Question: How should we monitor a child having general anesthesia for a MRI scan?
Answer: In monitoring patients during the performance of Magnetic Resonant Imaging (MRI, Nuclear Magnetic Resonance, NMR), the anesthetist must be aware of two physical forces that affect the performance of anesthesia equipment. One is constant, the other is not; both, however, require the anesthesiologist to adjust his expectations in the clinical situation.
The first is a constant, strong magnetic field from the large-magnet instruments. It is most powerful at the center of the machine and fade-s as the distance from the electromagnet increases. This prevents the use of ferromagnetic materials such as iron, cobalt, nickel and some forms of stainless steel. The other is radio frequency (RF signals) of 5-8 min duration directed towards the patient which produce a strong background noise in the monitoring equipment.
It is an irony that with this sophisticated, advanced technological procedure, simple devices are more suitable than complicated electronic equipment. Through trial and error, we have found that simple instruments made of nonferromagnetic material (Mapleson T-piece, precordial stethoscope, blood pressure dial), when used with common sense, will provide the basic clinical information before and during the anesthetic in this out-of-the-ordinary environment.
Anesthesiologist: As in any clinical procedure, the most important monitor is the anesthesiologist himself. One of the advantages of MRI is that magnetic fields cause no injury to living organisms. The anesthesiologist can therefore remain near the patient’s side without any fear that harm will come to him/her. However, individuals having pacemakers or ferromagnetic vascular clips should avoid these strong magnetic fields.
Anesthetic machine: There is a strong pull by the magnet on the typical anesthetic machine (especially on the gas cylinders) because of the presence of a large amount of ferromagnetic material. Some countermeasures may be employed to circumvent this problem. First, the machine should be outfitted with aluminum tanks. By experience, we have discovered that the magnetic pull does not affect the functioning of the anesthetic machine or the oxygen monitor if the machine is kept in the comer of the room. To this end we modified a simple Mapleson D-system by interposing a very long inspiratory limb (6 meters) to administer the necessary anesthetic gases and the supplementary oxygen. The expiratory valve is made of aluminum with a scavenging port.
Precordial -Stethoscope: A brass precordial stethoscope (available commercially) with plastic tubing may safely he used. Though the sounds of radio frequency partially interfere with the quality of the heart sounds, a discerning ear can reliably distinguish one from the other.
Blood Pressure: Oscillatory blood pressures can be easily obtained by adding long wide bore tubing to the cuff, and using brass or plastic adapters for the junctions. As long as the dial is kept away from the magnet no interference is detected. Electrocardiogram: Our unit is provided with a wireless electrocardiogram that provides adequate display in the absence of an RF signal. The RF signal does produce a background noise, but the QRS interval can be detected with some concentration.
C02 analyzer. C02 monitors that use a suctionin8 mechanism to sample gases need only minor modification. The motor should be switched to high power and a longer tubing introduced. In the sedated patient breathing spontaneously, the end of the tubing can be taped near the nostril. The endotracheal tube adapters are usually made of aluminum and present no problem.
Pulse oximeter: the pulse oximeter can be used during the initial stages of adjustment and then when the scout images are taken. During the actual scanning when the RF signal is on (5-8 min periods) the signal from the pulse oximeter will interfere with imaging. It must therefore be turned off during these times. During the critical periods of adjusting and positioning, the pulse oximeter is always on.
Temperature: Any of the commercially available liquid crystal thermometers can be used to read the skin temperature
Practical implications: Initially, we had difficulty in determining whether a material was ferromagnetic or not. A simple test was to hold the piece of equipment tightly in the hand and move it slowly near the magnet; if a pull was felt, we did not use that particular instrument. It was interesting to notice that similar looking equipment had different magnetic pulls. We also found that it was much easier and more comforting to anesthetize or sedate the patient at a distance from the magnet; at this critical time, one should not have to worry about every piece of equipment. Then, once the patient had been “stabilized” and the monitors checked, he can be moved near and into the magnetic field. As an extra safety measure, all loose equipment is taped to the frame that the patient is lying on.
Anesthetic Technique: Any anesthetic technique can be used during MM. In our practice, we found IM methohexital (30 mg/kg) to he satisfactory in children without any major neurological abnormalities. In children on anticonvulsant medications, IM methohexital (8-10 mg/W will provide a more predictable result. In patients with airway or major neurological problems, or in cases of failure with methohexital, general endotracheal anesthesia can be administered. Since such patients require only a light level of anesthesia, most can be discharged for home on the day of their procedure
Answer by N. Goudsouzian, M.D., Director, Pediatric Anesthesia, Mass. General Hospital, Boston.
Addendum:
The MRI scanner has presented us with unique challenges in patient monitoring. The most obvious difficulty is in closely observing the patient, because of his location in the long tunnel of the scanner that is not always well lit. The distance also makes it difficult to hear breath and heart sound through the necessarily long tubing of the precordial stethoscope. Auscultation is further complicated by the loud noise coming from the scanner.
Our use of electronic monitoring is also severely compromised, because the electromagnetic radiation emanation from the scanner tends to jam the monitoring equipment and the monitors tend to emit signals that interfere with the proper functioning of the scanner. Any electric monitoring cables that travel to the patient from the monitors act as antennae carrying stray signals to the scanner and thereby markedly reducing the quality of the image.
Use of television cameras will probably become commonplace in the scanner rooms. Because magnetic fields distort electron beams, traditional television cameras with vacuum tube image detection are not satisfactory. Solid state cameras will be useful because there is no electron beam that is deviated by the magnetic field. In addition, they can operate in low levels of light. By placing the camera in the right location, it could be possible to obtain a clear view of the patient’s face with the aid of a zoom lens. The television monitor has to relocated at a significant distance from the scanner. The future availability of large flat solid-state color monitors will make it possible to place the monitor in most convenient locations.
High quality heart and lung sounds can be obtained with electronic assistance. The precordial stethoscope can be connected to a length of wide bore tubing and the other end connected to a condenser microphone. The scanner may, however, make so much noise it would be difficult to hear the heart and lung sounds. The machine noise can easily be reduced by using two concentric layers of tubing and by electronically filtering out high frequency sounds. The patient sounds are then monitored over a speaker or can be transmitted to the anesthesiologist over an infra-red light beam.
Pulse oximetry also tends to interfere with the performance of the MRI scanner and is affected by the scanner. Our attempts to correct this by use of a fiber-optic pulse oximeter have been unsuccessful in that the fiber-optic cable acts as an antenna carrying stray signals in both directions. We hope that by shielding the cable of the oximeter, the monitor will be able to perform without distorting the image of the MRI scanner.
Addendum by Francis X. Vacanti, M.D., Mass. General Hospital, Boston