|dc.description.abstract||Increased temperature is common after brain trauma and stroke, considered to be
detrimental to outcome and usually treated with systemic cooling interventions.
However, targeting cooling interventions at the head may be more logical. In
addition to arterial blood, the human brain is cooled by heat loss through the skull
and heat loss from the upper airways. It is these two mechanisms of heat loss which
are the subject of this thesis.
The initial research aim was to find out if restoring ‘normal’ airflow through the
upper respiratory tracts of intubated, brain-injured patients could reduce brain
temperature. Air at room temperature and humidity replicating normal resting
minute volume was continuously administered nasally to 15 such patients. After a
30 minute baseline, they were randomised to receive airflow or no airflow for 6
hours and then crossed over for a further 6 hours. The airflow did not produce
significant reductions in intracranial temperature (Mean -0.13 °C, SD 0.55 °C, 95%
CI -0.43 to 0.17 °C). However, some evidence of heat loss through the skull was
serendipitously observed. This was investigated formally in a randomised factorial
trial, together with nasal airflow with enhancements (unhumidified air at twice
minute volume with 20 ppm nitric oxide gas) intended to overcome some of the
possible reasons for the neutral results with ‘normal’ airflow. After a 30 minute
baseline, 12 intubated, brain-injured patients received enhanced nasal airflow,
bilateral head fanning (8 m/s), both together and no intervention in randomised order.
Each intervention was delivered for 30 minutes followed by 30 minutes washout.
Mean brain temperature was reduced by 0.15 °C with nasal airflow (p=0.001, 95%
CI 0.06 to 0.23 °C) and 0.26 °C with head fanning (p<0.001, 95% CI 0.17 to 0.34
°C). The estimate of the combined effect of airflow and fanning on brain temperature
was 0.41 °C. Physiologically, this study demonstrated that heat loss through the
upper airways and through the skull can reduce parenchymal brain temperature in
brain-injured humans, that the effects are additive and the onset of temperature
reduction is rapid. The most promising mechanism appeared to be heat loss through the skull and the final piece of research involved developing and initial (phase I)
assessment of a convective head cooling device in healthy volunteers, with
intracranial temperature measured non-invasively by magnetic resonance
spectroscopy. After a 10 minute baseline, five healthy volunteers received 30
minutes head cooling followed by 30 minutes head and neck cooling via a hood and
neck collar delivering 14.5 °C air at 42.5 L/s. The net brain temperature reduction
with head cooling was 0.45 °C (SD 0.23 °C, p=0.01, 95% CI 0.17 to 0.74 °C) and
with head and neck cooling 0.37 °C (SD 0.30 °C, p=0.049, 95% CI 0.00 to 0.74 °C).
There was no significant reduction in cooling with progressive depth into the brain
i.e. core brain was cooled.
The main relevance of this research is physiological because it adds to knowledge
and understanding of mechanisms of heat loss from the upper airways and through
the skull in humans. Clinically, factors which enhance or inhibit these mechanisms
may have an effect on brain temperature but the therapeutic relevance of head
cooling by these methods requires further research.||en