The influence of thoracic epidural analgesia on transcapillary fluid balance in subcutaneous tissue

Clinical observation and calculation of fluid balance have shown that patients undergoing aortocoronary bypass surgery with thoracic epidural analgesia (TEA) in addition to general anesthesia retain less fluid than patients having general anesthesia only. The present study was designed to investigate whether this effect could be explained by thoracic epidural analgesia influencing the transcapillary fluid balance, i.e. the transcapillary forces (COPpl, COPif, Pif). Interstitial fluid colloid osmotic pressure (COPif) and interstitial fluid pressure (Pif) were measured subcutaneously at heart level by the blister suction technique and the wick-in-needle technique, respectively. Simultaneously plasma colloid osmotic pressure (COPpl) was recorded. Sixteen male patients were allocated to two groups, one having general anesthesia only (controls, n = 8). The other group (TEA, n = 8) received, at the induction of anesthesia, bupivacaine 50 mg via an epidural catheter as an adjunct to general anesthesia. TEA was maintained by continuous infusion for 24 h postoperatively. Preoperatively no intergroup differences were observed in "the Starling forces" (COPpl, COPif, Pif). At the start of extracorporeal circulation COPpl was significantly lower in the TEA-group than in controls. During extracorporeal circulation the transcapillary COP-gradient (COPpl-COPif) was reversed in both groups. At the end of extracorporeal circulation Pif increased to a minor degree in the TEA-group and remained significantly lower than in controls from 3 to 24 h postoperatively. The subcutaneous interstitial tissue could be less expanded postoperatively in the TEA-group, also reflected by a lower increase in Pif.