Spectrophotometry of carboxyhaemoglobin

This chapter is probably at least somewhat related to Section F8(ii) from the 2023 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the oxyhaemoglobin dissociation curve and factors that may alter it". It deals mainly with the detection of carboxyhaemoglobin.  Carboxyhaemoglobin concentration is measured using spectrophotometry, comparing the total absorption spectrum curve to the characteristic curve generated by pure COHb. The concentration is expressed by the ABG machine as a percentage (FCOHb) - a ratio of cCOHb to ctHb.

Spectrophotometry of carboxyhaemoglobin

The diagrams depicting the superimposed spectra of oxyhaemogobin and carboxyhaemoglobin are based on the findings from a 1950 paper by Klendshoj Feldstein and Sprague.

superimposed absorption spectra of oxyhaemoglobin and carboxyhaemoglobin

From the difference in absorption spectra, it is possible to calculate the concentration of carboxyhaemoglobin. Foetal haemoglobin interferes with this measurement by falsely increasing the measured FCOHb, but this is rarely an issue outside of neonatal burns resuscitation.

There is a normal endogenous production of carbon monoxide, which occurs as a result of haem catabolism and which usually does not generate an FCOHb greater than 1%. Any FCOHb beyond this level is likely due to the inhalation of exogenous carbon monoxide.

Another exotic endogenous source of CO is actually the metabolism of methylene chloride, a component of commerically available painstripper (this is why the can says "use in a well ventilated area"). In general, most dihalomethanes will have this effect.

Properties of carboxyhaemoglobin

The key problem with this haemoglobin species is its total unwillingness to acquire or release oxygen.

To be sure, its rate of binding to Hb is about 20% lower than for pure oxygen, but its chemical affinity for haemoglobin (the Haldane coefficient, M) is about 210-250 times greater than that of oxygen, and it will actively displace oxygen from its binding site in oxyhaemoglobin.

Carboxyhaemoglobin does not merely decrease the oxygen-carrying capacity of blood by removing some haemoglobin molecules from the equation; like methaemoglobin, it also interferes with "cooperativity", the property of haemoglobin which ensures the sigmoid shape of the normal oxygen-haemoglobin dissociation curve (which is discussed in greater detail elsewhere). The affinity of the unaffected normal haem units in the same molecule increases. The oxyhaemoglobin dissociation curve shifts to the left, and becomes a near-rectangular hyperbola.

oxygen-hemoglobin dissociation curve at different carboxyhaemoglobin concentrations

The deposition of oxygen in the tissues is obviously completely wrecked by this. Note how at around 60% FCOHb the p50 is about 12mmHg - how are your tissues supposed to get oxygen from this feral molecule?

As for the diagram; it seems almost everybody has a curve similar to this, but all the curves are disturbingly quite different in every textbook. The above curves I view as "gold standard" - they were extracted from an erratum to the original Roughton and Darling paper (1944). These original graphs - hand plotted- do not superimpose well on top of the Excel graphs, because the spaces between the pO2 lines are all different. In view of this, and in the interest of accuracy, I offer the original image, rather than some sort of modification.


Device-specific information in all these ABG pages refers to the ABG machine used in my home unit.

Other machines may have different reference ranges and different symbols.

For my ABG analyser, one can examine this handy operations manual.

There is also an even more handy reference manual, but one needs to be an owner of this equipment before one can get hold of it. Its called the "989-963I ABL800 Reference Manual"

Roughton, F. J. W., and R. C. Darling. "The effect of carbon monoxide on the oxyhemoglobin dissociation curve." Am J Physiol 141.1 (1944): 17-31.

Siggaard-Andersen, O., et al. "Measured and derived quantities with modern pH and blood gas equipment: calculation algorithms with 54 equations."Scandinavian Journal of Clinical & Laboratory Investigation 48.S189 (1988): 7-15.

Klendshoj, Niels C., Milton Feldstein, and Alice L. Sprague. "The spectrophotometric determination of carbon monoxide." Journal of Biological Chemistry 183.1 (1950): 297-303.

Vreman, H. J., D. K. Stevenson, and A. Zwart. "Analysis for carboxyhemoglobin by gas chromatography and multicomponent spectrophotometry compared." Clinical chemistry 33.5 (1987): 694-697.

Vreman, H. J., et al. "Interference of fetal hemoglobin with the spectrophotometric measurement of carboxyhemoglobin." Clinical chemistry34.5 (1988): 975-977.

Roughton, F. J. W., and R. C. Darling. "The effect of carbon monoxide on the oxyhemoglobin dissociation curve." Am J Physiol 141.1 (1944): 17-31.

Piantadosi, C. A. "Carbon monoxide poisoning." (2004). UHM Vol. 31, No 1

Piantadosi, C. A. "Carbon Monoxide, Oxygen Transport, and Oxygen Metabolism." (1987).

Fagin, James, Julian Bradley, and Derek Williams. "Carbon monoxide poisoning secondary to inhaling methylene chloride." British medical journal281.6253 (1980): 1461.

Kubic, V. L., et al. "METABOLISM OF DIHALOMETHANES TO CARBON MONOXIDE I. In Vivo Studies." Drug Metabolism and Disposition 2.1 (1974): 53-57.

Joels, N., and L. G. C. E. Pugh. "The carbon monoxide dissociation curve of human blood." The Journal of physiology 142.1 (1958): 63-77.

Meredith, Tim, and Allister Vale. "Carbon monoxide poisoning." British medical journal (Clinical research ed.) 296.6615 (1988): 77.

Maynard, Robert L., and Robert Waller. "Carbon monoxide." Air Pollution and health (1999): 749-796.