Effects of pressure- and volume-controlled inverse ratio ventilation on haemodynamic variables, intracranial pressure and cerebral perfusion pressure in rabbits: a model of subarachnoid haemorrhage under isoflurane anaesthesia.

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 (P(AW)) and arterial blood-gas status were measured. RESULTS: Both modes increased mean P(AW) (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.

Effects of normovolaemic haemodilution on middle cerebral artery blood flow velocity and oxygen delivery.

BACKGROUND AND OBJECTIVE: Assessment of the effects of normovolaemic haemodilution on middle cerebral artery blood flow velocity with transcranial Doppler ultrasonography, intracranial pressure, cerebral perfusion pressure, arterial oxygen content and cerebral oxygen delivery. METHODS: Normovolaemic haemodilution was induced in rabbits under general anaesthesia, and the haematocrit was allowed to decrease to 30% in Group 1 (n = 6) and to 20% in Group 2 (n = 6). Peak systolic and diastolic velocities, mean blood flow velocity, and pulsatility and resistance indices of the middle cerebral artery were measured by transcranial Doppler ultrasonography. Changes in intracranial pressure, cerebral perfusion pressure, arterial oxygen content and cerebral oxygen delivery were also assessed. RESULTS: In Group 2, middle cerebral artery blood flow velocity increased from 0.4 +/- 0.01 to 0.51 +/- 0.02 m s(-1) after the induction of normovolaemic haemodilution (P = 0.04), while arterial oxygen content decreased from 16.2 +/- 0.1 to 8.5 +/- 0.1 mLdL(-1) (P = 0.002). The decrease in cerebral oxygen delivery from 6.5 +/- 0.2 to 4.3 +/- 0.2 was also significant (P = 0.02). However, no associated changes in intracranial pressure and cerebral perfusion pressure could be demonstrated. CONCLUSIONS: Normovolaemic haemodilution resulted in an increase in the mean blood flow velocity of the middle cerebral artery. However, this increase did not compensate for the consequences of the altered oxygen delivery to the brain when the haematocrit was reduced to 20%.