Arterial oxygen content primarily reflects haemoglobin concentration in a cohort of hypoxaemic patients

Introduction: Understanding factors that reduce arterial oxygen content (CaO 2 ) thus increasing cardiac demands, is important for clinical practice. Currently, attention is primarily directed to the partial pressure of oxygen (PaO 2 ) and particularly haemoglobin saturation (SaO 2 ), but CaO 2 is also directly proportional to the haemoglobin concentration. Methods: Presentation data were evaluated on 497 consecutive patients with pulmonary arteriovenous malformations (AVMs) and hereditary haemorrhagic telangiectasia (HHT), at risk of both hypoxaemia and iron deficiency anaemia. Between 1999 and 2013, SaO 2 was measured by pulse oximetry in the supine and erect postures, and the mean SaO 2 calculated after 7, 8, 9 and 10 minutes standing. Same-day haemoglobin was measured in venous blood samples in 440 patients. Presentation CaO 2 was calculated by the equation oxygen saturation (SaO 2 , % ) x haemoglobin (gram/dL) x 1.34/100. Results: There was a four-fold difference in CaO 2 across the 440 patients (range 7.6-27.5, median 17.6) mls of oxygen per decilitre (dL) of arterial blood. SaO 2 ranged from 59-100% (median 94.8%), but CaO 2 did not change appreciably across the SaO 2 quartiles (median CaO 2 17.1; 18.1; 17.7; 17.8mls/dL; p=0.34). In contrast, CaO 2 was primarily determined by haemoglobin which ranged from 5.9-21.8g/dL (median 14.1g/dL). The median CaO 2 across quartiles of haemoglobin were 14.1; 16.7, 18.5; and 20.5mls/dL (p Conclusions: Minor reductions in haemoglobin substantially reduce CaO 2 and deserve greater attention in respiratory medicine.