Proper oxygen supply and metabolism are the main priority in the therapy of critically ill patients and are essential for safe anaesthesia. All the methods commonly used for this purpose have their assets and limitations; therefore, studies on new useful instruments are being carried out.

The standard element of anaesthesiological monitoring is pulse oximetry based on near infrared spectroscopy (NIRS). Thanks to the use of two wavelengths – 660 nm and 940 nm and analysis of optic density of blood elements, pulse oximetry measurements provide information about haemoglobin oxygen saturation in arterial blood [1].  

Spectroscopy involves the emission of near infrared photons and their penetration through the tissues where they are partially absorbed or reflected and finally detected by the light sensor measuring the amount of returning light. Spectroscopy was described over 150 years ago; a pulse oximeter was introduced for clinical use in the 70-ties of the previous century [2].

A relatively new device for monitoring oxygen metabolism is a cerebral oximeter used to measure regional cerebral saturation (rSO2).

Spectroscopy for monitoring cerebral oxygenation was introduced in hospitals towards the end of the 20th century [3]. The key element of a cerebral oximeter is an electrode consisting of several parts. One of them is the light-emitting diode (LED) emitting two light bundles of various wavelengths, i.e. 735 nm and 810 nm. The other relevant elements are the superficial and deep detectors, situated 3 and 4 cm from the LED. This distance between the detector and LED enables deeper tissue penetration, prevents disorganized dispersion of photons and provides better recordings by receiving devices [4]. Moreover, it eliminates extra-cerebral artefacts and minimizes the effects of skull bones on rSO2. Regional oxygen saturation is measured in the tissues 3-5 cm beneath the sensor (Fig. 1).

An important stage of spectroscopy is the absorption of photons by biological tissues. In NIRS modules, photons are absorbed by haemoglobin particles. The measurement of the type of each haemoglobin particle enables to define the amount of oxygen each particles carries, whereas data concerning the kind and extent of absorption returning to the detector reflect the amount of desoxyhaemoglobin and total haemoglobin, which calculations of rSO2 are based on.

The cerebral oximeter measures total optic density of the tissue beneath the detector. Additionally, it is based on the assumption that 70-80% of tissue blood is constituted by venous blood, 15-20% by arterial blood and only 5% by capillary blood. Hence, the final value of regional cerebral saturation is predominantly venous blood saturation (70-80%) in tissues beneath the detector.

Venous blood saturation is an important marker reflecting the balance between oxygen supply and its consumption in tissues. According to the current guidelines for septic shock treatment, proper saturation of mixed venous blood (SvO2) should be ≥65% or saturation in the superior vena cava (SvcO2) ≥70%, which emphasizes the relevance of measurements of venous blood saturation [5].

To make the measurement of regional cerebral saturation a sensitive marker of cerebral hypoxia, the range of rSO2 reference values was introduced. The proper cerebral saturation is within 65±9% [4].

The alarming signals of oxygen deficit are cerebral rSO2 <40%, which indicates general ischaemia of the organ. The cerebral rSO2 values in the range of 40-50% may indicate reduced cerebral flow and the need to undertake actions to increase the oxygen supply to the brain and cerebral flow or to reduce its metabolism or to initiate the anti-oedematous therapy.

The cerebral oximeter, although primarily used for metabolism monitoring and prevention of cerebellum hypoxia, is also applied to measure regional saturation of other organs, which is possible thanks to slight modifications in the cerebral electrode enabling its use for rSO2 measurements in peripheral tissues. Numerous clinical studies have assessed the relevance of measurements of regional saturation of peripheral tissues in various diseases.

The normative rSO2 values for peripheral tissues other than the cerebral ones have not been explicitly defined. For those tissues, the rSO2 change in time is important. The physiological rSO2 values for peripheral tissues are usually higher than for the brain. Contrary to peripheral tissues, the brain is characterized by high perfusion, quicker metabolism and high oxygen consumption – 25%. The norm of cerebral flow under resting conditions is 50 mL min-1/100 g brain, (i.e. 700-900 mL min-1 in an adult, higher – in a child); cerebral oxygen consumption is 3.2 mL min-1 /100 g tissue (higher in a child) [6].

In regional tissue saturation measurements, the cerebro-splanchnic oxygenation ratio (CSOR) should be considered, which is the quotient of regional cerebral saturation and regional splanchnic saturation (CSOR= cerebral rSO2 / splanchnic rSO2).The clinical value of this quotient has already been demonstrated [7].

