Lung stress and strain during mechanical ventilation in animals with and without pulmonary acute respiratory distress syndrome.

ABSTRACT

BACKGROUND: Mechanical ventilation is a life-saving therapy for acute respiratory distress syndrome (ARDS). However, unphysiological lung stress (transpulmonary pressure) and strain (the ratio between inflated volume and functional residual capacity) can induce lung injury. Unfortunately, both stress and strain are not measured directly because of technical limitations but predicted from airway plateau pressure (Pplat) and tidal volume (Vt). Recently, some literatures indicated that Pplat and Vt cannot be good surrogates without distinguishing pulmonary ARDS patients (ARDSp) from extrapulmonary ARDS patients. Analyzing them together might distort the truth. Thus, we established animal models of ARDSp to explore whether lung stress and strain can be surrogated precisely by Pplat and Vt. METHODS: Fifteen Beagle dogs were recruited to establish ARDSp models by injection of oleic acids. Esophageal manometry was performed to estimate pleural pressure and lung stress. Functional residual capacity was obtained through computed tomography. In the first stage, Vt was set at 10mL/kg body weight in healthy and injured lungs and then adjusted to achieve lung strain as 1.0, 1.5, and 2.0 in sequence. RESULTS: There was a good linear relationship between lung stress and Pplat in healthy and ARDS lungs (P<0.001). For a given Vt (10mL/kg body weight), the global lung strains varied from 0.197 to 0.416 and 0.467 to 0.715 in healthy individuals and different ARDS patients, respectively. On the contrary, Vt varied remarkably for a given lung strain. CONCLUSIONS: Pplat is an adequate surrogate for lung stress, but Vt cannot represent lung strain sufficiently.