Distribution of blood flow in the perfused tracheae of sheep: a search for arteriovenous anastomoses.

The possible existence of arteriovenous anastomoses (AVAs) in the tracheobronchial circulation has been largely ignored; however, their presence would have many implications. Here we have searched for AVAs in the tracheal circulation of seven anesthetized sheep that were ventilated through a low cervical tracheostomy. Arteries to the midcervical trachea were isolated and perfused in situ at systemic blood pressure; blood flow was measured with an electromagnetic flow probe. A vein draining the perfused segment was cannulated. Radiolabeled microspheres (16 microns diam) injected into the arterial inflow of the perfusion indicated the distribution of blood flow (cervical trachea, 69.9 +/- 4.55% (SE); esophagus, 15.1 +/- 3.9%; connective tissue and blood vessels near trachea, 9.5 +/- 1.8%; lymph nodes, 4.0 +/- 2.2%; skeletal muscle, 0.17 +/- 0.07%; trachea below intubation, 0.00%; larynx, 0.00%; other tissues 0.81 +/- 0.32%). Less than 0.6% of the activity was detected in the tracheal venous blood and lungs, indicating little shunting via AVAs. Within the trachea, blood flow (per wet weight of tissue) to the mucosa overlying smooth muscle was approximately 25% of that to the mucosa overlying cartilage. Intra-arterial infusion of methacholine doubled perfusion blood flow without altering the proportion of shunted blood. Within the trachea, blood flow increased most to the mucosa overlying smooth muscle and to smooth muscle. In conclusion the contribution of AVAs to total blood flow in the cervical trachea is < 1%.