Jugular Venous Oximetry

Anatomy of Venous Drainage of the Brain

The venous drainage of the brain flows into superficial (external or cortical) and deep (internal) cerebral veins. The superficial cerebral veins drain predominantly into the superior sagittal sinus and partially to the cavernous sinus or sphenoparietal sinus. The deep cerebral veins, on the other hand, drain into the straight sinus. Both the superior sagittal sinus and straight sinus then join to form the confluence of sinuses where the two lateral sinuses originate before continuing laterally to the right and left transverse (sigmoid) sinuses and right and left jugular bulbs. Although the dural sinuses join at the confluence of sinuses, mixing is incomplete. The blood from the superior sagittal sinus (venous drainage from cortical area) is more likely to drain into the right lateral sinus, whereas the blood from the straight sinus (venous drainage from subcortical area) flows into the left side. In the cavernous and circular sinuses, which are the two sinuses at the base of the brain, the venous blood drains freely and equally through the petrosal sinuses to both right and left jugular bulbs. Each side of the internal jugular veins comprises two dilated parts: the superior and inferior jugular bulbs. The superior bulb is a sampling site for jugular venous oxygen saturation (SjvO ₂ ) because it is less contaminated by extracerebral venous return.

Jugular Venous Oxygen Saturation (SjvO ₂ )

Jugular venous oxygen saturation directly reflects the global balance between cerebral oxygen supply and metabolic consumption per the Fick principle.

When CMRO ₂ = cerebral metabolic rate for oxygen, CBF = cerebral blood flow, CaO ₂ = arterial oxygen content, and CjO ₂ = jugular venous oxygen content.

Cerebral oxygen supply (delivery):

Cerebral oxygen demand (consumption):

Oxygen content is directly proportional to oxygen saturation with minor contributions from dissolved oxygen (0.003 × PO ₂ ) with hemoglobin being the constant. Therefore, the arterial-jugular oxygen content difference (AjvDO ₂ ) is largely determined by the gradient between SaO ₂ and SjvO ₂ .

From the preceding equation:

If CMRO ₂ remains constant, changes in AjvDO ₂ are inversely proportional to CBF. If AjvDO ₂ decreases, we assume that oxygen supply is excessive compared to demand (hyperemic state). In contrast, if AjDO ₂ increases, the brain is extracting more oxygen when the supply is too low for the metabolic requirement (hypoperfusion state). If CMRO ₂ increases without an appropriate increase in CBF, AjvDO ₂ is widened; thus, if hemoglobin and SaO ₂ remain constant, SjvO ₂ is an indicator of cerebral oxygen demand. In the clinical setting, the reduction in SjvO ₂ is an indicator of imbalance between cerebral oxygen demand and supply, which is either a relative increase in CMRO ₂ or a reduction in CBF. Assuming constant CMRO ₂ , low CBF may be due to hypotension or hypocarbia.

Jugular Bulb Monitoring Equipment

Jugular venous oximetry can be monitored by either intermittent sampling (using standard intravascular catheters) or continuous monitoring (using fiberoptic catheters). The fiberoptic technology used in a spectrophotometric catheter allows continuous displays of SjvO ₂ values based on the differential absorption of light at the different wavelengths between oxyhemoglobin and deoxyhemoglobin. The available catheters are the Baxter–Edwards system (Edslab Sat II, Baxter Edwards Critical Care Division, Irvine, California, USA) and the Abbott system (Opticath Oximetrix, Abbott Critical Care System, Abbott Park, Illinois, USA). Both catheters have two lumens, one for blood sampling and the other one has two optical fibers transmitting lights to and from the venous blood. Although the basic principle is similar, the Abbott system uses three wavelengths of light for reflectance spectrophotometry instead of two wavelengths allowing the automatic measurement of both hemoglobin concentration and oxygen saturation and minimizing artifact interference. This feature is important in patients with rapid changes in hemoglobin concentration, e.g., during cardiopulmonary bypass.

