MRI and Anesthesia

Sedation versus general anesthesia in MRI

M.P. Boidin MD, PhD, G.R. Wolff MD, C. Doelman MD
Afdeling Anesthesiologie
Amphia Ziekenhuis Breda
Breda, The Netherlands

Email: mpboidin@wanadoo.nl

The article also available in PDF: 91 KB

Introduction

Based on our observations, anesthetists tend to prefer general anesthesia for diagnostic procedures rather than only sedation, mostly because general anesthesia is regarded as safe, controllable, and relatively easy to perform. The effect of sedation, however, is sometimes unpredictable. From literature, it appears that sedation has a relatively high failure rate (15 %) [1-2], and it is most certainly not free of mortality and morbidity [3–4]. However, general anesthesia cannot be organized in every unit performing MRI investigations, and there would be a need for ventilators and monitors [5-6] specially designed for the MRI room.

In practise, anesthetists in small hospitals have to deal with both these patients and with requests from radiologists, pediatricians, and other clinical staff. There are various general protocols and standard operating procedures [7–13] which have been made by medical and nursing organizations and societies. They are often general in nature, and thus the anesthetists should design specific protocols to be used in their own hospitals for their personnel. This article, which reflects our own experiences in the clinic, presents a discussion of whether to perform these investigations under general anesthesia or under sedation.

Sedation or general anesthesia

Conscious sedation, as described by the American Academy of Pediatrics [9-10], is "a medically induced state of CNS depression in which communication is maintained so that the patient can respond to verbal commands".

Deep sedation is defined as "a medically induced state of CNS depression in which the patient is essentially unconscious and does not respond to verbal commands". In such a deep state, it is typical that the patient breathes spontaneously. However, protective reflexes may be lost and the maintenance of a free airway is not assured.

When sedation is concerned, there is a spectrum from light to deep, and it is possible to move from conscious sedation to deep sedation without recognition. In fact, the varying levels of deep sedation may, in the end, overlap with general anesthesia.

The effectiveness of orally and rectally administered sedatives is highly unpredictable [1–4]. Differences in absorption and a wide variety in individual sensitivity to drug effects cause 10 - 20 % of all sedation cases to be unpredictable. Hence, deep sedation also requires a rigid protocol with intensive monitoring and attending personnel [12-13], In general, there is no doubt that there is a fair place for properly organized conscious and deep sedation, providing that supervision of these activities has been arranged and personnel has been adequately trained. Background information on the subject can be obtained from the subsequent guidelines of the Royal College of Anaesthetists and of Radiologists and of the American Academy of Pediatrics [7–13].

Sedation

When sedatives are administered, patients should have a qualified person present, who should monitor patient’s physiologic parameters. For us, APLS or Advanced Pediatric Life Support training is a prerequisite. Before sedation is even considered, adequate interviews and explanations to parents should be a routine in every hospital. The contra-indications to sedation should be considered in every case. Monitors should be routinely in place when sedation is applied.

Some clinically relevant contraindications to sedation:

• conditions with an increased risk of pulmonary aspiration
• possibility of airway obstruction or respiratory irregularities
• raised intracranial pressure, or other conditions where increased PaCO2 could be dangerous
• conditions where respiratory center is desensitised to carbon dioxide
• renal or hepatic dysfunction, which may alter drug kinetics
• unpredictable drug effect, as sedatives may increase restlessness

In a study of a group of small (5-10 kg) children which was published in London [14], 1 % of the children needed additional intravenous sedatives during the investigation. In children of 3 - 7 years of age, the need of additional intravenous sedation increased to 30%. Understandably, children with educational problems and children with claustrophobia needed more sedation.

In the study of Malviya et. al. [15], it was concluded that increased morbidity during sedation was associated with age, ASA classification, and the use of benzodiazepines as the sole sedative agent. Sedation does not always work as expected, and sedation is certainly not without its risks – quite opposite to what many clinicians may believe. Our clinical observation is that general anesthesia usually results in a stable and controlled situation when we have a well-monitored and immobile patient in the hands of an anesthetic team in the MRI room.

Drugs for sedation

Several drugs have been used for sedation. Here we briefly discuss some of our own observations and experiences which we have found useful in our department. The following short list gives an overview of some options available, it does not try to be comprehensive recommendation and it is not intended as a guideline for any other clinical setting.

• For us, chloral hydrate seems to be very useful and safe when administered both orally or rectally. We have used it in children weighing up to 10 kilograms with good effect.
• The short acting benzodiazepine midazolam is frequently used for IV-sedation in adults and children. It appears to have much greater potential for respiratory depression in the elderly than in children. It should be used with great caution together with opioids. It is important to know that its sedative action is also increased when given at the same time as erythromycin.
• Flumazenil is the antidote to benzodiazepines, a competitive inhibitor in a similar manner like naloxone for opioids. It has been advised not to use it routinely to "reverse" sedation. It is good to bear in mind that the effect of flumazenil is generally shorter than the potential effect of the sedative drugs. Hence, prolonged observation of the patient is necessary.

MRI: sedation versus anesthesia

MRI investigations are frequently ordered by neurologists for all aged patients who have neurological disorders. Patients with neurological disorders are frequently restless, and sedation may be insufficient to immobilize patients in this pre-selected group. Children may have rather high ASA 3-4 classifications, because they may have complex syndromes and multiple organ dysfunction. Adequate monitoring and controlled circumstances are important in these cases.

When patients have a psychiatric history or fear syndrome, they may frequently be restless and may require deep sedation. Some patients may indeed be resistant to sedatives and their restlessness increases even despite sedation.

Concerning safety, it is important to keep in mind that during sedation, hypoxemia can also cause restlessness. The long duration of investigations may cause severe discomfort for the patients, resulting in a poor quality images. During MRI, the patients should remain in the same position without moving, like in the still photographs of the past. The measurement period can last up to 20 minutes. The better the patient is immobilized, the sharper the image. Movement of patient gives blurry pictures that do not result in a clear diagnosis.

Discussion

State of the art techniques enable the high level of care required by modern anesthesia, even in the proximity of a strong permanent magnet field. Today, MRI compatible monitors and ventilators are commercially available. Therefore, these instruments should be used in all MRI cases under general anesthesia. As a practical solution in our hospital, one of the routine OR ventilators has been selected for its MRI compatibility, and it is also used in anesthesia in the shielded MRI room.

When indicated, it is possible to consider sedation or to try a technique without sedation e.g. with healthy, normal children. Considering the high level of failures, morbidity and even mortality, adequate monitoring and trained personnel are always necessary when any sedation is applied. The MRI compatible monitor, which is mandatory in general anesthesia, is also well-suited for sedated patients.

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Last updated: 1 August 2002Created
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