Tidal Volume Selection in the ICU

Abstract

Background: Tidal volume (V_T) selection for adult patients should be based on ideal body weight (IBW); however, this method is not routinely practiced. Instead, the V_T is often set arbitrarily or calculated based on actual body weight (ABW). Inappropriate selection of V_T may result in hypoventilation and patient-ventilator asynchrony (V_T ≤ 6 mL/kg) or can induce lung injury (V_T ≥10 mL/kg). A low tidal volume ventilation (LTVV) or lung protective strategy has been defined as V_T 6-8 mL/kg based on IBW. The aim of this study was to evaluate if selection of V_T in a group of mechanically ventilated subjects was based on a LTVV strategy. The secondary aim was to determine if there was a correlation between V_T selected and higher oxygen requirement.

Methods: This was a retrospective cohort study performed on subjects admitted to a medical ICU at a level-1 trauma center in San Antonio, Texas during the month of March of 2021. The study evaluated subjects ≥18 years of age, intubated, and mechanically ventilated ≥ 48 h. Ventilator parameters recorded included: V_T, respiratory rate (RR), fraction of inspired oxygen (FIO₂), minute volume (MV), PEEP, and IBW. Higher oxygen requirement was arbitrarily defined as PEEP ≥ 5 cm H₂O and FIO₂ ≥ 0.5. Descriptive statistics (mean/SD) were used to report results and a Pearson test to report correlation. IRB approval was obtained.

Results: A total of 50 subjects meeting criteria were selected for analysis. Using IBW, the V_T selected was 7.2 ± 1.6 mL/kg (range: 5.3 - 9.8). Eight subjects (16%) received a V_T < 6 mL/kg (5.71 ± 0.2) while 13 patients (26%) received a V_T > 8 mL/kg (8.94 ± 0.58). The mean V_T selected for these patients based on ABW was 5.6 mL/kg (± 1.86). The single most often selected V_T ranged from 450 and 500 mL (n = 36; 72%). Fifteen patients (30%) had a PEEP > 5 cm H₂O while 38 patients (76%) required FIO₂ > 0.5. Patients on high oxygen requirement represented 28% (n = 14) of the study population. There was no correlation between high-oxygen requirement and V_T selected (r = 0.08).

Conclusions: This study suggests that a LTVV-strategy was routinely used to select the V_T in patients admitted to the medical ICU. This probably explains why patients with high oxygen requirements did not require additional adjustments on V_T. Although utilizing LTVV in lungs without injury appears theoretically attractive, more support from large-scale trials is necessary.