The Rate of Blood Withdrawal Affects the Accuracy of Jugular Venous Bulb : Oxygen Saturation Measurements

With approval from the institutional Human Subjects Committee, informed written consent was obtained from 10 patients (American Society of Anesthesiologists' [ASA] physical status 1 or 2; aged 31 +/- 6 yr; weight, 83 +/- 13 kg; hematocrit [Hct], 33 +/- 4%) scheduled to undergo general anesthesia for neurosurgical procedures. Patients with significant respiratory disease and those with cerebral arteriovenous malformations were excluded. With routine monitors in place (electrocardiograph [ECG], pulse oximetry, and noninvasive blood pressure), anesthesia was induced with propofol, 2.5 mg/kg, fentanyl, 3 micro gram/kg, and muscle relaxation was achieved with vecuronium, 0.1 mg/kg. After tracheal intubation, the lungs were ventilated with oxygen-air mixture and isoflurane (0.5–1.0% end-tidal). Repeated doses of fentanyl and vecuronium were administered when clinically indicated but not during the sampling periods. A radial arterial catheter was inserted for monitoring mean arterial pressure (MAP) and for blood gas sampling. A 5.25-inch, 16-gauge retrograde JVBC (Angiocath(R), Becton and Dickinson, Sandy, Utah) was placed using an aseptic technique described previously. [1] The position of the JVBCs was confirmed radiographically with the tip of the catheter seen at the level of and medial to the mastoid process. The catheter was connected to a pressure flush system (normal saline at 3 ml/h).

After the start of surgery and during stable anesthetic and surgical conditions, JVBC samples were drawn at 2, 4, and 10 ml/min using a calibrated pump (Harvard Pump model 900). An arterial blood sample was obtained simultaneously. This sampling was performed during moderate and mild hyperventilation (Pa^CO2, 26.0 +/- 0.5 and 33.0 +/- 0.5 mmHg) with a 15-min period of stabilization at each Pa^CO2(as reflected by unchanged end-tidal carbon dioxide). The order of hyperventilation was randomized. Blood samples were immediately analyzed using an automated blood gas analyzer (Nova Biomedical, Waltham, MA).

The cerebral arteriovenous oxygen content difference (AVD^O2) was calculated using the equation: AVD^O2= hemoglobin concentration (Hgb) x 1.39 x (Sa^O2- Sjv^O2)+ 0.003 x (Pa^O2- Pjv^O2) vol%, where Sa^O2is arterial oxygen saturation, Sjv^O2is the jugular bulb oxygen saturation, Pa^O2is arterial oxygen tension, and Pjv^O2is the jugular bulb oxygen tension. The Sjv^O2data were analyzed using two-way analysis of variance (ANOVA) for repeated measures. When significance was found, a post hoc test (Fisher's least significance difference) was used to delineate the differences. A P value < 0.05 was considered statistically significant.

The main findings of the study are shown in Table 1. There was no change in the patients' temperature, heart rate, MAP, or Hct during the study period. All JVBCs were inserted without complication and found to be in satisfactory position, with the tip at the level of and just medial to the mastoid bone.

Faster rates of withdrawal (4 and 10 ml/min) resulted in significantly higher Sjv^O2and lower AVD^O2values, which were evident at both levels of Pa^CO2. The magnitude of increase in Sjv^O2at 10 ml/min (compared with the rate of 2 ml/min) was greater during moderate hyperventilation than during mild hyperventilation (9.09 +/- 2.55% vs. 6.73 +/- 1.25%). This difference did not reach statistical significance.

In this study, we have shown that Sjv^O2measurements depend on the speed with which blood is drawn from the JVBC, with faster rates yielding higher Sjv^O2values.

