Mixed and central venous oxygen saturation monitoring in the ICU

Haemodynamic resuscitation should target goals that reflect the tissue oxygen needs of an individual patient. Venous oximetry may be such a tool. Oxygen saturation of blood in the pulmonary artery contains venous blood from the whole body and is referred to as mixed oxygen saturation (SvO2). Measurement of oxygen saturation in blood obtained from a central venous catheter is referred to as central venous oxygen saturation (ScvO2). Both values are not identical since a catheter placed into the superior vena cava only represents venous blood draining the upper body. While it is not possible, in the clinical setting, to predict SvO2 from ScvO2, changes in SvO2 are adequately mirrored by changes in ScvO2. Post-operative patients and patients admitted to intensive care with a low ScvO2 show a higher morbidity and mortality. Early goal-directed therapy (EGDT) combines several haemodynamic goals into a treatment algorithm, including a ScvO2 target. However, recent studies do not support the systematic use of this protocolized approach. A normal value of SvO2 or ScvO2 saturation does not always exclude tissue hypoxia, since it is not possible to identify an inadequate oxygen supply in single organs. A further limitation of this technique is that organ dysfunction can progress, or serum lactate increases, despite normal or even increased venous oximetry values.

Jugular venous oximetry

The measurement of saturation of venous blood as it drains out of brain by sampling it from the jugular bulb provides us with an estimate of cerebral oxygenation, cerebral blood flow and cerebral metabolic requirement. Arterio-jugular venous difference of the oxygen content (AVDO2) and jugular venous oxygen saturation (SjVO2) values per se helps clinicians in identifying the impairment of cerebral oxygenation due to various factors thereby prompting implementation of corrective measures and the prevention of secondary injury to the brain due to ischaemia. SjVO2 values are also used for prognostication of patients after traumatic brain injury and in other clinical situations. Sampling and measuring SjVO2 intermittently or continuously using fibreoptic oximetry requires the tip of the catheter to be placed in the jugular bulb, which is a relatively simple bedside procedure. In the review below we have discussed the relevant anatomy, physiology, techniques, clinical applications and pitfalls of performing jugular venous oximetry as a tool for measurement of cerebral oxygenation.