Under physiological conditions, CSOR may be lower than 1 due to lower value of cerebral rSO2. In shock, this ratio is reversed, i.e. higher than 1, which is associated with centralization of circulation and provision of perfusion of organs essential for survival. In shock, the cerebral flow and cerebral rSO2 are higher than for peripheral tissues, hence CSOR exceeds 1. Thanks to the therapy administered, CSOR = 1 is first achieved and the desirable moment is when CSOR <1 (Fig. 2).

During rSO2 measurements, absolute values as well as direction of rSO2 changes in time are important. There are reports comparing the clinical value of a regional cerebral oximeter and other  analysers of oxygen metabolism and consumption. It is stressed that regional oximetry is non-invasive, provides continuous measurements in time and is a bedside procedure. By measuring rSO2 of the tissue 3-5 cm beneath the detector, regional oximetry provides information evidencing changes in the currently examined organ. These changes may not be reflected, e.g. by measurements of lactate levels as their increased concentration is the overall index informing about anaerobic metabolism somewhere in the organism, whereas the rSO2 measurement is locally specific.

The comparison of values obtained using cerebral oximetry and classical global indices of oxygen metabolism, i.e. saturation of mixed venous blood (SvO2) or saturation of blood from the bulb of the internal jugular vein (SjO2) provides interesting data. It is emphasized that SjO2 is an invasive measurement – the cannula is introduced to the internal jugular vein retrograde until its tip reaches the bulb of the internal jugular vein. The measurement reflects overall oxygen metabolism for the entire brain hemisphere (2/3 of SjO2 is saturation of venous blood from the hemisphere on the same side +1/3 from the opposite hemisphere) [4]. Moreover, several studies have demonstrated positive correlation between cerebral rSO2 and SvO2 [8].

Compared to transcranial Doppler or EEG, measurements of cerebral rSO2 are more sensitive and their predictive value for detection of all cerebral incidents is higher . [9, 10].

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REFERENCES

1.    Z prac Zarządu i Sekcji Anestezjologii UEMS. http://www.ptaiit.org/news,news,8.html.

2.    Wahr J, Tremper K, Samra S, Delpy D: Near infrared spectroscopy: theory and applications. J Cardiothorac Vasc Anesth 1996; 10: 406-418.

3.    Jöbsis F: Noninvasive, infrared monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters. Science l977; 198: 1264-l267.

4.    Machała W, Śmiechowicz K, Patyk M, Lesiak P: Wybrane metody monitorowania czynności ośrodkowego układu nerwowego w sali operacyjnej. Anestezjol Inten Ter 2005; 37: 268-273.

5.    Dellinger R, Levy M, Carlet J, Bion J, Parker, Jaeschke R, Reinhart K, Angus DC, Brun-Buisson C, Beale R, Calandra T, Dhainaut JF, Gerlach H, Harvey M, Marini J, Marshall J, Ranieri M, Ramsay G, Sevransky J, Thompson B, Townsend S, Vender J, Zimmerman J, Vincent J: Surviving Sepsis Campaign: International guidelines for management of severe sepsis and septic shock: 2008. Intensive Care Med 2008; 34: 17-60.

6.    Udomphorn Y, Armstead W, Vavilala M: Cerebral blood flow and autoregulation after pediatric traumatic brain injury. Pediatr Neurol 2008; 38: 225-234.

7.    Fortune P, Wagstaff M, Petros A: Cerebro-splanchnic oxygenation ratio (CSOR) using near infrared spectroscopy may be able to predict splanchnic ischaemia in neonates, Intensive Care Med 2001; 27: 1401-1407.

8.    Simsic, J.Bradley S, Stroud M: Cerebral oximetry as noninvasive indicator of mixed venous oxygen saturation in newborns after cardiac surgery. Coll of Cardiol 2003; 41: 494.

9.    Tortoriello T, Stayer S, Mott A, McKenzie E, Fraser C, Andropoulos D, Chang A: A noninvasive estimation of mixed venous oxygen saturation using near-infrared spectroscopy by cerebral oximetry in pediatric cardiac surgery patients. Paediatr Anaesth 2005; 1: 495-503.

10.    Austin E, Edmonds H, Auden S, Seremet V, Niznik G, Sehic A, Sowell M, Cheppo C, Corlett K: Benefit of neurophysiologic monitoring for pediatric cardiac surgery. J Thorac Cardiovasc Surg 1997; 114: 707-715.

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address:

*Marcin Owczarek,
Oddział Kliniczny Anestezjologii
i Intensywnej Terapii dla Dzieci
Szpitala Uniwersyteckiego w Bydgoszczy,
Collegium Medicum w Bydgoszczy
ul. Marii Skłodowskiej-Curie 9, 85-095 Bydgoszcz
e-mail: owczarekmarcin@o2.pl;
tel.: +48 607 529 588

received: 12.05.2009
accepted: 03.11.2010