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

Area of Science:

• Critical Care Medicine
• Respiratory Physiology
• Mechanical Ventilation

Background:

• The ventilatory ratio (VR) is a common surrogate for ventilatory efficiency in Acute Respiratory Distress Syndrome (ARDS).
• Its reliability in reflecting alveolar ventilation (V̇Talv/VT) under modified respiratory mechanics is not well-established.
• Understanding this relationship is crucial for optimizing mechanical ventilation strategies.

Purpose of the Study:

• To investigate the correlation between VR and V̇Talv/VT.
• To assess if VR accurately reflects changes in alveolar ventilation during controlled alterations in respiratory mechanics, tidal volume (VT), and minute ventilation (V̇E) in ARDS patients.

Main Methods:

• Secondary analysis of a quasi-experimental, repeated-measures study in an adult ICU.
• Twenty-two ARDS patients underwent three 60-minute periods with adjusted trunk inclination to modify VT.
• VR was calculated, and V̇Talv/VT was measured using volumetric capnography; 66 paired measurements were analyzed.

Main Results:

• Tidal volume (VT) was intentionally altered, increasing then decreasing.
• No significant differences in VR were observed between measurement periods.
• Alveolar ventilation ratio (V̇Talv/VT) showed significant changes, increasing and then decreasing.
• No significant relationship was found between VR and V̇Talv/VT (β = -0.056, p = 0.111).

Conclusions:

• In ARDS patients, VR did not correlate with V̇Talv/VT despite controlled changes in respiratory mechanics.
• These findings indicate that VR may not be a dependable measure of ventilatory efficiency when ventilatory conditions are modified.
• Caution is advised when using VR as a surrogate for ventilatory efficiency in dynamic clinical scenarios.