NEURO-ICU MONITORING TECHNIQUES
Intracranial Pressure Monitoring
A common complication of many serious neurologic conditions is an elevation of the pressure within the skull, the intracranial pressure or ICP. In adults, the average ICP ranges from 0-10 mm Hg. 20 mm Hg is considered to be the maximal upper limit of desirable ICP and pressures exceeding 40 mm Hg are considered extremely elevated. Intracranial pressure may be high for several reasons. ICP can be elevated if there is a rise in the pressure of the fluid circulating around the brain (the cerebrospinal fluid). This condition is known as hydrocephalus. Alternatively, the blood vessels supplying the brain can leak fluid into the brain causing the brain to swell, a situation referred to as cerebral edema. Whatever the underlying cause an increase in intracranial pressure is extremely dangerous.
The type of monitor used is dependent on a number of clinical factors, not the least of which is the neurologic disease causing the pressure increase. The following is a list of the instruments commonly used:
These are the most widely used ICP monitoring devices. A catheter (a tubular instrument) is placed inside fluid filled cavities within the brain called ventricles. Cerebrospinal fluid is synthesized within these cavities and then flows out of the ventricles to circulate over the surface of the brain. Intraventricular catheters can be installed during brain surgery or in the ICU under local anesthesia. They are considered among the most accurate devices used to measure ICP. Intraventricular catheters also are unique in that they can simultaneously monitor and treat increased intracranial pressure. The catheter allows cerebrospinal fluid to be drained thus lowering the pressure within the skull.
These are devices which employ fiberoptic technology to measure intracranial pressure. The fiber-optic probe has a transducer at the tip which can be inserted into the brain itself, the ventricles which it surrounds, or the subdural space. Fiber-optic monitors are easily inserted and their usage is increasing. At the Columbia Neuro-ICU, the Camino system of fiberoptic monitors is used.
These are metallic cylindrical instruments which are inserted such that the tip protrudes into the subarachnoid space. The most common device used is the Becker bolt (also known as the Richmond screw). These devices have the advantage of being easy to instal. However, problems with accuracy have limited their use.
These are recording devices which are placed in the epidural space. This is a potential space between the inner surface of the skull and the dura mater.
Transcranial Doppler (TCD) Ultrasonography
Transcranial Doppler (TCD) monitoring devices are used to asses blood flow in vessels supplying blood to the brain. This technique is used to detect a blockage of blood flow to the brain (cerebral ischemia). Cerebral ischemia can result in a stroke. In this case blood flow is arrested by the presence of a clot in a blood vessel. Patients also can have cerebral ischemia as the result of vasospasm, the contraction of the blood vessels supplying the brain. Vasopasm of cerebral blood vessels is a common complication of a subarachnoid hemorrhage (a bleeding into the subarachnoid space between the brain and the skull). Blood vessel contraction can result in the obstruction of the lumen of the vessel, leading to cerebral ischemia and a subsequent number of disabilities. In a Neuro-ICU TCD can be used to detect vasospasm before it can give rise to deleterious effects of cerebral ischemia.
Transcranial Doppler ultrasonography operates in the following manner. TCD devices use an ultrasonic signal which penetrates areas of the skull where the bone is thinnest. Upon meeting flowing blood cells this signal is altered and reflected back to a recording device. When an artery within the skull contracts due to vasospasm the blood flow velocity within this vessel increases, an increase which can be recorded by a TCD instrument.
Unlike other techniques used to asses cerebral blood flow (CBF) a TCD monitor is non-invasive. It is completely painless, easily performed at the patient's bedside and it gives immediate results.
Single Photon Emission Computerized Tomography(SPECT)
Single Photon Emission Computerized Tomography or SPECT is a method used to create a computer generated image of the brain in order to asses blood flow. The SPECT device generates a computer image of the brain based on its detection of photons emitted by radionuclides administered to the patient. SPECT images are of resolution comparable to that of Positron Emission Tomography (PET) scans. SPECT scans are cheaper, faster, and more accessable than PET scans.
There are specific medical conditions especially amenable to SPECT monitoring. Following stroke, SPECT can give an accurate assessment of the adequacy of blood flow to the brain and the presence of neurologic injury. SPECT can also accurately detect reductions in blood flow related to vasopasm, blood vessel contraction which is a common complication following subarachnoid hemorrhage. Lastly, SPECT is used to asses brain damage following head injury. Following head injury an area of the brain may become poorly perfused with blood. This area may not be visible on Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans. Even subtle brain abnormalities can be picked up in head injury patients using SPECT.
There are a number of clinical situations in which the use of a SPECT machine is invaluable in assessing blood flow and associated brain damage. This makes SPECT a valued component of the Neuro-ICU's arsenal of monitoring techniques.
Continuous Electroencephnalogram (EEG) Monitoring
An electroencephalogram or EEG is a device used to measure electrical potentials in the brain by attaching electrodes to the scalp. An EEG is often employed to monitor brain wave patterns during seizures and to give critical information concerning the level of functioning of the central nervous system. A Neuro-ICU is specialized to take continuous EEG recordings. A single recording is a mere sampling and often does not accurately reflect the patient's overall condition. Continuous EEG monitoring has several advantages over a single EEG reading. A continuos EEG profile often leads the ICU staff to a correct diagnosis and gives a much better indication of the patient's prognosis. Furthermore, continuous recording allows physicians to monitor a patient's response to drug therapy as well as identify complications.
