How the body controls brain temperature: the temperature shielding effect of cerebral blood flow

Abstract

Normal brain functioning largely depends on maintaining brain temperature. However, the mechanisms protecting brain against a cooler environment are poorly understood. Reported herein is the first detailed measurement of the brain-temperature profile. It is found to be exponential, defined by a characteristic temperature shielding length, with cooler peripheral areas and a warmer brain core approaching body temperature. Direct cerebral blood flow (CBF) measurements with microspheres show that the characteristic temperature shielding length is inversely proportional to the square root of CBF in excellent agreement with a theoretical model. This "temperature shielding effect" quantifies the means by which CBF prevents "extracranial cold" from penetrating deep brain structures. The effect is crucial for research and clinical applications; the relationship between brain, body, and extracranial temperatures can now be quantitatively predicted.

Fig. 1

Data sets showing difference between brain and body temperature (T_brain - T_body) vs. brain depth. At each depth, the temperature was averaged over the final 1 min (30 data points) before the probe was moved to the next depth. Symbols, experimental data; solid lines, curves representing the theoretical model, Eq. 2. Inset shows a transverse magnetic resonance image of a rat head with a cartoon that represents the cylindrical model in transverse plane with a radius R. Parameter x is the depth from the brain surface. The variation in the data confirms temperature shielding effect of blood flow as the characteristic shielding length depends on cerebral blood flow (CBF).

Fig. 2

Graphical depiction of CBF correlation with both mean arterial blood pressure (MABP) and arterial P_CO2 (Pa_CO2) as determined by linear multiregression analysis, Eq. 4. Abscissa is CBF calculated from regression expression with MABP and Pa_CO2 values as inputs. Ordinate is CBF obtained either from the theoretical modeling of measured brain temperature distribution (first group rats, ■) or from microsphere measurement of CBF (second group rats, ○). Continuous lines (on both sides) show 95% prediction limit based on multiregression analysis of group 1 subjects. Dotted line is y = x. Units of CBF for both axes are ml·g^-1·min^-1.

Fig. 3

Dependence of the brain temperature characteristic shielding length on the CBF. In general, the greater the latter, the shorter the former, hence the better the temperature shielding of the brain by the blood flow. CBF values corresponding to normal adult humans (25), term and premature (27 wk) infants (31), and nonanesthetized rats (16) are marked with arrows. Characteristic shielding length values are derived from Eq. 1 with physical parameters the same as in Table 1.