Conventional monitoring equipment and accessories were not designed to operate in the harsh magnetic resonance (MR) environment that utilizes electromagnetic fields that can adversely affect or alter the operation of these devices. Fortunately, various monitors and other patient support devices have been developed or specially-modified to perform properly during MRI procedures.
MR healthcare professionals must consider the ethical and medico-legal ramifications of providing proper patient care that includes identifying patients that require monitoring in the MRI environment and following a proper protocol to ensure their safety by using appropriate equipment, devices, and accessories. The early detection and treatment of complications that may occur in high-risk, critically-ill, sedated, or anesthetized patients undergoing MRI procedures can prevent relatively minor problems from becoming life-threatening situations.
GENERAL POLICIES AND PROCEDURES
Monitoring during an MRI examination is indicated whenever a patient requires observations of vital physiologic parameters due to an underlying health problem or is unable to respond or alert the MRI technologist or other healthcare worker regarding pain, respiratory problem, cardiac distress, or difficulty that might arise during the examination. In addition, a patient should be monitored if there is a greater potential for a change in physiologic status during the MRI procedure. Table 1 summarizes the patients that may require monitoring and support during MRI procedures. Besides patient monitoring, various support devices and accessories may be needed for use in high-risk patients to ensure safety.
Because of the widespread use of MRI contrast agents and the potential for adverse effects or idiosyncratic reactions to occur, it is prudent to have appropriate monitoring equipment and accessories readily available for the proper management and support of patients who may experience side-effects. This is emphasized because adverse events, while extremely rare, may be serious or life threatening.
In 1992, the Safety Committee of the Society for Magnetic Resonance Imaging published guidelines and recommendations concerning the monitoring of patients during MRI procedures. This information indicates that all patients undergoing MRI procedures should, at the very least, be visually (e.g., using a camera system) and/or verbally (e.g., intercom system) monitored, and that patients who are sedated, anesthetized, or are unable to communicate should be physiologically monitored and supported by the appropriate means.
Severe injuries and fatalities have occurred in association with MRI procedures. These may have been prevented with the proper use of monitoring equipment and devices. Importantly, guidelines issued by The Joint Commission (TJC) indicate that patients receiving sedatives or anesthetics require monitoring during administration and recovery from these medications. Other professional organizations similarly recommend the need to monitor certain patients using proper equipment and techniques.
Table 1. Patients that require monitoring and support during MRI procedures.
-Physically or mentally unstable patients.
-Patients with compromised physiologic functions.
-Patients who are unable to communicate.
-Neonatal and pediatric patients.
-Sedated or anesthetized patients.
-Patients undergoing MR-guided interventional procedures.
-Patients who may have a reaction to an MRI contrast agent.
-Critically ill or high-risk patients.
SELECTION OF PARAMETERS TO MONITOR
The proper selection of the specific physiologic parameter(s) that should be monitored during the MRI procedure is crucial for patient safety. Various factors must be considered including the patient's medical history, present condition, the use of medication and possible side effects, as well as the aspects of the MRI procedure to be performed. For example, if the patient receives a sedative, respiratory rate, apnea, and/or oxygen saturation should be monitored. If the patient requires general anesthesia during the MRI procedure, monitoring multiple physiologic parameters is required.
Policies and procedures for the management of the patient in the MRI environment should be comparable to those used in the operating room or critical care setting, especially with respect to monitoring and support requirements. Specific recommendations for physiologic monitoring of patients during MRI procedures should be developed in consideration of "standard of care" issues as well as in consultation with anesthesiologists and other similar healthcare specialists.
PERSONNEL INVOLVED IN PATIENT MONITORING
Only healthcare professionals with appropriate training and experience should be permitted to monitor patients during MRI procedures. The healthcare professional must be experienced with the operation of the monitoring equipment and accessories used in the MRI environment and should be able to recognize equipment malfunctions, device problems, and recording artifacts. Furthermore, the person responsible for monitoring the patient should be well-versed in screening patients for conditions that may complicate the procedure. For example, patients with asthma, congestive heart failure, obesity, obstructive sleep apnea, and other conditions are at increased risk for having problems during sedation. Also, the healthcare professional must be able to identify and manage adverse events using appropriate equipment and procedures in the MRI environment.
