The need for deep sedation

As movement interferes with effective MRI, a challenging problem is provided by patients unable to lie still. Conscious sedation is unable to guarantee patient compliance and therefore a deeper level of sedation is required. Infants may go to sleep with a feed or be adequately sedated with oral chloral hydrate. A recent series of 1155 nurse led sedations for MRI at Great Ormond Street Hospital (London, UK) had a 1% failure rate for children weighing 5–10 kg with only 8% of this group needing intravenous in addition to oral sedative drugs.6 Children older than 7 years can often comply with instructions to remain still. Intravenous sedation is thus required for many of those between 1 and 7 years, and some older children with learning difficulties or claustrophobia; this was needed in almost 30% of children heavier than 10 kg in the aforementioned study.6

The nature of the MRI hardware precludes easy access to the patient whose head lies over one metre inside the housing of the electromagnetic coils of the scanner. This differentiates MRI scanning from other situations in which deep sedation can be used, such as minor procedures in the accident and emergency department or endoscopy in which immediate access to the child and its airway is not an issue. As a consequence, difficulty of access is the principal argument concerning the safety of deep sedation compared to general anaesthesia. Additionally, MRI requires specially designed equipment that can function within a powerful magnetic field, that does not degrade the image by interference, and that does not cause injuries from currents induced by strong magnetic fields. Although such specialised equipment is expensive, this should not be a significant consideration when safety is of paramount importance.

It is worth remembering that the definition of conscious sedation given by the American Academy of Pediatrics4 is “a medically induced state of CNS depression in which communication is maintained so that the patient can respond to verbal command”. Protective reflexes are preserved and the patient can independently maintain a patent airway. Meanwhile deep sedation is defined as “a medically induced state of CNS depression in which the patient is essentially unconscious, and so does not respond to verbal command”. The patient breathes spontaneously but protective reflexes may be lost and the ability to maintain an airway is not assured. The potential complications of deep sedation include hypoventilation, apnoea, airway obstruction, aspiration, hypotension, bradycardia, and increased intracranial pressure. A spectrum of sedation may exist, but with oral and intramuscular regimens in older children it is possible to move from conscious sedation to deep sedation without ready recognition. Intravenous sedation is more predictable in this group as it has an immediate effect and is much less reliant on other factors such as absorption. However, it has also been suggested that there are varying levels of deep sedation at the end of which is an overlap with general anaesthesia.4

Safe practice guidelines

Present American paediatric guidelines4 augmented by a literature search suggests that if deep sedation is required then it should be performed by someone:

• who is working to an accepted guideline

• with sole responsibility for the sedation

• who has been trained to an acceptable level (such as, Advanced Paediatric Life Support provider status)

• who is familiar with the drugs, dosages, monitoring equipment, and requirements of the procedure

• who is supported by other skilled staff such as a children's nurse.

Contraindications to sedation exist6 7 and include:

• potential airway obstruction—for example, sleep apnoea

• respiratory centre abnormalities—for example, brain stem tumours

• respiratory centre desensitised to carbon dioxide—for example, conditions with chronically raised Paco ₂

• renal or hepatic dysfunction leading to altered drug kinetics

• conditions in which a rise in Paco ₂ would be detrimental—for example, raised intracranial pressure

• conditions with high risk of pulmonary aspiration of gastric contents.

Children should be prepared in a similar way to a child undergoing anaesthesia:

• informed consent for sedation taken

• children fasted by withholding milk and solids for four hours before sedation

• reliable intravenous access essential before, during, and after the procedure if intravenous drugs are used.

There has been much debate over appropriate drugs and their dosage, and those who sedate children will have their favourite regimens. From the literature it appears important that the person administering the drugs is familiar with them, and that cocktails of more than two drugs are to be avoided because of the unpredictability of drug interactions and the increased incidence of important side effects.8 9 Minimum doses should be used to achieve the necessary level of sedation and yet it is preferable to give a reasonable bolus of drug rather than attempting to titrate the dose by small repeated increments over a prolonged period.

