Association between ventilation parameters and outcomes in acute respiratory failure patients

Background

In recent years, several studies have described different strategies for ventilatory support, some of them associated with lower morbidity and mortality. Although lower tidal volume ventilation seems to be preferred over traditional volumes, there is a great variability in clinical practice. Scarce data exist describing daily practice in patients managed in intensive care units outside clinical trials and in nonacademic institutions.

Objectives

(1) To describe mechanical ventilation parameters in patients admitted to our intensive care unit, and (2) to evaluate the effect of lower volume ventilation compared with traditional ventilation on clinical outcomes.

Method

Consecutive patients admitted between June and November 2000 in the medical-surgical ICU, who required mechanical ventilation for more than 24 h because of acute respiratory failure, were included in this observational study. Clinical and ventilatory parameters were recorded at baseline, soon after initiation of mechanical support, and within 36-48 h. Patients were stratified into two groups: group I, patients ventilated with lower tidal volumes (<8 ml/kg); and group II, patients ventilated with higher volumes (≥ 8 ml/kg). Multivariate analyses for repeated measures were performed to evaluate the independent effect of lower minute volume ventilation on mortality and duration of mechanical ventilation.

Results

A total of 58 patients were enrolled in the study (mean age 66 ± 18 [20-98] years, 39 [66%] were male and mean APACHEII score of 20 ± 7). Primary modes of mechanical ventilation were pressure-control ventilation (73%), pressure-support ventilation (25%) and synchronized intermittent mandatory ventilation (2%). Sixty-one per cent of the patients were initially ventilated with volumes greater than 8 ml/kg (Table). Tidal volumes did not differ from baseline to 36 h (8.9 ± 2 versus 8.9 ± 3 ml/kg; P = 0.89). Both groups were similar in their demographics and causes of respiratory failure.

Table

In multivariate analysis, after adjustment for the clinical differences between groups, tidal volumes were not independently associated with mortality and ventilator time. Patients managed with higher tidal volumes (≥ 10 ml/kg) at 36 h had prolonged mechanical ventilation (9 ± 8 versus 6 ± 4 days; P = 0.05). Other ventilator settings such as FiO₂, pressure control mode and maximal inspiratory pressure at 36 h were significantly associated with increased mortality.

Conclusion

In this heterogeneous cohort of mechanically ventilated patients, pressure control and pressure support were the preferred modes of mechanical ventilation, and traditional tidal volumes (greater than 8 ml/kg) were utilized in the majority of the cases. Although our results showed a nonsignificant difference in mortality, there was a trend towards shorter ventilator times in patients ventilated with lower tidal volumes.