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Jugular bulb oximetry is the first continuous and bedside brain monitoring method estimating cerebral perfusion adequacy. The fact that most of the regulatory mechanisms of cerebral perfusion are failing in patients suffering from severe neurologic lesions explains the growing interest in monitoring cerebral perfusion adequacy in these critically ill patients. Moreover, brain ischaemia has been observed in not less than 80% of all patients dying from severe head injury (*68), illustrating the frequent occurence of brain ischaemia after severe head injury. An important mainstay of the intensive management is accordingly to avoid the occurence of cerebral ischaemia. A distinction is often made between primary and secondary ischaemia, with primary ischaemia considered as resulting from the ischaemic insults that occurred at the scene of the accident and secondary ischaemia as the ischaemic insults occuring during the further intensive care management. To avoid secondary ischaemia during the intensive care management, monitoring of the cerebral perfusion state seems obligatory.
In this work, a new cerebral monitoring technique, providing information concerning the actual cerebral perfusion, was described. The monitoring technique itself and the validation of this technique are extensively reported. The clinical use of this new parameter, and its subsequent place in the management of severe traumatic brain injury is analyzed in a homogenous group of 50 severely head injured patients. Starting in january 1990, we have an almost 5 years experience with jugular bulb oximetry monitoring and a total of almost 200 patients were treated, partially guided by jugular bulb oxygen saturation data. In january 1990, very few centers performed jugular bulb oximetry and most of them used it as part of study protocols, but nowadays any major neurotrauma center considers jugular bulb oximetry as an absolute part of cerebral monitoring in severe head injury. We have extended the indications for jugular bulb oximetry and were the first to report on its use in patients suffering from severe hepatic encephalopathy (*130,*131,*132,*133,*134). We also reported on it's use in patients suffering from subarachnoid hemorrhage and during cerebral aneurysm clipping (*135,*136). We are moreover convinced that in the future still more patients suffering from severe neurologic dysfunction such as hypoxic encephalopathy or severe stroke could benefit from this new monitoring technique.
The guarantee of an adequate cerebral perfusion is of extreme importance for all patients suffering from severe neurologic dysfunction. Bedside monitoring of the adequacy of cerebral perfusion can be performed by monitoring the arterial-jugular difference in oxygen content, referring to the Fick equation. This difference in oxygen content is however only intermittently available, and this is of course a major disadvantage when rapidly changing events are to be monitored. Thanks to recent technology, invasive measurement of the jugular venous O2 saturation became available. This technique may provide a continuous estimation of the cerebral perfusion adequacy, by monitoring the jugular bulb venous oxygen saturation.
Jugular bulb catheterization has been practiced for already a long time in study protocols using invasive cerebral blood flow measurements. Early measurements of jugular bulb oxygen content were made by repeated direct punctures of the jugular bulb. However, because of the technical complexity of repeated direct punctures of the jugular bulb, retrograde cannulation of the jugular vein with a percutaneously inserted catheter was subsequently favoured. Nevertheless, questions still arise as to the exactitude of using jugular bulb venous blood as representative of the cerebral venous effluens. Extracerebral contamination of the blood in the jugular bulb by the cavernous system should amount about 3 per cent. Errors attributable to mixing of the cerebral venous drainage with extracerebral venous blood should occur especially when CBF is low. Extracerebral contamination could be a good reason to prefer trend analysis of the SjO2 values, above the absolute values of SjO2, as a guide for the clinical management.
A second methodological shortcoming of jugular bulb catheterization is the problem of an unilateral sampling. This is indeed a problem because there is a considerable mixing of the venous effluens from both hemispheres, as two thirds of the blood supplied to one hemisphere is drained through the ipsilateral jugular vein, whereas one third drains contralaterally. This implies that blood samples from one jugular bulb may not be representative for the drainage of the entire brain. Moreover, there are even recent reports concerning discrepancies in O2 saturation between the two jugular bulbs in a higher than suspected proportion of patients. As a third shortcoming, it should be emphasized that the information gained from the SjO2 is only a global information and that not all regional changes will be equally reflected in the final value of the SjO2. The knowledge of these 3 major methodological shortcomings will for sure not invalidate the use of SjO2-monitoring, but should call for vigilance.
There are a lot of questions about the pure technical aspects of jugular bulb catheterization and about the reliability of the fibre-optic catheter, retrogradely inserted in the internal jugular vein with its tip in the jugular bulb. As already mentioned, there are only very few clinical studies published on these technical aspects and the study group mentioned in this work is, by our knowledge, the largest one.