Policies and procedures must be implemented to continue appropriate physiologic monitoring and management of the patient by trained personnel after the MRI procedure is performed. This is especially needed for a patient recovering from the effects of a sedative or general anesthesia.
The monitoring of physiologic parameters and management of the patient during an MRI procedure may be the responsibility of one or more individuals depending on the level of training for the healthcare worker and in consideration of the condition, medical history, and procedure that is to be performed for the patient. These individuals include anesthesiologists, nurse anesthetists, nurses, and properly trained and certified MR technologists/radiographers, or radiologists.
The development, implementation, and regular practice of an emergency plan that addresses and defines the activities, use of equipment, and other pertinent issues related to a medical emergency are important for patient safety. For example, a plan needs to be developed for handling patients if there is the need to remove them from the MR system room to perform cardiopulmonary resuscitation. Obviously, taking necessary equipment such as a cardiac defibrillator, intubation instruments, or other similar devices near the MR system could pose a substantial hazard to the patients and healthcare workers if these are not safe for use in the MRI environment. Healthcare professionals that are members of the cardiopulmonary resuscitation (i.e., code blue) team must be trained to conduct their activities in the MRI environment.
For out-patient or mobile MR facilities, it is necessary to educate outside emergency personnel (e.g., paramedics, firefighters, etc.) regarding the potential hazards associated with the MRI environment. Typically, MRI facilities not affiliated with or in close proximity to a hospital must contact paramedics to handle medical emergencies and to transport patients to the hospital for additional care. Therefore, personnel responsible for summoning the paramedics, notifying the hospital, and performing other integral activities must be designated beforehand to avoid problems and confusion during an actual emergent event.
TECHNIQUES AND EQUIPMENT USED TO MONITOR AND SUPPORT PATIENTS
Physiologic monitoring and support of patients is not a trivial task in the MRI environment. A variety of potential problems and hazards exist. Furthermore, the types of equipment for patient monitoring and support must be considered carefully and implemented properly to ensure the safety of both patients and MR healthcare professionals.
Several potential problems and hazards are associated with the performance of patient monitoring and support in the MRI environment. Physiologic monitors and accessories that contain ferromagnetic components (e.g., transformers, outer casings, etc.) can be strongly attracted by the static magnetic field used of the MR system, posing a serious missile or projectile hazard to patients and healthcare workers.
If possible, necessary or critical devices that have ferromagnetic components should be permanently fixed to the floor or tethered to the wall and properly labeled with warning information to prevent them from being moved too close to the MR system. All personnel involved with MRI procedures should be aware of the importance of the placement and use of the equipment, especially with regard to the hazards of moving portable equipment too close to the MR system.
Electromagnetic fields associated with the MR system can significantly effect the operation of monitoring equipment, especially those with displays that involve electron beams (i.e., CRTs) or video display screens (with the exception of those with liquid crystal displays or LCDs). In addition, the monitoring equipment, itself, may emit spurious noise that, in turn, produces distortion or artifacts on the MR images.
Physiologic monitors that contain microprocessors or other similar components may leak RF, producing electromagnetic interference that can substantially alter MR images. To prevent adverse radiofrequency-related interactions between the MR system and physiologic monitors, RF-shielded cables, RF filters, special outer RF-shielded enclosures, or fiber-optic techniques can be utilized to prevent image-related or problems in the MRI environment.
During the operation of MR systems, electrical currents may be generated in the conductive materials of monitoring equipment that are used as part of the interface to the patient. These currents can be of sufficient magnitude to cause excessive heating and thermal injury to the patient. Numerous first, second, and third degree burns have occurred in association with MRI procedures that were directly attributed to the use of monitoring devices. These injuries were related to the use of electrocardiographic lead wires, plethysmographic gating systems, pulse oximeters, and other types of monitoring equipment and accessories comprised of wires, cables, or similar components made from conductive materials.
