Rosendo A. Rodriguez, MD, PhD
Popham Clinical Research Fellow,
Division of Cardiovascular Surgery,
Children's Hospital of Eastern Ontario and University of Ottawa
Objectives:
Introduction
In the last decade, improvements in surgical techniques and life-supporting methods have increased the number of survivors following complex surgical procedures [1]. At the same time, refined diagnostic techniques have shown evidence of neurologic sequelae in those survivors suggesting that this damage might have occurred during or immediately after the surgical procedure [2]. This concern has resulted in intensive research for developing monitoring techniques in the operating room for the early detection of brain dysfunction and the assessment of new strategies of cerebral protection.
The potential mechanisms of such brain injuries during surgical anaesthesia relates primarily to the interaction of vascular anatomic factors and the intrinsic vulnerability of the central nervous system in the human brain to hypoxia, ischemia and metabolic changes[1]. The concepts expressed herein are based both on the reports found in the literature and the author's own monitoring experience in more than 500 cases of adult and pediatric surgical procedures.
Although the estimates of brain injury observed after complex vascular, cardiac and neurological surgery vary considerably from study to study and according to the outcome measure, the incidence of neurologic complications has been reported in approximately 10 to 40% depending on the type of surgical procedure [1-3]. The major neurologic deficits most commonly observed have been cognitive impairment, seizures, choreoathetosis, bilateral motor deficits and hemiparesis [2,3].
Anaesthetic agents and manipulations affect multiple areas of the patient's physiology, particularly the brain.These effects include metabolic rate and oxygen consumption, electrical activity in the brain, cerebral blood flow, systemic vascular resistance and arterial perfusion pressure, and the hormonal and metabolic stress responses [4]. Because of these particular effects, anaesthetic agents may have important implications on the patient neurologic outcome.
In complex vascular or neurological procedures, anaesthetic management not only must ensure that the patient is anaesthetized deeply enough to prevent recall or awareness, but impending risk of brain damage must be recognized, and treament should be properly instituted for minimizing this type of injury.
Currently, the most effective means of protecting the brain from ichemia is hypothermia. Hypothermia reduces cerebral blood flow and metabolism and preserves cellular stores of high energy phosphates [5]. Volatile and intravenous anaesthetics have also been used as neuroprotective agents, but previous investigations have demonstrated that not all of these agents are equally effective [4]. Factors such as haemodilution, hypothermia, organ blood flow and decreased effectiveness of the blood-brain barrier, modify the normal patterns of cerebral distribution for these anaesthetic agents [4]. Consequently, continuous monitoring of these cerebral effects is desirable, because it allows to understand the nature of the brain insult and the cerebral response to specific interventions.
Current neuromonitoring techniques comprises several distinct continuous functional measures that complement each other in a comprehensive strategy [6]. This allows the integration of physiologic and hemodynamic information from various sources for detection of cerebral ischemia, assessment of causality and proper intervention. On-line information derived from hemodynamic indicators of cerebral perfusion and measurements of cerebral venous oxygen saturation are now integrated in parallel to electrophysiologic recordings for providing a "wider" picture of the functional changes in the brain.
Transcranial Doppler sonography is a non-invasive technique which allows real- time continuous measurements of blood flow velocity in major intracranial vessels within the circle of Willis [7].This method is used to monitor cerebral circulation under cardiac, vascular and neurological surgeries and to detect cerebral emboli [6,7]. Variations in brain blood flow velocities under normothermic conditions have been correlated with changes in cerebral blood flow as measured by positron emission tomography [8] and/or Xenon-techniques [9]. In contrast, a larger variation in the correlation of these two parameters has been found under hypothermic conditions in adults [6,7]. This device can evaluate cessation of cerebral perfusion in cases of brain ischemia or death, and provides information about the status of colateral circulation to the brain [7].
New transcranial Doppler equipment has been specifically designed for meeting the demands of intraoperative monitoring. It displays continuous and simultaneous time trends of the flow velocities from specific arteries [7]. In addition, Doppler waveforms indicate changes in the direction of the blood flow (e.g. anterograde vs retrograde), variations in the flow pattern (e.g. pulsatile to non-pulsatile), which represent hemodynamic features that can be used as feedback during surgery. This technique has proven useful in the management of patients undergoing repair of complex congenital heart lesions, neurosurgery and during cardiac catheterizations [6-11].
