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Englewood, NJ
Dr. Shander is Chief, Department of Anesthesiology, Critical Care Medicine, Pain Management and Hyperbaric Medicine, and Dr. Lobel is a Pediatric Anesthesiologist, Department of Anesthesiology and Critical Care Medicine, at Englewood Hospital and Medical Center.
Correspondence to: Aryeh Shander, MD, FCCP, Chief, Department of Anesthesiology, Critical Care Medicine, Pain Management and Hyperbaric Medicine, Englewood Hospital and Medical Center, 350 Engle St, Englewood, NJ 07631; e-mail: aryeh.shander{at}ehmc.com
Whether a "lumper" or a "splitter," physicians have always battled with a way to measure organ function. Besides the difficulty in obtaining accuracy of function, outcome predictions have been and continue to be a significant challenge. Determination of global as well as local measure of organ function is a large part of daily patient care. Does monitoring of bodily and organ function help in improving the care and outcome of our patients? On examination of this question, it becomes apparent that a culture has developed that has harnessed us to monitors despite their inherent lack of accuracy and, at times, little impact on positive outcomes. For example, the measurement of oxygen saturation in the critically ill and anesthetized patient would be considered heresy if not employed in every occasion when supplemental oxygen is needed or ordered. To date, uncertainty remains if this monitor improves survival or improves quality of life in those patients who are intermittently or continuously monitored.1
To monitor cardiac function in many instances, invasive procedures must be entertained in order to achieve accuracy and minute-to-minute assessment of interventions. This monitor may result in a worse outcome for patients.2 Noninvasive equipment and procedures may help our therapy but suffer from reduced accuracy and increased operator margin of error. Issues remain, including the validity of these noninvasive monitors as they relate to patient outcome, the ability to obtain accurate noninvasive information that directly reflects organ function and, lastly, a well-defined standardized methodology of interpretation.
Right-heart catheterization as a monitor has baffled the medical community, necessitating a review of this procedure and monitor. The fact that a monitor could be associated with negative results (in this case, suspicion of increased mortality) was difficult and still troubles many physicians. Is it the monitor, the patient’s acuity, or the operator’s inability to interpret the information in a standardized, comprehensive, and accurate means? Or is it poor outcome of patients whose acuity of illness is so great that they would perish regardless of monitors or intervention? These are some of the dilemmas facing the medical community each and every time we are introduced to new methods to monitor organ function.
Diagnosing disturbances in brain function has had a long history. The EEG, an established standard, in its raw form presents most clinicians with difficulty in interpretation. Specifically, the EEG is difficult, if not impossible, to interpret by most nonneurologically trained physicians. The EEG represents both local and global brain function and can alert us to the etiology and, possibly, the impact of therapeutic intervention, specifically with seizure activity.3 Global assessment of brain function relying on clinical findings and physical examination, as it relates to outcome, was adopted by trauma clinicians (the Glasgow coma scale)45 and is related to survival. This serves as a reminder that as we improve our technology, clinical assessment remains a significant element.
Cerebral function monitors currently in use show either electrical activity in a global or local fashion and at times are a reflection of blood flow and tissue oxygenation. Measurement of awareness and/or cognitive function represents integration of multiple cerebral activities that are difficult to predict or measure with accuracy. In critical illness, as well as during surgery, sedation level and comfort are sometimes confused and used interchangeably. Measurement of sedation may be more complex than comfort since the clinician can no longer rely on the patient’s report. Conversely, comfort is easily reported in many situations but cannot be measured in patients who have lost capacity to report. To date, measuring both analgesia and sedation present a formidable challenge to the clinician. Increasing our ability to measure one or the other with new methods may represent better survival and improved quality of life for our patients.
For adult patients, participation in intensive care activity such as ventilatory support during respiratory distress and potential anoxia has been associated with a posttraumatic syndrome months after discharge.6 The need for effective sedation in this population has become a pressing reality, as our ability to reduce mortality with aggressive measures improves annually.
While much attention has been paid to the adult in distress, only lately has attention shifted to the pediatric population. Neonates, infants, and children may need effective sedation as well, and could have significant deficits later in life attributed to unnecessary participation in aggressive invasive support. Current measures of nonsedation, such as increased heart rate and BP, are nonspecific and can mislead the clinician to add medicinal or other interventions, which may increase risk unnecessarily.
The BIS Monitor (Aspect Medical Systems; Newton, MA) is one of the more recent attempts to simplify the monitoring of higher cerebral activity in a sedated patient. The initial assumption in the article in this issue of CHEST (see page 303) by Trope et al was that the bispectral index (BIS) met the goal of a reliable monitor for level of sedation in critically ill pediatric patients. They reference several studies that correlate the BIS with previously validated sedation scoring systems. The study by Crain et al7 noted that there were favorable correlations between the BIS and the COMFORT (Calmness, Movement, Facial Tension, Respiratory Response, and Muscle Tone) scale in certain ranges but correlated less when comparing measurements at isolated moments during a prolonged pediatric ICU course. Courtman et al8 concluded that BIS scores correlated with COMFORT scores to a "moderate" degree. Lack of accuracy at some levels of sedation does not reject the BIS as a useful method to evaluate levels of sedation in critically ill children, especially those receiving neuromuscular blocking drugs. The many different scoring systems for sedation have advantages and disadvantages, but none detect changes in sedation over time. In pediatric patients, these sedation scores rely on variables that are impossible to measure in patients receiving neuromuscular blocking agents, often in use in the intensive care setting. Hence, the authors looked for a correlation between the BIS Monitor and changes in autonomic variables, the method most commonly used to assess sedation in patients receiving neuromuscular blocking agents, for lack of a better alternative. Another point to remember is that level of sedation does not necessarily correlate with level of comfort: a patient could be awake with a high BIS and be comfortable, or be sedated with a low BIS and be in pain.
This retrospective chart review is a good first step in a population that is often ignored. There are many limitations on the conclusions that can be drawn from this review, many of which are recognized by the authors. One limitation that was not mentioned is the inclusion of patients receiving fentanyl and pancuronium, which could affect the heart rate. Another is the restriction of inclusion criteria, which eliminates a significant portion of the critically ill pediatric population. In fact, the BIS Monitor itself may have limitations. For example, during the lag time between EEG processing and BIS number determination, the physician can miss detecting an early change in sedation. As a retrospective review, the only points that could be evaluated were those where there were a concomitant recording of the BIS, the heart rate, and the mean arterial pressure. Continuous monitoring of all the variables in a prospective trial may provide more data. Currently, the ideal level of sedation for pediatric patients receiving neuromuscular blockers has not been established. Future studies need to evaluate other scenarios that can affect the autonomic variables in an ICU patient, as well as continue to validate the BIS as a monitor for sedation.
Monitoring sedation in adults and pediatric patients remains a challenge. BIS in the pediatric population may not be as predictive as in the adult since the EEG may differ and the BIS algorithm was derived from data collected from adults.910 A study by Rodriguez et al11 found a large interindividual variability of BIS at different levels of anesthetic depth, which they concluded might limit the applicability to pediatric anesthesia. Although vital signs may not be accurate at all times in this population, they should not be ignored in favor of a monitor, and changes in both deserve reassessment of the patient’s condition. Despite the significant limitations and narrow population in this study, this topic deserves our attention since the need for noninvasive accurate measurement of level of sedation cannot be overemphasized. This is true for the adult population but more so for the pediatric patients who have been "left behind" numerous times. Lastly, we must still ask, what is the impact of "accurate" sedation on outcome? Will better monitoring lead to better sedation and reduce negative outcomes? Large, long-term randomized trials will be needed to answer this question.
References
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