Selecting the Right Tidal Volume for Patients With ARDS: Are We Staying Within the Recommended Range for a Lung Protective Strategy?

Abstract

Background:

Low tidal volumes (V_T) may improve the outcomes of patients with ARDS outcomes; however, it may also promote atelectasis and patient-ventilator asynchrony. Tidal volume (V_T) selection for adult patients should be based on ideal body weight (IBW), but sometimes is set arbitrarily or calculated based on actual body weight (ABW) in adult ICUs. Since the publication of the Acute Respiratory Distress Syndrome (ARDS) Network Trial almost 20 years ago on the benefit of low V_T for patients with ARDS, there have been reports on how V_T selection is inconsistently used. The aims of this study were to determine how was V_T selected in a group of adult mechanically ventilated patients and to determine if a lung protective strategy (V_T 6-8 mL/kg) was used, particularly in patients with P/F ratio < 150 mm Hg.

Methods:

Retrospective cohort study in several university-affiliated medical ICUs. Patients admitted to the ICU with a diagnosis of ARDS, 18 y of age and older, intubated and mechanically ventilated between January of 2017 and March of 2018 were included for analysis. Patient and ventilator parameters collected included IBW, V_T, frequency, FIO₂, PEEP, P/F ratio, and oxygenation index (OI). Lung protective strategy defined as V_T 6-8 mL/kg based on IBW.

Results:

The V_T selected for these 61 patients based on IBW was 7.48 mL/kg (± 1.51; range 3.87-11.21). A total of 6 patients (9.84%) received a V_T < 6 mL/kg (5.12 ± 0.92) while 19 patients (31.15%) received a V_T > 8mL/kg (9.08 ± 0.95). The mean V_T selected for these patients based on ABW was 6.07 mL/kg (± 1.93). The single most often selected V_T was 500 mL (n=12; 60%) and 36.7% of patients were placed on a V_T of 500 mL or greater. Twenty-five patients (41%) were on PEEP > 5 cm H₂O and 72% of those received a PEEP ≥ 10 cm H₂O. Mean FIO₂ was 63.3% (± 27.0). P/F ratio for 54% of the patients with ABGs was 175.80 (± 128.77; range 28.75-603; IQR 85.79-226.5) and OI 14.56 (± 13.78; range 1.33-66.09; IQR 4.72-16.36). Almost 61% of patients had a P/F ratio < 150. For these patients V_T set was 7.97 mL/kg (± 2.02).

Conclusions:

Although the average V_T selected for this group of patients admitted to the ICU with ARDS was within the 6-8 mL/kg based on IBW, a considerably large group of patients (almost 1 out of 3) received V_T > 8 mL/kg. Furthermore, patients with moderate-to-severe hypoxemia, based on P/F ratio and OI, did not receive V_T consistent with a more aggressive lung protective strategy.