Chloral hydrate is an extremely useful and safe oral drug and can be used with good effect in children up to 10 kg.6 For those requiring intravenous drugs midazolam is a short acting benzodiazepine that is frequently used for sedation in adults and children alike. In general it “seems to have much greater potential for respiratory depression in the elderly than in children”10; however, it should be used with extreme caution with fentanyl because of adverse interactions. It also has increased sedative action when given at the same time as erythromycin. It is often combined with pethidine or pentazocine, which potentiate its sedative effect. Flumazenil is the antidote to benzodiazepines and naloxone for opiates but they should not be routinely used to “reverse” the sedation. Their effect is generally shorter than the potential effect of the sedative drugs, which could lead to children becoming more sedated following discharge. Table 1 shows the usual and maximum dosages for commonly used sedatives and their antidotes.

After giving the sedative drugs it is helpful if the head is extended to avoid forward flexion, which may result in airway obstruction. Thereafter, monitoring the patient during and following the procedure is the cornerstone of safe practice.4 Pulse oximetry and close physical observation are essential. During sedation, oxygen saturation should stay above 93%. Observations including time of administration of drugs, time of achievement of sedation, and time to recovery should be recorded. Vital signs such as level of consciousness, pulse, respiratory rate, and oxygen saturation readings should be taken at five minute intervals during the procedure and any adverse events must be recorded.

Resuscitation equipment should be readily available and should include:

• oxygen

• bag and mask with oral airways

• suction apparatus

• intubation tubes and laryngoscope.

After the scan, observations should be continued and recorded until recovery of consciousness has been achieved and the child is responding to verbal commands in an appropriate way. Resuscitation equipment should be readily available in the recovery area.

Adverse events with sedation in children

Several case reports of adverse events are to be found in the literature; however, none provides data on the denominator, which would give an indication of the relative risk of sedation. In a poster publication by Coté et al, American data over 28 years for patients younger than 21 years was analysed.11 From 69 reports (39 patients < 4 years) there were 52 deaths (22 in patients < 4 years). Fourteen occurred in association with “computed tomography/MRI/radiology”. Of the common causes at all ages, drug overdose (n = 38) was most frequent followed by inadequate monitoring (n = 25), followed by premature discharge (n = 14), inadequate help (n = 9), drug interaction (n = 6), and drug error (n = 4). Twenty six occurred in hospital. No denominator is stated and an acknowledgement is given regarding the inherent limitations of such data.

General anaesthesia

Advantages may be achieved by using general anaesthesia instead of deep sedation. There should be fewer failures and there may be a faster turn round. However data are not available to determine whether it is a safer than deep sedation. The disadvantages of general anaesthesia include the need for dedicated anaesthetic equipment and a greater availability of paediatric anaesthetists.6 Indeed, it could be argued that children could have significant delays in gaining results from important investigations through long waiting lists for procedures carried out under general anaesthesia rather than sedation. This in turn might lead to an adverse outcome, which might have been avoided by a more timely investigation done under sedation.

Conclusion

The final argument as to whether deep sedation in children is safe for MRI is finely balanced. The only data on significant adverse reactions other than anecdotal evidence are offered in a poster by the same author11 who, in reviewing the subject in 1994, concluded that safe sedation was possible in children providing previously stated guidelines were followed.10 His poster's conclusion is not that sedation be abandoned but that “Our specialty's [anaesthesiology] involvement in the development and enforcement of sedation guidelines within institutions is critical.” Indeed, if the guidelines for safe practice had been followed, most if not all of the adverse events might have been avoided.

Ferguson and Ball,12 themselves senior registrars in anaesthesia, stated “ . . .but in special circumstances or complex patients . . .the paediatrician should feel comfortable in approaching a paediatric anaesthetist for advice and assistance.” British paediatric anaesthetists are opposed to deep sedation in principle but do not have objective evidence to unequivocally support their case. I suspect they have grave misgivings about any situation in which they do not have good control of the airway. However, their dogmatic stance is not contributing to the debate and is inhibiting otherwise good relationships based on a common desire to achieve what is best for the child.

If one accepts that deep sedation can only be acceptable if it causes no more deaths than general anaesthesia and that one death or episode of serious morbidity from sedation is unacceptable if anaesthesia is safer, then resolution of this argument must come from a national confidential inquiry into adverse events. As part of this, the denominator for all children undergoing sedation or anaesthesia must be known if these figures are to be of value. Until then a pragmatic stance would seem to be to allow deep sedation for MRI and other procedures, providing that the previously stated guidelines are observed closely. Enter the National Institute for Clinical Excellence police and some firm clinical governance.