Flap flow and cardiac output as functions of pulmonary artery wedge pressure: experimental study in the pig

Cardiac output (CO) is redistributed during hypovolemia, but it has not been clearly documented how this influences flap perfusion. Simultaneous changes in perfusion of a muscle flap and CO as modulated by reductions in cardiac filling pressure were compared in a pig model. The hypothesis was that flap flow (FF) would remain relatively constant as CO was reduced, according to the Frank-Starling curve. Under general anesthesia, five domestic pigs were utilized. Following right carotid artery cannulation, measurement of systemic blood pressure and blood gas analysis were carried out. An oximetric Swan-Ganz catheter insertion via the right internal jugular vein was used to measure CO, temperature, and pulmonary artery wedge pressure (PAWP). In the lateral decubitus position, a right latissimus dorsi muscle flap was elevated in the animal, using standard technique. The insertion of the muscle was divided, and a 1-cm section of thoracodorsal artery was mobilized to measure flap perfusion with a transonic Doppler flowmeter. After baseline measurements of PAWP, CO, and FF were obtained, blood volume was reduced in 350-cc aliquots at 10-min intervals. After each aliquot was removed, the hemodynamic parameters were again recorded. This procedure was repeated until the CO was less than 1.0 liter/min; at this point the pig was sacrificed. The mean baseline PAWP was 15.6+/-4.0 mmHg, which was reduced to 2.2+/-1.1 mmHg (p<0.05) by the end of the experiment. The mean baseline FF was 5.6+/-1.9 cc/sec, and the mean baseline CO was 4.6+/-1.0 l/min; these were reduced to 0.4+/-0.4 cc/sec (p< 0.05) and 1.1+/-0.1 l/min (p<0.05), respectively, by the end of the experiment. FF and CO were plotted as a function of PAWP and the two regression curves were evaluated by analysis of covariance. The slope of these curves was not different, p=0.4. The hypothesis that FF would be preserved in the face of reductions in CO did not prove to be correct in this experiment. Changes in FF paralleled changes in CO as the PAWP was reduced. Inhalation anesthesia and local sympathectomy may influence flap perfusion during hypovolemia. These results underscore the importance of avoiding hypovolemia during flap surgery, in order to maintain flap perfusion.