Introduction
History and Background
By most accounts the modern era of neuromonitoring began in the 1950s with reports of continuous intracranial pressure (ICP) measurements in humans by Janny et al. ICP was measured by connecting a strain gauge pressure transducer to a fluid-filled tube connected to a catheter with its distal end implanted in the patient’s cerebral ventricle. The use of a strain gauge, as opposed to a manometer, to measure pressure made ICP monitoring safe, easy, and accurate. The frequency response of the sensor and associated electronics was sufficient to allow visualization of the pulsatile ICP waveform. In addition periodic measurements could be plotted over time, leading to the discovery of the well-known and clinically useful Lundberg waves. Continuous ICP monitoring has become the cornerstone of critical care monitoring for patients admitted to neurocritical care units (NCCUs).
From an engineering point of view, it is interesting that the application of a simple electrical sensor and analog electronics resulted in a significant improvement in clinical practice. This is a pattern that has repeated itself in the years since Lundberg’s original work in the 1960s. The remarkable developments in electronics and sensing technologies driven by the computer, telecommunications, and aerospace and defense industries have been applied to many aspects of medicine including monitoring devices used in neurocritical care.
Clinical Background
An examination of engineering issues associated with neuromonitoring devices must be made in the context of the goals and challenges of neurocritical care. Neurocritical care patients commonly suffer from traumatic brain injury (TBI), from neurovascular diseases such as subarachnoid hemorrhage, or have undergone a neurosurgical procedure such as resection of a brain tumor. In addition to the original injury or disease, patients are at risk of “secondary” injury because of the unique nature of the head and brain. One of the primary goals of neurocritical care is the prevention of these secondary injuries. The purpose of monitoring equipment used in neurocritical care is to enable the clinician to identify signs and symptoms of the primary disease, to warn of impending secondary insults, and to help judge the efficacy of treatment.
Because the brain is encased in the rigid skull, there is limited room for the brain to swell when injured. Left unchecked, swelling (edema) can lead to serious morbidity or death. For this reason ICP monitoring is important in neurocritical care. Nutrients such as oxygen and glucose are supplied and waste products removed from the brain through the blood circulation. Although the brain is a relatively small organ at 2% of body weight, it receives 20% of the body’s blood flow and accounts for 20% of oxygen and 25% of glucose consumption. Significantly, the brain does not store oxygen. Alterations in blood flow are common in neurocritical care patients, and there is much interest in monitoring blood flow in both the large vessels leading to and from the brain and the microvasculature within the brain parenchyma. Systemic blood pressure is commonly monitored because it is the driving force for cerebral blood flow (CBF). The concentration of carbon dioxide (CO 2 ), which is a powerful vasodilator and affects CBF is indirectly monitored in the expired breath. CO 2 also has been monitored directly in the brain. Oxygen tension can be measured in both the circulation and in brain tissue. The oxygen saturation of systemic blood is routinely monitored, and estimates of regional saturation of cerebral blood also are possible.
It is common for alterations in cerebral metabolism to occur in neurocritical care patients. The brain normally produces energy in the form of adenosine triphosphate (ATP) through the Krebs cycle; oxygen and glucose are necessary for this ATP production. When cerebral metabolism is disturbed, for example, by a lack of oxygen availability, the brain relies more on anaerobic glycolysis that produces less ATP. Under these conditions, brain cells may not function normally and may even lose structural integrity. This condition is sometimes called hyperglycolysis . This condition as well as other metabolic disturbances can be inferred by various chemicals released by the brain. A portable analyzer can test samples collected from the interstitial fluid by dialysis probes implanted directly in the brain to determine the concentration of the chemical in question. The clinical utility of these measurements is the subject of significant research.
Neuromonitoring Systems: An Engineering Prospective
The Ideal Monitor
The ideal monitoring system would be noninvasive, provide continuous information, interrogate the entire brain, present the information in a way that is easily understood, be compatible with other monitoring and imaging systems including magnetic resonance imaging (MRI), take up little or no space, meet all regulatory requirements both medical and technical, and cost very little. Few if any currently marketed devices meet all of these requirements. The following sections describe the basic components of most common existing monitoring systems, give an overview of the types of sensing techniques that are or have been used, and assess the pros and cons of each technique.
Most monitoring systems used in neurocritical care consist of a sensor in contact with the scalp or directly implanted into the brain, some kind of fixation device that keeps the sensor in place during the monitoring period, connecting cables, and a stand-alone electronic monitor to operate the sensor and display data or connect to a bedside monitor.
