Anaesthesia outside the operating theatre

General anaesthesia outside the operating theatre suite is often challenging for the anaesthetist. Although the principles of remote site anaesthesia are common to many situations, each specialised environment poses its own unique problems. In hospital the anaesthetist must provide a service for patients with standards of safety which are equal to those in the main operating theatre department. Outside the hospital, this level of service may be more dependent on location and available resources.

Anaesthesia in remote locations within the hospital

In-hospital remote locations include radiology, radiotherapy and emergency departments, and wards with areas designated for procedures such as electroconvulsive therapy (ECT), assisted conception, cardioversion and intrathecal chemotherapy administration.

General considerations and principles

Anaesthetists are often required to use their skills (e.g. administer anaesthesia, analgesia, sedation, resuscitate, cannulate, etc.) outside the familiar operating theatre environment. When requests are made for anaesthetic intervention in remote locations, there are multiple considerations for the anaesthetist. These include the following:

1.
Appropriate personnel . Only senior experienced anaesthetists, who are also familiar with the particular environment and its challenges, should normally administer anaesthesia in remote locations. Patients are often challenging, and additional skilled anaesthetic help may not be readily available compared with an operating theatre suite.
2.
Equipment . The remote clinical area may not have been designed with anaesthetic requirements in mind. Anaesthetic apparatus often competes for space with bulky equipment such as scanners, and in general, conditions are less than optimal, including poor lighting. Monitoring capabilities and anaesthetic equipment should be of the same standard as those used in the operating department and should meet the minimum standards set by the Association of Anaesthetists. The updated 2015 guidelines include guidance on monitoring neuromuscular blockade, depth of anaesthesia and cardiac output. The anaesthetist who is unfamiliar with the environment should spend time becoming accustomed to the layout and equipment. Compromised access to the patient requires careful consideration, and advanced planning helps to prepare for unanticipated scenarios. Basic requirements such as i.v. fluids should be located, as these may be stored in an area away from the procedure room, and the presence of general and specific emergency drugs and equipment should be verified before starting the procedure.
3.
Patient preparation . Preparation of the patient may be inadequate because the patient is from a ward where staff are unfamiliar with preoperative protocols, or patients may be unreliable, such as those presenting for ECT.
4.
Assistance . An anaesthetic assistant (e.g. operating department practitioner) should be present, although this person may be unfamiliar with the environment, and maintenance and stocking of anaesthetic equipment may be less than ideal. Consequently the anaesthetist must be particularly vigilant in checking the anaesthetic machine, particularly because it may be disconnected and moved when not in use.
5.
Communication . Communication between staff from other specialties and the anaesthetist may be poor. This may lead to failure in recognising each other's requirements. The use of the WHO checklist is particularly important in these areas and often highlights issues that may have been overlooked.
6.
Recovery . Recovery facilities are often non-existent, and anaesthetists may have to recover their own patients in the suite. Consequently, they must be familiar with the location of recovery equipment, including suction, supplementary oxygen and resuscitation equipment. Alternatively, patients may be transferred to the main hospital recovery area. This requires the use of routine transfer equipment, which should ideally be available as a grab-bag kept alongside monitoring equipment, and a portable oxygen supply. This ensures that nothing is forgotten and avoids delays in searching for various pieces of equipment. The bag should be regularly checked and maintained.

There should be a nominated lead anaesthetist responsible for remote locations in which anaesthesia is administered in a hospital. This individual should liaise with the relevant specialties (e.g. radiologists, psychiatrists) to ensure that the environment, equipment and guidelines are suitable for safe, appropriate and efficient patient care.

Anaesthesia in the radiology department

In most hospitals, members of the anaesthetic department are called upon to anaesthetise or sedate patients for diagnostic and therapeutic radiological procedures. These procedures include angiography and subsequent intervention, CT scanning and MRI. The major requirement of all these imaging techniques is that the patient remains almost motionless. Thus general anaesthesia may be necessary when these investigations and interventions are performed in children, the critically ill or uncooperative patient and/or if the procedure is likely to be painful or prolonged.