Jugular bulb oximetry is in fact an in vivo reflectance spectrophotometry technique. Reflectance spectrophotometry relies upon the absorbance and reflectance characteristics of red blood cells in motion. The wavelengths of light used for reflectance spectrophotometry are based upon the reflectance characteristics of oxyhaemoglobin and deoxyhaemoglobin within red blood cells. The principles of in vivo reflectance spectrophotometry were first applied to measure the mixed venous oxygen saturation in the pulmonary artery. To perform jugular bulb oximetry, we used the Opticath Oximetrix system, which is a three-wavelengths fibre-optic reflectance spectrophotometry system. The catheter used is a 4Fr, 40 cm length, double- lumen, polyurethane catheter, originally designed to be used in the umbilical artery of the neonate.
The puncture of the internal jugular vein in cephalad direction seemed quite easy and the only complication encountered was an inadvertent puncture of the carotid artery (in 10 of 165 patients). Although no adverse sequelae from these carotid artery punctures were observed, there was a potential risk of carotid artery spasm. We noticed that most problems in locating the internal jugular vein in the retrograde way occurred in the hypovolaemic patients and that assuring haemodynamic stability by adequate volume repletion might avoid difficult jugular bulb catheterization.
We adopted a puncture technique without head rotation, in contrast to other investigators, as head rotation in patients suffering from severe head trauma may create some problems. It is contraindicated in patients with suspected or known cervical spine injury and head rotation impairs cerebral venous drainage by obstruction of the internal jugular vein and may so potentially cause intracranial hypertension.
After catheterization, we confirmed the exact positioning of the catheter tip by x-ray of the cervical spine. We could experience that uncorrect positioning of the catheter tip could still occur after a 3 years experience and as we know that the correct positioning of the catheter tip is of crucial importance to avoid important extracerebral contamination, x-ray confirmation seems obligatory.
Assuring a correct positioning of the catheter tip in the jugular bulb during the further monitoring course appeared to be thé major problem of jugular bulb oximetry. In an attempt to reduce the frequent dislocations of the catheter tip, we maintained the introducing sheath to provide, if possible, more stability to the extremely flexible fibre-optic catheter.
All the small dislocations resulted in altered light intensity signals and were detected as light intensity alarm messages on the display screen of the fibre-optic system. Therefore, special vigilance is indicated for the maintenance of an exact light intensity signal during the whole monitoring course. But perhaps future developments of a specially designed "jugular bulb catheter" are needed to overcome definitively these positioning problems.
However, it is possible that all these technical problems occur as a result of the specific anatomy of the jugular bulb, so that in some patients we will always have technical problems, although special designed jugular bulb catheters would be available. A major argument in favour of this anatomical variable is that we observed in some patients a perfect longterm functioning of the jugular bulb catheter, while in others repeated technical problems were noticed.
The fibre-optic system used for this new monitoring technique was initIally designed to function in the pulmonary artery or in the umbilical artery. In these "arterial" settings its reliability has been widely proven. However, before promoting the use of jugular bulb oximetry, we had to make an evaluation of the reliability of the oximetrix system, when used in the "venous" setting of the jugular bulb. Due to the fact that jugular bulb oxygen saturation measurements are made in totally different conditions of blood flow, the reliability of the fibre- optic system can be questioned.
Whenever evaluating the reliability of the invasive oximetry system, it is extremely important to exclude possible sampling errors of jugular bulb venous blood. We repeatedly noticed that too rapid aspiration of venous blood samples from the jugular bulb resulted in extremely, falsely elevated saturation readings obtained by the cooximetry analysis. Too vigourous aspiration of blood out of the jugular bulb induces important extracerebral contamination of the blood sample and induces therefore a "false" difference between oxygen saturation measured on the blood sample and oxygen saturation measured in the jugular bulb. Great attention has althus to be paid to gentle, smooth aspiration of blood from the jugular bulb, to reduce the possibility of extracerebral contamination of the blood sample to almost zero.
It must also be reminded that only blood samples taken in periods of acceptable light intensity signals on the monitor screen could be retained for comparison with co-oximetry values, as in the presence of light intensity alarms, displayed O2 saturation values were unreliable. When checking the displayed saturation values combined with normal light intensity signals every 6 hours by cooximetry control, we found a quite acceptable accuracy of the oximetry system and this especially during the first 24 hours of monitoring. In our observations there was no need for repeated in vivo recalibrations, in contrast to other investigators, but we advise a regular checking of the fibre-optic measurements by repeated co-oximetry control (e.g. every 6 hours), as we did observe sudden drifts of fibre-optic catheter readings.