Journal of Toxicology and Environmental Health
ISSN: 0098-4108 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/uteh19
Effects of altitude on endogenous carboxyhemoglobin levels James J. McGrath To cite this article: James J. McGrath (1992) Effects of altitude on endogenous carboxyhemoglobin levels, Journal of Toxicology and Environmental Health, 35:2, 127-133, DOI: 10.1080/15287399209531601 To link to this article: http://dx.doi.org/10.1080/15287399209531601
Published online: 19 Oct 2009.
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Date: 13 November 2015, At: 06:57
EFFECTS OF ALTITUDE ON ENDOGENOUS CARBOXYHEMOGLOBIN LEVELS James J. McGrath
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Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, Texas
The effects of carbon monoxide (CO) on blood carboxyhemoglobin (COHb) levels, though well studied at sea level, have not been investigated in populations at altitude. COHb levels were measured in laboratory rats following 6 wk exposure to either clean air or air containing 9 ppm CO at ambient altitude (3300 ft), 10,000 ft, or 15,000 ft simulated high altitude. In animals breathing clean air, COHb levels increased with increasing altitude from 0.68 ± 0.09% at 3300 ft to 1.16 ± 0.28% and 1.68 ± 0.14%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude is linear with a correlation coefficient of 0.90 (p < .001). In animals breathing 9 ppm CO, COHb levels also increased with increasing altitude from 0.99 ± 0.06% at 3300 ft to 1.77 ± 0.17% and 2.10 + 0.08%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude in animals breathing CO is also linear with a correlation coefficient of .92 (p < .001). These data indicate that, compared with animals at sea level, animals at altitude have an increased body burden of COHb and will attain the COHb level associated with the National Ambient Air Quality Standard for CO more quickly when breathing CO.
INTRODUCTION Carbon monoxide (CO) exerts its toxic effects by combining with hemoglobin to form carboxyhemoglobin (COHb), which, by decreasing transport and release of oxygen to the tissues, produces tissue hypoxia (Root, 1965). The Environmental Protection Agency (EPA) has set 9 ppm CO in inhaled air as the National Ambient Air Quality Standard for 8 h exposure (National Research Council, 1977). This corresponds to an equilibrium percentage saturation of hemoglobin with CO of less than 2% The author wishes to thank David Smith for his excellent technical assistance and Debbie Hays for reviewing and typing this manuscript. Research described in this article is conducted under contract to the Health Effects Institute (HEI), an organization that supports the conduct of independent research, and is jointly funded by the U.S. Environmental Protection Agency (EPA) and automotive manufacturers. Publication here implies nothing about the view of the contents of HEI or its research sponsors. HEI's Health Review Committee may comment at any time and will evaluate the final report of the project. Additionally, although the work described in this document has been funded in part by the U.S. Environmental Protection Agency under assistance agreement X8088S9 with HEI, the contents do not necessarily reflect the view and policies of the agency; nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Requests for reprints should be sent to James J. McGrath, Ph.D., Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, TX 79430. 127 Journal of Toxicology and Environmental Health, 35:127-133, 1992 Copyright © 1992 by Hemisphere Publishing Corporation
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). J. McGRATH
COHb in blood (EPA, 1979), the level at which detrimental effects may occur in patients with cardiac impairment. COHb levels of about 0.60.7% are normally present in the blood; however, because a small amount of CO is formed in the body by catabolism of hemoglobin and other heme proteins (Coburn et al., 1963). Moreover, because of competition between O2 and CO for the available hemoglobin, the equilibrium level of COHb in the blood depends on the partial pressure of O2 (PO2) as well as on the level of CO (Forster, 1970). Because the inspired PO2 and the percent of hemoglobin saturated with O2 both decrease with increasing altitude, these studies were conducted to compare the effects of increasing altitude on COHb concentrations in rats breathing air or 9 ppm CO. METHODS Exposure System The altitude chamber system used in these studies has been described previously (McGrath, 1988). It consists of six cylindrical chambers and a system of interconnected valves, which maintains the separate chambers at either control (ambient) or low barometric pressure (simulated high altitude). Each chamber is constructed from a 200-I steel barrel and is fitted with a clear Plexiglas door for viewing the animals. The door is machined and fitted with an O-ring to ensure a gas tight seal. Air, supplied from a central air duct, enters the system through a HEPA filter and flows through each chamber at 55 l/min (16 air changes/h). In the altitude studies, pressure in the chambers is reduced by a water-sealed pump (Atlantic Fluidic, Inc., Darien, Conn.), and changes in pressure are measured with an altimeter. In the altitude-CO studies, CO is provided in cylinders and introduced into the chambers through a multiple mass flow controller (Matheson Gas, Inc., Horsham, Pa.). A switching solenoid (Chronotrol-Lindberg Enterprises, Inc., San Diego, Calif.) allows the air in each chamber to be sampled and analyzed in sequence. CO concentrations, measured with an infrared analyzer (Beckman Instruments, Inc., Fullerton, Calif.), are recorded on a chart recorder (Houston Instruments, Inc., Austin, Tex.). Chamber CO2 concentrations are measured with an LB-2 CO2 analyzer (Sensormedics, Inc., Anaheim, Calif.). The average ambient temperature and relative humidity for these studies were 22 ± 1°C and 50 ± 5%, respectively. Exposures Male Fischer-344 rats, six animals to a cage, were placed in the chambers and exposed for 6 wk to (1) ambient altitude (3300 ft), (2) 10,000 ft, (3) 15,000 ft, (4) 3300 ft and 9 ppm CO, (5) 10,000 ft and 9 ppm CO, or (6) 15,000 ft and 9 ppm CO. The chambers were opened two to three times a week to replenish food and water, clean cages, and weigh the animals.
