Can death at a low COHb concentration frequently observed in fire victims be explained by hypoxic hypoxia?

Fire deaths at low COHb (carboxyhemoglobin) concentrations are frequently observed, but it is difficult to determine the causative factors. Participation of hypoxic hypoxia was examined using rats and rabbits exposed to various low-O2 and CO gas mixtures. The ranges of O2 and CO concentration were 3.0-22.2% and 0.3-2.9%, respectively. The concentration of CO2 was fixed at about 5%. Rats were individually exposed to the test gas in a plastic chamber. Rabbits inhaled the test gas through a tracheal cannula. The survival time of rats ranged from 3.9-7.7 min, and that of rabbits was 8.0-22.5 min. Rats exposed to the most hypoxic gas mixture (O2 3%-CO 0.3%) died with COHb values below 40%. Rabbits also died with a low COHb concentration under this condition, but the COHb concentration was not below 50%. Rabbits were considered to be more resistant to hypoxic hypoxia than rats. From the literature on the relationship between the grade of burns and the concentration of COHb, as well as the findings obtained in the present experiment, hypoxic hypoxia did not appear to be a main factor causing death at low COHb concentrations. In the rabbit the COHb concentration increased exponentially and reached plateau levels within 10 min in many groups. The time required to reach a plateau COHb level was shorter when the concentration of CO was high and that of O2 was low.