Radiological studies may require administration of conscious sedation; chloral hydrate may be used in young children and benzodiazepines, opioids or propofol in adults. This is discussed in more detail in Chapter 4 , but essential practice elements include the following:

Personnel responsible for the sedation should be familiar with the effects of the medication and skilled in resuscitation (including airway management).
All the equipment and drugs required for resuscitation should be readily available and checked regularly.
It is undesirable for a single operator to be responsible for both the radiological procedure and administration of sedation because there is the potential to be distracted from one responsibility and to allow adverse events to go untreated.
Guidelines for prescribing, evaluating and monitoring sedation should be readily available.
Particular care should be taken with high-risk groups such as frail and older patients and those with cardiovascular or respiratory disease.
Patients should be starved before sedation, vital signs monitored and documented, and appropriate recovery and discharge criteria used.

Iodine-containing intravascular contrast agents are used routinely during angiographic and other radiological investigations. The anaesthetist must always be aware of the risk of an anaphylactoid contrast reaction. In recent years, low-osmolar contrast media have been introduced; these cause less pain on injection and have fewer adverse effects than the older contrast agents (3% vs. 15%).

Factors contributing to the development of adverse reactions include:

speed of injection;
type and dose of contrast used;
intra-arterial contrast injection (coronary and cerebral angiography are associated with a greater risk of anaphylactoid contrast reaction); and
patient susceptibility (e.g. allergy, asthma, extremes of age (younger than 1 and older than 60 years), cardiovascular disease and a history of previous reaction to contrast medium)

Nausea and vomiting are common and reactions may progress to urticaria, hypotension, arrhythmias, bronchospasm and cardiac arrest. Fatal reactions are rare, occurring in about 1 in 100,000 procedures. Treatment depends on the severity of the reaction but usually consists of i.v. fluids, supplemental oxygen and careful monitoring (see Chapter 27 ). An anaphylaxis protocol and kit should be readily available containing appropriate drugs and fluids.

Adequate hydration is important because high-osmolarity contrast dye can potentially cause dose-dependent contrast-induced nephropathy (CIN) (see Chapter 11 ). Risk factors include: dehydration; pre-existing acute kidney injury; and repeated administration of contrast. The circulating volume should be maintained or expanded using oral fluids and i.v. saline 0.9% or bicarbonate. There is little evidence to support therapies such as N -acetylcysteine or diuretics, but they are sometimes used. The incidence and severity of CIN has decreased with the use of lower-osmolar contrast media, and severe CIN requiring renal replacement therapy is rare. A urinary catheter may be useful for patients undergoing long procedures.

Healthcare workers are exposed to X-rays in the radiology and imaging suites. The greatest source is usually from fluoroscopy and digital subtraction angiography. Exposure to ionising radiation from a CT scanner is relatively low, although the patient dose is high because the X-rays are highly focused. Radiation intensity and exposure decrease with the square of the distance from the emitting source. The recommended distance is 2–3 m. This precaution, together with lead aprons, thyroid shields and movable lead-lined glass screens, keeps exposure at a safe level. A personal-dose monitor should be worn by personnel who work frequently in an X-ray environment. All female patients between the ages of 12 and 55, along with those who have experienced earlier menarche or later menopause, should be offered pregnancy testing before exposure to ionising radiation. In the emergency situation this may be deferred in the interests of time and the risk/benefit ratio to the patient.

Computed tomography

General principles

A CT scan provides a series of tomographic axial ‘slices’ of the body. Each image is produced by computer integration of the differences in radiation absorption coefficients between different normal tissues and between normal and abnormal tissues. The image of the structure under investigation is generated by a cathode ray tube and the brightness of each area is proportional to the absorption value. One rotation of the gantry produces an axial slice, or ‘cut’. A series of cuts is made, usually at intervals of 7 mm, but this may be larger or smaller depending on the diagnostic information sought. The circular scanning tunnel contains the X-ray tube and detectors, with the patient lying in the centre of the doughnut during the study.