Therefore, in consideration of the various problems and hazards associated with the use of monitoring equipment and accessories, it is important to follow the instructions and recommendations from the manufacturers with regard to the use of the devices in the MRI environment.
MONITORING EQUIPMENT AND SUPPORT DEVICES
This section describes the physiologic parameters that may be assessed in patients during MRI procedures using appropriate monitoring equipment for the MRI environment. In addition, various devices and accessories that are useful for the support and management of patients are presented.
Electrocardiogram and Heart Rate
Monitoring the patient's electrocardiogram (ECG) in the MRI environment is particularly challenging because of the inherent distortion of the ECG waveform that occurs using MR systems operating at high field strengths. This effect is observed as blood, a conductive fluid, flows through the large vascular structures in the presence of the static magnetic field of the MR system. The resulting induced biopotential is seen primarily as an augmented T-wave amplitude, although other non-specific waveform-changes are also apparent on the ECG. Since altered T-waves or ST segments may be associated with cardiac disorders, static magnetic field-induced ECG-distortions may be problematic for certain patients. For this reason, it may be necessary to obtain a baseline recording of the ECG prior to placing the patient inside the MR system along with a recording obtained immediately after the MRI procedure to determine the cardiac status of the patient.
Additional artifacts caused by the static, gradient, and RF electromagnetic fields of the MR system can severely distort the ECG, making observation of morphologic changes and detection of arrhythmias quite difficult. ECG artifacts that occur in the MRI environment may be decreased substantially by implementing several simple techniques that include, the following:
-Use ECG electrodes that have minimal metal or those recommended by the manufacturer
-Select electrodes and cables that contain nonferromagnetic metals
-Place the limb electrodes in close proximity to one another
-Position the line between the limb electrodes and leg electrodes parallel to the magnetic field flux lines
-Maintain a small area between the limb and leg electrode
-Twist or braid the ECG leads
-Position the area of the electrodes near or in the center of the MR system
The use of proper ECG electrodes (i.e., those tested and deemed to be acceptable for patients) is required to ensure patient safety and proper recording of the electrocardiogram in the MRI environment. Accordingly, ECG electrodes have been specially developed for use during MRI procedures to protect the patient from potentially hazardous conditions. These ECG electrodes were also designed to reduce MRI-related artifacts.
The use of standard ECG electrodes, leads, and cables may cause excessive heating that could burn the patient during an MRI procedure. This occurs as electrical current is generated in the ECG cable or leads during the operation of the MR system. Accordingly, monitoring equipment has been modified to record the ECG while ensuring patient safety in the MRI environment.
Fiber-optic ECG recording techniques may be used to prevent burns during MRI procedures. For example, one such fiber-optic system acquires the ECG waveform using a special transceiver that resides in the MR system bore along with the patient and is located very near the ECG electrodes. A module digitizes and optically encodes the patient's ECG waveform and transmits it out from the MR system to the monitor using a fiber-optic cable. The use of this fiber-optic ECG technique eliminates the potential for burns associated with hard-wired ECG systems by removing the conductive patient cable and its antenna effect that are typical responsible for excessive heating.
Besides using an ECG monitor, the patient's heart rate may be determined continuously during the MRI procedure using various types of acceptable devices including a photoplethysmograph and a pulse oximeter. A noninvasive, heart rate and blood pressure monitor (see section below) can also be utilized to obtain intermittent or semi-continuous recordings of heart rate during the MRI procedure.
Conventional, manual sphygmomanometers may be adapted for use during MRI procedures. This is typically accomplished by lengthening the tubing from the cuff to the device so that the mercury column and other primary components may be positioned an acceptable distance (e.g., 8 to 10 feet from the bore of a 1.5-Tesla MR system, at about the 200 gauss level) from the fringe field of the MR system.
