Lower transfer factor of the lung for carbon monoxide in women with a patent foramen ovale

New Findings What is the central question of this study? Do individuals with a patent foramen ovale (PFO+) have a lower lung transfer factor for carbon monoxide than those without (PFO−)? What is the main finding and its importance? We found a lower rate constant for carbon monoxide uptake in PFO+ compared with PFO− women, which was physiologically relevant (≥0.5 z‐score difference), but not for PFO+ versus PFO− men. This suggests that factors independent of the PFO are responsible for our findings, possibly inherent structural differences in the lung. The transfer factor of the lung for carbon monoxide (TLCO) measure assumes that all cardiac output flows through the pulmonary circuit. However, right‐to‐left blood flow through a shunt can result in a lower transfer factor than predicted. A patent foramen ovale (PFO) is a potential source of right‐to‐left shunt that is present in ∼35% of the population, but the effect of PFO on TLCO is unknown. We sought to determine the effect of PFO on the TLCO. We conducted a retrospective analysis of TLCO data from 239 (101 women) participants. Anthropometrics and lung function, including spirometry, plethysmography and TLCO, were compiled from our previously published work. Women, but not men, with a PFO had a significantly lower TLCO and rate constant for carbon monoxide uptake (KCO) (percentage of predicted and z‐score) than women without a PFO. Women and men with a PFO had normal alveolar volumes that did not differ from those without a PFO. Correcting the data for haemoglobin in a subset of subjects did not change the results (n = 58; 25 women). The lower KCO in women with versus without a PFO was physiologically relevant (≥0.5 z‐score difference). There was no effect of PFO in men. This suggests that factors independent of the PFO are responsible for our findings, possibly inherent structural differences in the lung.