Background Patients with pulmonary arteriovenous malformations are often quoted as displaying orthodeoxia, that is, a fall in oxygen saturation on standing. It is unclear how common this phenomenon is, and how patients would compensate for any acute fall in SaO₂.

Methods Postural changes in oxygen saturation and pulse were examined in a series of 165 prospectively-recruited patients with radiologically-proven PAVMs. Self-reported exercise at presentation was graded using a modified MRC dyspnoea scale. SaO₂ and pulse measurements were made in both erect and supine postures on 1–8 separate occasions through presentation to post-embolisation follow up. These 522 sets of postural measurements displayed very high within-patient reproducibility.

Results Age ranged from 17–87 (median 49) ys, 62 (37.6%) were male, and for 159 (96%), PAVMs were attributable to HHT. 18.9% were obese with a body mass index (BMI) >30, including one with morbid obesity (BMI>40). At presentation, the SaO₂ fell by at least 2% on standing in 51(33%) patients, compared to the equivalent average supine reading. A smaller fall of 1–2% was present in a further 28 (17%) of patients. Patients with higher BMI had significantly higher supine SaO₂ for their erect SaO₂, and significantly lesser falls in SaO₂ on standing. A postural tachycardia consistently exceeded the increment required to sustain oxygen delivery at rest, across all SaO₂.12/163 (7.4%) of individuals met the pulse definition for postural orthostatic tachycardia syndrome with an increase on standing of more than 30 beats per minute. Once adjusted for baseline supine pulse and age, the erect pulse was lower in patients with higher BMI (adjusted regression coefficient 0.23 (95% confidence interval 0.026, 0.48), p = 0.011). Using the pre-specified 5 group grading system, exercise tolerance was worse in patients with lesser postural tachycardias (regression coefficient -2.19 (-3.74, -0.65) p = 0.006)).

Conclusions Orthodeoxia and postural tachycardia are common in PAVM patients. More pronounced postural tachycardias were associated with improved exercise tolerance. Further studies will be required to assess if this is because it is a surrogate of lower BMI, with obesity effectively limiting a further fall in SaO₂ on standing because of obesity-related lower supine SaO₂.