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Awareness in retrieval medicine.

SUMMARY

Awareness is the spontaneous recall of an event(s) that occurred during general anaesthesia and surgery. The incidence of awareness is approximately 0.2% of cases where neuromuscular blockers are used and half that where they are omitted. The majority of data relating to awareness is from anaesthetic practice. We report a case of awareness associated with an out-of-hospital transportation of a critically ill patient requiring a medical escort (retrieval). We discuss the risk factors associated with awareness during retrieval, in particular the trend toward excessive administration of neuromuscular blockers, and the unique challenges for the prevention of awareness within the retrieval environment.

Key Words: awareness, retrieval, emergency medicine

Awareness is the spontaneous recall (explicit) of an event(s) that occurred during general anaesthesia and surgery (1). It may occur days later in up to half of cases (2,3). Unconscious (implicit) memories of events occurring during anaesthesia can manifest late as post traumatic stress disorder (PTSD) (3), or as an unexplained change in a person's behaviour, especially in the presence of cues. Awareness was first described in 1960 (4). The following year, the psychological effects of awake paralysis (5), later referred to as PTSD were published. The incidence of awareness varies widely, occurring in approximately 0.2% of anaesthetic cases where neuromuscular blockers (NMB) are used and half that where they are omitted (6). Awareness accounts for less than 2% of closed claims (7), but the problem remains underreported (2,8). There is wide public knowledge (and anxiety) of the problem (9).

Although primarily seen as an anaesthetic complication, there is a growing appreciation amongst other critical care areas such as intensive care (10-12) and emergency medicine (13). For example, of intensive care unit (ICU) patients undergoing mechanical ventilation, 66% are aware of the endotracheal tube, tracheal suction and extubation, and found this experience distressing 10,14). Of 26 patients extubated in the emergency department and able to converse post extubation, four had memory of the time whilst entubated (13). In contrast, based upon Medline/EMBASE searches (using keywords: awareness, recall, retrieval, interhospital, patient transportation and emergency medical services), there is no reported case of explicit awareness following out-of-hospital transportation of critically ill patients requiring a medical escort (retrieval). We present a case of awareness during retrieval, discuss the known contributing factors and how they relate to the retrieval setting, suggest ways to reduce its occurrence during retrieval and contrast the retrieval patient population for our organization in relation to known risk factors for awareness.

CASE HISTORY

A 23-year-old, previously well primigravida at 38 weeks gestation, presented to hospital following spontaneous rupture of membranes. Soon after an uncomplicated lower segment caesarian section (LSCS) under general anaesthesia was performed. Shortly following extubation, she became tachypnoeic and reliant on non-invasive ventilatory support via continuous positive airways pressure (CPAP) at 10 cm [H.sub.2]O and high inspired oxygen concentration (Fi[O.sub.2] 0.7). A presumptive diagnosis of acute pulmonary oedema complicating pre-eclampsia was made. As the local hospital did not have the appropriate intensive care resources, retrieval to another hospital ICU was arranged. The retrieval team, two paramedic ambulance officers and two doctors responded by road ambulance to the referring hospital. One doctor was a senior anaesthetic trainee, undergoing orientation to retrieval, supervised by a more experienced retrieval doctor.

The retrieval team found the patient to be conscious, hypertensive (170/105 mmHg) and in respiratory distress. The retrieval team, having no effective means of administering non-invasive ventilation over the expected interhospital transport time, felt it would be safest to sedate, intubate and mechanically ventilate the patient prior to transport. The patient and her partner were informed of this decision.

Anaesthesia was induced with intravenous fentanyl 200 [micro]g, lignocaine 100 mg, thiopentone 400 mg, suxamethonium 100 mg and following intubation, vecuronium 8 mg. Intubation was uneventful and good haemodynamic control was sustained throughout. Neuromuscular blockade was maintained with intermittent intravenous boluses of vecuronium. Anaesthesia was maintained with a propofol infusion alone. Magnesium sulphate (1 g/hour) was also being infused.

