Anaesthesia Outside the Operating Theatre
G eneral anaesthesia outside the operating theatre suite is often challenging for the anaesthetist, because specialized environments pose unique problems. In hospital, the anaesthetist must provide a service for patients with standards of safety and comfort which are equal to those in the main operating department. Outside the hospital, this level of service is more dependent on location and available resources.
In-hospital remote locations include radiology, radiotherapy, the Accident and Emergency (A & E) department and wards with areas designated for procedures such as ECT, assisted conception, cardioversion and intrathecal chemotherapy administration.
Anaesthetists are frequently 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 which the anaesthetist must be aware of. 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. Additional skilled anaesthetic help may not be readily available compared with an operating theatre suite and patients are often challenging, e.g. paediatric or critically ill.
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 (e.g. scanners) and, in general, conditions are less than optimal. Monitoring capabilities and anaesthetic equipment should be of the same standard as those used in the operating department. In reality, such equipment may not be readily available and the equipment used is often the oldest in the hospital. Nevertheless, the monitoring equipment should meet the minimum standards set by the Association of Anaesthetists of Great Britain and Ireland (AAGBI, 2006). The anaesthetist who is unfamiliar with the environment should spend time becoming accustomed to the layout and equipment. Compromised access to the patient and the type of monitors used during the procedure require careful consideration. Advanced planning helps to prepare for unanticipated scenarios. Clinical observation may be limited by poor lighting.
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, e.g. 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. Maintenance 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. Empty gas cylinders need to be replaced in older suites without piped gases, and also the anaesthetist must ensure the presence of drugs, spare laryngoscope and batteries, suction and other routine equipment.
5. Communication. Communication between staff of other specialities and the anaesthetist may be poor. This may lead to failure in recognizing each other’s requirements. Education programmes for non-anaesthesia personnel regarding the care of anaesthetized patients may be of benefit.
6. Recovery. Recovery facilities are often non-existent. 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 ‘pack’ kept alongside monitoring equipment and a portable oxygen supply. This avoids searching for various pieces of equipment which may delay transfer, and ensures that nothing is forgotten. The pack 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 specialities, e.g. radiologists, psychiatrists, to ensure that the environment, equipment and guidelines are suitable for safe, appropriate and efficient patient care.
In most hospitals, members of the anaesthetic department are called upon to anaesthetize or sedate patients for diagnostic and therapeutic radiological procedures. These procedures include ultrasound, angiography, computed tomography (CT) scanning and magnetic resonance imaging (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 the uncooperative patient. The presence of pain or prolonged procedures may also be an indication for anaesthesia.
Radiological studies may require administration of conscious sedation. This term describes the use of medication, often given by a non-anaesthetist, to alter perception of painful and anxiety-provoking stimuli while maintaining protective airway responses and the ability to respond appropriately to verbal command. Medical 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 side-effects to go untreated. Ideally, different individuals should be responsible for each of these tasks. Guidelines for prescribing, evaluating and monitoring sedation should be readily available. Chloral hydrate may be used in young children and benzodiazepines, opioids or propofol in adults. Patients should be starved before sedation and vital signs monitored and documented.
Iodine-containing intravascular contrast agents are used routinely during angiographic and other radiological investigations. The anaesthetist must always be aware of the risk of adverse reactions to contrast dyes. In recent years, low-osmolarity contrast media have been introduced; these cause less pain and have fewer toxic effects than the older contrast agents, but are more expensive. Factors contributing to the development of adverse reactions include speed of injection, type and dose of contrast used and patient susceptibility.
Coronary and cerebral angiography are associated with a high risk of reaction. Other major risk factors include allergies, asthma, extremes of age (under 1 and over 60 years), cardiovascular disease and a history of previous contrast medium reaction. Fatal reactions are rare, occurring in about 1 in 100 000 procedures. Nausea and vomiting are common and reactions may progress to urticaria, hypotension, arrhythmias, bronchospasm and cardiac arrest. Treatment of allergic reactions depends on the severity of the reaction. This usually consists of fluids, oxygen and careful monitoring. An anaphylaxis protocol should be readily available along with adrenaline, antihistamines, steroids and fluids, preferably as an ‘anaphylaxis kit’. Adequate hydration is important, because patients undergoing contrast dye procedures usually have an induced osmotic diuresis, which may exacerbate pre-existing renal dysfunction. Patients at particular risk are dehydrated patients, those with chronic renal dysfunction and patients in the recovery phase of acute renal failure for which renal replacement therapy has been required. In these patients, even small doses of contrast may cause sufficient deterioration to necessitate further haemofiltration. Some intensive care units have a policy of giving acetylcysteine with a fluid load up to 1 h before the scan, with a further dose after the scan to protect against contrast-induced nephropathy. 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. Although the patient dose is high, ionizing radiation from a CT scanner is relatively low 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.
General Principles: A CT scan provides a series of tomographic axial ‘slices’ of the body. It is used most frequently for intracranial imaging and for studies of the thorax and abdomen and is the investigation of choice in the evaluation of major trauma when whole body CT may be used in place of plain X-rays. Each image is produced by computer integration of the differences in the 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 first-generation scanners took 4.5 min per cut, but the newest scanners take only 2–4 s.
