Left lower lobe ventilation is reduced in patients with cardiomegaly in the supine but not the prone position

To determine the effect of the heart on regional ventilation, Krypton-81m (^81mKr) tomographic (SPECT) ventilation scans were recorded in seven patients with cardiomegaly and four normal subjects in the supine and prone positions. All patients had a cardiothoracic ratio of greater than 0.50 and clear lung fields radiographically. Using standard gamma camera tomographic reconstruction techniques, images of transaxial slices were obtained during a 360 degree rotation around the thorax of the subject breathing the radioactive gas ^81mKr. The transaxial images, acquired over 10 min were aligned in each posture at the level of the cardiac apex, mid-heart, and aortic arch and were matched in relation to a radioactive marker on the chest wall and to anatomic landmarks. A horizontal line (gravity independent and parallel to the couch) was drawn on the transaxial section through the dorsal regions of the right and left lung. Counts per resolution element (12 to 15 mm) were plotted along this line and the ratios of the peak values in right and left lung compared. These ratios represent differences in regional ventilation per unit lung volume. In controls the mean left-to-right (L/R) peak count ratio varied from 0.91 to 1.00 at the three levels (range: 0.76 to 1.04); there were no significant differences between supine and prone. In patients wtih cardiomegaly the mean (± SEM) L/R peak count ratio at cardiac apex, mid-heart, and aortic arch was 0.46 (± 0.08), 0.55 (± 0.07), and 0.89 (± 0.08) when supine and 1.04 (± 0.07), 1.05 (± 0.05), and 1.08 (± 0.07) when prone, respectively. There was a significant increase in L/R ratio from supine to prone at the cardiac apex and mid-heart levels. These increases were also present when percent total counts for each lung in the whole transaxial slice were compared. In patients with cardiomegaly, there was a reduction in left mid and lower zone ventilation in the supine posture that was probably related to the mass of the heart compressing the lung, leading to airway and/or alveolar closure.