CARBOXYHEMOGLOBIN AT ALTITUDE
129
Hemoglobin concentrations and COHb levels were measured on blood samples obtained by snipping the tails immediately after the animals were removed from the chambers and collecting the blood in capillary blood collection tubes (Microtainer) coated with ammonium heparin. The animals, fed a commercial diet and provided tap water ad libitum, gained weight progressively throughout the exposure. Only in the animals exposed to 15,000 ft was there a significant decrease in body weight. These data were reported earlier (McGrath, 1988,1989).
Downloaded by [University of Florida] at 06:57 13 November 2015
Analytical Methods COHb determinations were based on the gas-chromatographic method of Rodkey (1970). In this procedure, 100 /x\ of blood is introduced via a gas-tight syringe into a reaction vessel containing 1% citric acid monohydrate (89 /*l) and 10% sterox (178 /*l). After introducing the sample, 3% potassium ferricyanide (133 (JL\) is added and the reaction is allowed to proceed for 10 min. At the end of this time, the reaction vessel is switched into the gas chromatograph system through a four-way valve. CO is separated from oxygen, nitrogen, and methane by a 6-ft molecular sieve 5A column operating at 100°C. Water, carbon dioxide, and organic substances that interfere with the performance of the column are removed by a liquid-nitrogen trap. The effluent from the column is mixed with hydrogen, catalytically reduced to methane by a nickel catalyst at 400°C, and passed through a flame ionization detector. The peak area in the chromatogram is determined by integration and, by comparing the peak area with a standard calibration curve, the amount of CO in the sample is determined. The concentration of CO in the blood, expressed as %COHb, is obtained from the following relationship: %COHb = A/Hb x (1.39 ml/g Hb) where A is the volume of CO gas corrected to standard temperature and pressure (STP) conditions, Hb is the amount of hemoglobin in the sample, as determined by the cyanmethemoglobin method, and 1.39 ml/g Hb is the conversion factor that accounts for the fact that 1 M of hemoglobin with a molecular weight of 64,458 combines with 4 x 22,414 ml of CO under STP conditions. Statistics The data were analyzed on a Macintosh computer by means of a Statworks software package. One-way analysis of variance (ANOVA) followed by a Newman-Keuls test was used to assess significant differences between means (Steel and Torrie, 1960). Spearman rank correlation coefficients were calculated to assess the relationship between COHb levels and altitude. The statistical criterion for a significant difference was p < .05.
