- Medical Science, Somanetics Corporation, Troy, USA
- Department of Neurosurgery, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA
- Department of Physiology, Wayne State University School of Medicine, Detroit, MI, USA
Erin A. Booth
Medical Science, Somanetics Corporation, Troy, USA
Department of Physiology, Wayne State University School of Medicine, Detroit, MI, USA
DOI:10.4103/2152-7806.73316© 2010 Booth EA This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
How to cite this article: Booth EA, Dukatz C, Ausman J, Wider M. Cerebral and somatic venous oximetry in adults and infants. Surg Neurol Int 27-Nov-2010;1:75
How to cite this URL: Booth EA, Dukatz C, Ausman J, Wider M. Cerebral and somatic venous oximetry in adults and infants. Surg Neurol Int 27-Nov-2010;1:75. Available from: http://sni.wpengine.com/surgicalint_articles/cerebral-and-somatic-venous-oximetry-in-adults-and-infants/
Background:The development in the last decade of noninvasive, near infrared spectroscopy (NIRS) analysis of tissue hemoglobin saturation in vivo has provided a new and dramatic tool for the management of hemodynamics, allowing early detection and correction of imbalances in oxygen delivery to the brain and vital organs.
Description:The theory and validation of NIRS and its clinical use are reviewed. Studies are cited documenting tissue penetration and response to various physiologic and pharmacologic mechanisms resulting in changes in oxygen delivery and blood flow to the organs and brain as reflected in the regional hemoglobin oxygen saturation (rSO2). The accuracy of rSO2 readings and the clinical use of NIRS in cardiac surgery and intensive care in adults, children and infants are discussed.
Conclusions:Clinical studies have demonstrated that NIRS can improve outcome and enhance patient management, avoiding postoperative morbidities and potentially preventing catastrophic outcomes.
Keywords: INVOS, near infrared spectroscopy, noninvasive monitoring, Hemodynamic management, CO2 reactivity, tissue oxygenation
Noninvasive, transcutaneous oximetry based on near infrared (NIR), diffuse reflectance spectroscopy has rapidly become a standard of care. While pulse oximetry is used to indicate arterial hemoglobin saturation, venous oximetry is used to measure tissue hemoglobin saturation in the capillary beds, reflecting O2 delivery and demand in the tissues. This simple, noninvasive technology is based on complex and in-depth chemical principles that have been refined over the last 80 years to provide unique and invaluable insights into oxygen biology.
The use of NIR light of 700–1000 nm wavelength for spectroscopic analysis of hemoglobin saturation in vivo is based on the fact that very few substances in tissue absorb NIR light, allowing for deeper light penetration. The absorption of electromagnetic radiation from X-ray to infrared wavelengths by chemical compounds is specific to the structure of the molecule being analyzed, with specific molecular bonds absorbing specific wavelengths of radiation. Accurate analysis of a compound normally requires that it be in a highly pure state without any contaminants, unless the contaminants do not absorb the specific wavelengths of radiation.
Metalloproteins with prophyrin rings are the only biologic structures that absorb much NIR light and the hemoglobin molar absorptivity at these overtone (harmonic) wavelengths is extremely low. The cytochrome enzymes absorb NIR light but the tissue concentration is an order of magnitude lower than hemoglobin. Additionally, myoglobin desaturation is limited, allowing the analysis of saturation to be hemoglobin specific.[
Millikan coined the term “oximeter” in the 1940s[
The early machines measured hemoglobin saturation in the blood just below the skin surface since visible light cannot penetrate tissue very far, but were surprisingly accurate for their simplicity. Though they were accurate, they could only provide information about capillary blood in the skin. It was not until the 1950s when Chance[
The spectra in
The extremely low level of absorption of NIR light by hemoglobin is the dominant factor in achieving accurate measurements, requiring low light intensity in order to avoid overwhelming the signal and a very low level of noise in the system to produce a high signal to noise ratio. While benchtop co-oximeters utilize multiple wavelengths to differentiate various dyshemoglobins in vitro, noise reduction remains the most important factor in improving accuracy and precision in vivo.
Near infrared spectroscopy (NIRS) has been used extensively over the past decade to monitor oxygen delivery to the brain and spine in adults, children and infants during cardiac and vascular surgeries. It has been shown to improve outcomes and prevent potentially catastrophic results from incidents such as accidental cannula misplacement.[
The accuracy of NIRS for monitoring brain O2 delivery has been validated by comparison to internal jugular vein hemoglobin saturation (SijvO2) levels reported in independent studies in adults, children and infants,[
There are a number of factors that need to be taken into account when validating the accuracy of any cerebral oximeter, including the significant incidence of gross anatomical variability of the vascular anatomy of the brain,[
Furthermore, rather than relying on global hypoxia, which affects all tissues, validation of specificity requires alteration of O2 delivery to that specific tissue. This was done for brain by altering partial pressure of carbon dioxide in the arterial blood (PaCO2) levels to increase cerebral perfusion in normal adults[
The issues of accuracy and precision are essential components of analytical techniques and are requisites for validating any monitoring technology but are different from target physiologic values. While pulse oximetry measures the saturation of arterial blood (SpO2) by monitoring the pulse interval and has a defined target value, tissue oximetry is a venous weighted measure and hence reflects the arteriovenous (AV) difference and the adequacy of oxygen delivery. Tissue oximetry provides a range of “normal” saturation that is associated with venous outflow which is patient specific and is known to have wide variability.[
Studies reporting SijvO2 means and range in normal subjects and cardiac patients using co-oximetry of blood samples from the internal jugular vein[
It is critical to understand the value and accuracy of rSO2 to remember that it is venous weighted and hence responsive to all the physiologic factors that influence oxygen availability such as anatomical variability, hemoglobin dissociation, cardiac output, dyshemoglobinemias, blood pH, vascular permeability and metabolic demand, resulting in an AV difference. The greatest value of rSO2 monitoring results from the fact that it is impacted by all these variables and, hence, unlike SpO2, reflects the amount of O2 that was actually available to, and consumed by, the tissues.
