Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Abstract

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

Electrocortical and Hemodynamic Changes within the Brain during Incremental Bicycle Exercise in Normoxia and Hypoxia—A Combined EEG/NIRS Study

“…Regarding absolute intensity, exercise test ended earlier under hypoxic than normoxic conditions (decrease of about 19.1%) as expected [4,27]. After adjusting the data to the last three exercise stages respectively for both conditions instead of regarding absolute intensity levels, heart rate, lactate concentration and ratings of perceived exertion did not differ between conditions.…”

Reduced Regional Gray Matter Volume in Patients with Chronic Obstructive Pulmonary Disease: A Voxel-Based Morphometry Study

“…1 A greater proportion of regions showing GM loss located in the limbic/paralimbic cortices in patients with COPD found in the present study may be due to the fact that those phylogenetically older regions of the brain showed sharper vascular responses to hypoxia than evolutionarily younger regions. 27 Although there is evidence that cigarette smoking may cause neuronal damage in the brains of patients with COPD, nicotine has no selective effect on some brain regions. In our study, we did not detect a correlation of the duration or the amount of smoking with the regional GM volume among all smokers.…”

Kinetics of brain nicotine accumulation in dependent and nondependent smokers assessed with PET and cigarettes containing 11 C-nicotine

“…Although the brain is one of the most perfused organs in the body, the normal cerebral blood flow (CBF) is only 0.45 to 0.50 mL/g/min (0.0075-0.0083 mL/g/sec) (20)(21)(22), which is not sufficient to accomplish the fast washout nicotine from the brain and to sustain short-duration (tens of seconds) spikes in brain nicotine concentration. This conclusion can be illustrated by the following calculations.…”