The relationship of tidal volume and driving pressure with mortality in hypoxic patients receiving mechanical ventilation

PURPOSE: To determine whether tidal volume/predicted body weight (TV/PBW) or driving pressure (DP) are associated with mortality in a heterogeneous population of hypoxic mechanically ventilated patients. METHODS: A retrospective cohort study involving 18 intensive care units included consecutive patients ≥18 years old, receiving mechanical ventilation for ≥3 days, with a PaO(2)/FiO(2) ratio ≤300 mmHg, whether or not they met full criteria for ARDS. The main outcome was hospital mortality. Multiple logistic regression (MLR) incorporated TV/PBW, DP, and potential confounders including age, APACHE IVa(®) predicted hospital mortality, respiratory system compliance (C(RS)), and PaO(2)/FiO(2). Predetermined strata of TV/PBW were compared using MLR. RESULTS: Our cohort comprised 5,167 patients with mean age 61.9 years, APACHE IVa(®) score 79.3, PaO(2)/FiO(2) 166 mmHg and C(RS) 40.5 ml/cm H(2)O. Regression analysis revealed that patients receiving DP one standard deviation above the mean or higher (≥19 cmH(2)0) had an adjusted odds ratio for mortality (OR(mort)) = 1.10 (95% CI: 1.06–1.13, p = 0.009). Regression analysis showed a U-shaped relationship between strata of TV/PBW and adjusted mortality. Using TV/PBW 4–6 ml/kg as the referent group, patients receiving >10 ml/kg had similar adjusted OR(mort), but those receiving 6–7, 7–8 and 8–10 ml/kg had lower adjusted OR(mort) (95%CI) of 0.81 (0.65–1.00), 0.78 (0.63–0.97) and 0.80 0.67–1.01) respectively. The adjusted OR(mort) in patients receiving 4–6 ml/kg was 1.26 (95%CI: 1.04–1.52) compared to patients receiving 6–10 ml/kg. CONCLUSIONS: Driving pressures ≥19 cmH(2)O were associated with increased adjusted mortality. TV/PBW 4-6ml/kg were used in less than 15% of patients and associated with increased adjusted mortality compared to TV/PBW 6–10 ml/kg used in 82% of patients. Prospective clinical trials are needed to prove whether limiting DP or the use of TV/PBW 6–10 ml/kg versus 4–6 ml/kg benefits mortality.