Factors influencing the R-R interval during central venous injection in newborn swine

Seven Yorkshire swine, ages 7-11 days and weighing 1.4-2.8 kg were studied to determine the effects of temperature and volume of injectate, depth of anaesthesia, position of the central venous catheter tip and vagotomy on the R-R interval after central venous injection of saline. The swine were anaesthetized with halothane in 100 per cent oxygen and their lungs ventilated to normocapnia. The length of the R-R varied inversely with the temperature of the injectate between 0 and 20 degrees C reaching a maximum prolongation of 152 per cent above control values with 0 degrees C saline. Injecting saline at 37 degrees C did not affect the R-R interval. The length of the R-R interval varied directly with the volume of injectate between 1.5 and 4.5 ml.kg-1 (P less than 0.05). The R-R interval response also varied directly with the depth of anaesthesia: the post-injection R-R interval increased from 185 per cent to 341 per cent as the end-tidal halothane concentration increased from 0.45 to 1.20 per cent. The position of the tip of the central venous catheter that produced the maximum increase in the R-R interval, as determined radiographically, was at the junction of the superior vena cava and the right atrium. Neither bilateral vagotomy nor atropine (50 micrograms.kg-1) affected the R-R interval after injecting 3 ml.kg-1 saline 0 degrees C. We conclude that the increases in R-R interval after injection of fluid into the right atrium are due to direct effects on the nerve conduction system of the heart, possibly on the sino-atrial node.