The relationship between polycythaemia, P50 and SaCO (saturation in carboxyhaemoglobin) has been studied in 50 patients who were hypoxaemic due to chronic respiratory insufficiency. These patients were divided into two groups according to their haemoglobin concentration and haematocrit: 21 polycythaemic patients with haemoglobin greater than or equal to 16 g/dl and haematocrit greater than or equal to 50% and 29 patients without polycythaemia. PaO2, PaCO2, plasma and erythrocyte pH, haemoglobin, haematocrit, and carbon monoxide saturation and intraerythrocytic 2-3 diphosphoglycerate concentration were measured during steady-state ventilation. All polycythaemic patients were smokers and their carbon level was significantly higher than that observed in patients without polycythaemia. Additionally, their P50 and 2-3 DPG concentration were significantly lower than in patients without polycythaemia. The correlations between P50 and HbCO and between Hb and HbCO were significant (r=--0-672; r=0-552 respectively: P less than 0-001). Eleven non-polycythaemic patients who were smokers had a high level of HbCO but normal P50. A group of 29 normoxic subjects was also studied, 14 non-smokers and 15 smokers with a high HbCO level. The mean value of P50 was lower in smokers and their haematocrit was higher although the difference was not significant for the latter. The HbCO increase by tobacco seems to be a factor in the occurrence of polycythaemia in patients with chronic respiratory insufficiency. The level of increase of HbCO and/or its duration and perhaps other individual factors and explain why all patients with high HbCO level and hypoxaemia were not polycythaemic.
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