Background: Bedside assessment of lung volume in clinical practice is crucial to adapt ventilation strategy. We\ncompared bedside measures of lung volume by helium multiple-breath washout technique (EELVMBW,He) and\neffective lung volume based on capnodynamics (ELV) to those assessed from spiral chest CT scans (EELVCT) under\ndifferent PEEP levels in control and surfactant-depleted lungs.\nMethods: Lung volume was assessed in anaesthetized mechanically ventilated rabbits successively by measuring\ni) ELV by analyzing CO2 elimination traces during the application of periods of 5 consecutive alterations in inspiratory/\nexpiratory ratio (1:2 to 1.5:1), ii) measuring EELVMBW,He by using helium as a tracer gas, and iii) EELVCT from CT scan\nimages by computing the normalized lung density. All measurements were performed at PEEP of 0, 3 and 9 cmH2O in\nrandom order under control condition and following surfactant depletion by whole lung lavage.\nResults: Variables obtained with all techniques followed sensitively the lung volume changes with PEEP. Excellent\ncorrelation and close agreement was observed between EELVMBW,He and EELVCT (r = 0.93, p < 0.0001). ELV\noverestimated EELVMBW,He and EELVCT in normal lungs, whereas this difference was not evidenced following surfactant\ndepletion. These findings resulted in somewhat diminished but still significant correlations between ELV and EELVCT\n(r = 0.58, p < 0.001) or EELVMBW,He (0.76, p < 0.001) and moderate agreements.\nConclusions: Lung volume assessed with bedside techniques allow the monitoring of the changes in the lung\naeration with PEEP both in normal lungs and in a model of acute lung injury. Under stable pulmonary haemodynamic\ncondition, ELV allows continuous lung volume monitoring, whereas EELVMBW,He offers a more accurate estimation, but\nintermittently.
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