Independent studies in humans and validation data for FDA clearance as well as data from animal studies shown in
Somatic monitoring presents a greater challenge since the NIR light penetrates potentially thick muscle and fascial layers when applied to the body surface, making shallow compensation essential to limit the impact of intervening tissues. Animal experiments have demonstrated that when the skin to organ distance is less than 1.4 cm with the INVOS system (INVOS 5100C, Somanetics Corporation, Troy, MI, USA), changes in flow to the organ are reflected in immediate and sensitive change in the rSO2.
The rSO2 response to occlusion of the renal artery, stopping flow to the kidney in isoflurane anesthetized piglet, is shown in
The specificity of the response to organ ischemia enables somatic rSO2 to be used as a measure of peripheral perfusion and oxygen delivery and has been demonstrated to be very sensitive to changes in organ blood flow.[
Tissue oximetry is complex and highly informative but is not a standalone diagnostic. It is a valuable part of a differential diagnosis and has been shown to provide an early alert to occult hemodynamic changes not indicated by other physiologic parameters including arterial blood gases and mean arterial pressure.[
The relationship between cerebral and somatic rSO2 has been defined and reduced to practice in congenital heart surgery and the cardiovascular intensive care unit (CVICU) with the perirenal rSO2 kept higher than cerebral to ensure adequate peripheral perfusion[
Cerebral saturation thresholds for intervention have been established in adults and neonates based on clinical[
The management of hemodynamics in neonates presents a significant challenge. Blood pressure is adjusted with pressors and inotropes to the gestational age, which is standard of practice in many NICUs but is not well substantiated in relation to outcome.[
The animal experiments cited above demonstrated drug specific responses in oxygen delivery between the brain and the kidney, gut and muscle, which would not be obvious from other clinical measures since the pressure and SpO2 remained unchanged. Simple adjustment of the mean arterial pressure to improve circulation has the potential to expose patients to the risk of increased cerebral flow, potentially leading to intracranial bleeds or conversely to hypoxic conditions from vasoconstriction that could cause vascular or neuronal damage.
NIRS holds great promise for managing the impact of pressors and inotropes in infants and neonates to ensure that peripheral vasoconstriction or inotropic stimulation results in appropriate perfusion balancing and does not cause organ morbidities while resulting in too much or too little blood flow to the brain. Conversely, there will be situations where it is necessary to deny the visceral organs to provide enough O2 to prevent neurological compromise despite the cost of organ damage. NIRS currently provides the only means of monitoring the distribution of perfusion in these instances.
While cerebral rSO2 below 40% has been shown to present significant risk as discussed above, there is no target level rSO2 for the organs. Multisite monitoring, however, provides insight into the tissue response to interventions allowing the clinician to asses the impact of those interventions on the distribution of blood flow.
In addition to hemodynamic management, NIRS can provide insight into potential cerebral perfusion response to changes in ventilation. Premature neonate’s lungs are frequently kept functional with high frequency ventilation or continuous positive airway pressure, both of which can impact PaCO2 levels. Normal SpO2 does not ensure appropriate oxygen delivery to the brain which is responsive to changes in circulating CO2. C-rSO2 reflects blood flow to the brain, and hence O2 delivery, and responds rapidly to changes in PaCO2 as shown in
C-rSO2 tracking of cerebral O2 delivery response to hypercapnia followed by hypocapnia in a 2.0 kg, term, normothermic piglet, demonstrating the ability of NIRS to track the cerebral blood flow response to changes in CO2 (CC flow, common carotid flow; MBP, mean blood pressure; etCO2, end tidal CO2). SpO2 of 100% and blood pressure remained constant through the entire experiment. Periodic blood gas analysis was run to validate the etCO2
Advances in the use of NIRS to determine cerebral and somatic oxygen delivery has provided a new and powerful tool for obviating frequently occult changes in oxygen biology that can result in serious morbidities and even death. The information provided by NIRS is a significant and unique addition to hemodynamic management and has been shown to improve the outcome and reduce the length of stay. While a threshold for cerebral rSO2 has been established in children and adults, the use of somatic monitoring provides a new component to differential diagnosis and intervention. The inclusion of multisite rSO2 in a diagnostic assessment provides insight into perfusion distribution and directs the clinician to assess the potential causes of change in cerebral or somatic O2 delivery, enhancing patient management and improving outcomes.
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