Decreased oxyhemoglobin affinity in patients with sleep apnea syndrome

Oxyhemogloblin affinity (P50 at pH 7.4, PaCO2 = 40 mm Hg, temperature = 37 degrees C) and 2,3-DPG concentration were assessed in 15 nonsmokers (14 men and one woman 46 to 63 yr of age) with sleep apnea syndrome (SAS) and in 10 normal subjects (eight men and two women 22 to 48 yr of age). In patients with SAS, mean nocturnal apnea index was 46 +/- 20/h, and mean nocturnal SO2 was 86 +/- 6% versus 94.6 +/- 1.8% during the daytime. Daytime mean P50 of the patients was 28.5 +/- 1.2 mm Hg versus 27.1 +/- 0.3 mm Hg in the normal subjects (p less than 0.05). Daytime mean 2.3-DPG was 1.23 +/- 0.25 moles DPG/mole hemoglobin versus 0.80 +/- 0.15 (p less than 0.05). Significant correlations were found in patients between P50 and mean nocturnal SO2 (r = -0.62, p less than 0.01) and between P50 and 2,3-DPG (r = 0.68, p less than 0.01). The measurements were repeated in five patients after surgical or positive-pressure treatment. P50 and 2,3-DPG both decreased and returned to normal values. In conclusion, the oxyhemoglobin dissociation curve is shifted to the right in patients with SAS and there is an increase in 2,3-DPG. These could be protective mechanisms against the development of polycythemia, pulmonary hypertension, and cor pulmonale.