Pulmonary arteriovenous malformation
Pulmonary arteriovenous malformations are caused by abnormal communications between pulmonary arteries and pulmonary veins, which are most commonly congenital in nature. Although these lesions are quite uncommon, they are an important part of the differential diagnosis of common pulmonary problems such as hypoxemia and pulmonary nodules. Since their first description at autopsy in 1897 (1), these abnormal communications have been given various names including pulmonary arteriovenous fistulae, pulmonary arteriovenous aneurysms, hemangiomas of the lung, cavernous angiomas of the lung, pulmonary telangiectases, and pulmonary arteriovenous malformations (2). The term "pulmonary arteriovenous malformations" (PAVM) will be used in this review to describe these lesions.
The fraction of cardiac output that shunts from right-to-left (shunt fraction, normal = 5% by arterial blood gas determination. However, neither of these studies reported whether shunt fraction was calculated while breathing 100% oxygen. A shunt fraction of > 5% using the 100% oxygen method showed a sensitivity of 87.5% and specificity of 71.4% in 12 patients with PAVM and HHT who were discovered by formal screening of 98 relatives of HHT patients (19). Haitjema and coworkers (51) reported a series of 32 patients who underwent embolotherapy and found that 30 of 31 patients had a shunt fraction of > 5% (mean 16.6%, range 3.5 to 35%) by the 100% oxygen method [assuming an arteriovenous oxygen content difference, C(a-v)O2, of 5 ml%]. Other smaller studies of patients prior to embolotherapy have found shunt fraction by the 100% oxygen method to be uniformly increased at between 14 to 55% (49, 52, 61, 62, 93). Combined, these studies found that 119 of 122 (97.5%) patients with PAVM had a shunt fraction > 5% (15, 19, 48, 49, 51, 52, 61, 62, 93). Although the true sensitivity of a shunt fraction > 5% in detecting all PAVM is likely lower than 97.5%, this is a reasonable estimate of its sensitivity in detecting clinically important PAVM.
It is important to understand the natural history of an illness in order to approach its treatment. Unfortunately, the natural history of untreated PAVM is not well delineated as there are no prospective reports of patients who were randomized to treatment versus observation only. When followed over time, PAVM typically remain unchanged in size, although about 25% enlarge gradually (14, 17). Serial chest radiographs over a median observation time of 18.9 yr in 16 patients with PAVM and HHT showed enlargement in four patients and near total regression in one patient (10 mm decrease over 14 yr) (17). The growth rate tended to be slow---about 5 to 10 mm every 5 to 15 yr; one of the four patients whose PAVM enlarged showed a subsequent decrease of 10 mm over 6 yr (17). Dines and coworkers (14) followed 27 patients who were not treated and found PAVM enlargement in seven patients over a mean of 6 yr. They also found that the incidence of PAVM enlargement was greater in the patients with HHT versus those without HHT (14).