The following set of arterial blood gases were obtained from a patient admitted to the ICU after a suicide attempt.

Test

Value

Normal Range

pH*

6.84

(7.36 – 7.44)

PCO2*

94 mm Hg

(36 – 44)

PO2

140 mm Hg

P50

24 mm Hg

Standard base excess*

-16.0 mmol/L

(-2.0 – +2.0)

a)     What  anomaly  do  you  notice  in  the  blood  gas  report?  (Apart  from  the hypercapnia and the acidosis).

b)     List 2 other investigations you would perform to elucidate the cause of the anomaly.

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College Answer

a)     What  anomaly  do  you  notice  in  the  blood  gas  report?  (Apart  from  the hypercapnia and the acidosis).

A left shifted curve despite a high PCO2 and a low pH.

b)     List 2 other investigations you would perform to elucidate the cause of the anomaly.

•    CoHb
•    Measure temperature
•    Measure 2,3 DPG

Discussion

This ABG represents a mixed respiratory and metabolic acidosis.

By the standard equation of CO2 to pH relationship (Change in pH = 7.40 - (pCO2 x 0.008)) the pH should be 7.04.

Thus, there is also a metabolic acidosis in play, driving the pH lower.

The anomaly, as pointed out by the college answer, is in the low p50 value.

The p50 is the partial pressure of oxygen which will result in a 50% oxygen saturation of hemoglobin.

Acidosis tends to drive this value higher; this "right shift" of the oxyhemoglobin dissociation curve is known as the Bohr effect.

In essence, the lower the pH, the lower the affinity of hemoglobin for oxygen, and the higher the partial pressure of oxygen required to saturate 50% of the hemoglobin. This facilitates the transport of oxygen out of hemoglobin and into the tissues.

Thus, a low P50 value suggests that the affinity of hemglobin for oxygen is actually quite high (because only a small amount of oxygen is required to saturate it to 50%).

This, in the context of acidosis, is odd.

Thus we come to the real question the college is asking: "What are the causes of a left-shift of the oxygen-hemoglobin dissociation curve?"

The answer is temperature and 2,3-DPG.

Hypothermia and low 2,3-DPG levels can force the curve to the left in spite of acidosis.

Additionally, the p50 can be affected by the presence of any hemoglobin type which has an altered affinity for hemoglobin, whatever the pH. These are the methemoglobins, carboxyhemoglobin, sulfhaemoglobin, foetal hemoglobin, and so on and so forth. If the proportion of such altered haemoglobin is high enough, the total oxyhemoglobin dissociation curve will look weird. The college has only opted to test for one type (COHb) and perhaps for now one should leave it at that.

Other causes of a shifted p50 are listed elsewhere.

References

References

LIFL have an excellent summary.

Another excellent summary of how this can be easily taught can be found at PubMed:

Julian Gomez-Cambronero "The Oxygen Dissociation Curve of Hemoglobin: Bridging the Gap between Biochemistry and Physiology." Journal of chemical education 78, no. 6 (2001): 757.

 

A specific article related to critical illness is available from CICM itself

Morgan, T. J. "The oxyhaemoglobin dissociation curve in critical illness." Critical Care and Resuscitation 1.1 (1999): 93.

 

The association of oxygen-hemoglobin dissociation and carboxyhemoglobin can be found here:

Rovida, E., et al. "Carboxyhemoglobin and oxygen affinity of human blood."Clinical chemistry 30.7 (1984): 1250-1251.

 

Seminal papers on this subject are still available.

Aberman, A., et al. "An equation for the oxygen hemoglobin dissociation curve."Journal of applied physiology 35.4 (1973): 570-571.

 

Forbes, W. H., and F. J. W. Roughton. "The equilibrium between oxygen and hæmoglobin I. The oxygen dissociation curve of dilute blood solutions." The Journal of physiology 71.3 (1931): 229-260.