Anaesthetic management

Computed tomography is non-invasive and painless, requiring neither sedation nor anaesthesia for most adult patients; however, it is noisy and claustrophobic, and a few patients may require conscious sedation to alleviate fears or anxieties. Patients who cannot co-operate (most often paediatric and head trauma patients or those who are under the influence of alcohol or drugs) or those whose airway is at risk may need general anaesthesia to prevent movement, which degrades the image. Anaesthetists may also be asked to assist in the transfer from the ICU and in the care of critically ill patients who require CT scans.

General anaesthesia is preferable to sedation when there are potential airway problems or when control of ICP is critical. Because the patient's head is inaccessible during the CT scan, the airway needs to be secured. The scan itself requires only that the patient remains motionless. If ICP is high, controlled ventilation is necessary to ensure tight control of P a co ₂ . Because these patients are often in transit to or from critical care areas or the emergency department (ED), a total intravenous technique with neuromuscular blockade is usually the technique of choice, with tracheal intubation and controlled ventilation. Use of volatile anaesthetic agents during the scan is acceptable but may involve changing from one technique to another for transfer. In addition, the anaesthetic machine may be left unplugged when not in use in the scanner, and reconnecting and checking it may be distracting and time consuming. A portable ventilator with end-tidal CO ₂ monitoring removes the need to change breathing systems. If the scan is likely to take a long time, it may be advisable to change from cylinder to piped oxygen supply to conserve supplies for transfer. If during the scan the anaesthetist is observing the patient from inside the control room, it is imperative that alarms/monitors have visual signals which may be seen easily.

Anaesthetic complications while in the CT scanner include:

kinking of the tracheal tube or disconnection of the breathing system, particularly during positioning and movement of the gantry;
hypothermia in paediatric patients;
disconnection of drips and lines during transfer; and
haemodynamic instability during movement on to the scanning table (e.g. in the trauma setting).

Magnetic resonance imaging

General principles

Magnetic resonance imaging (MRI) is an imaging modality that depends on magnetic fields and radiofrequency pulses to produce its images. The imaging capabilities of MRI are superior to those of CT for examining intracranial, spinal and soft tissue lesions. It may display images in the sagittal, coronal, transverse or oblique planes and has the advantage that no ionising radiation is produced.

An MRI imaging system traditionally requires a large magnet in the form of a tube, which is capable of accepting the entire length of the human body. Wider bore and open scanners have more recently been developed, which allow the patient to stand up to reduce the feeling of claustrophobia. This also means that more obese patients or those with significant deformities (e.g. severe kyphosis) can now be scanned. A radiofrequency transmitter coil is incorporated in the tube which surrounds the patient; the coil also acts as a receiver to detect the energy waves from which the image is constructed. In the presence of the magnetic field, protons in the body align with the magnetic field in the longitudinal axis of the patient. Additional perpendicular magnetic pulses are applied by the radiofrequency coil; this causes the protons to rotate into the transverse plane. When the pulse is discontinued, the nuclei relax back to their original orientation and emit energy waves which are detected by the coil. The magnet is more than 2 m in length and weighs approximately 500 kg. The magnetic field is applied constantly even in the absence of a patient. It may take several days and great expense to re-establish the magnetic field if it is removed, and so the magnet is only quenched in an emergency.

The magnetic field strength is measured in tesla units (T). One tesla is the field intensity generating 1 newton of force per 1 ampere of current per 1 metre of conductor. One tesla equals 10,000 gauss; the Earth's surface strength is 0.5–1.0 gauss. MRI strengths usually vary from 1 to 3 T, although some research facilities have scanners which may produce fields up to 9.4 T. The force of the magnetic field decreases exponentially with distance from the magnet, and a safety line at a level of 5 gauss is usually specified. Higher exposure may result in pacemaker malfunction, and unscreened personnel should not cross this safety line. At 50 gauss, ferromagnetic objects become dangerous projectiles. The magnetic fields present are strong static fields, which are present all around the magnet area, and fast-switching and pulsed radiofrequency fields in the immediate vicinity of the magnet.