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J. J. McGRATH
RESULTS In rats breathing air, hemoglobin levels were significantly higher at 15,000 ft than at 3300 or 10,000 ft (Table 1). Hemoglobin concentrations did not differ in rats breathing air at 10,000 and 3300 ft. In rats breathing 9 ppm CO, hemoglobin levels were significantly higher at 15,000 ft than at 3300 or 10,000 ft. Hemoglobin concentrations did not differ in rats breathing CO at 10,000 and 3300 ft. In rats breathing air, endogenous COHb levels rose with altitude (Fig. 1), increasing from 0.68 ± 0.09% at 3300 ft to 1.16 ± 0.28% at 10,000 ft and to 1.68 ± 0.14% at 15,000 ft. COHb levels were significantly greater at 10,000 than at 3300 ft and significantly greater at 15,000 ft than at 3300 or 10,000 ft. The relationship between COHb levels and altitude is linear (Fig. 2) and is described by the regression equation: y = 0.372 + (8.477 x 10~5)x The Spearman correlation coefficient between COHb and altitude of 0.90 is significant (p < .001). In rats breathing 9 ppm CO, COHb levels rose with altitude (Fig. 1) from 0.99 ± 0.06% at 3300 ft to 1.77 ± 0.17% at 10,000 ft and to 2.10 ± 0.8% at 15,000 ft. COHb levels were significantly greater at 10,000 than at 3300 ft and significantly greater at 15,000 ft than at 3300 or 10,000 ft. This relationship between COHb levels and altitude in rats breathing CO is also linear (Fig. 2) and is described by the regression equation: y = 0.7403 + (9.120 x 10~5)x The Spearman correlation coefficient between COHb and altitude in rats breathing 9 ppm CO of 0.92 is significant (p < .001). COHb levels are compared in rats breathing air or 9 ppm CO at 3300 ft and air at 10,000 ft and 15,000 ft (Table 2). COHb levels are higher in rats breathing air at 10,000 or 15,000 ft than in rats breathing CO at 3300 ft. TABLE 1 Hemoglobin Levels in Rats Breathing Air or 9 ppm CO at 3300,10,000, and 15,000 ft
A. Air
B. CO
a
Group
Altitude (ft)
CO ppm
n
Hemoglobina
1 2 3 4 5 6
3300 10,000 15,000 3300 10,000 15,000
0 0 0 9 9 9
12 5 5 12 5 5
18.0 17.7 22.5 18.7 18.6 23.4
Values are mean ± 1 SD. ^Significantly different group 3 versus 1, p < .05. c Significantly different group 6 versus 4, p < .05.
± ± ± ± ± ±
1.1 3.0 2.36 1.2 3.1 1.5C
CARBOXYHEMOGLOBIN
TABLE 2
AT ALTITUDE
131
COHb(%) Levels after 6 Weeks in Rats Breathing Air at 3300,10,000, and 15,000 ft Altitude
or Breathing 9 ppm (CO) at 3300 ft Percent of EPA
CO ppm
n
COHB 3 (%)
action level6
3300
0
6
0.68 ± 0.09
34
3300 10,000 15,000
9 0 0
6 5 5
0.99 ± 0.06 1.16 ± 0.28 1.68 ± 0.14
50 58 84
Altitude (ft)
a
Values are mean
± SD.
b
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2% COHb.
DISCUSSION The EPA has established 2% COHb as an action level because it is near this level that symptoms have been reported in exercising angina patients. The data base is composed of the earlier angina studies of Aronow (1973, 1981) followed by the recent work of Allred et al. (1989). The Aronow studies and the Allred study were all conducted in laboratories at altitudes below 500 ft; the COHb level in the subjects breathing room air was 0.6%. In the United States, however, an estimated 35,000,000 people live or sojourn at altitudes above 5000 ft. In the studies reported herein we observed in rats that ascent to altitude (in the absence of inhaling CO) is sufficient to raise endogenous COHb levels to within 58% of the critical COHb value at 10,000 ft and to within 84% of the critical value at 15,000 ft (Table 2). In rats breathing 9 ppm CO, the COHb levels were increased to within 89% of the critical value at 10,000 ft and exceeded the critical value at 15,000 ft. It is particularly striking that COHb levels in animals breathing air at 10,000 ft (1.16 ± 0.28%) and 15,000 ft (1.68 ± 0.14%) equaled or exceeded the COHb levels of animals breathing CO at ambient altitude (0.99 ± 0.06%). These results are in accord with the predictions of Collier and Goldsmith (1983), who transformed and rearranged the Coburn-Forster-Kane equation (Coburn et al., 1965) and derived an equation expressing COHb in terms of endogenous and exogenous sources of CO: VCOZ
+
~T
where COHb is the percent COHb, Flco the fraction inspired CO (ppm), PB the barometric pressure (torr), Vco the rate of CO production (mL/min STPD), and K = PC-O2/(M x SO2), Z = 1/DLCO + (PB-47/VA)
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132
J.). McGRATH
