Introduction and objectives It is recognised that age-associated changes in the chest wall and lung parenchyma lead to decreased efficiency of ventilation and gas exchange, resulting in reduced arterial partial pressure of oxygen (PaO2) and haemoglobin saturation (SaO2). The total oxygen content of arterial blood (CaO2) depends upon SaO2, as well as haemoglobin concentration. Our goal was to examine serial changes in arterial oxygen content with age in a cohort with hypoxaemia due to pulmonary arteriovenous malformations (PAVMs).
Methods Retrospective longitudinal follow-up data was collected for 100 consecutive PAVM patients presenting to a tertiary care institutional clinic between 1984 and 2001, and reviewed until 2015. Subjects provided up to 30 (median 9) separate annual datasets. SaO2 was measured by pulse oximetry in the supine and erect postures, and the mean SaO2 was calculated after 7, 8, 9 and 10 min standing. Haematological and biochemical blood indices evaluated haemoglobin, haematinics, and iron indices. CaO2 in mls of oxygen per dL (ml/dL) of blood was calculated using the equation: [SaO2 (%) x haemoglobin (g/dL) X 1.34]/100. Data were analysed using STATA IC v13.1.
Results Age and PAVM-treatment associated changes in SaO2 were mostly accompanied by opposing changes in haemoglobin levels that maintained the CaO2. Two major patterns were observed. The first was the expected increase in haemoglobin with lower SaO2, due to secondary erthyrocytosis and polycythaemia. The second, less well recognised, was an increase in SaO2 when haemoglobin fell, most commonly when subjects developed iron deficiency and anaemia. Nevertheless, excluding participants with iron deficiency, CaO2 decreased with age (Figure 1, r2 = -0.0654; p < 0.001, Figure 1).
Conclusions The body maintains arterial oxygen content within a normal range using well-known erythropoeitic mechanisms in response to hypoxaemia. Despite this feedback mechanism, in patients with pulmonary arteriovenous malformations, overall arterial oxygen content still decreases with age.