The electroencephalogram (EEG) reflects the electrical activity localized in the cerebral cortical mantle and is one of the older neurophysiologic tests available. Today, it remains an important diagnostic tool for lesions in the central nervous system [12].
In the literature, there is adequate agreement that evidence of neuronal dysfunction due to inadequate cortical perfusion or oxygenation is detected by EEG slowing [6]. Consequently, extensive research has been focused on improving the quality of EEG recordings in the operating room and the establishment of EEG guidelines for the detection of cerebral ischemia during surgery. These applications are expanded to evaluate depth of anaesthesia, cerebral protection, and as an outcome predictor in a variety of situations associated with brain damage [12-15].
Recent technological advances in this field have resulted in the use of computer processed quantitative EEG analyses [13-14]. By using this method, in addition to continuously recording the EEG, the magnitude of those EEG changes are quantitatively estimated and the trend of the critical periods easily visualized. The most clear examples of the utility of these techniques for intraoperative monitoring, have been the recent application of quantitative EEG measures during cardiac and neurological surgeries or carotid endarterectomies [12,13,15,16]. It has been our experience, as well as that of others [6], that in the absence of persistent focal EEG slowing intraoperatively, patients recovering from anaesthesia are free of new neurologic deficits.
Evoked potentials. Stimulation of a sensory pathway through its peripheral receptor results in generation of small electrical signals that are recordable over the human scalp [17-19]. Because these signals are intermixed within the random EEG activity, monitoring uses computer averaging techniques [17]. Monitoring equipment is specifically designed for the hostile electrical environment of the operating room and new software strategies can provide faster data acquisition with automatic display of the measurements.
These techniques have been applied for monitoring during anaesthesia when the neural structures of specific sensory pathways are at risk of damage [17]. This technique relies on adequate waveform identification. The time from initial stimulus to every individual peak-wave is calculated as the wave latency, and the size of the waveform as the peak amplitude. By monitoring the real time changes of these two parameters we can evaluate the electrical conduction at the sensory pathway from the peripheral receptor to the sensory cortex [17-18].
Clinical interpretation of this type of monitoring is based on the concept that presence of a normal evoked response indicates integrity of an specific sensory pathway (e.g. visual, auditory or somesthetic) [18]. However, during this type of monitoring, special considerations should be given to a balanced anaesthetic management and patient temperature during such procedures [17-19].
The most recent non-invasive "window" to the brain applied to surgical procedures is the measurement of cerebral oxygen saturation. This procedure is based on the principle of the near-infrared light and relies on the difference of absorption spectra between the oxidated and reduced hemoglobin and the total hemoglobin for calculating estimates of oxygen saturation [20].
This technique requires the use of two sensors separarated by a fixed distance. The proximal sensor records infrared light reflected from superficial tissues while the distal signal represents the brain tissue saturation [20]. The substraction between these two signals represents a venous weighted estimate of the regional cerebral oxygen saturation. Using proper sensor separations this technique detects variations in venous oxygen saturation of the brain under conditions such as cerebral ischemia, circulatory arrest or hypothermia in adults [6,20].
A potential advantage with the near-infrared spectroscopy is that it does not require pulsatile blood flow as in the case of pulse oxymetry for providing continuous reading [6]. This feature makes it ideal as an indicator of the mixed venous oxygen saturation under cardiopulmonary bypass or circulatory arrest.
Although disagreement has been found regarding the extent of correlation between cerebral venous oxygen saturation obtained by this method and the jugular bulb O2 saturation [21], results of this technique has been consistently correlated with direct brain tissue polarographic oxygen measurements [22-24]. A more extensive investigation is required to determine its utility as intraoperative monitor for the pediatric population.
Prevention of cerebral injury under surgical anaesthesia will require first, an understanding of its basic nature, pathogenesis and timing, and second, the evaluation of protective measures addressing the different aspects of its pathogenesis [22].
Several distinct functional measures of the brain for intraoperative monitoring are advised in the context of detection for brain ischemia. The indicators of hemodynamic assessment and venous oxygen saturation may document the nature of these changes, but electrophysiologic evaluations are the only means of assessing the functional changes in the brain and its recovery.
Prevention of brain damage and the duration of any intervention following termination of the insult remains to be established under clinical settings, but an early identification of "signs" of cerebral dysfunction is central to obtaining best results. The information provided by these strategies and the improvement of intraoperative neuromonitoring techniques may be crucial in the design of future clinical trials.
Questions.
Answers: c, e, false