Blood pressure measuring devices that incorporate a spring-gauge instead of a mercury column may be adversely affected by magnetic fields, causing them to work erroneously in the MR setting. Therefore, spring-gauge blood pressure devices should undergo pre-clinical testing before being used to monitor patients undergoing MRI procedures.
Blood pressure monitors that use other noninvasive techniques, such as the oscillometric method, may be used to obtain semi-continuous recordings of systolic, diastolic, and mean blood pressures as well as pulse rate. These devices can be utilized to record systemic blood pressure in adult, pediatric, and neonate patients, selecting the appropriate blood pressure cuff size for a given patient. The intermittent inflation of the blood pressure cuff by an automated, noninvasive blood pressure monitor may disturb lightly sedated patients, especially infants or neonates, causing them to move and disrupt the MRI procedure. For this reason, the use of a noninvasive blood pressure monitor may not be the best instrument to conduct physiologic monitoring in every patient.
Respiratory Rate and Apnea
Because respiratory depression and upper airway obstruction are frequent complications associated with the use of sedatives and anesthetics, monitoring techniques that detect a decrease in respiratory rate, hypoxemia, or airway obstruction should be used during the administration of these drugs. This is particularly important in the MRI environment because visual observation of the patient's respiratory efforts is often difficult.
Respiratory rate monitoring can be performed during MRI procedures by various techniques. The impedance method that utilizes chest leads and electrodes (similar to those used to record the ECG) can be used to monitor respiratory rate. This technique of recording respiratory rate measures a difference in electrical impedance induced between the leads that correspond to changes in respiratory movements. Unfortunately, the electrical impedance method of assessing respiratory rate may be inaccurate in pediatric patients because of the small volumes and associated motions of the relatively small thorax.
Respiratory rate may also be monitored using a rubber bellows placed around the patient's thorax or abdomen (i.e., for chest or belly breathers). The bellows is attached to a pressure transducer that records body movements associated with inspiration and expiration. However, the bellows monitoring technique, like the electrical impedance method, is only capable of recording body movements associated with respiratory efforts. Therefore, these respiratory rate monitoring techniques do not detect apneic episodes related to upper airway obstruction (i.e., absent airflow despite respiratory effort) and may not provide sufficient sensitivity for assessing patients during MRI procedures. For this reason, assessment of respiratory rate and identification of apnea should be accomplished using more appropriate monitoring devices.
Respiratory rate and apnea may be monitored during MRI procedures using an end-tidal carbon dioxide monitor or a capnometer. These devices measure the level of carbon dioxide during the end of the respiratory cycle (i.e., end-tidal carbon dioxide). Additionally, capnometers provide quantitative data with respect to end-tidal carbon dioxide that is important for determining certain aspects of gas exchange in patients. The waveform provided on the end-tidal carbon dioxide monitors is also useful for assessing whether the patient is having difficulties breathing. The interface between the patient for the end-tidal carbon dioxide monitor and capnometer is a nasal or oro-nasal cannula that is made from plastic. This interface prevents potential adverse interactions between the monitor and the patient during an MRI procedure.
Oxygen saturation is a crucial variable to measure in sedated and anesthetized patients. This physiologic parameter is measured using pulse oximetry, a monitoring technique that assesses the oxygenation of tissue. Because oxygen saturated blood absorbs differing quantities of light compared with unsaturated blood, the amount of light that is absorbed by the blood can be readily used to calculate the ratio of oxygenated hemoglobin to total hemoglobin and displayed as the oxygen saturation. Additionally, the patient's heart rate may be calculated by measuring the frequency that pulsations occur as the blood moves through the vascular bed. Thus, a pulse oximeter can be used to determine oxygen saturation and pulse rate on a continuous basis by measuring the transmission of light through a vascular site such as the ear lobe, finger-tip, or toe. Notably, anesthesiologists consider the use of pulse oximetry to be the standard practice for monitoring sedated or anesthetized patients.
