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Editor
Courtney A. Hardy, MD
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Co-Editors
Mark Twite, MD, BCh
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Stuart R. Hall, MD
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Call for CCAS Directors Nominations
Preview of the CCAS
Program at Pediatric Anesthesiology 2010
An Interesting Case
The Anesthetic and Cardiopulmonary Bypass Management of an 800g Premature Neonate for the Arterial Switch Operation
PRO vs. CON
Should NIRS Monitoring Be Considered Standard of Care on Every Case Involving CPB?
Literature Reviews
1. Perioperative stroke in infants undergoing open heart operations for congenital heart disease.
2.
Brain maturation is delayed in infants with complex congenital heart defects.
CON: Cerebral Near-Infrared Spectroscopy Monitoring SHOULD NOT Be Considered Standard of Care on Every Case Involving Cardiopulmonary Bypass
Susan C. Nicolson MD
David R. Jobes MD
Division of Cardiothoracic Anesthesia
The Cardiac Center at The Children’s Hospital of Philadelphia
In 2010 the expected outcome for pediatric cardiac surgery is survival, even for the most complex forms of congenital heart disease (CHD) necessitating repair in the neonatal period. With survival expected, the focus of clinicians and investigators has shifted from examining factors that impact mortality to those that impact the quality of life of the survivors. One, if not the most important, factor in determining quality of life is the presence and extent of neurological injury. Neurodevelopmental dysfunction has been identified as the most common and potentially most disabling long term complication of CHD and its treatment. Close examination of the growing population of survivors indicates that a significant percentage of these children have abnormal neurologic findings after cardiac surgery. The clinical findings that have been the focus of attention include: seizures, stroke and coma, in the immediate post-operative period and neuro-cognitive development, including school performance, in the long term. The incidence of these abnormal findings spans a wide range, between 2-25%, depending on the cardiac lesion, gestational age, age at surgical intervention, strategies imposed to address the lesion, peri-operative clinical course, associated non-cardiac abnormalities, including genetic syndromes, who is evaluating the patient, what tools are being used with what sensitivity over what time course.
The etiology of abnormal neurologic findings appears to be multi-factorial. Up until a few decades ago, the brain of a child with CHD was presumed to be normal at birth and it was assumed that all the brain injury occurred intra-operatively. There is an increasing amount of evidence to show that many neonates with complex CHD are born with co-existing structural brain abnormalities and that the brains of neonates with complex CHD are immature compared with neonates of the same gestational age without CHD. A number of studies have now shown that some neonates acquire white matter injury in the form of peri-ventricular leukomalacia (PVL) between birth and correction of the cardiac defect. After acknowledging pre-existing neurologic disease that cannot be treated, the goal is prevention of further permanent undesirable alteration in brain cell metabolism (neurologic complication) that occurs as the byproduct of strategies used to treat the heart and great vessels. These strategies include: low flow bypass, regional cerebral perfusion, and deep hypothermic circulatory arrest each with accompanying bypass management of temperature, hematocrit and blood gases. For many years, there was a focus in the surgical treatment of CHD on the use of cardiopulmonary bypass (CPB) as the mechanism of injury and on changing the conduct of CPB to prevent injury. The outcome benefits of these changes and “optimal” management have been significantly less than predicted. Injury can also occur secondary to low cardiac output states in the peri-operative period. It is believed that most neurologic complications are mediated by lack of adequate oxygen delivery causing cell damage directly or stimulating abnormal apoptosis. Some injury may be due to direct or indirect neurocellular toxins, e.g. high levels of cytokines. There is data to indicate that exposure to pharmaceuticals, including anesthetics, may impact the neonate’s neurologic outcome favorably via ischemic protection or negatively via toxicity.
Reduction in the incidence of neurologic complications requires, in part, identification of one or more parameters that predict neurologic outcome. Once the parameter(s) is identified there needs to be a monitor that allows easy, continuous, reliable and reproducible data about this parameter. To date, no parameter has been identified which predicts neurological outcome and no intervention influencing a potential parameter has been shown to impact neurologic outcome. Clinicians often measure what can be measured, surrogate markers, not what we need to measure. Device manufacturers market precision and accuracy of what their device measures, not evidence that measuring the parameter has predictable effect on a clinical outcome.
The available monitors examine one of three surrogate markers: EEG, cerebral oxygenation (NIRS), or cerebral blood flow velocity and each have significant limitations. Each monitors only one element of brain integrity and looks at only a small portion of the brain at any point in time. The operating room is a hostile environment and many of the devices are affected by intraoperative conditions including use of electrocautery, extremes of temperature, motion artifact, and the use of volatile anesthetics. Given the limitations of each a number of clinicians have advocated using more than one modality simultaneously to get a better understanding of the brain’s integrity during the process of heart repair and recovery.
A number of studies have raised concerns about NIRS technology which have been summarized in a comprehensive review by Hirsch and colleagues in 2009 (1). The tissue bed assessed by the NIRS probe depends on the tissue which lies within the optical pathway. Small differences in the placement of the probe can result in altered optical pathways with different tissue absorption and altered contribution of extracranial tissue absorption to the NIRS measurement. A recent study examined the reproducibility of data from the probe. One sensor was removed and replaced with another sensor in the same position. There was poor agreement of the ScO2 measurement before and after sensor exchange. There are technical issues including precise placement of the monitor on a small child’s head and sufficient training on the part of anesthesiologists, surgeons and intensivists to interpret the data.
The “safe” range for the parameter, both level and duration needs to be identified to enable one to determine when intervention is required. The threshold between “safe” and injury needs to be defined for pediatric patients of all ages under a variety of conditions including extremes of temperature and flow. Extremes are often predictors of bad outcomes, but intermediary changes are not clearly related to outcome. Some changes may be reversible without intervention, whereas others may require rescue. Hirsch et al. concluded that “the significant variability in NIRS measurements, temporally and between individual patients, precludes the establishment of absolute threshold values for tissue ischemia. Relative values and individual patient trends have been used rather than absolute values.” (1). Once conditions exist that require intervention there needs to be evidence that the intervention returns the parameter to the “safe” range, which in turn influences neurologic outcome. Treatment options are often limited and all interventions come at some cost which is hard to justify in the absence of proven benefit. If one is to intervene often the execution of the procedures limits what can be done and when. Some interventions can have CNS consequences in their own right. Increasing the mean arterial blood pressure on bypass in the small heparinized neonate with an immature brain may result in intracranial hemorrhage which would result in brain injury. Any intervention that extends the support time or compromises the surgical repair may result in low cardiac output in the post-operative period which, in turn, may impact neurologic outcome.
In January 2010 there is insufficient data to support the routine use of NIRS or any other neurologic monitor in all children undergoing procedures requiring CPB. We as a group of practitioners do have an obligation to conduct well designed observational studies to determine if any measureable parameter or combination of parameters is predictive of CNS injury. During these observational studies the data acquired should not be used to treat the patient. Once such observational trials have shown which parameters are predictive of injury, then and only then should interventional trials be designed and undertaken to show what interventions impact clinical outcomes. This logical progression will result in evidence based support for application of routine neurological monitoring into clinical practice.
Reference:
- Hirsch JC, Charpie JC, Ohye RG, Gurney JG: Near-infrared spectroscopy: what we know, what we need to know – a systematic review of the congenital heart disease literature. J Thorac Cardiovasc Surg 137(1): 154-9, 2009
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