Various studies have suggested that the measurement of Sjv^O2may be useful in the treatment of those patients at risk for neurologic injury. [1–6] Recent attempts at determining the accuracy of continuous fiberoptic oximetry of jugular bulb venous blood have yielded conflicting results. Although readings from the fiberoptic oximetric catheter compared favorably with blood gas analysis of simultaneously drawn samples from the catheter in the intensive care unit, [7–8] at least one study showed a poor correlation during cardiopulmonary bypass. [9] The reasons for this inconsistency are unclear and may include the presence of anesthetic gases, head positioning, the type of catheters studied, or the rate at which the blood is withdrawn from the JVBC. In this study, we have attempted to highlight a possible source for this discrepancy.

Blood drained to the internal jugular veins from the sigmoid sinus is to a minor degree admixed with extra-cerebral blood. Sources of extracranial blood are the emissary and frontal veins draining into the superior sagittal sinus, the cavernous sinus connection to the sigmoid sinus and the superior bulb of the internal jugular vein through the petrosal sinuses, and the cavernous sinus communication with the ophthalmic veins and pterygoid plexus. Despite all these connections, there is good evidence that all but a small proportion (3–7%) of the jugular venous bulb blood drains the cerebral hemispheres. [10] However, below the jugular bulb, the internal jugular vein receives facial and retromandibular venous blood, causing a substantial extracranial admixture. Jackobsen et al. [11] have shown that if the JVBC tip is placed > 2 cm from the bulb, major extracerebral contamination can occur. It is possible that faster rates of withdrawal of blood may increase the blood admixture from the extracranial contributory veins, giving a falsely elevated Sjv^O2reading.

The increase in Sjv^O2value, although statistically significant, may not in the majority of patients be clinically significant. However, because AVD^O2or its reciprocal of cerebral blood flow equivalent (CBFE = 1/AVD^O2) are used as a measure of brain energy metabolism and are used to guide therapy in the neurologically injured patient, correct estimation of these variables is important. For example, if the Sjv^O2value is overestimated by 5%(which may not be clinically significant at Sjv^O2of 65%), the error in calculating the AVD^O2(assuming an arterial oxygen saturation of 100%) can be as much as 14%. The error is further increased during hyperventilation and at higher cerebral metabolic rates. During marked hyperventilation, cerebral blood flow (CBF) is reduced, and therefore, the risk of extracranial contamination increases. This is evident from our study wherein the errors in Sjv^O2(and AVD^O2) appeared to be greater at the lower Pa^CO2. Similarly, with high cerebral metabolic rates (if CBF is normal or low) because the difference between cerebral arterial and venous blood oxygenation is great, the error is further highlighted. The estimation errors in AVD^O2and CBFE are particularly important when the effects of drugs or therapeutic maneuvers are being monitored. In some situations, the true change in AVD^O2may be smaller than the measured ones because of contamination with extracerebral blood. Although not studied, other measured variables such as glucose consumption or lactate production can be expected to be affected in a similar manner. Calculation of cerebral metabolic rate for glucose or oxygen (CMR^O2) using the Fick's Principle (CMR^O2= CBF x AVD^O2) critically depends on accurate determination of Sjv^O2.

With the increase in the use of continuous fiberoptic monitoring of Sjv^O2and as all available catheters require calibration against a simultaneously drawn blood sample, it is equally important to use a blood sample that is as representative as possible of the true Sjv^O2. The inconsistent rate of blood sample withdrawal during recalibration may account for some of the inaccuracies observed in the use of continuous fiberoptic jugular venous oximetry.

We should point out that our study did not prove that the rate of 2 ml/min is free of extracranial blood contamination. In choosing these rates of withdrawal, we try to use rates that are feasible and practical. Although a rate of 1 ml/min may further improve accuracy, it is not practical and does not allow rapid assessment of changes.

In conclusion, the rate with which blood is withdrawn from JVBC affects Sjv^O2and CBFE values. To avoid overestimation of Sjv^O2and CBFE, blood samples should be drawn slowly, at approximately 2 ml/min, particularly when the patient is hyperventilated or CBF is otherwise reduced pharmacologically (e.g., during barbiturate therapy).