Mild hypothermia reduces ventilator-induced lung injury, irrespective of reducing respiratory rate

In the era of lung-protective mechanical ventilation using limited tidal volumes, higher respiratory rates are applied to maintain adequate minute volume ventilation. However, higher respiratory rates may contribute to ventilator-induced lung injury (VILI). Induced hypothermia reduces carbon dioxide production and might allow for lower respiratory rates during mechanical ventilation. We hypothesized that hypothermia protects from VILI and investigated whether reducing respiratory rates enhance lung protection in an in vivo model of VILI. During 4 h of mechanical ventilation, VILI was induced by tidal volumes of 18 mL/kg in rats, with respiratory rates set at 15 or 10 breaths/min in combination with hypothermia (32°C) or normothermia (37°C). Hypothermia was induced by external cooling. A physiologic model was established. VILI was characterized by increased pulmonary neutrophil influx, protein leak, wet weights, histopathology score, and cytokine levels compared with lung protective mechanical ventilation. Hypothermia decreased neutrophil influx, pulmonary levels, systemic interleukin-6 levels, and histopathology score, and it tended to decrease the pulmonary protein leak. Reducing the respiratory rate in combination with hypothermia did not reduce the parameters of the lung injury. In conclusion, hypothermia protected from lung injury in a physiologic VILI model by reducing inflammation. Decreasing the respiratory rate mildly did not enhance protection.