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SjO2

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Cerebral venous oxygen saturation monitoring (SjO2)

Anatomy

  • Virtually all of the blood from the brain drains into the internal jugular veins

  • Blood from subcortical areas tends to flow into the left internal jugular vein while blood from the cortical areas tends to drain into the right

  • Approximately 2/3 of the blood in the internal jugular vein is thought to come from the ipsilateral hemisphere and 1/3 from the contralateral hemisphere

  • Contribution of the extra-cranial circulation is estimated to range from 0-6.6%. This is exacerbated if the blood is contaminated with blood from the facial vein which joins the internal jugular vein a few cm. below the jugular bulb.

Physiology

  • The Fick equation describes the relationship between cerebral blood flow (CBF), arterial (CaO2) and venous oxygen content (CjO2) and cerebral metabolic rate for oxygen (CMRO2):
    CMRO2 = CBF x CaO2-CjO2
    ̃ CMRO2 » CBF x 1.34 x Hb(SaO2-SjO2) (assuming dissolved oxygen is negligible)
    ̃ SjO2 µ CBF/CMRO2 (assuming Hb and SaO2 are constant) 

  • Thus SjO2 is dependent predominantly on SaO2, CMRO2 and CBF.

  • Lower limit of normal range thought to be about 50-54% with an upper limit of 75%

  • As with all global measures of cerebral oxygenation a normal SjO2 does not exclude localized areas of cerebral ischaemia with associated areas of luxury perfusion.

Insertion

  • Position the patient head down or horizontal (do not allow the ICP to exceed 20 mmHg)

  • Cannulate internal jugular vein between the two heads of sternocleidomastoid or at the level of the cricoid ring

  • Advance catheter into the jugular bulb (about 12-15 cm)

  • Check position of tip of catheter with lateral or AP neck X-ray (tip should be above the C1/C2 intervertebral disc)

Measurement

  • SjO2 can be measured continuously using a fibreoptic catheter and reflectance oximetry or intermittently by drawing samples from the catheter and measuring saturation with a co-oximeter (NB arterial blood gases machines do not provide sufficiently accurate readings)

  • If the intermittent method is used care must be taken with sampling to prevent aspiration of blood from the facial vein (1-2 ml/min is optimal).

Interpretation

Low SjO2

  • Increased brain oxygen extraction as a result of

    • Systemic arterial hypoxia

    • Low CBF from hypotension, vasospasm or intracranial hypertension

  • Increased brain metabolic requirements due to:

    • Pyrexia

    • Fits (may be subclinical)

Increased SjO2

  • Hyperaemia

  • Failure of oxygen extraction

  • Grossly reduced cerebral blood flow due to high ICP with shunting of arterial blood (pre-terminal event)

Clinical uses

  • Part of multi-modality monitoring of brain injured patients

  • As a guide during hyperventilation

    • Although hyperventilation reduces intracranial pressure it is important that this does not occur at the expense of cerebral ischaemia. The PaCO2 at which this occurs varies between patients depending on cerebral perfusion pressure, baseline PaCO2 and other factors such as atherosclerosis

Complications

  • Carotid artery puncture (1-4.5%)

  • Thrombosis (incidence of subclinical thrombosis may be as high as 40%)

  • Haematoma formation

  • Raised ICP (rare)

  • Infection

Further reading

Macmillan CSA, Andrews PDA. Cerebrovenous oxygen saturation monitoring: practical considerations and clinical relevance. Intensive Care Medicine, 2000; 26:1028-1036

 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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