Uneven distribution of ventilation in acute respiratory distress syndrome

Abstract

Introduction: The aim of this study was to assess the volume of gas being poorly ventilated or non-ventilated within the lungs of patients treated with mechanical ventilation and suffering from acute respiratory distress syndrome (ARDS).

Methods: A prospective, descriptive study was performed of 25 sedated and paralysed ARDS patients, mechanically ventilated with a positive end-expiratory pressure (PEEP) of 5 cmH2O in a multidisciplinary intensive care unit of a tertiary university hospital. The volume of poorly ventilated or non-ventilated gas was assumed to correspond to a difference between the ventilated gas volume, determined as the end-expiratory lung volume by rebreathing of sulphur hexafluoride (EELVSF6), and the total gas volume, calculated from computed tomography images in the end-expiratory position (EELVCT). The methods used were validated by similar measurements in 20 healthy subjects in whom no poorly ventilated or non-ventilated gas is expected to be found.

Results: EELVSF6 was 66% of EELVCT, corresponding to a mean difference of 0.71 litre. EELVSF6 and EELVCT were significantly correlated (r2 = 0.72; P < 0.001). In the healthy subjects, the two methods yielded almost identical results.

Conclusion: About one-third of the total pulmonary gas volume seems poorly ventilated or non-ventilated in sedated and paralysed ARDS patients when mechanically ventilated with a PEEP of 5 cmH2O. Uneven distribution of ventilation due to airway closure and/or obstruction is likely to be involved.

Figure 1

Concentration of the tracer gas sulphur hexafluoride (SF₆) plotted during 30 s of rebreathing in a supine healthy subject.

Figure 2

Linear regression between EELV measurements by rebreathing of sulphur hexafluoride (EELV_SF6) and by computed tomography (EELV_CT) obtained in 25 ARDS patients. The dotted line is the regression line EELV_SF6 = 0.4EELV_CT + 0.3 (r² = 0.72; P < 0.001).

Figure 3

Linear regression between FRC measurements by rebreathing of sulphur hexafluoride (FRC_SF6) and by computed tomography (FRC_CT) in 20 healthy subjects. The dotted line is the regression line: EELV_SF6 = 0.9FRC_CT + 0.1 (r² = 0.83; P < 0.001).

Figure 4

Bland-Altman plot [24] of supine functional residual capacity measured by rebreathing of sulphur hexafluoride (FRC_SF6) and by computed tomography (FRC_CT) in 20 healthy subjects. The individual differences of paired measurements (y axis) did not depend on the magnitude of their average values (x axis). The mean difference (solid line; dotted lines represent the mean ± 2SD) was small.