V/Q distribution and correlation to atelectasis in anesthetized paralyzed humans

Regional ventilation and perfusion were studied in 10 anesthetized paralyzed supine patients by single-photon emission computerized tomography. Atelec tasis was estimated from two transaxial computerized tomography scans. The ventilation-perfusion (over dotV/over dotQ) distribution was also evaluated by multiple inert gas elimination. While the patients were awake, inert gas over dotV/over dotQ ratio was normal, and shunt did not exceed 1% in any patient. Computerized tomography showed no atelectasis. During anesthesia, shunt ranged from 0.4 to 12.2%. Nine patients displayed atelectasis (0.6-7.2% of the intrathoracic area), and shunt correlated with the atelectasis (r = 0.91, P < 0.001). Shunt was located in dependent lung regions corresponding to the atelectatic area. There was considerable over dotV/over dotQ mismatch, with ventilation mainly of ventral lung regions and perfusion of dorsal regions. Little perfusion was seen in the most ventral parts (zone I) of caudal (diaphragmatic) lung regions. In summary, shunt during anesthesia is due to atelectasis in dependent lung regions. The over dotV/over dotQ distributions differ from those shown earlier in awake subjects.