PCP SBC

Question 1

What is the hypothesis for the point and the counterpoint?

Answer 1

Point: Humans selectively cool their brains when they are in hyperthermia

Counter Point: humans do not selectively cool their brains in hyperthermia

Question 2

What is SBC?

Answer 2

Selective Brain Cooling (SBC) is the process in which brain and body temps remain separate when the body is under hyperthermic conditions; e.g. during exercise.

Question 3

Evidence for counterpoint (humans do not show SBC when they become hyperthermic)

• Patients with intubation removed, they showed inc in core temp but braina nd esophageal actually decreased. This si due to evapouration and conudtion of muscus in nasal cavity.

Face fanning and evapouration of head sweat resulted in cooling the brain

Answer 3

1. Since direct measurement of the brain temp is very difficult, SBC hasn’t been demonstrated by direct temp measurement in human brain. Instead, tympanic temp is used as an inference of brain temp. But there’s no certainty that tympanic temp is an accurate reflection of actual brain temp.
2. Major structure that has been shown to be responsible for SBC in animals, the carotid rete, does not exist in humans. It is the countercurrent heat exchanger between arterial and venous blood flowing in and out of the brain found in other mammals that undergo SBC.
3. Humans do not use panting as a mechanism to dissipate heat: rabbits, dogs, and sheep, all use panting as a way to cool their bodies when body temp is elevated (??)
4. Human airway not long enough or close enough to the arterial blood supply entering the brain to be an effective cooling mechanism, even though evaporative heat loss occurs most in the upper airway closer to brain
5. Relatively low tympanic temp recorded during hyperthermia may not indicate SBC, but rather contamination of brain temp by low skin temp of the head and face.
6. SBC has only been documented in humans with clinical interventions, such as inducing brain cooling through the use of perfused cooling helmets. So, although the brain does have the ability to be cooled independently from the body, it has not been conclusively shown to occur without external intervention.
7. Humans have natural adaptive mechanisms of lowering body temp during states of hyperthermia, such as evaporation and convection during running. Thus, humans can experience heat loss from nearly all areas of the body uniformly, mainly through sweating.
8. All organs benefit equally from cooling due to vasodilation combined with evaporation from the skin. This allows blood to be delivered to all of the organs at roughly the same temperature
9. Humans have the unique capacity for flow inc in vessels throughout the skin, again, allowing all organs to benefit equally from the heat exchange between cool venous blood and warm arterial blood. This process of cooling is referred to as countercurrent heat exchange and is believed to be one of the
10. SBC has only been observed in smaller mammals, e.g. rodents, rabbits. Not been conclusively shown in humans

Question 4

Evidence for point (humans do show SBC when they become hyperthermic)

Answer 4

1. In one study, after tracheal extubation from patient undergoing surgery, their Tcore increased. However, the brain and esophagus temps decreased while the patient had mild hyperthermia. In one patient, the brain temp was even lower than the esophageal temp. Therefore, concluded that this is due to evaporation and conduction of the mucus in the nasal cavity.
2. In support of the definition of SBC as a process in which brain and body temps remain separate when the body is under hyperthermic conditions, one study looked at fanning during hyperthermia. Tympanic temp was used to represent brain temp and the esophageal temp was used to represent core temp. When arms of subject were fanned during hyperthermia, only Tcore dec but the tympanic temp continued to inc. When the face was fanned, the tympanic, skull skin and angularis oculi vein temps decreased. Demonstrating that indeed, body and brain temp were separated when hyperthermic. The evaporation of sweat created a similar stimulus as face fanning to dec the skull skin and tympanic temp. The results of the study also showed that the tympanic temp was consistently lower than the esophageal temp, even during the normothermic (no fanning) condition.
3. A study by Mariak et al indicated that during face fanning, the change in tympanic temp was highly correlated with change in intracranial temp. The change of tympanic and subdural temp during facial fanning was positive with a correlation close to 1 (p-value <0.05). Thus, tympanic temp can reflect brain temp and these temps are generally higher than the core temp.
4. When humans are in hyperthermia, the sweat rate in the skull skin increases dramatically. The evaporation of the sweat decreases the skull skin and tympanic temps.