The oxygen affinity of sickle hemoglobin

The right-shifted oxyhemoglobin dissociation curve of sickle cell disease (SCD) has been thought to result in abnormally low arterial oxygen saturation (S_O2), even when oxygen partial pressure (P_O2) is normal. However, without polymer formation (minimal under normoxic conditions), HbS oxygen affinity is normal. We hypothesized that in SCD, in vivoS_O2 is normal when P_O2 is normal. We retrospectively examined 50 blood gas and COoximetry samples from SCD patients and from controls matched for pH, P_O2, and carboxyhemoglobin. Control data fell close to the Severinghaus curve, as did non-hypoxemic (S_O2 > 92.5 %) SCD data. In contrast, hypoxemic (S_O2 < 92.5 %) SCD data fell well below the standard curve. Thus, although SCD patients' oxygen affinity is low under hypoxic conditions, it is normal at normal arterial S_O2. Therefore, a finding of abnormally low saturation demonstrates that P_O2 is abnormally low. Given our previous finding that pulse oximetry faithfully reflects saturation in SCD, low pulse oximeter readings in SCD constitute reliable evidence of impaired gas exchange.