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Hypoxaemia, generalised or local, causes sickling of red cells in people with sickle cell disease and there is evidence that nocturnal hypoxaemia may be a cause of central nervous system complications and vaso-occlusive crises. Researchers in Philadelphia (
) have shown that children with severe sickle cell disease may be hypoxaemic both by night and by day and that the hypoxaemia is related to the degree of anaemia and associated with changes likely to lead to tissue damage.
They studied 37 patients aged 3–19 years with sickle cell disease (28 HbSS, 9 Hb SC) and ten controls, measuring daytime and overnight blood oxygen saturation (SaO2) by pulse oximetry. Blood was taken for measurement of concentrations of vascular cell adhesion molecule 1 (VCAM-1) and markers of activation of neutrophils (L-selectin and leukotriene B4), and platelets (P-selectin). Fifteen (54%) of the children with HbSS, but none of the Hb SC group, had sleeping hypoxaemia, defined as mean sleeping SaO2 93% or less. There was no significant difference between sleeping and waking SaO2.
Daytime and night-time hypoxaemia was demonstrated in over half of the children with HbSS and was associated with a greater degree of anaemia, increased red cell adhesion to endothelium, and an increase in markers of neutrophil and platelet activation. Is the hypoxaemia the cause or the result of the observed changes? These researchers have previously published evidence that the hypoxaemia in their patients is not due to lung disease. In severe sickle cell disease greater anaemia is associated with a greater shift to the right of the haemoglobin dissociation curve and they propose that it is this that leads to cellular activation and sickle cell complications. Clinicians, they say, may need to take more notice of modest levels of hypoxaemia and be prepared to intervene more readily.