Background
Both atelectasis and fibrosis may decrease pulmonary compliance in patients with acute respiratory distress syndrome (ARDS). Together with hypoxic pulmonary vasoconstriction they may increase pulmonary vascular resistance and cause pulmonary hypertension. ARDS may thus lead to an acute cor pulmonale. Right ventricular dysfunction occurs in 61% of patients with ARDS submitted to mechanical ventilation using conventional tidal volumes. It is estimated to be present in only 25% of patients with ARDS submitted to mechanical ventilation using lower tidal volumes.
N–terminal proB–type natriuretic peptide (NT–proBNP) is secreted by the ventricles of the heart in response to excessive stretching of heart muscle cells. Although the right ventricle secretes less NT–proBNP than the left ventricle serum levels of NT–proBNP can be elevated in patients with an acute cor pulmonale due to ARDS. Notably, stretching of heart muscle cells could also be the result of fluid overload, frequently seen in patients with ARDS. In addition, NT–proBNP levels can be elevated due to kidney failure and elevated levels are seen in patients with acute brain injury.
It can be questioned whether the size of tidal volumes used with mechanical ventilation influences the right ventricular afterload and therefore has an effect on serum levels of NT–proBNP. In the present study we measured levels of NT–proBNP in serial samples taken from patients without ARDS who were randomized to mechanical ventilation with lower tidal volumes or conventional tidal volumes in a previous trial. We hypothesized that conventional tidal volumes may lead to increased pulmonary vascular resistance as the incidence or ARDS is higher compared to lower tidal volumes. In this way, increased pulmonary vascular resistance may lead to increased stretching of the right ventricle and therefore increased NT-proBNP levels. To investigate this possibility we studied the relation between tidal volumes and NT–proBNP levels. The purpose of this study was (a) to investigate whether serum levels of NT–proBNP are dependent on tidal volume size in patients without ARDS, and (b) serum levels of NT–proBNP to increase parallel to severity of lung injury, when controlling for acute kidney injury, cumulative fluid balance and presence of brain injury.