Low f/Vt ratio is associated with successful extubation in our pulmonary patients

Background

Weaning from mechanical ventilation is usually conducted in an empirical manner and a standardized approach has not been developed. A variety of criteria have been proposed as predictors of the success or failure of weaning attempts, among them rapid shallow breathing – the ratio of respiratory frequency to tidal volume (f/Vt).

Aims

The aim of the study was to evaluate whether low f/Vt ratio (< 105) is associated with successful extubation in our pulmonary patients. We also wanted to find out whether there is a correlation between low f/Vt ratio and our previous criteria: stable clinical state, maximal inspiratory pressure > 25 cmH₂O, vital capacity >1000 ml, respiratory frequency < 35/min and also arterial blood gas analysis.

Methods

A prospective trial was conducted in 37 patients receiving mechanical ventilation for more than 24 hours because of acute respiratory failure in our intensive care unit because of pulmonary disease: pneumonia (15 patients), chronic obstructive pulmonary disease (nine patients), asthma (four patients), bronchiectasis (four patients), fibrothorax (four patients), pyopneumothorax (one patient). When patients were able to breathe spontaneously we measured the respiratory frequency (f), tidal volume (Vt), maximal inspiratory pressure (MIP), vital capacity (VC), minute ventilation (MV), PaCO₂, PaO₂ and calculated the f/Vt. We measured criteria many times in each patient until extubation.

Criteria for extubation were a stable clinical state of patient and all three or at least two of three criteria: MIP > 25 cmH₂O, VC >1000 ml and/or f < 35/min. Successful extubation was determined as spontaneous breathing for at least 72 hours after extubation.

Results

Data from 37 patients were obtained (51% women, age 30–84 years). Median duration of mechanical ventilation was 13 days, range 2–30 days. Thirty patients (30/37, 82%) were extubated, all had ratio f/Vt less than 105. Among them 28 patients (28/30, 93%) underwent successful extubation, two patients (2/30, 7%) were reintubated because of laryngeal edema. Seven patients (7/37, 18%) were not extubated, all of them had high f/Vt ratio (>105). There was positive correlation between f/Vt and respiratory frequency, and negative correlation between f/Vt and vital capacity and between f/Vt and maximal inspiratory pressure. No correlation was found between f/Vt ratio and PaCO₂ (P = 0.7), PaO₂ (P = 0.7), FiO₂ (P = 0.4).

Conclusions

Low f/Vt ratio (<105) was associated with successful extubation in our pulmonary patients. There was good correlation between low f/Vt ratio and maximal inspiratory pressure, vital capacity and respiratory frequency. There was no correlation between f/Vt ratio and PaCO₂ and PaO₂ – blood gas analysis only should not be a criterion for extubation.