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Hazards of Advanced Neuromonitoring
Physiologic monitors are tools that enable the “vigilance” described in the motto of the American Society of Anesthesiologists (ASA) and guide the patient “safety” ( securitas ) in the motto of the Association of the Anesthetists of Great Britain and Ireland. The term is derived from monere , which in Latin means to warn, remind, or admonish. In perioperative care, monitoring implies the following four essential features: observation and vigilance, instrumentation, interpretation of data, and initiation of corrective therapy when indicated. The principal objectives of intraoperative monitoring are to improve perioperative outcome, facilitate surgery, and reduce adverse events using continuously corrected data of cardiopulmonary, neurological, and metabolic functions to guide pharmacologic and physiologic therapy. Although sophisticated and reliable apparatuses may be used to collect these data, they are useless or even harmful without proper interpretation.
Advanced neuromonitoring has been broadly used in most operating rooms. This multimodal monitoring has evolved good outcomes with the advent of new and updated monitoring techniques in neurocritical intensive care. Advanced neuromonitoring allows sophisticated and individually focused treatment, thus contributing to patient safety. They not only provide real time information regarding the affected site but also guide us toward goal-directed, proactive preventive strategy with the interpretation of the available physiologic end points. But these are not without any disadvantages, as it always is with any advanced method.
The hazards of neuromonitoring include complications associated with the particular system, apart from its disadvantages. The distinguished limitation of these monitors is that it gives either an overall global measure or a regional measure of the cerebral function monitored.
Standard monitors used commonly are invasive lines for arterial and central venous pressure (CVP) monitoring. The complications associated with these monitors include:
- 1.
Invasive arterial lines: Complications include thrombosis, infection, and disconnection. Inappropriate administration of medications into the arterial line may exacerbate thrombosis, sometimes leading to amputation of the limb. Mismanagement of arterial lines during positioning may lead to disconnection and unexpected blood loss. Sometimes, over- and underdamping of arterial lines may lead to false readings and inappropriate management. The incidence of serious complications with radial, femoral, and axillary artery cannulations like permanent ischemic damage, sepsis, and pseudoaneurysm formation is very low, i.e., <1%.
- 2.
Central venous and pulmonary artery catheters (PACs): The complications include arterial puncture, infection, thrombosis, kinking and knotting of the catheter, retained guidewire, entrapment with sutures or by cannulas, irritation of the right heart leading to arrhythmias, heart block, ventricular tachycardia, and ventricular fibrillation. Fatal and rare complications include perforation of the sinus coronarius with resultant cardiac tamponade, and laceration of the subclavian artery. PACs are rarely indicated in neurosurgical cases with significant cardiac dysfunction. Apart from the complications seen with CVP catheters, this also includes some serious complications like pulmonary artery (PA) rupture, air embolism, torsion of the catheter, endocarditis, and pulmonary infarction.
Intracranial Pressure Monitors
These are most valuable in managing critically ill neurosurgical patients in the intensive care unit. The various intracranial pressure (ICP) monitors are seldom used intraoperatively. Potential complications of intraventricular ICP catheters include bleeding and infection leading to intracerebral hematomas and meningitis, respectively ( Table 1 ). On the other hand, intraparenchymal fiber optic catheters are costlier with the disadvantages of inability to zero and not able to drain cerebrospinal fluid (CSF) at times of emergency. Technical complications include dislocation of the transducer, breakage of the fiber optic cable, dislocation of the fixation screw, and defective probe for unknown reasons. Rare but disastrous complications include injury to vital structures like brain, important nerves, and vessels. The main disadvantage of these devices is that they cannot be recalibrated in situ. Another limitation is that they cannot be used for CSF drainage or compliance testing unless inserted in conjunction with a ventriculostomy. Subdural devices are easily inserted but can malfunction if they are not coplanar to the brain surface or if they become loose. The major complication of this procedure is infection, commonly meningitis, osteomyelitis, or a localized infection. Epidural bleeding and focal seizures, if the bolt is inserted too deeply, can also occur. With an epidural transducer , technical problems in positioning and calibrating the transducer in situ can also occur. Another shortcoming of the epidural transducer is that intracranial compliance testing and therapeutic CSF drainage cannot be performed.
Table 1
Comparison of Intracranial Pressure Monitors
| Intracranial pressure monitoring | Intraventricular catheter (IVC) | Subarachnoid bolt | Fiber optic sensor |
|---|---|---|---|
| Accuracy of readings | Good | Less accurate | Better |
| Drainage of CSF | Possible | May or may not | Not possible |
| Risk of infection | Increased risk | Less risk compared to IVC | No risk |
| Recalibration | Possible | Possible | Not possible |
| Disruption of brain tissue | Yes | No | Yes; comparatively less than IVC |
Cerebral Perfusion Monitoring
Either the functionality of parts of the brain are monitored (and the assumption is made that continued function implies an adequate oxygen supply) or the blood flow or pressure at one or more points in the brain are measured (and the assumption is made that the flow or pressure is equivalent elsewhere). Neither assumption is always correct, and despite monitoring, ischemia may sometimes occur without detection and may result in stroke.
Noninvasive methods of monitoring cerebral blood flow (CBF): This involves injection of intravascular radioactive isotope followed by measurement of radioactivity using gamma detectors. Drawbacks of the methods are the patient’s exposure to radioactive compounds and a need for externally placed, potentially cumbersome detector equipment, which may interfere with the surgery itself in case of intracranial surgery.
Transcranial Doppler (TCD): An important limitation of TCD results from the feeling that most of the examination is done through the temporal bone, which may be thick enough to preclude an adequate examination in 10–20% of patients. Blood flow velocity is directly related to blood flow only if the diameter of the artery where it is measured and the measurement angle of Doppler probe remains constant. The difficulty is to find a means to affix the probe in a way that prevents dislodgement or movement during monitoring. Limitations of this technique include between-subject variation of TCD velocities, within-subject variation if vessel diameter changes in response to vasoactive agents or conditions, and error from changing the angle of insonation.
Near infrared spectroscopy: The main disadvantage is increase of the temperature because of the heating of the semiconductor junction. The increase in temperature is seen around 1–10 °C. This may sometimes lead to intracranial heat injuries. Its major limitations include intersubject variability, variable optical path length, potential contamination from extracranial blood, and lack of a definable threshold. At present, it is considered a trend monitor, with each patient acting as his or her own control. In situations of potential regional ischemia—for example, carotid endarterectomy and temporary clip application during intracranial aneurysm surgery—bilateral monitoring should be used.
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