The correlation between the inspiratory to expiratory ratio (I:E ratio) and intracranial pressure (ICP) has not been clarified. This study was undertaken to determine the effects of the I:E ratio=1:2 to 4:1 on ICP in dogs with normal ICP and elevated ICP. Animals in the first group (n=10) were ventilated with a constant inspiratory flow pattern, in the second group (n=10) with an Increasing inspiratory flow pattern, and In the third group (n=10) with a constant inspiratory flow pattern under elevated ICP. In each animal, following the control ventilation (I:E ratio=1:2), three different I:E ratios of inverse ratio ventilation (IRV) were applied in the order of I:E=1.7:1, 2.3:1, and 4:1. ICP, mean airway pressure (mean Paw), peak airway pressure (peak Paw) and such hemodynamics as mean arterial pressure, were measured at 30 mm under every I:E ratio ventilatory condition. Changes in the I:E ratio had no effect on hemodynamics in any of the groups. The mean Paw was higher during IRV than
that during control ventilation (p<0.05), but there was no difference in peak Paw between control ventilation and the different ratios of IRV. Despite the higher mean Paw, ICP remained unchanged during IRV as compared with control ventilation in the three groups (12.7±3.1, 12.4±4.6, 12.3±3.3 and 14.4±3.5 cmH2O in the first group, 12.0±6.6, 14.0±6.0, 12.0±6.3 and 13.8±2.8 cmH2O in the second group, 29.4±7.0, 28.6±7.0, 28.1±7.1 and 27.0±6.2 cmH2O in the third group, at I: E=1:2, 1.7:1, 2.3:1 and 4:1, respectively). This study showed that IRV (at I:E ratios of 1.7:1, 2.3:1, and 4:1) does not influence ICP. Therefor IRV may be a good strategy in the mechanical ventilation of patients with intracranial hypertension.