Distribution of alveolar edema in ventilated and unventilated canine lung lobes

Pulmonary edema frequently is treated with ventilation but its effects on the distribution of edema, including gravity-dependent gradients as determined by computed tomography (CT) scanning, are unclear.

To study this, 30 to 50 mL 5% albumin in dextran were instilled in both caudal lobes of supine dogs. They were ventilated only on the left side for 1 minute (n = 4), 30 minutes (n = 6), or 60 minutes (n = 6), and the lobes excised, frozen, and imaged in a CT scanner. Regions of interest were outlined on regional CT slices and tissue from corresponding regions taken for measurements of extravascular lung water (quantity of wet lung [Qwl]/dry quantity of lung [dQl] and for histology to grade interstitial and alveolar edema.

After ventilation for 30 and 60 minutes, the CT density of the left caudal lobes was significantly lower than the right caudal lobes (P < 0.05), with no significant differences in their Qwl/dQl. Although gravity-dependent gradients of Qwl/dQl were demonstrated, they were unaffected by ventilation. Histology showed a trend for more interstitial edema in left caudal lobes ventilated for 60 minutes compared with lobes ventilated for 1 minute (P = 0.054).

Ventilation appears to act primarily by maintaining lung aeration and may play a minor role in alveolar fluid clearance.