Commercially-available, specially-modified pulse oximeters that have hard-wire cables have been used to monitor sedated patients during MRI procedures with moderate success. Unfortunately, these pulse oximeters tend to work intermittently during the operation of the MR system, primarily due to interference from the gradient and/or radio frequency electromagnetic fields. Of greater concern is the fact that pulse oximeters with hard-wire cables have been responsible for many patient burn injuries, presumably as a result of excessive current being induced in the conductive cables.
Pulse oximeters have been developed that use fiber-optic technology to obtain and transmit the physiologic signals from the patient. These devices operate without interference by the electromagnetic fields used for MRI procedures. It is physically impossible for a patient to be burned by the use of a fiber-optic pulse oximeter during an MRI procedure because there are no conductive pathways formed by any metallic materials that connect to the patient. There are several different fiber-optic pulse oximeters that are commercially available for use in the MRI environment.
There are several reasons to monitor skin and/or body temperatures during MRI procedures. These include recording temperatures in neonates with inherent problems retaining body heat (a tendency that is augmented during sedation), in patients during MRI procedures that require high levels of RF power, and in patients with underlying conditions that impair their ability to dissipate heat.
Skin and body temperatures may be monitored during MRI procedures using a variety of techniques. However, it should be noted that the use of hard-wire, thermistor or thermocouple-based techniques to record temperatures in the MRI environment may cause artifacts or erroneous measurements due to direct heating of the temperature probes. A more effective and easier technique of recording temperatures during MRI procedures is with the use of a fluoroptic thermometry system. Notably, monitoring skin temperature, alone, is insufficient to ensure patient safety as this parameter does not provide proper information relative to deep body temperature.
The fluoroptic monitoring system has several important features that make it particularly useful for temperature monitoring during MRI procedures. For example, the device incorporates fiber-optic probes that are small but efficient in carrying optical signals over long paths, it provides noise-free applications in electromagnetically hostile environments, and has fiber-optic components that will not pose a risk to patients.
Multi-Parameter Physiologic Monitoring Systems
In certain cases, it is necessary to monitor several different physiologic parameters simultaneously in patients undergoing MRI procedures. While several different stand-alone units may be used to accomplish this task, the most efficient means of recording multiple parameters is by utilizing a monitoring system that permits the measurement of different physiologic functions such as heart rate, respiratory rate, blood pressure, and oxygen saturation.
Currently, there are a number of multi-parameter patient monitoring systems that are acceptable for use in the MRI environment. Typically, these devices are designed with components positioned within the MR system room and incorporate special circuitry to substantially reduce the artifacts that effect the recording of ECG and other physiologic variables, making them also useful for the performance of gated MRI procedures.
Devices used for mechanical ventilation of patients typically contain mechanical switches, microprocessors, and ferromagnetic components that may be adversely effected by the electromagnetic fields used by MR systems. Ventilators that are activated by high pressure oxygen and controlled by the use of fluidics (i.e., no requirements for electricity) may still have ferromagnetic parts that can malfunction as a result of interference from MR systems.
Some ventilators only operate properly and are approved at a “safe” distance from the MR system due to the presence of ferromagnetic parts, including batteries. Thus, these devices are labeled “MR Conditional’ and the conditions for use must be followed carefully. Therefore, it is advisable to permanently fix these ventilators to the floor or tether them to prevent missile-related accidents. Furthermore, these devices must be properly labeled with warning information to prevent them from being moved too close to the MR system. Healthcare workers should be trained regarding the specific MR conditional aspects of these ventilators.
Additional Devices and Accessories
A variety of devices and accessories are often necessary for support and management of high-risk or sedated patients in the MRI environment. Gurneys, oxygen tanks, stethoscopes, suction devices, infusion pumps, power injectors, and other similar devices and accessories shown to be acceptable for the MRI environment may be obtained from various manufacturers and distributors. Additionally, there are gas anesthesia systems available that have been designed for use in patients undergoing MRI procedures.
