Brain temperature changes during selective brain cooling

Introduction

Induced hypothermia is used in patients with brain injury [1, 2]. During selective brain cooling (SBC), only the brain temperature (T_B) is reduced while the core temperature (T C) remains unchanged [3]. Under normal conditions T_B ≤ T_C; however, in some diseased states T_B >T_C is found [4]. Since SBC renders T_B <<T_C, heat deposition from incoming blood to brain tissue can occur. The purpose of our animal study was 1) to investigate the temperature changes that can arise during SBC and 2) relate them to commonly encountered clinical situations (i.e. seizure activity, hypercapnia) with respect to brain temperature.

Methods

Experiments were conducted in artificially ventilated rats under sedation with α-chloralose (40 mg/kg per hour IP) and d-tubocurarine (0.05 mg/kg per hour). Bicuculline (1 mg/kg; Sigma-Aldrich, St Louis, MO, USA) was infused for generalized seizure induction. The animal was placed on a heating blanket with its head in a stereotaxic holder. Through a small burr hole a thermocouple wire (Oxylite™; Oxford Optronix, Oxford, UK) was inserted to measure the brain temperature in the cortex (n = 18). The head was covered with cotton balls to minimize heat loss to the environment. SBC was achieved by a new approach: Tygon™ Ultra-Soft tubing (R-1000, ID 3.2 mm; SGPP Corp, Akron, OH, USA) was perfused with water at +4°C. A V-shaped part of the tubing with inflow and outflow was placed into the pharynx of the animal (n = 8). The temperature in the mouth during SBC was measured (n = 2). To raise pCO₂ to ca 80 mmHg, the ventilatory rate was halved.

Results

With SBC T_B could be lowered to 33.1 ± 1.23°C (mean ± SD) from 36.9 ± 0.67°C (P < 0.001). There was a trend towards a lower T_C during SBC (from 36.90 to 36.44, P = 0.22). The temperature in the pharynx during SBC was 29.1 ± 2.19°C. From the lowest achieved SBC temperature T_B rose during CO₂challenge by 1.22 ± 0.67°C which is significantly higher than the increase of T_B seen without SBC (0.85 ± 0.34°C, P < 0.05). The increase in T_B from the lowest SBC level during seizure was 2.08 ± 0.35°C (1.15 ± 0.55°C in non-SBC animals [P = 0.001]).

Conclusion

Significant cooling of the cortex could be achieved by SBC from the pharynx in a rat model. Marked changes during hypercapnia and with seizure activity were seen that partially reversed the cooling effect of SBC. It therefore seems advisable to avoid, for example, permissive hypercapnia in patients where SBC is to be achieved. Whether the proposed approach to cool the brain from the pharynx can be applied in a clinical setting needs to be tested in larger animal species because of different anatomical compositions.