Interstitial fluid volume during cardiac surgery measured by means of a non-invasive conductivity technique

Fluid accumulation in the interstitium is frequently found after cardiac surgery. In extreme this can lead to pulmonary and myocardial oedema. The origin of this accumulation is not exactly known and may be twofold. It is probably a combination of the noninfectious whole body inflammatory response and a change in Starling forces due to a decrease in colloid osmotic pressure (COP) which is caused by the primed extracorporeal circuit. To study the changes in interstitial fluid volume (ISFV) a non-invasive conductivity technique was used. The relationship between temperature and conductivity was first investigated in vitro . A linear relationship was found between conductivity and different saline solutions and temperature. From the in vitro experiments it can be concluded that temperature corrected conductivity does not depend on haematocrit. After the in vitro experiments eleven patients undergoing cardiac surgery were studied. During the first minutes of cardiopulmonary bypass (CPB) a steep significant decrease in COP to 61.4±6.9% (from 19.6±1.1 to 12.0±1.2 mmHg), and a rise in ISFV to 105.5±2.8% (from 12.3±1.4 mS to 14.0±1.3 mS) was noticed. After this decrease COP increased significantly, till the end of the operation, but did not reach the pre-operative level. An increase in ISFV was noticed till the rewarming point. After this point no significant change in ISFV was noticed. Furthermore, a significant correlation was found between the fluid balance and the ISFV increase at the start, at the end of CPB, and at the end of the operation. From the in vivo experiments it can be concluded that the non-invasive conductivity technique in a valuable acquisition for the investigation of ISFV changes during cardiac surgery. It shows that the changes in ISFV are mainly disturbed during the first part of CPB probably due to a marked decrease in COP.