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Excerpt
Cerebral blood flow (CBF) is a key variable to a better understanding of the pathological process responsible for cerebral ischaemia, and is of great interest for diagnostic and therapeutic purposes in the management of brain injury. The classical Kety-Schmidt method using nitrous oxide as a tracer, and subsequent modifications of the technology using radioisotopes such as Kr85 or Xe133 have given interesting and fundamental information on the physiology of cerebral circulation and metabolism [1]. Many investigations have focused on acute brain injury [2-4]. However, a more widespread clinical application has been limited by several disadvantages, including elevated cost, exposure to radioisotopes, and requirement for complicated analyses and calculations. Furthermore, these techniques cannot be performed easily at the bedside. More sophisticated techniques, such as single-photon emission computed tomography, positron emission tomography, angiography with magnetic resonance imaging or stable Xenon-enhanced computed tomography have been developed recently. These represent powerful research tools, but again do not yield bedside measurements [5].
The exploration of the cerebral circulation in the intensive care unit (ICU) requires fast, easy and accurate bedside procedures at a relatively low cost. The method must be reproducible, allowing repeated or continuous measurements. Recently, several techniques meeting these requirements have been developed, including transcranial Doppler, jugular bulb oximetry, near-infrared spectroscopy or laser Doppler [6-8]. Unfortunately, these techniques do not measure true flows but indices that can be related to flow changes.
Jugular blood flow: The estimation of venous blood flow by continuous thermodilution is a relatively simple technique that has been initially developed to measure coronary sinus blood flow [9]. This method has also been occasionally used to estimate CBF by measuring jugular blood flow [10-12].
Because of the confluence of sinuses into the transverse sinus ending in the jugular bulb, the major part of the CBF passes through both internal jugular veins, while venous outflow via the emissary veins and vertebral venous plexus is relatively small [13, 14]. There is contamination from the contralateral hemisphere and each bulb represents roughly one third of the contralateral hemisphere drainage in normal subjects [13]. In addition, an average of 2.7% (range 0-6.6%) of the blood in the jugular bulb is collected from extracerebral structures [13]. Therefore, jugular blood flow is only slightly contaminated by extracerebral blood and can be considered as a real measure of global CBF [5].
Jugular venous blood flow is measured using a thermodilution catheter (CCS-7U-90B 7 Fr, Cordis-Webster, Baldwin Park, CA) [15]. The catheter is retrogradely inserted in an internal jugular vein with the tip placed in the jugular bulb [16]. Body temperature (tempB) is recorded from the catheter thermistors before injection. Then the indicator (isotonic saline), at room temperature, is injected at a constant rate through the distal lumen into the jugular bulb. During indicator infusion, the external thermistor measures the temperature of the blood-indicator mixture (tempM), and the internal thermistor measures the temperature of the indicator (tempI). Flow in the jugular bulb(JBF) is calculated as: Equation 1 where FI is the indicator flow rate (38 ml min−1) and C a constant based on the thermal properties of blood and indicator (1.10 for isotonic saline).
To compare measured values to those obtained with other methods, CBF is expressed in ml min−1 100 g−1 of brain tissue. JBF which reflects the venous outflow of one hemisphere is doubled and divided by brain weight 100−1:Equation 2 where the brain weight is estimated as k× height (cm) with k = 8.3 for male and k = 8.0 for female [17].
