THE RATE OF BLOOD WITHDRAWAL AFFECTS THE ACCURACY OF JUGULAR VENOUS BULB - OXYGEN-SATURATION MEASUREMENTS

Background: Accuracy of jugular venous oxygen saturation (Sjv(O2)) mea surement depends on sampling of cerebral venous outflow blood not cont aminated by systemic venous blood. The influence of the rate of blood withdrawal has not been determined. Methods: The authors examined the effect of withdrawing blood at different rates from jugular venous bul b catheters (JVBC) on Sjv(O2) in 10 mechanically ventilated patients u ndergoing neurosurgical procedures. All patients received a standardiz ed anesthetic consisting of propofol, fentanyl, vecuronium, and isoflu rane. Routine monitors included electrocardiograph (EGG), invasive blo od pressure, pulse oximetry, and a JVBC. During a period of stable ane sthetic and surgical conditions, JVBC blood samples were drawn at 2, 4 , and 10 ml/min using a calibrated pump (Harvard Pump model 900, Harva rd Apparatus, South Natick, MA) during mild and moderate hypocapnia (a rterial carbon dioxide tension [Pa-CO2], 26.0 +/- 0.5 and 33.0 +/- 0.5 mmHg). Results: Faster rates of withdrawal (10 and 4 ml/min vs. 2 ml/ min) resulted in significantly higher Sjv(O2) values at both levels of Pa-CO2 (66.0 +/- 3% and 61.2 +/- 3% vs. 56.9. +/- 3% at Pa-CO2 = 26.0 +/- 0.5 mmHg, and 75.0 +/- 3% and 71.3 +/- 3% vs. 68.0 +/- 3% at Pa-C O2 = 33.0 +/- 0.5 mmHg, respectively; P < 0.01). Conclusions: The auth ors conclude that the Sjv(O2) values measured with intermittent sampli ng are affected by the rate of withdrawing blood from JVBC, probably a s a result of extracranial contamination. They recommend blood samples should be drawn slowly.