ticipated in several exercise levels and their alveolar ventilation rates (6.2-17 liters per minute [L/min]) were measured. All of these individual variables were used for the calculation of COHb values using the CFK equation; the affinity constant, M, was assumed to be 218 and the endogenous CO production rate to be 0.007 milliliters per minute (mL/min) for all subjects. Peterson and Stewart concluded that the CFK equation predicts COHb concentrations equally well for sedentary subjects and exercising individuals; during exercise, the alveolar ventilation rate changed by a factor of 2.5 from the sedentary level but did not alter the fit of the CFK equation to the experimental data. The authors further noted that the work rate (300 kpm/min) for this level of exercise is equivalent to the work rate of an individual who consumes oxygen at 10 L/min or to the work rates of many industrial workers. The CFK equation also predicts values for men and women equally well. To test the goodness of fit of the CFK equation to predict COHb for other exposure scenarios, Peterson and Stewart extrapolated from these experimental data with CO concentrations of 8.7, 25, 35, 50, 200, 500, and 1,000 ppm and concluded that the CFK equation fit the resulting data very well. From these observations and the results of their previous studies investigating discontinuous or intermittent CO exposures, Peterson and Stewart concluded, “Even though the CFK equation has not been completely tested at all levels of all parameters (and such testing is, in fact, impossible), present indications are that it describes uptake and excretion of CO extremely well. This equation even appears suitable for summing (integrating) long-term exposures to varying concentrations of CO in air.”
If the CFK equation is valid for predicting COHb in CO-exposed subjects, then an exposure to x ppm (concentration) for y minutes will produce the same COHb level as that produced by an exposure to y ppm for x minutes. This hypothesis was tested by Tikuisis et al. (1987a) on 11 nonsmoking men exposed to two exposure regimens, both of which gave the same total concentration (c) × exposure time (t) values of 37,500 ppm-min. In regimen I, the subjects were exposed to five sessions of 1,500 ppm for 5 min per session; each pair of sessions was separated by 3 min. In regimen II, each session consisted of exposure to 7,500 ppm for 1 min; sessions were separated by 7 min. The COHb values measured for regimens I and II were 11.46 ± 0.41% and 11.13 ± 0.45%, respectively. These values agreed well with the values (11.63 ± 0.59% and 11.46 ± 0.49%) predicted using the CFK equation. However, Tikuisis et al. stressed the importance of using the subject’s alveolar ventilation rate in the CFK equation.
Having the same objective as the Army about assessing exposures of personnel to CO in armored vehicles, Tikuisis and colleagues at the Canadian Defense and Civil Institute of Environmental Medicine (DCIEM) exposed test subjects to CO in a series of experiments that simulated the environment in an armored vehicle during weapons firing (that is, CO concentrations were transient and their peak was as high as 4,000 ppm). The test subjects were at rest and exposed to varying concentrations of CO in a symmetric stepwise fashion beginning with 500 ppm for 60 seconds (sec), followed by steps of 1,000, 2,000, 4,000, 2,000, and 1,000 ppm CO for 30 sec each and ending with 500 ppm for 60 sec (Tikuisis et al. 1987b). The transient exposure gave the subjects a nominal CO dosage of 6,000 ppm-min in a 4.5-min period. The second series of experiments included exercise patterns to imitate the workload of soldiers in armored vehicles before or during CO exposures. The overall results showed that the exposures raised the subjects’ COHb saturation from 1.7% to 17.3%. The CFK equation was solved using parameters (such as affinity and alveolar ventilation) used by the DCIEM or those recommended by the National Institute of Occupational Safety and Health (NIOSH). Tikuisis et al. (1987b) concluded that when DCIEM values were used with the CFK equation, the predicted values compared favorably (regression coefficient, b = 1.04) with the measured COHb data, but when the NIOSH values were used, the CFK equation significantly (b = 1.28) overpredicted the COHb concentrations.
This review shows that the CFK equation has been validated for various exposure concentrations, durations, and conditions. All of these validation studies collectively show that the values predicted by the CFK equation agree well with experimental data. However, the above studies used inspired CO concentrations in excess of those found in armored-vehicle cabin air. Further experimentation, as described in Chapter 3 and Appendix C, is needed to assess whether the CFK prediction equation is valid (1) at low and or spiking levels of CO or (2) under conditions of rapid changes in ventilation.