In health the majority of venous drainage from both cerebral hemispheres is via venous sinuses through the sigmoid sinus to the right internal jugular vein, although significant drainage may occur through the left. In 75 per cent of subjects there is free venous cross-flow between the transverse sinuses, and therefore there is little gradient in SjO2 between right and left jugular bulbs. When cerebral blood flow is normal, about 3 to 5 per cent of jugular bulb blood arises from extracranial venous sources.
If focal intracranial pathology is present, the patterns of venous drainage may change and differences in SjO2 between right and left become evident. Some investigators choose to monitor SjO2 from the side of the focal pathology, while others suggest using the effects of sequential unilateral compression of the internal jugular veins on intracranial pressure to guide the side for cannulation. In those patients with cerebral aneurysms undergoing angiography, the side of predominant venous drainage can be determined angiographically. If no difference is detected and in patients with diffuse brain injury, the right side is used for ease of insertion.
The jugular venous bulb is found immediately inferior to the jugular foramen and is the dilated initial part of the internal jugular vein. The internal jugular vein descends adjacent to the lateral aspect of the internal and common carotid arteries together with the vagus nerve, and all these structures are enclosed within the carotid sheath.
Blood from extracranial sources may lead to contamination and erroneously high SjO2 readings; thus the correct position of the catheter tip, i.e. above the top of the C2 vertebra, should be confirmed on a lateral cervical radiograph ( CD Figure.!).
CD Figure 1. Lateral radiograph of the head showing the ideal position of a jugular bulb catheter with its tip (indicated by the arrow) just below the jugular foramen at the base of the skull.
Contraindications to this technique include local infection or trauma, a bleeding diathesis, or pre-existing impairment to cerebral venous drainage. The effect of insertion of an SjO2 catheter on intracranial pressure is minimal if monitoring is confined to 7 days or less, and the risk of cerebral venous thrombosis is below 5 per cent when the catheter is continuously flushed with neutral saline (3 ml/h).
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