with PjO2 the mean partial pressure of pulmonary capillary O2 (torr), M the Haldane constant, SO2 the O2Hb (%), DLCO the carbon monoxide diffusing capacity (mL/min/torr), and VA the alveolar ventilation (mL/ min"1 STPD). At high altitudes inspired PO2 and PjO2 are reduced and, assuming V^co remains constant, a given Flco will result in a higher percent COHb. Moreover, Collier and Goldsmith (1983) predict an increase in COHb at altitude, even in the absence of inhaled CO, due to endogenous production of CO. These predictions are supported by the results of this study. Because blood COHb levels increase with altitude in the absence of ambient CO exposure, it might be expected that human populations in high-altitude cities have increased body stores of CO; this increase is independent of local CO levels and results from the physiological process by which the body excretes CO produced endogenously. Endogenous CO is produced by catabolism of hemoglobin and other heme proteins. In humans, the normal catabolism of hemoglobin and the breakdown of nonhemoglobin heme proteins produce approximately 0.4 ml/h of CO (Coburn et a!., 1963); this CO causes an average background COHb level of 0.4-0.7%. Certain drugs, chemical exposures, and disease states (e.g., hemolytic anemia) can increase CO production within the body and may produce COHb levels of 4-6% (Stewart, 1976). Typical COHb levels reported for urban dwelling nonsmokers are 1.2% for Milwaukee and 2.2% for Chicago. These levels represent a rise above the background levels of 0.4-0.7% and, presumably, the additional COHb derives from exposure to exogenous CO in the ambient air of the city. Animals undergo pronounced physiological changes in their attempt to adapt to higher altitude (Barbashova, 1964), and these changes may also affect blood COHb levels. Early in the adaptation process, hyperventilation and hemoconcentration increase the O2-carrying capacity of the blood and also affect the affinity of hemoglobin for oxygen. Later, erythropoiesis is stimulated, which further increases the O2-carrying capacity. All of these changes may affect COHb levels. While an increase in erythropoiesis will increase the size of the circulating hemoglobin pool, it will also provide a greater sink for CO. Conversely, an increased rate of hemoglobin destruction will cause an increase in the rate of CO production while decreasing the size of the circulating hemoglobin pool. We have not measured hemoglobin production or destruction, which were beyond the scope of this study, but these would be key measurements to make in future studies.
REFERENCES Allred, E. N., Bleecker, E. R., Chaitman, B. R., Dahms, T. E., Gottlieb, S. O., Hackney, J. D., Pagano, M., Selvester, R. H., Walden, S. M., and Warren, J. 1989. Short-term effects of carbon
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CARBOXYHEMOGLOBIN AT ALTITUDE
133
monoxide exposure on the exercise performance of subjects with coronary artery disease. N. Engl. J. Med. 321(21):1426-1432. Aronow, W. S. 1981. Aggravation of angina pectoris by two percent carboxyhemoglobin. Am. Heart J. 101:154-157. Aronow, W. S., and Isbell, M. W. 1973. Carbon monoxide effect on exercise-induced angina pectoris. Ann. Intern. Med. 79:392-395. Barbashova, Z. I. 1964. Cellular level of adaptation. In Handbook of Physiology. Adaptation to the Environment, pp. 37-54. Washington, D.C.: American Physiological Society. Coburn, R. F., Blakemore, W. S., and Forster, R. E. 1963. Endogenous carbon monoxide production in man. J. Clin. Invest. 42(7):1172-1178. Coburn, R. F., Forster, R. E., and Kane, P. B. 1965. Considerations of the physiological variables that determine the blood carboxyhemoglobin concentration in man. J. Clin. Invest. 44(11):18991910. Collier, C. R., and Goldsmith, J. R. 1983. Interactions of carbon monoxide at altitude. Atmos. Environ. 17:723-728. Environmental Protection Agency. 1979. Air Quality Criteria for Carbon Monoxide, EPA-60018-79002. Washington, D.C.: EPA. Forster, R. E. 1970. Carbon monoxide and the partial pressure of oxygen in the tissue. Ann. N.Y. Acad. Sci. 174:233-241. McGrath, J. J. 1988. Body and organ weights of rats exposed to carbon monoxide at high altitude. J. Toxicol. Environ. Health 23:303-310. McGrath, J. J. 1989. Cardiovascular effects of chronic carbon monoxide and high altitude exposure. Health Effects Institute Research Report 27, July. National Research Council. 1977. Carbon Monoxide, Committee on Medical and Biological Effects of Environmental Pollutants, p. 166. Washington, D.C.: National Academy of Science. Rodkey, F. L. 1970. Carbon monoxide estimation in gases and blood by gas chromatography. Ann. N.Y. Acad. Sci. 174:261-267. Root, W. S. 1965. Carbon monoxide. In Handbook of Physiology, eds. W. O. Fenn and H. Rahn, p. 1087. Washington, D.C.: American Physiology Society. Steel, R. G., and Torrie, J. H. 1960. Principles and Procedures of Statistics. New York: McGraw-Hill. Stewart, R. D. 1976. The effect of carbon monoxide on humans. J. Occup. Med. 18(5):304-309. Received April 22, 1991 Accepted August 31, 1991
ISSN: 0098-4108 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/uteh19
Effects of altitude on endogenous carboxyhemoglobin levels James J. McGrath To cite this article: James J. McGrath (1992) Effects of altitude on endogenous carboxyhemoglobin levels, Journal of Toxicology and Environmental Health, 35:2, 127-133, DOI: 10.1080/15287399209531601 To link to this article: http://dx.doi.org/10.1080/15287399209531601
Published online: 19 Oct 2009.
