Unmatched ventilation and perfusion measured by electrical impedance tomography predicts the outcome of ARDS

Abstract

Background: In acute respiratory distress syndrome (ARDS), non-ventilated perfused regions coexist with non-perfused ventilated regions within lungs. The number of unmatched regions might reflect ARDS severity and affect the risk of ventilation-induced lung injury. Despite pathophysiological relevance, unmatched ventilation and perfusion are not routinely assessed at the bedside. The aims of this study were to quantify unmatched ventilation and perfusion at the bedside by electrical impedance tomography (EIT) investigating their association with mortality in patients with ARDS and to explore the effects of positive end-expiratory pressure (PEEP) on unmatched ventilation and perfusion in subgroups of patients with different ARDS severity based on PaO₂/FiO₂ and compliance.

Methods: Prospective observational study in 50 patients with mild (36%), moderate (46%), and severe (18%) ARDS under clinical ventilation settings. EIT was applied to measure the regional distribution of ventilation and perfusion using central venous bolus of saline 5% during end-inspiratory pause. We defined unmatched units as the percentage of only ventilated units plus the percentage of only perfused units.

Results: Percentage of unmatched units was significantly higher in non-survivors compared to survivors (32[27-47]% vs. 21[17-27]%, p < 0.001). Percentage of unmatched units was an independent predictor of mortality (OR 1.22, 95% CI 1.07-1.39, p = 0.004) with an area under the ROC curve of 0.88 (95% CI 0.79-0.97, p < 0.001). The percentage of ventilation to the ventral region of the lung was higher than the percentage of ventilation to the dorsal region (32 [27-38]% vs. 18 [13-21]%, p < 0.001), while the opposite was true for perfusion (28 [22-38]% vs. 36 [32-44]%, p < 0.001). Higher percentage of only perfused units was correlated with lower dorsal ventilation (r = - 0.486, p < 0.001) and with lower PaO₂/FiO₂ ratio (r = - 0.293, p = 0.039).

Conclusions: EIT allows bedside assessment of unmatched ventilation and perfusion in mechanically ventilated patients with ARDS. Measurement of unmatched units could identify patients at higher risk of death and could guide personalized treatment.

Keywords: Acute respiratory distress syndrome; Electrical impedance tomography; Positive end expiratory pressure; Ventilation/perfusion matching.

Fig. 1

Ventilation and perfusion measured by EIT in a representative patient. A Electrical impedance tomography signals during the infusion of saline bolus. The white arrow indicates the duration of the end-inspiratory hold during which the saline bolus is administered. Note that changes in impedance due to tidal ventilation are interrupted; the drop in impedance due to the bolus pass is indicated by the red arrow. b Representative image of the perfusion (red-colour map) and ventilation (blue-colour map) distribution. Note the gray circle indicating the cardiac region that has been removed from the analysis. c Top: representative map with the percentage perfusion (red numbers) and ventilation (blue numbers) in the three lung regions. Bottom: representative map obtained by integrating ventilation and perfusion maps: the grey area indicates matched units which are both ventilated and perfused, while red area indicated only perfused units and blue area only ventilated units

Fig. 2

Representative EIT images. a Patient with 15% of unmatched units. b Patient with 57% of unmatched units

Fig. 3

Percentage of unmatched units and clinical outcome. a Unmatched units in survivors versus non-survivors. For each boxplot, the line within the box indicate the median value, the box boundaries indicate 25th and 75th percentiles, and the error bars indicate the 5th and 95th percentiles. P-values for Wilcoxon-signed rank test. b ROC curve for SAPS II and unmatched units. c Correlation between unmatched units and ventilator-free days

Fig. 4

Regional distribution of ventilation, perfusion and ventilation/perfusion ratio. *p < 0.05 versus dorsal perfusion; **p < 0.05 versus dorsal ventilation; §p < 0.05 versus dorsal ventilation/perfusion ratio

Fig. 5

Correlation between percentage of only perfused units and dorsal ventilation and between percentage of only perfused unit and PaO₂/FiO₂. Higher percentage of only perfused units were correlated with lower dorsal ventilation (r =  − 0.486, p < 0.001) and with lower PaO₂/FiO₂ ratio (r =  − 0.293, p = 0.039)

Fig. 6

Correlation between PEEP and percentage of unmatched units in patients with low and high PaO₂/FiO₂. Higher PEEP values tend to correlate with lower percentage of unmatched units in the more severe subgroup with lower PaO₂/FiO₂ ratio (r =  − 0.387, p = 0.056), but not in the subgroup with higher PaO₂/FiO₂ ratio (r = 0.152, p = 0.465). The two subgroups were defined based on the median value of PaO₂/FiO₂ in the study population (lower or equal to 165 mmHg versus higher than 165 mmHg)

Fig. 7

Correlation between PEEP and only ventilated/only perfused units in patients with low and high compliance. Higher PEEP was associated with higher only ventilated/only perfused units ratio) in the subgroup of patients with higher compliance (r = 0.386, p = 0.056), but not in the subgroup with lower compliance (r =  − 0.193, p = 0.351). The two subgroups were defined based on the median value of compliance of the respiratory system in the study population (lower or equal to 41 ml/cmH₂O versus higher than 41 ml/cmH₂O)