The final magnetic resonance (MR) image is made from very weak electromagnetic signals, which are subject to interference from other modulated radio signals. Therefore, the scanner is contained in a radiofrequency shield (Faraday cage). A hollow tube of brass is built into this cage to allow monitoring cables and infusion lines to pass into the control room. This is termed the waveguide.

Anaesthetic management

Staff safety.

Staff safety precautions are essential. The supervising magnetic resonance (MR) radiographer is operationally responsible for safety in the scanner, and anaesthetic staff should defer to him or her in matters of safety. Screening questionnaires identify those at risk, and training should be given in MR safety, emergency procedures arising from equipment failure and evacuation of the patient. Anaesthetists should also understand the consequences of quenching the magnet and be aware of recommendations on exposure and the need for ear protection. Long-term effects of repeated exposure to MRI fields are unknown, and pregnant staff should be offered the option not to work in the scanner. All potentially hazardous articles should be removed (e.g. watches, mobile telephones bleeps, pens and stethoscopes). Bank cards, credit cards and other belongings containing electromagnetic strips become demagnetised in the vicinity of the scanner, and personal computers, pagers, mobile telephones and calculators may also be damaged.

Patient safety.

Ferromagnetic objects within or attached to the patient pose a risk.

Jewellery, hearing aids or drug patches should be removed.
Absolute contraindications to MRI include implanted surgical devices such as cochlear implants, intraocular metallic objects and metal vascular clips.
Pacemakers remain an absolute contraindication in most settings, although MRI-conditional pacemakers have now been developed.
Programmable shunts for hydrocephalus may malfunction because the pressure setting may be changed by the magnetic field, leading to over- or underdrainage.
The use of neurostimulators such as spinal cord stimulators for chronic pain is increasing. These devices may potentially fail or cause thermal injury on exposure to the magnetic field. Each must be considered individually; some may be safe if strict guidelines are adhered to.
Joint prostheses, artificial heart valves and sternal wires are generally safe because of fibrous tissue fixation. Patients with large metal implants should be monitored for implant heating. A description of the safety of various devices is available on dedicated websites.
All patients should wear ear protection because noise levels may exceed 85 dB.

In most scanners the body cylinder of the scanner surrounds the patient totally; manual control of the airway is impossible, and tracheal intubation or use of a supraglottic airway device (SAD) is essential when general anaesthesia is necessary. The patient may be observed from both ends of the tunnel and may be extracted quickly if necessary. Because there is no hazard from ionising radiation, the anaesthetist may approach the patient in safety.

Technological advances mean that intraoperative MRI is now possible, and MRI scanners are becoming integrated into operating theatres to allow real-time imaging during complex neurosurgery. There are additional risks associated with its use including contamination of the surgical field, the need for patient repositioning and the requirement for non-ferromagnetic surgical instruments (see Chapter 40 ).

Equipment.

The magnetic effects of MRI impose restrictions on the selection of anaesthetic equipment. Any ferromagnetic object distorts the magnetic field sufficiently to degrade the image. It is also likely to be propelled towards the scanner and may cause a significant accident if it makes contact with the patient or with staff. Equipment used in the MRI scanner is designated ‘MR-conditional’, ‘MR-safe’ or ‘MR-unsafe’.

MR-conditional equipment pose no hazards in a specified MR environment with specified conditions of use. The conditions in which it may be used must accompany the device, and it may not be safe to use it outside these conditions. Consideration needs to be given to replacing equipment if a scanner is replaced by one of higher field strength.
MR-safe equipment pose no safety hazard in the MR room, but may not function normally or may degrade the image quality.