Submit your article to this journal
Article views: 1
View related articles
Citing articles: 9 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=uteh20 Download by: [University of Florida]
Date: 13 November 2015, At: 06:57
EFFECTS OF ALTITUDE ON ENDOGENOUS CARBOXYHEMOGLOBIN LEVELS James J. McGrath
Downloaded by [University of Florida] at 06:57 13 November 2015
Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, Texas
The effects of carbon monoxide (CO) on blood carboxyhemoglobin (COHb) levels, though well studied at sea level, have not been investigated in populations at altitude. COHb levels were measured in laboratory rats following 6 wk exposure to either clean air or air containing 9 ppm CO at ambient altitude (3300 ft), 10,000 ft, or 15,000 ft simulated high altitude. In animals breathing clean air, COHb levels increased with increasing altitude from 0.68 ± 0.09% at 3300 ft to 1.16 ± 0.28% and 1.68 ± 0.14%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude is linear with a correlation coefficient of 0.90 (p < .001). In animals breathing 9 ppm CO, COHb levels also increased with increasing altitude from 0.99 ± 0.06% at 3300 ft to 1.77 ± 0.17% and 2.10 + 0.08%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude in animals breathing CO is also linear with a correlation coefficient of .92 (p < .001). These data indicate that, compared with animals at sea level, animals at altitude have an increased body burden of COHb and will attain the COHb level associated with the National Ambient Air Quality Standard for CO more quickly when breathing CO.
INTRODUCTION Carbon monoxide (CO) exerts its toxic effects by combining with hemoglobin to form carboxyhemoglobin (COHb), which, by decreasing transport and release of oxygen to the tissues, produces tissue hypoxia (Root, 1965). The Environmental Protection Agency (EPA) has set 9 ppm CO in inhaled air as the National Ambient Air Quality Standard for 8 h exposure (National Research Council, 1977). This corresponds to an equilibrium percentage saturation of hemoglobin with CO of less than 2% The author wishes to thank David Smith for his excellent technical assistance and Debbie Hays for reviewing and typing this manuscript. Research described in this article is conducted under contract to the Health Effects Institute (HEI), an organization that supports the conduct of independent research, and is jointly funded by the U.S. Environmental Protection Agency (EPA) and automotive manufacturers. Publication here implies nothing about the view of the contents of HEI or its research sponsors. HEI's Health Review Committee may comment at any time and will evaluate the final report of the project. Additionally, although the work described in this document has been funded in part by the U.S. Environmental Protection Agency under assistance agreement X8088S9 with HEI, the contents do not necessarily reflect the view and policies of the agency; nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Requests for reprints should be sent to James J. McGrath, Ph.D., Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, TX 79430. 127 Journal of Toxicology and Environmental Health, 35:127-133, 1992 Copyright © 1992 by Hemisphere Publishing Corporation
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128
). J. McGRATH
COHb in blood (EPA, 1979), the level at which detrimental effects may occur in patients with cardiac impairment. COHb levels of about 0.60.7% are normally present in the blood; however, because a small amount of CO is formed in the body by catabolism of hemoglobin and other heme proteins (Coburn et al., 1963). Moreover, because of competition between O2 and CO for the available hemoglobin, the equilibrium level of COHb in the blood depends on the partial pressure of O2 (PO2) as well as on the level of CO (Forster, 1970). Because the inspired PO2 and the percent of hemoglobin saturated with O2 both decrease with increasing altitude, these studies were conducted to compare the effects of increasing altitude on COHb concentrations in rats breathing air or 9 ppm CO. METHODS Exposure System The altitude chamber system used in these studies has been described previously (McGrath, 1988). It consists of six cylindrical chambers and a system of interconnected valves, which maintains the separate chambers at either control (ambient) or low barometric pressure (simulated high altitude). Each chamber is constructed from a 200-I steel barrel and is fitted with a clear Plexiglas door for viewing the animals. The door is machined and fitted with an O-ring to ensure a gas tight seal. Air, supplied from a central air duct, enters the system through a HEPA filter and flows through each chamber at 55 l/min (16 air changes/h). In the altitude studies, pressure in the chambers is reduced by a water-sealed pump (Atlantic Fluidic, Inc., Darien, Conn.), and changes in pressure are measured with an altimeter. In the altitude-CO studies, CO is provided in cylinders and introduced into the chambers through a multiple mass flow controller (Matheson Gas, Inc., Horsham, Pa.). A switching solenoid (Chronotrol-Lindberg Enterprises, Inc., San Diego, Calif.) allows the air in each chamber to be sampled and analyzed in sequence. CO concentrations, measured with an infrared analyzer (Beckman Instruments, Inc., Fullerton, Calif.), are recorded on a chart recorder (Houston Instruments, Inc., Austin, Tex.). Chamber CO2 concentrations are measured with an LB-2 CO2 analyzer (Sensormedics, Inc., Anaheim, Calif.). The average ambient temperature and relative humidity for these studies were 22 ± 1°C and 50 ± 5%, respectively. Exposures Male Fischer-344 rats, six animals to a cage, were placed in the chambers and exposed for 6 wk to (1) ambient altitude (3300 ft), (2) 10,000 ft, (3) 15,000 ft, (4) 3300 ft and 9 ppm CO, (5) 10,000 ft and 9 ppm CO, or (6) 15,000 ft and 9 ppm CO. The chambers were opened two to three times a week to replenish food and water, clean cages, and weigh the animals.
CARBOXYHEMOGLOBIN AT ALTITUDE
129
Hemoglobin concentrations and COHb levels were measured on blood samples obtained by snipping the tails immediately after the animals were removed from the chambers and collecting the blood in capillary blood collection tubes (Microtainer) coated with ammonium heparin. The animals, fed a commercial diet and provided tap water ad libitum, gained weight progressively throughout the exposure. Only in the animals exposed to 15,000 ft was there a significant decrease in body weight. These data were reported earlier (McGrath, 1988,1989).
Downloaded by [University of Florida] at 06:57 13 November 2015
Analytical Methods COHb determinations were based on the gas-chromatographic method of Rodkey (1970). In this procedure, 100 /x\ of blood is introduced via a gas-tight syringe into a reaction vessel containing 1% citric acid monohydrate (89 /*l) and 10% sterox (178 /*l). After introducing the sample, 3% potassium ferricyanide (133 (JL\) is added and the reaction is allowed to proceed for 10 min. At the end of this time, the reaction vessel is switched into the gas chromatograph system through a four-way valve. CO is separated from oxygen, nitrogen, and methane by a 6-ft molecular sieve 5A column operating at 100°C. Water, carbon dioxide, and organic substances that interfere with the performance of the column are removed by a liquid-nitrogen trap. The effluent from the column is mixed with hydrogen, catalytically reduced to methane by a nickel catalyst at 400°C, and passed through a flame ionization detector. The peak area in the chromatogram is determined by integration and, by comparing the peak area with a standard calibration curve, the amount of CO in the sample is determined. The concentration of CO in the blood, expressed as %COHb, is obtained from the following relationship: %COHb = A/Hb x (1.39 ml/g Hb) where A is the volume of CO gas corrected to standard temperature and pressure (STP) conditions, Hb is the amount of hemoglobin in the sample, as determined by the cyanmethemoglobin method, and 1.39 ml/g Hb is the conversion factor that accounts for the fact that 1 M of hemoglobin with a molecular weight of 64,458 combines with 4 x 22,414 ml of CO under STP conditions. Statistics The data were analyzed on a Macintosh computer by means of a Statworks software package. One-way analysis of variance (ANOVA) followed by a Newman-Keuls test was used to assess significant differences between means (Steel and Torrie, 1960). Spearman rank correlation coefficients were calculated to assess the relationship between COHb levels and altitude. The statistical criterion for a significant difference was p < .05.