There are two clinical situations in which continuos EEG monitoring has proved especially useful. First, an EEG can accurately detect ongoing seizures in a patient with status epilepticus. Second, an EEG can provide important data on the flow of blood to the brain. Prolonged EEG recordings can detect a cessation of blood flow to part of the brain (a situation known as cerebral ischemia). Such ischemia commonly follows strokes and is extremely dangerous if not rapidly detected.
Therefore, continuos EEG monitoring is critical in establishing a correct diagnosis of the patient, predicting the outcome of the patient, assessing the patient's condition during drug therapy, and detecting problems early before they can lead to serious neurologic complications.
The Columbia-Presbyterian Neuro-ICU has available to it a group of consulting epileptologists. These are physicians who are specialized in monitoring EEGs and treating serious seizure disorders. The combination of epileptologists and continuos EEG monitoring allows the Neuro-ICU to rapidly and adeptly treat life-threatening seizure disorders.
Invasive Hemodynamic Monitoring
Invasive hemodynamic monitoring refers to the assessment of blood flow and blood pressure via devices placed within blood vessels. In many medical settings these devices are used to measure circulatory function in the presence of heart or lung disease. A Neuro-ICU uses these techniques to ensure that the brain is being properly perfused with blood. The instruments used to measure circulatory function are known as catheters. Blood pressure is recorded by a fluid-filled catheter and this pressure is transmitted to a transducer which converts the information into an electrical signal visible on a monitor. There are two general techniques to monitor blood pressure:
These are catheters placed into a major artery (such as the radial artery in the wrist or the femoral artery in the thigh) to measure blood pressure. Arterial catheterization is indicated in a Neuro-ICU setting when abnormal blood pressure threatens to compromise blood flow to the brain, exacerbate high pressure within the skull (intracranial pressure), or worsen bleeding within the skull. Low blood pressure (hypotension) can cause inadequate blood flow to the brain, especially if intracranial pressure is already elevated. High blood pressure (hypertension) can increase intracranial pressure and/or increase bleeding into the brain and surrounding compartments. Furthermore, arterial catheterization can be used in patients with neuromuscular respiratory failure, (due to myasthenia gravis or Guillain-Barré syndrome) in order to measure blood gases.
These are catheters placed into the pulmonary artery which carries blood from the heart to the lungs to be oxygenated. A Swann-Ganz catheter has an inflatable balloon near its tip. The catheter is capable of measuring a number of clinical variables including pulmonary wedge pressure and cardiac output. Pulmonary wedge pressure is obtained when the balloon near the tip of the Swann-Ganz catheter is inflated thus "wedging" it into a branch of the artery entering the lungs. The resulting "wedge" pressure recorded is approximately equivalent to the pressure in the left atrium, that chamber of the heart into which blood flows after exiting the lungs. Cardiac output is the volume of blood ejected by the heart in a one minute period. The use of a Swann-Ganz catheter is mandated in the presence of shock due to low blood pressure. It is also recommended to asses cardiovascular status when the patient is on a mechanical ventilator. However, a Swann-Ganz is primarily used in a neuro-ICU to monitor a patient's condition during vasoactive drug therapy. Thus the Swann-Ganz catheter is used to ensure that drugs used to increase the brain's perfusion do not cause cardiovascular complications.
Near Infrared (NIR) Spectroscopy
The brain is nourished by oxygen carried to it in the blood. Many imaging techniques can asses the blood supply to the brain but near infrared (NIR) spectroscopy is unique in being able to provide information about the amount of oxygen present in this blood supply, the cerebral oxygen saturation.
Cerebral oxygen saturation is measured by a device which emits light of a near infrared wavelength. Once in the blood the light is absorbed by the blood protein which carries oxygen, hemoglobin. The signal is then reflected back to two detectors. The signal will differ based on whether the light was absorbed by hemoglobin containing oxygen or by hemoglobin not carrying oxygen. As a result the machine is capable of determining the ratio of oxygen carrying hemoglobin to hemoglobin which is not bound to oxygen. It is this ratio which reveals the cerebral oxygen saturation.
NIR is a new technique with several technical pitfalls. NIR is currently under investigation as a potential monitoring technique for use in the neuro-ICU.
Jugular Venous Oxygen Saturation Monitoring
In order to understand this monitoring technique one must first have knowledge of the blood supply to the brain. The brain is supplied with oxygenated blood via the internal carotid arteries. Once blood from these vessels has perfused the brain the blood is drained back to the heart in the internal jugular veins. Jugular Venous Oxygen Saturation (SjvO2) Monitoring is the assessment of the amount of oxygen dissolved in the internal jugular vein. This is the amount of dissolved oxygen in the blood returning from the brain which was not used by the brain. As such SjvO2 monitoring records the balance of the supply of oxygen available to the brain and the brain's demand for oxygen. The knowledge of such a balance (sometimes referred to as the cerebral oxygen balance) is important as it indicates the brain's oxygen consumption. For example, a low SjvO2 value indicates that the brain needs more oxygen. This is a dangerous situation since without oxygen brain tissue dies.
Jugular venous oxygen saturation is monitored by the placement of a fiber-optic catheter into the internal jugular vein. This is a catheter (a tubular instrument) which employs fiber-optic technology to measure the amount of oxygen dissolved in a blood vessel.
Measuring SjvO2 is currently investigational but may be useful in certain clinical settings. For instance, studies have shown that during a cardiopulmonary bypass operation jugular venous oxygen saturation monitoring can be useful to detect cerebral oxygen deprivation. In addition SjvO2 monitoring can be used to detect cerebral oxygen deprivation following head injury. It is common for the brain to be deprived of oxygen following head injury. A SjvO2 monitoring device can detect such deprivation before it can give rise to permanent and severe neurological injury.
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August 27, 2014