Jugular Bulb Catheter Placement

Unlike central line placement, the operator stands in the axillary space facing the cephalad position to facilitate retrograde placement of the jugular venous catheter. The anatomical relationship between the jugular vein and carotid artery is visualized, and the jugular vein is punctured under ultrasound-guided technique at the apex of the triangle formed by the sternal head, the clavicular head of the sternocleidomastoid muscle, and the clavicle. However, in contrast to that of the central line placement, the needle, the guidewire, and the catheter are pointed in the cephalad direction. The final position of the catheter needs to be as proximal to the jugular bulb as possible to facilitate blood sampling but minimize extracranial blood contamination. Patients may be placed in the Trendelenburg position to improve the visualization of the internal jugular vein if they do not have poor intracranial compliance.

We routinely use a 16G, 5.25-inch-long venous cannula for this procedure in the operating room. Alternatively, a fiberoptic catheter can also be placed using the Seldinger technique. Here a J-shaped guidewire is passed into the jugular bulb and advanced no further than 2–3 cm beyond the needle insertion site if needed. In adults, an introducer sheath (5–6 French size) is then inserted before an oximetry catheter (4.5–5 French) is advanced until resistance is met at the jugular bulb (approximately 15–20 cm in adults). Resistance felt is due to a slight bend in the vessel lumen distal to the jugular foramen just below the skull exit. In the awake patient, a sensation in the ipsilateral jaw or ear may be noted when the catheter reaches this point. The catheter is then pulled back about 0.5–1 cm distally to avoid continuously abutting the roof of the jugular bulb. Alternatively, the catheter tip can be inserted at a distance measured from the insertion point to the level of mastoid process (approximately the level of the jugular bulb) until resistance is met.

The desired final position of the catheter is as close to the roof of the jugular bulb as possible to minimize extracranial venous blood contamination. As little as a 2-cm difference can result in as high as 10% contamination. Radiographic assessment can be used to confirm position of the catheter tip and to detect any kinking, and it is commonly practiced in the ICU. On a lateral X-ray of the skull, the catheter tip should be just above the lower border of C1, medial to the mastoid process. An anteroposterior (AP) neck radiographic view can also be used as an alternative. On AP view, the tip should be more cephalad to a line extending from the atlantooccipital joint and caudal to the inferior margin of orbit. The catheter tip should be cephalad to an imaginary line connected between the tips of mastoid processes. We do not routinely use X-ray to confirm jugular venous catheter placement.

Side of Jugular Bulb Monitoring

Approximately 70% of the blood from one side of the internal carotid artery drains to the ipsilateral jugular bulb, and 30% is drained contralaterally. Although each side of the jugular bulbs receives venous drainage from both cerebral hemispheres, generally most patients have a dominant side (usually the right). We recommended that SjvO ₂ should be obtained from the patient's dominant side, especially in patients with bilateral brain injury, unless it interferes with surgical procedure. The cerebral venous dominant side can be determined by several imaging techniques, including examining the venous caliber size on the venous phase of a cerebral angiogram, size of internal jugular vein by ultrasonography, or computed tomography assessment of the jugular foramen size. Moreover, the dominant side can be determined by the effect on ICP after compression of the internal jugular veins. A greater increase in ICP is an indicator for a more prominent venous drainage.

It is controversial in patients with unilateral brain lesion as to which side the jugular venous catheter should be placed if the affected side is not the dominant side. In a study comparing between SjvO ₂ from the jugular veins of the 32 patients with traumatic brain injury (TBI), the authors reported only 8 (25%) patients with the difference of SjvO ₂ < 5% between right and left sides, while 15 (47%) had a maximal right-to-left difference of >15%. Another study by Beards et al. also reported that asymmetry between right and left SjvO ₂ > 10% occurred in 65% of monitored time. Therefore, it is reasonable to choose the dominant side as it has greater venous flow for jugular venous catheter insertion.