Effects of duration of isoflurane anesthesia and mode of ventilation on intracranial and cerebral perfusion pressures in horses.

The study analyzes changes in blood flow within a horse’s brain under prolonged isoflurane anaesthesia, comparing two methods of ventilation. The study finds that over time, the method of ventilation can change the blood flow in the brain, potentially leading to decreased blood flow to the brain during extended anaesthesia.

Methods and Procedure

• The experiment involved six healthy adult horses, all put under anesthesia using 1.57% isoflaurane in Oxygen for a duration of five hours.
• Two different ventilation techniques were used: controlled ventilation (maintaining normal levels of carbon dioxide in blood vessels, or normocapnia) and spontaneous ventilation (leading to higher levels of carbon dioxide in blood, or hypercapnia). The use of these techniques was randomized through a crossover design to minimize any bias.
• Intracranial pressure – pressure inside the skull and thus, indirectly, the brain – was monitored using a subarachnoid strain-gauge transducer, a device used to measure changes in pressure within the cerebrospinal fluid space.
• Other measurements collected included carotid artery pressure, central venous pressure, airway pressures, blood gases, and minute ventilation – each of these readings can provide further insights into the horse’s physiological state.

Results

• Results indicated that throughout controlled ventilation, the intracranial pressure significantly increased even though the dosage of isoflurane anaesthesia was consistent. This rise in pressure inside the skull implies a decrease in the pressure gradient, reducing overall blood flow to the brain (cerebral perfusion pressure).
• During spontaneous ventilation, the initial intracranial pressure was found to be higher than under controlled ventilation; however, this difference faded over time. Intriguingly, the cerebral perfusion pressures showed no significant differences between horses under controlled or spontaneous ventilation.

Conclusions

• The research concluded that over time, the cerebral hemodynamics – the circulation and flow patterns of blood within the brain – tend to alter in horses anesthetized with isoflurane.
• The mode of ventilation has a significant impact on these changes, and these changes could lead to decreased blood flow to the brain during prolonged periods of anaesthesia.
• These findings could potentially have implications in veterinary medicine, especially surgical procedures requiring prolonged anesthesia.