Very rare. Anomaly of hemoglobin due to the replacement of tryptophan by the arginine at position 37 on the  ß chain of hemoglobin.

This mutation results in a reduction in the affinity of hemoglobin for O2 (deviation to the right of the oxyhemoglobin dissociation curve: P50 = 35 mmHg vs 25 mmHg for HgA).

Because the mutation affects the β-chain of hemoglobin, desaturation does not appear until after 3 to 6 months of age, when HbF has been replaced by HbA.

Anesthetic implications:

SPO2 measures are artificially low (pulse oximetry); monitoring by use of transcutaneous paO2 or arterial blood gases

References : 

-        Bruns CM, Thet LA, Woodson RD, Schultz J, Hla KM.
Hemoglobinopathy case finding by pulse oximetry.
Am J Hematol 2003; 74: 142-3.

-        Burstal R, Ross B.
Unexpectedly low oxygen saturation in a child with a variant hemoglobin.
Pediatr Anesth 2023 ; 33 : in press. doi:10.1111/pan.14650

Updated: April 2023