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J. J. McGRATH
RESULTS In rats breathing air, hemoglobin levels were significantly higher at 15,000 ft than at 3300 or 10,000 ft (Table 1). Hemoglobin concentrations did not differ in rats breathing air at 10,000 and 3300 ft. In rats breathing 9 ppm CO, hemoglobin levels were significantly higher at 15,000 ft than at 3300 or 10,000 ft. Hemoglobin concentrations did not differ in rats breathing CO at 10,000 and 3300 ft. In rats breathing air, endogenous COHb levels rose with altitude (Fig. 1), increasing from 0.68 ± 0.09% at 3300 ft to 1.16 ± 0.28% at 10,000 ft and to 1.68 ± 0.14% at 15,000 ft. COHb levels were significantly greater at 10,000 than at 3300 ft and significantly greater at 15,000 ft than at 3300 or 10,000 ft. The relationship between COHb levels and altitude is linear (Fig. 2) and is described by the regression equation: y = 0.372 + (8.477 x 10~5)x The Spearman correlation coefficient between COHb and altitude of 0.90 is significant (p < .001). In rats breathing 9 ppm CO, COHb levels rose with altitude (Fig. 1) from 0.99 ± 0.06% at 3300 ft to 1.77 ± 0.17% at 10,000 ft and to 2.10 ± 0.8% at 15,000 ft. COHb levels were significantly greater at 10,000 than at 3300 ft and significantly greater at 15,000 ft than at 3300 or 10,000 ft. This relationship between COHb levels and altitude in rats breathing CO is also linear (Fig. 2) and is described by the regression equation: y = 0.7403 + (9.120 x 10~5)x The Spearman correlation coefficient between COHb and altitude in rats breathing 9 ppm CO of 0.92 is significant (p < .001). COHb levels are compared in rats breathing air or 9 ppm CO at 3300 ft and air at 10,000 ft and 15,000 ft (Table 2). COHb levels are higher in rats breathing air at 10,000 or 15,000 ft than in rats breathing CO at 3300 ft. TABLE 1 Hemoglobin Levels in Rats Breathing Air or 9 ppm CO at 3300,10,000, and 15,000 ft
A. Air
B. CO
a
Group
Altitude (ft)
CO ppm
n
Hemoglobina
1 2 3 4 5 6
3300 10,000 15,000 3300 10,000 15,000
0 0 0 9 9 9
12 5 5 12 5 5
18.0 17.7 22.5 18.7 18.6 23.4
Values are mean ± 1 SD. ^Significantly different group 3 versus 1, p < .05. c Significantly different group 6 versus 4, p < .05.
± ± ± ± ± ±
1.1 3.0 2.36 1.2 3.1 1.5C
CARBOXYHEMOGLOBIN
TABLE 2
AT ALTITUDE
131
COHb(%) Levels after 6 Weeks in Rats Breathing Air at 3300,10,000, and 15,000 ft Altitude
or Breathing 9 ppm (CO) at 3300 ft Percent of EPA
CO ppm
n
COHB 3 (%)
action level6
3300
0
6
0.68 ± 0.09
34
3300 10,000 15,000
9 0 0
6 5 5
0.99 ± 0.06 1.16 ± 0.28 1.68 ± 0.14
50 58 84
Altitude (ft)
a
Values are mean
± SD.
b
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2% COHb.
DISCUSSION The EPA has established 2% COHb as an action level because it is near this level that symptoms have been reported in exercising angina patients. The data base is composed of the earlier angina studies of Aronow (1973, 1981) followed by the recent work of Allred et al. (1989). The Aronow studies and the Allred study were all conducted in laboratories at altitudes below 500 ft; the COHb level in the subjects breathing room air was 0.6%. In the United States, however, an estimated 35,000,000 people live or sojourn at altitudes above 5000 ft. In the studies reported herein we observed in rats that ascent to altitude (in the absence of inhaling CO) is sufficient to raise endogenous COHb levels to within 58% of the critical COHb value at 10,000 ft and to within 84% of the critical value at 15,000 ft (Table 2). In rats breathing 9 ppm CO, the COHb levels were increased to within 89% of the critical value at 10,000 ft and exceeded the critical value at 15,000 ft. It is particularly striking that COHb levels in animals breathing air at 10,000 ft (1.16 ± 0.28%) and 15,000 ft (1.68 ± 0.14%) equaled or exceeded the COHb levels of animals breathing CO at ambient altitude (0.99 ± 0.06%). These results are in accord with the predictions of Collier and Goldsmith (1983), who transformed and rearranged the Coburn-Forster-Kane equation (Coburn et al., 1965) and derived an equation expressing COHb in terms of endogenous and exogenous sources of CO: VCOZ
+
~T
where COHb is the percent COHb, Flco the fraction inspired CO (ppm), PB the barometric pressure (torr), Vco the rate of CO production (mL/min STPD), and K = PC-O2/(M x SO2), Z = 1/DLCO + (PB-47/VA)
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J.). McGRATH
