Effect of hyperventilation on oxygenation of the brain cortex of neonates

Compulsory hyperventilation and hypocapnia of patients with Leigh syndrome associated with SURF1 gene mutations as a cause of low serum bicarbonates

Experimental data show that elevation of intracellular pH leads to severe lesions of brain cells. Acidification of intracellular fluid by accumulation of lactate may compensate the effect of respiratory alkalosis. Increased serum pH, and low PCO2, associated with hyperlactataemia (sometimes incorrectly called 'acidosis') have been reported in children with Leigh syndrome (LS). The aim of the study was to determine whether respiratory alkalosis is characteristic of patients with LS due to SURF1 mutations. All venous blood gas data (88 samples) of 18 spontaneously breathing LS patients with recently established SURF1 mutations, hospitalized during 1986-2000, were retrospectively reviewed. The data of an affected boy who survived on a respirator for more than 3 months (79 daily samples) were analysed separately. In spontaneously breathing patients, the data indicated that the patients had compensated or partially compensated respiratory alkalosis (pH 7.388+/-0.060, Pco2 29.2+/-5.7 mmHg, HCO3- 17.4+/-3.0 mmol/L, BE -6.7+/-3.2 mmol/L). Bicarbonate excretion was detected in urine of two examined LS cases in spite of decreased serum HCO3-. In the affected child maintained on a respirator, simple manipulation of the inspired CO2 tension to establish a normal pressure of 35-45 mmHg automatically caused an increase of serum HCO3- concentration to a normal value of 26.3+/-2.9 mmol/L (and BE to +2.2+/-3.1 mmol/L), in spite of cytochrome oxidase (COX) deficiency due to a confirmed SURF1 mutation. We suggest that respiratory alkalosis (hypocapnia) of Leigh syndrome patients with SURF1 mutations results from compulsory hyperventilation and speculate that hypocapnia may contribute to Leigh-like brain damage in the SURF1-deficient patients as well as in other patients presenting with Leigh-like syndrome. The supposition that accumulation of lactate may protect the brain of LS patients from alkalosis-related damage requires further study. Avoidance of any factors stimulating hyperventilation of LS patients and caution when attempting to correct low plasma bicarbonate are suggested.

The use of hyperventilation and its impact on cerebral ischemia in the treatment of traumatic brain injury

Traumatic brain injury is a common occurrence in the United States, leading to approximately 190,000 deaths or long-term disabilities. Following the primary insult, secondary disturbances in cerebral blood flow (CBF) and metabolism may have deleterious effects on potentially viable neurons. Recent studies evaluating CBF immediately following head injury have revealed flows low enough to produce cerebral ischemia. Hyperventilation is used routinely to lower suspected increased intracranial pressure (ICP). Aggressive hyperventilation produces a marked reduction in CBF, which may give rise to or exacerbate cerebral ischemia, thus enhancing rather than reducing secondary injury. This article reviews the role of hyperventilation in the treatment of increased ICP and its impact on cerebral ischemia following traumatic brain injury.

Oxygen dependent quenching of phosphorescence: a perspective

Oxygen quenches phosphorescence by energy transfer from the phosphor when oxygen molecules collide with molecules of the phosphor in the excited triplet state. Thus increasing oxygen pressure causes an increase in the rate of decay of phosphorescence (shorter lifetimes) and a decrease in total phosphorescence intensity. Phosphors have been selected which decay with a single exponential and for which the relationship between phosphorescence lifetime and oxygen pressure is quantitatively described by the Stern-Volmer equation. The use of phosphorescence lifetime as the measure of oxygen pressure makes the method insensitive to the absorbance changes of other chromophores in the system. This method has permitted quantitative, rapid (less than 10 msec) and sensitive (to less than 10(-8) Torr) measurements of oxygen pressure in suspensions of cells or subcellular organelles. In tissues, oxygen pressure has been evaluated by measuring phosphorescence using an intensified CCD camera. Maps of oxygen pressure in the vasculature of the cortex of the brain and of other tissues demonstrate the method is limited only by the optics of the system and resolutions of a few microns are readily attained.