Ventilatory Ratio Threshold for Unassisted Breathing: A Retrospective Exploratory Analysis

Abstract

Background: The ventilatory ratio (VR) is a simple index of ventilatory efficiency and dead space. Because increased dead space and high ventilatory demands impose a limitation to unassisted ventilation, and may predispose patients to injurious strong efforts during assisted ventilation, evaluation of the VR could provide helpful information during weaning. We hypothesize that there is a threshold of VR associated with tolerance of unassisted breathing.

Methods: In a retrospective analysis, we included subjects ventilated in a control mode for at least 24 h, who were successfully liberated from mechanical ventilation, without use of noninvasive ventilation, and discharged alive from the ICU. We focused on the successful weaning attempts (the last, if more than one was performed) and evaluated the VR at the beginning and at the end of the assisted ventilation period.

Results: We examined 2,000 medical records and included in our analysis 572 subjects (age: 68 y, R_5-95 = 25-85, 68% male) with main admission diagnosis of respiratory failure (23%), sepsis (11%), brain injury (34%), and postoperative (14%). The VR at the beginning and the end of the assisted ventilation period was 1.5 (R_5-95 = 1-2.1) and 1.4 (R_5-95 = 1-2), respectively. The median duration of assisted ventilation in subjects with a VR ≥ 2 at the beginning of the assisted ventilation period was 3 d (R_5-95 = 0-14 d), significantly longer than in those with a VR < 2, 0.5 d (R_5-95 = 0-8 d, P < .001).

Conclusions: Successful liberation from assisted ventilation was associated with a VR < 2. A VR > 2 was associated with longer duration of weaning. The VR could be used as an additional tool to facilitate the decision-making process during weaning.

Keywords: assisted ventilation; dead space; mechanical ventilation; weaning.