Fixation Techniques
Fixation is an extremely important aspect of monitoring systems. Fixation systems must ensure consistent alignment or contact between the sensor and the patient. Two primary fixation techniques are used for indwelling sensors: bolt fixation and tunneling. Most indwelling sensors take the overall physical shape of a long slender cylinder usually referred to as a catheter or probe with the sensor located at the tip of the probe ( Fig. 39.1 ).
Bolt Fixation
The bolt device is composed of a short metal cylinder with self-tapping threads at one end and some kind of compression fitting at the other end. The bolt is usually threaded to a depth of approximately the thickness of an adult skull of approximately 0.7 cm. The thread pitch is usually sized such that at least three or four fully formed threads will be in contact with the skull with the bolt in place. If the thread pitch is too large, not enough contact is made by the thread crests to ensure a secure and leakproof fit. The lead-in threads are usually tapered in a manner similar to pipe threads with the crest height of the initial one to three pitches smaller than the diameter of the twist drill used to cut the insertion hole in the skull. The compression fitting must resist axial movement of the probe and provide a leakproof seal around the outer surface of the probe. The maximum compressive force that can be exerted must be limited to an amount that will not damage the probe. Providing strain relief at the junction between the top of the bolt and the probe is a good practice. The ultimate strength of both the bolt and probe in bending and tension must be carefully considered because the probe and bolt often are subjected to significant loading when the probe or a connecting cable is inadvertently pulled when it catches on a fixed object during patient transport, the patient falls out of bed with the cable wrapped on the bed frame or other stationary object, or the patient pulls on the probe in a semiconscious state. It is far better for the bolt or probe (outside the patient) to break than the bolt to break in the skull ( Fig. 39.2 ).
Tunneling
Perhaps the oldest fixation method is tunneling. Tunneling refers to the surgical technique of routing an elongated catheter under the scalp toward the insertion site. This method has the advantage of good fixation to the patient using stitches in the scalp and better infection control than simply routing the catheter out of the scalp directly over the insertion site. This technique is commonly used with fluid-filled ventriculostomy ICP monitoring. It also has become common for transducer-tipped probes to be affixed to the patient via tunneling. It is important to recognize that the probe must be capable of making a right-angle turn into the skull into the insertion site under the scalp without being damaged. Because the connectors on the proximal end of most probes are too large to be tunneled under the scalp, many of these devices are not tunneled per se but are routed through a plastic sleeve that has itself been tunneled under the scalp. This “tunneling sleeve” usually takes the form of a plastic tube connected at one end to a solid trocar. After the sleeve has been tunneled under the scalp, the trocar is cut from the sleeve and the catheter is passed through the sleeve toward the insertion site. The tip of the catheter is then implanted and the catheter and/or sleeve are sutured to the scalp via loops ( Fig. 39.3 ).
Other Fixation Methods
Noninvasive monitoring devices such as electroencephalograph (EEG) electrodes, near infrared (NIR) oximeters, and transcranial Doppler (TCD) probes are affixed by hand, by mechanical means, or by the use of some kind of adhesive tape–like substance. Heavy devices that are not usually used to make continuous measurements such as TCD transducers are simply held to the head by the technician or are sometimes fitted to a device that looks like an eyeglass frame. Lighter transducers like EEG electrodes or oximeter leads usually are held in place by adhesive patches. The adhesives used must not irritate the skin during monitoring durations of up to several days, must not cause hair to be removed when the patch is removed, and must securely hold the transducer in place over the monitoring period.
The Bedside Monitor
Most if not all neurocritical care patients are connected to bedside monitors (BSMs). The BSM collects signals from external monitors and displays them on a single screen. Generally each parameter is connected to the BSM via detachable “modules” that are part of the BSM and accept analog or serial input from an external monitor. Each module is tailored to a specific parameter type such as pressure or temperature. Commonly displayed parameters include electrocardiogram (ECG), heart rate, temperature, blood oxygen saturation, cardiac output, and mean arterial blood pressure among other options. Most physiologic pressures such as arterial pressure, pulmonary artery pressure, and pulmonary capillary wedge pressure are monitored using strain gauges. BSMs are therefore designed to provide electrical excitation of the strain gauge and to interpret pressure from the electrical signals returned form the strain gauge. Neurologic-related parameters, other than ICP signals measured by fluid filled systems, are generally not included in the current generation of BSMs. Devices that measure parameters that are not included as standard features of the BSM commonly connect to it via analog strain gauge emulation or by serial means when available. Cable connectors must accommodate the various makes and models of BSMs ( Fig. 39.4 ).
