January 2002 / Volume 47 / Number 1 / Page 61
Modification of a Critical Care Ventilator for Use During Magnetic Resonance Imaging
Since the first description of normal human magnetic resonance imaging (MRI) in 1981, the use of MRI has increased dramatically. MRI images have been particularly valuable for detecting abnormalities in the brain, spinal cord, spine, and major joints not readily apparent by conventional imaging techniques.
For mechanically ventilated patients, the electromagnetic environment of the MRI scanner raises several technical issues regarding safety and monitoring. The strong magnetic and radio frequency fields produced by the MRI scanner, for example, have been hypothesized to prevent the electronic components of most critical care ventilators from functioning properly. Electromagnetic interference from intensive care ventilators may also interfere with the MRI process, potentially producing artifacts in the finished scans. Finally, the strong magnetic field generated by the MRI magnet raises the possibility that the ventilator could be physically drawn toward the magnet and thus present a projectile hazard to the patient or to care personnel.
Abnormalities in ventilator monitoring and alarm systems induced by electromagnetic interference may also increase the risk of patient harm. In pressure-control mode, tidal volume (VT) varies because of changes in lung compliance or airways resistance, so VT must be precisely monitored to avoid catastrophic hypoventilation. A malfunction of critical alarm or monitoring functions because of electromagnetic interference from the MRI scanner could be disastrous.
Some ventilators have been designed specifically for use during MRI, but most of them lack the flexibility in mode and air flow characteristics, sophisticated monitoring and alarm systems, flow delivery with high airway pressures, and familiarity of operation typical of standard critical care ventilators. For patients who require high flows or pressures to maintain adequate gas exchange, use of a critical care ventilator during MRI scanning would provide a greater margin of safety throughout the procedure and possibly reduce errors resulting from unfamiliarity with the ventilators designed for MRI use. We therefore systematically evaluated the Siemens Servo 900C ventilator for use under electromagnetic conditions typical of the MRI environment. We then evaluated ventilator performance in the MRI suite, using a test lung and with critically ill patients during active scanning conditions.