Mechanical Power Correlates With Stress, Strain, and Atelectrauma Only When Normalized to Aerated Lung Size in Patients With Acute Respiratory Distress Syndrome

Abstract

Objectives: First, to investigate whether the severity of acute respiratory distress syndrome (ARDS) influences ventilator-induced lung injury (VILI) risk in ventilated patients with similar mechanical power of respiratory system (MP_RS). Second, to determine whether, under these circumstances, there is a relationship between transpulmonary mechanical power (MP_Tp) normalized to the aerated lung (specific lung mechanical power or SLMP) and VILI risk, and third, to determine whether normalizing MP_RS to compliance of respiratory system (CRS) can replace SLMP to bedside.

Design: Prospective cohort study.

Setting: The study was conducted in a tertiary academic ICU.

Patients: The study included 18 patients with ARDS.

Interventions: Ventilatory settings were adjusted to achieve a similar MP_RS.

Measurements and main results: Mechanical power was normalized to CRS (specific mechanical power or SMP = MP_RS/CRS), and SLMP was calculated as the ratio between MP_Tp and end-expiratory lung volume (SLMP = MP_Tp/EELV). The strain was defined as the ratio between tidal volume and EELV (strain = Vt/EELV), stress as transpulmonary pressure at the end of inspiration, and atelectrauma as the difference between expiration and inspiration in the nonaerated lung. Although patients had been ventilated with similar MP_RS = 23.75 (23-24) J/min and MP_Tp = 11.6 (10.8-12.8) J/min, SLMP increased linearly with the fall in Pao₂/Fio₂ (R = -0.83, p = 0.0001). MP_RS only correlated positively with VILI-associated mechanisms when normalized to aerated lung size: correlations between SLMP and stress (R = 0.9, R² = 0.84, p = 0.00004), strain (R = 0.97, R² = 0.94, p < 0.00001) and atelectrauma (R = 0.82, R² = 0.70, p = 0.00002), and correlations between SMP and stress (R = 0.86, R² = 0.75, p = 0.00001), strain (R = 0.68, R² = 0.47, p = 0.001) and atelectrauma (R = 0.67, R² = 0.46, p = 0.002).

Conclusions: The results suggest that normalizing mechanical power to lung-aerated size or CRS may correlate positively with stress, strain, and atelectrauma.

Keywords: acute respiratory distress syndrome; mechanical power; mechanical ventilation and respiratory failure; ventilator-induced lung injury.

Figure 1.

Correlation was positive between specific lung mechanical power (SLMP) and Pao₂/Fio₂. SLMP linearly increased from mild to severe acute respiratory distress syndrome. SMP = specific mechanical power.