The circular scanning tunnel contains the X-ray tube and detectors, with the patient lying stationary in the centre during the study. The procedure is noisy and patients are occasionally frightened or claustrophobic.
Anaesthetic Management: Computed tomography is non-invasive and painless, requiring neither sedation nor anaesthesia for most adult patients. A few patients may require conscious sedation to relieve fears or anxieties. However, patients who cannot cooperate (most frequently 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 intracranial pressure (ICP) is critical. Because the patient’s head is inaccessible during the CT scan, the airway needs to be secured. In the majority of situations, tracheal intubation is more appropriate than the use of a laryngeal mask airway (e.g. full stomach). The scan itself requires only that the patient remains motionless and tolerates the tracheal tube. If ICP is high, controlled ventilation is essential to maintain normo- or hypocapnia.
Because these patients are often in transit to or from critical care areas or A & E, 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 also 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 CO2 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. Anaesthetic complications while in the 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 and disconnection of drips and lines during transfer. In addition, in the trauma setting, patients may become markedly unstable during movement on to the scanning table, and emergency drugs and fluids should be readily available. 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.
Stereotactic-guided surgery is possible using CT scanners although this has largely been replaced by frameless image guidance systems which allow tumour localization in the theatre suite from an existing scan. Most procedures involve aspiration or biopsy of intracranial masses in eloquent or important motor areas to give a tissue diagnosis with minimal risk to surrounding structures. The patient may either be anaesthetized in the theatre suite and then transferred to the CT scanner or anaesthetized in the scanner itself. Advantages of the former are induction of anaesthesia in a familiar environment with assistance and equipment readily available; the disadvantage is that the patient then must be transferred to the CT scanner and back again to theatre. The advantage of inducing anaesthesia in the CT scanner is that the need for initial transfer is removed, but there are the risks already outlined of anaesthesia in a remote location. Pins are used to hold a radiolucent frame around the head and then coordinates are mapped onto the frame from the scan to allow precise location of the tumour. Application of the frame via the pins is painful and access to the patient and airway with the frame attached inside the scanner is difficult.
General Principles: Magnetic resonance imaging (MRI) is an imaging modality which depends on magnetic fields and radiofrequency pulses for the production of its images. The imaging capabilities of MRI are superior to those of CT for examining intracranial, spinal and soft tissue lesions. MRI differentiates clearly between white and grey matter in the brain, thus making possible, for example, the in vivo diagnosis of demyelination. It may display images in the sagittal, coronal, transverse or oblique planes and has the advantage that no ionizing radiation is produced.
An MRI imaging system requires a large magnet in the form of a tube which is capable of accepting the entire length of the human body. 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; these cause 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 over 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 to establish the magnetic field if it is removed and this is only done in an emergency because it is very expensive to shut down the field. 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 between 0.5 and 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 level. 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 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.
STAFF SAFETY: Staff safety precautions are essential. The supervising MR radiographer is operationally responsible for safety in the scanner and anaesthetic staff should defer to him or her in matters of MR safety. Screening questionnaires identify those at risk and training should be given in MR safety, electrical 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, bleeps and stethoscopes. Bank cards, credit cards and other belongings containing electromagnetic strips become demagnetized within the vicinity of the scanner and personal computers, pagers, phones and calculators may also be damaged.
PATIENT SAFETY: Metal objects within or attached to the patient pose a risk. Jewellery, hearing aids or drug patches should be removed. Absolute contraindications include implanted surgical devices, e.g. cochlear implants, intraocular metallic objects and metal vascular clips. Pacemakers remain an absolute contraindication in most settings although some patients with a pacemaker have undergone scanning under tightly controlled conditions when the benefit has been deemed to outweigh the risk. Metallic implants, e.g. intracranial vascular clips, may be dislodged from blood vessels. 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 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.
Other unique problems presented by MRI include relative inaccessibility of the patient. 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 laryngeal mask airway 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 ionizing radiation, the anaesthetist may approach the patient in safety.
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. Terminology regarding equipment used in the MRI scanner has now changed from ‘MR compatible’ or ‘MR incompatible’ to ‘MR conditional’, ‘MR safe’ or ‘MR unsafe’. MR conditional equipment is that which poses 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, e.g. higher field strength or rate of change of the field. MR safe equipment is that which poses no safety hazard in the MR room but it may not function normally or may degrade the image quality. Consideration needs to be given to replacing equipment if a scanner is replaced by one of higher field strength.
The layout of the MRI room/suite determines whether the majority of equipment needs to be inside the room (and therefore MR conditional or 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 minimize the length of the breathing system. They require piped gases or special aluminium oxygen and nitrous oxide cylinders. Consideration also needs to be given to intravenous 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. Laryngeal mask airways without a metal spring in the pilot tube valve should be available.
All monitoring equipment must be appropriate for the environment. Technical problems with non-compatible monitors include interference with imaging signals, resulting in distorted MRI pictures, 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. leads may be fibreoptic or carbon fibre cable). Ambient noise levels are such that visual alarms are essential. The 2010 AAGBI 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, which interferes with arrhythmia and ischaemia monitoring. 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.