Effects of inverse ratio ventilation on intracranial pressure in dogs*
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. Thirty animals were divided into three groups. 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. Arterial carbon dioxide (PaC02) was constantly maintained normocapnia by adding or subtracting the dead space tube throughout the experiment. Tidal volume and ventilatory frequency were unaltered. ICP, mean airway pressure (mean Paw), peak airway pressure (peak Paw) and such hemodynamics as mean arterial pressure, mean pulmonary artery pressure, central venous pressure and cardiac output, were measured at 30 min 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 cmH20 in the first group, 12.0±6.6, 14. 0±6. 0, 12.0± 6. 3 and 13.8 ± 2. 8 cmH20 in the second group, 29. 4 ±7.0, 28. 6 ±7.0, 28.1 ±7.1 and 27. 0± 6. 2cmH20 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. Therefore IRV may be a good strategy in the mechanical ventilation of patients with intracranial hypertension. (Accepted on April 30, 1998) Kawasaki Igakkaishi 24(1): 17-24, 1998
