) or EELV(MBW,He) (0.76, p < 0.001) and moderate agreements. CONCLUSIONS: Lung volume assessed with bedside techniques allow the monitoring of the changes in the lung aeration with PEEP both in normal lungs and in a model of acute lung injury. Under stable pulmonary haemodynamic condition, ELV allows continuous lung volume monitoring, whereas EELV(MBW,He) offers a more accurate estimation, but intermittently.