Introduction
Jugular bulb venous monitoring provides information about global cerebral hemodynamics and metabolism and has become an important tool in the clinical management of neurologically injured patients. Other techniques to measure cerebral oxygenation include near infrared spectroscopy (NIRS), which noninvasively detects changes in blood hemoglobin concentrations associated with neural activity and multiparameter intraparenchymal probes that are used to measure brain tissue and cerebrospinal fluid PO 2 , PaCO 2 , and pH. This chapter focuses on jugular bulb venous oximetry.
Anatomy
The venous drainage from the brain forms six major cranial sinuses (superior and inferior sagittal, occipital, right and left transverse, and the straight sinus). These venous channels, located between the dura mater and the periosteum lining the cranium, are devoid of valves or muscle in their walls. The final common pathway for the majority of blood returning from the brain is through the right and left sigmoid sinus, which curves down the posterior fossa, over the mastoid portion of the temporal bone, and then runs forward to form the right and left internal jugular veins in the posterior part of the jugular foramen.
The internal jugular vein (IJV) begins as a small dilation called the jugular bulb at the jugular foramen, and runs down the neck in the carotid sheath to form the brachiocephalic vein by joining the subclavian vein behind the medial end of the clavicle. Within the carotid sheath, the IJV is lateral to the vagus nerve and the carotid artery.
Posterior to the vein lie the transverse processes of the cervical vertebra, the cervical plexus, phrenic nerve, vertebral vein, the first part of the subclavian artery, and the thoracic duct toward the left. Anterolateral to the vein are superficial cervical fascia, platysma, transverse cutaneous nerve, deep cervical fascia, and the sternocleidomastoid muscle. The lower end of IJV drains into the right atrium.
The jugular bulb is the final common pathway for venous blood that drains from the cerebral hemispheres, cerebellum, and brainstem. Therefore the jugular venous oxygen saturation (SjvO 2 ) reflects the balance between supply and consumption of oxygen by the brain. Contribution of the extracranial circulation is estimated to range from zero to 6.6%. This is exacerbated if the blood is contaminated with blood from the facial vein that joins the IJV a few centimeters below the jugular bulb.
Physiologic Basis of SjvO 2 Interpretation
Jugular bulb oximetry has evolved from a combination of old and new technologies. The first measurements taken from the jugular bulb were used to calculate cerebral blood flow (CBF) using the Fick principle.
Cerebral metabolic rate of oxygen (CMRO 2 ) is calculated from the CBF and the difference between the arteriovenous difference in oxygen (AVDO 2 ) using the equation:
CMRO 2 = CBF × ( arterial oxyge n content minus jugular venous bulb oxygen content )
The oxygen content can be determined using co-oximetry, or calculated using the equations:
Arterial oxygen cont ent = [ ( Hgb × 1.34 × SaO 2 ) + ( PaO 2 × 0.0031 ) ]
Jugular venous oxyg en content = [ ( Hgb × 1.34 × SjvO 2 ) + ( PvO 2 × 0.0031 ) ]
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