Impaired left lower lobe ventilation in patients with cardiomegaly. An isotope study of mechanisms.

Reduced left lower lobe ventilation (V) in patients with enlarged hearts has been commonly observed on routine isotope ventilation-perfusion lung scanning, and there is evidence to show that this reduction is dependent on posture. Clinically, it may have a role in posture-dependent dyspnea and postoperative left lower lobe changes in cardiomegaly. Previous studies have shown improvement in left lower lobe V in the prone compared with the supine position. In 11 patients showing this phenomenon in krypton 81m V scanning, a mean relative reduction in V of 53 percent occurred at the left base. No significant change in perfusion or in the signal from preinjected technetium 99m macroaggregated albumin was observed between the two positions. No significant change in ventilatory turnover (measured with intravenous xenon 133) was seen either, suggesting that no air trapping takes place. By combining the data from the intravenous 133Xe (which gives the ratio, V/lung volume) and continuously inhaled 81mKr (which reflects regional V), an index of relative volume between the two sides was derived and shown to be significantly reduced at the left base on moving from the prone to supine positions (a mean reduction of 40 percent; p less than 0.02 by Wilcoxon signed rank test). Thus, the mechanism of postural left lower lobe hypoventilation in cardiomegaly is predominantly a regional loss of alveolar volume.