The layout of the MRI room or suite determines whether the majority of equipment needs to be inside the room (and therefore MR-conditional or MR-safe), or outside the room with suitable long circuits, leads and tubing to the patient. Suitable anaesthetic machines and ventilators are manufactured and may be positioned next to the magnetic bore to minimise the length of the breathing system. They require piped gases or special aluminium cylinders for oxygen and nitrous oxide. Consideration also needs to be given to i.v. fluid stands, infusion pumps and monitoring equipment, including stethoscopes and nerve stimulators. Laryngoscopes may be non-magnetic, but standard batteries should be replaced with non-magnetic lithium batteries. Supraglottic airway devices without a metal spring in the pilot tube valve should be available.

Monitoring.

All monitoring equipment must be appropriate for the environment. Technical problems with non-compatible monitors include interference with imaging signals and radiofrequency signals from the scanner inducing currents in the monitor which may give unreliable monitor readings. Special monitors are available or unshielded ferromagnetic monitors may be installed just outside the MRI room and used with long shielded or non-ferromagnetic cables (e.g. fibre-optic or carbon fibre). Ambient noise levels are such that visual alarms are essential. The 2010 Association of Anaesthetists guidelines on services for MRI suggest that monitoring equipment should be placed in the control room outside the magnetic area. A non-invasive automated arterial pressure monitor, in which metallic tubing connectors are replaced by nylon connectors, should be used. Distortion of the ECG may occur. Interference may be reduced by using short braided leads connected to compatible electrodes placed in a narrow triangle on the chest. There should be no loops in cables because these may induce heat generation and lead to burns. Side-stream capnography and anaesthetic gas concentration monitoring require a long sampling tube, which leads to a time delay of the monitored variables. The use of 100% oxygen during the scan should be indicated to the radiologist reporting the images because this may produce artefactually abnormal high signal in CSF spaces in some scanning sequences.

Conduct of anaesthesia.

The indications for general anaesthesia during MRI are similar to those for CT. A complex scan may take up to 20 min, and an entire examination more than 1 h. Most MRI scans are performed within normal working hours; exceptions may be neuraxial MRI scanning for acute evaluation of the brain or spinal cord.

Anaesthesia is usually induced outside the MRI room in an adjacent dedicated anaesthetic area where it is safe to use ferromagnetic equipment (outside the 5 gauss line). Short-acting drugs should be used to allow rapid recovery with minimal adverse effects. Sedation of children by organised, dedicated and multidisciplinary teams for MRI has been shown to be safe and successful. However, general anaesthesia allows more rapid and controlled onset, with immobility guaranteed. All patients must be transported into the magnet area on MRI-appropriate trolleys. During the scan, the anaesthetist should ideally be in the control room but may remain in the scanning room in exceptional circumstances if wearing suitable ear protection. If an emergency arises, the anaesthetist needs to be aware of the procedure for rapid removal of the patient to a safe area.

Increasingly, ICU patients require MRI scanning. Careful planning is required and screening checklists should be used. Non-essential infusions should be discontinued and essential infusions may need to be transferred to MRI-safe pumps. This may induce a period of instability in the patient while the infusions are being moved, and high requirements for drugs such as vasopressors may be a relative contraindication to scanning. The tracheal tube pilot balloon valve spring should be secured away from the scan area. Pulmonary artery catheters with conductive wires and epicardial pacing catheters should be removed to prevent microshocks. Simple CVCs appear safe if disconnected from electrical connections.

Gadolinium-based contrast agents are used in MRI and are generally safe, with a high therapeutic ratio. However, the use of these contrast agents in patients with renal failure may precipitate a life-threatening condition called nephrogenic systemic fibrosis. In patients with a glomerular filtration rate (GFR) less than 30 ml min ⁻¹ 1.73 m ^–2 , only minimal amounts of contrast should be given (if deemed absolutely necessary) and not repeated for at least 7 days.

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