Oxygen delivery and oxygen consumption

The oxygen extraction ratio, which is given by oxygen consumption divided by oxygen delivery, is reflected by a large arteriovenous oxygen difference. Thus the elevated partial pressure of venous oxygen in heatstroke victims leads to a depressed oxygen extraction ratio. In a state of energy depletion, as is the case in heatstroke, the oxygen extraction ratio is expected to be high. The association of heatstroke with increased glucose utilization, and marked production of cytokine

(Bouchama. eL§L 1993) and lactate, supports the assumption of impaired oxygen extraction in these patients. Victims of heatstroke also appear to have decreased hemoglobin concentrations (Kas.hmeery...1995b). Strenuous physical stress can, in combination with anemia and impaired oxygen extraction, lead to type A lactic acidosis, which is characterized by an increased load of hydrogen ions. Elevated urine pH in heat exhaustion patients might reflect impaired hydrogen ion secretion and/or impaired bicarbonate reabsorption in the kidney tubules.

The secretion of hydrogen ions, which takes place in the distal convoluted tubule of the nephron, is an active process requiring a constant energy supply. The source of this supply is oxidative phosphorylation, which requires adequate oxygen extraction. Minimizing hydrogen ion production is essential in managing type A lactic acidosis. This is achieved by improving tissue oxygen extraction. In both cases oxygen is a common factor in the manifestation of the disorder. However, it is not conclusively determined whether it is an aberration of oxygen delivery or oxygen consumption that is causing this manifestation.

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