OXYGEN AND TC-99M-MAA SHUNT ESTIMATIONS IN PATIENTS WITH PULMONARY ARTERIOVENOUS-MALFORMATIONS - EFFECTS OF CHANGES IN POSTURE AND LUNG-VOLUME

Background - Patients with arteriovenous malformations are routinely m onitored with arterial oxygen saturation (SaO(2)) estimations (breathi ng air) from which an oxygen shunt fraction can be calculated. This si mple estimation has been compared with an anatomically defined estimat e of the right to left shunt using a radioisotopic method. The fall in Sao, which occurs in patients with pulmonary arteriovenous malformati ons in the erect posture and at high lung volumes was used to test the ability of Sao, alone to follow changes in right to left shunt. Metho ds - Radiolabelled albumin macroaggregates (Tc-99m-MAA) or microsphere s (MS) were injected intravenously and kidneys and lungs were imaged. The shunt fraction (QS/QT(Tc)) in the supine position at functional re sidual capacity (baseline) was obtained by quantifying right kidney ra dioactivity. On standing or while breath holding at total lung capacit y, shunt fraction was calculated from baseline QS/QT(Tc) and from lung counts and the injected dose. Arterial oxygen saturation (SAO(2)) was recorded by a pulse oximeter for calculation of the oxygen shunt (QS/ QT(O2)) (breathing air). Results - In the postural study (n = 8) SaO(2 ) decreased from a mean (SD) value of 89 (5)% supine to 80 (6)% erect, corresponding to as QS/QT(O2) 28 (8)% and 44 (8)% respectively. QS/QT (Tc) increased from 28.7 (10.3)% to 39 (14.3)%. In the lung volume stu dy (n = 8) QS/QT(Tc) increased from 16.6 (11.5)% at functional residua l capacity to 23.3 (11.9)% at total lung capacity while QS/QT(O2) incr eased from 19.5 (7.5)% to 25.9 (10.6)% respectively. When all measurem ents were compared for QS/QT(Tc)% and QS/QT(O2)% (n = 32) the differen ce in the mean values was 2.5% (absolute) and the limits of agreement between the two methods were +38% to - 18% (relative). In neither the postural nor the volume study did Delta(QS/QT(O2)) reliably predict De lta(QS/QT(Tc))%. Conclusions - In pulmonary arteriovenous malformation s the simple physiological shunt calculated from SaO(2) breathing air agreed well with the anatomical right to left shunt measured with Tc-9 9m-MAA, but predicted poorly the changes in anatomical shunt induced b y postural or lung volume changes.