effects of inversed ratio ventilation (IRV) on intracranial pressure (ICP) in dogs with pulmonary edema *
Although correlation between variations in the inspiratory to expiratory ratio (I : E ratio) and intracranial pressure (ICP) has not been clarified, the study of Mihira showed that IRV (at I : E ratios of 1.7 : 1, 2.3 : 1, and 4 : 1) does not influence ICP in dogs with normal or elevated ICP. In order to estimate the influence of lowered lung compliance on ICP during IRV, an additional study was designed to observe the effects of the I : E ratio = 1 : 2 to 4 : 1 on ICP in 10 dogs with pulmonary edema induced by oleic acid. Following baseline measurement of control ventilation (I : E ratio =1 : 2), lung edema was induced by venous injection of oleic acid (0.05 mL/kg). After verifying the reduction of lung compliance, four different I : E ratios were applied in the order of I : E = 1 : 2, 1.7 : 1, 2-3 : 1, and 4 : 1. Throughout the period of these measurements, PaC02 constantly maintained normocapnia and arterial blood pressure was kept within normal range. Intracranial hemodynamics (ICP, cerebral perfusion pressure), lung mechanics (mean airway pressure (mAWP), peak inspiratory pressure (PIP), lung compliance), systemic hemodynamics (mean arterial pressure, mean pulmonary artery pressure, central venous pressure, cardiac output), and blood gases were measured at 30 min under every I : E ratio ventilatory mode. In these dogs with pulmonary edema, mAWP significantly increased during IRV in comparison with that during control ventilation (p<0.05), but there was no significant difference in PIP between control ventilation and IRV. ICP remained unchanged during IRV (12.5 ± 4.2, 10.0 ± 2.9, 11.1 ± 2.2, 11.3 ± 2.7 at I : E = 1 : 2, 1.7 : 1, 2.3 : 1 and 4 : 1, respectively). This study suggested that IRV (at I : E ratios of 1.7 : 1, 2-3 : 1, and 4 : 1), which can minimize ventilator-induced lung injury, has no influence on ICP. Therefore, IRV may be one beneficial option as ventilation strategy for acute respiratory distress syndrome with intracranial hypertension. (Accepted on October 15, 2002) Kawasaki Igakkaishi 28 (4 ) : 269 -278, 2002