with PjO2 the mean partial pressure of pulmonary capillary O2 (torr), M the Haldane constant, SO2 the O2Hb (%), DLCO the carbon monoxide diffusing capacity (mL/min/torr), and VA the alveolar ventilation (mL/ min"1 STPD). At high altitudes inspired PO2 and PjO2 are reduced and, assuming V^co remains constant, a given Flco will result in a higher percent COHb. Moreover, Collier and Goldsmith (1983) predict an increase in COHb at altitude, even in the absence of inhaled CO, due to endogenous production of CO. These predictions are supported by the results of this study. Because blood COHb levels increase with altitude in the absence of ambient CO exposure, it might be expected that human populations in high-altitude cities have increased body stores of CO; this increase is independent of local CO levels and results from the physiological process by which the body excretes CO produced endogenously. Endogenous CO is produced by catabolism of hemoglobin and other heme proteins. In humans, the normal catabolism of hemoglobin and the breakdown of nonhemoglobin heme proteins produce approximately 0.4 ml/h of CO (Coburn et a!., 1963); this CO causes an average background COHb level of 0.4-0.7%. Certain drugs, chemical exposures, and disease states (e.g., hemolytic anemia) can increase CO production within the body and may produce COHb levels of 4-6% (Stewart, 1976). Typical COHb levels reported for urban dwelling nonsmokers are 1.2% for Milwaukee and 2.2% for Chicago. These levels represent a rise above the background levels of 0.4-0.7% and, presumably, the additional COHb derives from exposure to exogenous CO in the ambient air of the city. Animals undergo pronounced physiological changes in their attempt to adapt to higher altitude (Barbashova, 1964), and these changes may also affect blood COHb levels. Early in the adaptation process, hyperventilation and hemoconcentration increase the O2-carrying capacity of the blood and also affect the affinity of hemoglobin for oxygen. Later, erythropoiesis is stimulated, which further increases the O2-carrying capacity. All of these changes may affect COHb levels. While an increase in erythropoiesis will increase the size of the circulating hemoglobin pool, it will also provide a greater sink for CO. Conversely, an increased rate of hemoglobin destruction will cause an increase in the rate of CO production while decreasing the size of the circulating hemoglobin pool. We have not measured hemoglobin production or destruction, which were beyond the scope of this study, but these would be key measurements to make in future studies.
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monoxide exposure on the exercise performance of subjects with coronary artery disease. N. Engl. J. Med. 321(21):1426-1432. Aronow, W. S. 1981. Aggravation of angina pectoris by two percent carboxyhemoglobin. Am. Heart J. 101:154-157. Aronow, W. S., and Isbell, M. W. 1973. Carbon monoxide effect on exercise-induced angina pectoris. Ann. Intern. Med. 79:392-395. Barbashova, Z. I. 1964. Cellular level of adaptation. In Handbook of Physiology. Adaptation to the Environment, pp. 37-54. Washington, D.C.: American Physiological Society. Coburn, R. F., Blakemore, W. S., and Forster, R. E. 1963. Endogenous carbon monoxide production in man. J. Clin. Invest. 42(7):1172-1178. Coburn, R. F., Forster, R. E., and Kane, P. B. 1965. Considerations of the physiological variables that determine the blood carboxyhemoglobin concentration in man. J. Clin. Invest. 44(11):18991910. Collier, C. R., and Goldsmith, J. R. 1983. Interactions of carbon monoxide at altitude. Atmos. Environ. 17:723-728. Environmental Protection Agency. 1979. Air Quality Criteria for Carbon Monoxide, EPA-60018-79002. Washington, D.C.: EPA. Forster, R. E. 1970. Carbon monoxide and the partial pressure of oxygen in the tissue. Ann. N.Y. Acad. Sci. 174:233-241. McGrath, J. J. 1988. Body and organ weights of rats exposed to carbon monoxide at high altitude. J. Toxicol. Environ. Health 23:303-310. McGrath, J. J. 1989. Cardiovascular effects of chronic carbon monoxide and high altitude exposure. Health Effects Institute Research Report 27, July. National Research Council. 1977. Carbon Monoxide, Committee on Medical and Biological Effects of Environmental Pollutants, p. 166. Washington, D.C.: National Academy of Science. Rodkey, F. L. 1970. Carbon monoxide estimation in gases and blood by gas chromatography. Ann. N.Y. Acad. Sci. 174:261-267. Root, W. S. 1965. Carbon monoxide. In Handbook of Physiology, eds. W. O. Fenn and H. Rahn, p. 1087. Washington, D.C.: American Physiology Society. Steel, R. G., and Torrie, J. H. 1960. Principles and Procedures of Statistics. New York: McGraw-Hill. Stewart, R. D. 1976. The effect of carbon monoxide on humans. J. Occup. Med. 18(5):304-309. Received April 22, 1991 Accepted August 31, 1991
