Physiological dissociation between ventilatory ratio and ventilatory efficiency in patients with ARDS

The ventilatory ratio (VR) is frequently used as a surrogate marker of ventilatory efficiency in patients with ARDS. However, its ability to reflect changes in alveolar ventilation (V̇Talv/VT) when respiratory mechanics are modified remains unknown. This study aimed to evaluate the relationship between VR and V̇Talv/VT during sequential changes in respiratory mechanics=, tidal volume (VT), and minute ventilation (V̇E) in patients with ARDS.

This was a secondary analysis of a quasi-experimental, repeated-measures study conducted in a single-center adult ICU. Twenty-two patients with ARDS were evaluated across three sequential 60 min controlled periods, during which trunk inclination was adjusted to induce changes in VT. At the end of each period, VR was calculated, and V̇Talv/VT was measured using volumetric capnography. A total of 66 paired measurements were analyzed in this study.

By design, VT increased from Time 1 to Time 2 by + 62 mL and decreased from Time 2 to Time 3 by - 68 mL. These changes in VT were associated with the following: VR was not significantly different between Time 1 and Time 2 [- 0.23 (95% CI: - 0.44 to - 0.02; p = 0.071)] or between Time 2 and Time 3 [+ 0.17 (95% CI: - 0.04 to + 0.38; p = 0.086)]. The alveolar ventilation ratio (V̇Talv/VT) increased significantly from Time 1 to Time 2 by + 0.080 (95% CI: + 0.039 to + 0.121; p < 0.001), and decreased from Time 2 to Time 3 by - 0.060 (95% CI: - 0.101 to - 0.019; p < 0.001). Association between VR and V̇Talv/VT: no significant relationship was found (β =  - 0.056, marginal R² = 0.052, conditional R² = 0.205, p = 0.111).

In this cohort of patients with ARDS, VR did not correlate with V̇Talv/VT following controlled modifications of respiratory mechanics. These findings suggest that VR may not reliably represent ventilatory efficiency under changing ventilatory conditions, and its use as a surrogate variable should be approached with caution.