Introduction and objectives Our goal was to use a long term model of human hypoxaemia to evaluate factors that reduce arterial oxygen content (CaO2) and therefore demand higher cardiac outputs to maintain tissue oxygen delivery. This is important for clinical practice; for clinical trials that use cardiac index as a primary outcome measure; and particularly relevant for patients with pulmonary and systemic arteriovenous malformations (AVMs) due to hereditary haemorrhagic telangiectasia (HHT).
Methods Presentation data were evaluated on 497 consecutive patients with pulmonary AVMs due to HHT, reviewed between 1999 and 2013. SaO2 was measured by pulse oximetry in the supine and erect postures, and the mean SaO2 calculated after 7, 8, 9 and 10 min standing. Same-day haemoglobin was measured in venous blood samples in 440 patients. Presentation CaO2 was calculated by the equation oxygen saturation (SaO2, %) x haemoglobin (gram/dL) x 1.34/100.
Results There was a four-fold difference in CaO2 across the 440 patients (range 7.6–27.5, median 17.6) mls of oxygen per decilitre (dL) of arterial blood. SaO2 ranged from 59–100% (median 94.8%), but CaO2 did not change appreciably across the SaO2 quartiles (median CaO2 17.1; 18.1; 17.7; 17.8 mls/dL; p = 0.34, Figure 1A). In contrast, CaO2 was primarily determined by haemoglobin which ranged from 5.9–21.8 g/dL (median 14.1 g/dL). The median CaO2 across quartiles of haemoglobin were 14.1; 16.7, 18.5; and 20.5 mls/dL (p < 0.0001, Figure 1B). For each 1 g/dL rise in haemoglobin, there was a 10% increase in mls of oxygen per unit blood volume.
Conclusions Currently, in long term conditions, more attention is paid to modest differences in SaO2 than to haemoglobin.1 It has been shown that patients with PAVMs maintain CaO2, and deliver the same amount of oxygen per heart beat (oxygen pulse) before and after correction of hypoxaemia by PAVM embolisation.2,3 For patients where higher cardiac outputs may be detrimental, further attention should be given to minor incremental falls in haemoglobin that substantially reduce arterial oxygen content.
References 1 Hardinge M, Annandale J, Bourne S, et al. BTS guidelines for home oxygen use in adults. Thorax 2015:70:i1–i43
2 Santhirapala V, Williams LC, Tighe HC, et al. Arterial oxygen content is precisely maintained by graded erythrocytotic responses in settings of high/normal serum iron levels, and predicts exercise capacity: an observational study of hypoxaemic patients with pulmonary arteriovenous malformations. PLoS One 2014;9(3):e90777
3 Howard L, Santhirapala V, Murphy K, et al. Chest 2014;146(3):709–18