Role of rebreathing in determination of the blood-breath ratio of expired ethanol.

To settle certain discrepancies about the correct value of the blood-breath ratio of ethanol, I analyzed samples of end-expired air, rebreathed air, and venous blood during the postabsorptive phase of ethanol metabolism in healthy men. Ethanol was determined in blood and breath by gas chromatography. After rebreathing one to five times the concentrations of ethanol rose by 5.3, 11.2, 13.7, 14.8, and 16.9%, respectively, above the levels in end-expired air. The average increase after three cycles of rebreathing was about 10% after adjusting for the higher airway temperatures associated with rebreathing. Both methods of breath sampling correlated well with blood ethanol. The mean blood-breath ratio of ethanol was 2,225 +/- 111 (SD, n = 23) for end-expired breath and this value decreased by 5.7%/1 degree C rise in expired air temperature (P less than 0.05). The ratio was 1,947 +/- 110 (n = 23) after five cycles of rebreathing, and it was not significantly influenced by temperature, 0.5%/1 degree C (P greater than 0.05). Ethanol vapor is highly soluble in water and can rapidly equilibrate between airway fluids and respiratory air. During rebreathing the concentration of ethanol in breath rises leading to lower blood-breath partition ratios.