Background and objective: An inverse I : E ratio (inspiratory time > expiratory time) may have benefits in patients suffering trauma who requiring lung ventilation. However, this application may be deleterious if there is concomitant head injury. We aimed to determine the physiological effects of pressure- and volume-controlled modes of inverse ratio (I : E = 2 : 1) ventilation of the lungs, while maintaining normocapnia, in a rabbit model of raised intracranial pressure (ICP).
Methods: New Zealand White rabbits were anaesthetized with isoflurane and a tracheostomy was performed. Subarachnoid haemorrhage was simulated in two groups by injecting blood into the cisterna magna. Groups 1 and 2 (n = 6, each), controls, were compared with Groups 3 and 4 (n = 6, each) with the simulated subarachnoid haemorrhage. Each ventilation mode was used with an I : E ratio of 2 : 1 for 30 min. Mean arterial pressure (MAP), ICP, cerebral perfusion pressure (CPP), mean airway pressure (PAW) and arterial blood-gas status were measured.
Results: Both modes increased mean PAW (P < 0.02). This increase was greater with the volume-controlled mode (P < 0.02). The baseline value averaged 5.8 ± 0.4 and 5.6 ± 0.3 mmHg in Groups 3 and 4, respectively, and increased to 7.8 ± 0.3 and 10.8 ± 0.4 mmHg. Inducing subarachnoid haemorrhage increased ICP and MAP (P < 0.02). Baseline ICPs were 10.3 ± 0.5 and 10.3 ± 0.4 mmHg in Groups 1 and 2, respectively, whereas they were 25.4 ± 1.2 and 25.8 ± 0.8 mmHg in Groups 3 and 4. However, ICP, MAP and CPP did not differ significantly according to the mode.
Conclusions: An already raised ICP was altered by the application of induced mean PAW increases as a consequence of inverse ratio ventilation of the lungs with normocapnia.