[Hypoxemia and the hematocrit in pathologic polycythemia due to altitude]

81 patients living at La Paz (3,600-4,000 m altitude) and suffering from chronic polycythaemia, with an haematocrit greater than 57% were studied. They were selected on clinical and spirometric criteria to exclude from the study patients with associated pulmonary diseases. 45% of them were overweight (group O). The group of patients having a normal weight was divided into two equal groups: "young" patients (less than 35 years, J) and "old" patients (greater than 35 years, V). The important findings of this study were: 1) the existence of hypoxaemia in all groups: compared to the control group, the mean differences are -2 kPa for O and V groups and -1.3 kPa for J group; 2) the presence of slight hypercapnia (+0.3 kPa in J and V groups; +0.5 kPa in O group), excluding diurnal hypoventilation as the major source of hypoxaemia; 3) a significant linear correlation between the increase in PaCO2 and haematocrit in O and (J + V) groups; 4) a significant linear correlation between hypoxaemia and the increase in haematocrit, particularly in group J, but also in O and (J + V) groups; hypoxaemia is also well correlated with age in (J + V) group: PaO2 (kPa) = 11.42 -0.025 Age (yr) -0.061 Hct (%) The classical assumption of a chronic hypoventilation syndrome producing a progressive increase in haematocrit is discussed. Haematocrit is suspected as a causative factor of hypoxaemia, itself a well known factor producing polycythaemia. This mechanism could be the source of a vicious circle.