Whenever sedatives are used, it is imperative to perform physiologic monitoring to ensure patient safety. In addition, it is important to have the necessary equipment readily available in the event of an emergency. These requirements should also be followed for patients undergoing sedation in the MRI environment.
There is controversy regarding who should be responsible for performing sedation of patients in the MRI environment. (For the sake of discussion, the terms "sedation" and "anesthesia" are used interchangeably since they are actually part of the same continuum.) Obviously, there are medical, regulatory, administrative, and financial issues to be considered.
In general, for patients with conditions that complicate sedation procedures, a nurse under the direction of an anesthesiologist or a specially trained radiologist may be responsible for preparing, sedating, monitoring, and recovering these cases. However, for patients with serious medical or other unusual problems, it is advisable to utilize anesthesia consultation to properly manage these individuals before, during, and after MRI procedures.
In addition, the MRI facility should establish policies and guidelines for patient preparation, monitoring, sedation, and management during the post-sedation recovery period. These policies and guidelines should be based on standards established by the American Society of Anesthesiologists (ASA), the American College of Radiology (ACR), the American Academy of Pediatrics Committee on Drugs (AAP-COD) and the Joint Commission on Accreditation of Healthcare Organizations (JCAHO).
For example, Practice Guidelines for Sedation and Anesthesia from the American Society of Anesthesiologists indicate that a person must be present that is responsible for monitoring the patient if sedative or anesthetic medications are used. Furthermore, the following aspects of patient monitoring must be performed:
1) Visual monitoring
2) Assessment of the level of consciousness
3) Evaluation of ventilatory status
4) Evaluation of oxygen status assessed via the use of pulse oximetry
5) Determination of hemodynamic status via the use of blood pressure monitoring and electrocardiography if significant cardiovascular disease is present in the patient
Healthcare professionals must be able to recognize complications of sedation such as hypoventilation and airway obstruction as well as be able to establish a patent airway for positive-pressure ventilation.
Special patient screening must be conducted to identify conditions that may complicate sedation in order to properly prepare the patient for the administration of a sedative. This screening procedure should request important information from the patient that includes the following: major organ system disease (e.g., diabetes, pulmonary, cardiac, or hepatic disease), prior experience or adverse reactions to sedatives or anesthetics, current medications, allergies to drugs, and a history of alcohol, substance, or tobacco abuse.
In addition, the nothing by mouth (NPO) interval for the patient must be determined to reduce the risk of aspiration during the procedure. The ASA "Practice Guidelines Regarding Preoperative Fasting" recommend a minimum NPO periods of two hours for clear liquids, four hours for breast milk, six hours for infant formula, and six hours for a light meal. The NPO period is extremely important because sedatives may depress the patient's gag reflex.
Administration of Sedation
A thorough discussion of sedation techniques especially with regard to the use of various pharmacologic agents is outside the scope of this monograph. Therefore, interested readers are referred to the excellent, comprehensive review of this topic written by Reinking Rothshild (2000), an anesthesiologist with extensive experience sedating patients in the MRI environment. A review of more recent peer-reviewed papers on this topic is also recommended.
During the use of sedation, written records should be maintained that indicate the patient's vital signs as well as the name, dosage, and time of administration for all drugs that are given to the patient. The use of a time-based anesthesia-type record, such as that recommended by Reinking Rothschild (2000), is the best means of maintaining written documentation for sedation of patients in the MRI environment.
After sedation, the medical care of the patient must continue. This is especially important for pediatric patients because certain medications have relatively long half-lives (e.g., chloral hydrate, pentobarbitol, etc.). Therefore, an appropriate room with monitoring and emergency equipment must be available to properly manage these patients.
Prior to allowing the patient to leave the MRI facility, the patient should be alert, oriented, and have stable vital signs. In addition, a responsible adult should accompany the patient home. Written instructions that include an emergency telephone number should be provided to the patient.
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