Patient monitoring consisted of electrocardiogram (ECG), invasive blood pressure, pulse oximetry, capnography and ventilator parameters (Oxylog 1000, Drager Medical AG, Lubeck, Germany). The transport vehicle was a road ambulance. The patient's husband did not accompany the patient. Soon after leaving the referring hospital the patient became hypertensive and boluses of hydrallazine and propofol were given to control the blood pressure. These episodes were followed by periods of hypotension and a subsequent reduction in the propofol infusion rate. The dose range of propofol was 5-60 ml/hour. Due to persistent and wide fluctuations in blood pressure, pulse and saturation recordings the urgency of the retrieval was upgraded and proceeded under 'lights and sirens' to the receiving ICU.

At the receiving ICU, sedation was changed to, and maintained with, a morphine and midazolam infusion. There was no further cardiovascular instability. The inspired oxygen concentration and ventilatory support were weaned and the patient extubated 16 hours after admission to the ICU. A transthoracic echocardiogram showed no abnormality.

Soon after extubation, following discussion with the duty intensivist, it became apparent the patient had experienced explicit awareness during retrieval. The patient could clearly recall the mode of transportation, the presence of the endotracheal tube and explicit details of the conversation between the retrieval staff whilst in transit. The patient was aware that she could not open her eyes, could not move and had no experience of pain.

The patient was discharged from ICU later that day. She was reviewed daily over the next few days by the intensivist who gave her and her partner an account of the transport and a detailed explanation of her recollections. The patient showed no signs of distress, was grateful that both she and her baby were well and for the retrieval. She declined any follow-up care other than obstetric. Attempts were made to obtain consent, however the patient was unable to be contacted by either telephone or mail.

DISCUSSION

The NRMA CareFlight/New South Wales Medical Retrieval Service is a doctor and ambulance paramedic-staffed service that utilizes helicopter, fixed wing and road vehicles for interhospital patient transfers and emergency response to accident scenes. Patient medical record details are entered by retrieval doctors onto a retrieval database. Over the five-year period January 1, 1998 to December 31, 2002, of the 3984 retrievals, 85% were interhospital, and of those 3370 patients, 75% required mechanical ventilation and 69% required NMB.

We reviewed the literature in relation to awareness and list the known risk factors for awareness associated with anaesthesia:

* nitrous oxide-opioid relaxant technique and the omission of a volatile agent in the setting of intra-operative hypotension

* high-dose opioid anaesthetic technique, despite the addition of benzodiazepines

* no premedication

* use of ultra short acting intravenous induction agents

* difficult or prolonged intubation

* excessive or unnecessary use of neuromuscular blocking agents

* equipment failure

* patients with an American Society of Anesthesiologists risk score of 4 or 5

* type of surgery (obstetric, gynaecological, bronchoscopy or cardiac)

* morbid obesity

* anaesthetic staff inexperience

* anaesthetic staff ignorance of the risk of awareness

* anaesthesia at high altitude

* reliance on intravenous boluses of anaesthetic agents

* patients with perceived high anaesthetic requirements

* failure to monitor inhalational anaesthetic agent concentration

* female gender

* patients with cardiorespiratory instability.

Retrieval patients are potentially at risk of awareness due to their higher severity of illness, haemodynamic instability" imposing limits on doses of sedative drugs, risk of equipment failure in the remote environment", high use of neuromuscular blocking drugs, environmental limits of monitoring equipment performance, inadequate clinical performance by inexperienced staff 9,2[degrees] and a lack of awareness of the problem by retrieval staff and follow-up of 'at-risk' retrieval patients. The patient we describe shared many of those risks. What differentiated this patient from other retrieval patients who are admitted directly to an ICU", was her relatively shorter length of ICU stay.

Reduction in the incidence of awareness begins with an appreciation of its occurrence, identification of at-risk patients and at-risk settings and vigilance for early detection and intervention. There appears to be however, varying concern for awareness even amongst anaesthetists, and despite 52% having had the experience of a patient with awareness, most rated it only a moderate problem's and underestimated the true incidence by a factor of five- to tenfold (46).

The required depth of sedation/anaesthesia to avoid awareness is difficult to accurately ascertain, even with existing monitors. The objective is to provide an individualized sedative/anaesthetic regimen appropriate to the level of stimulation and sufficient to suppress both extrinsic and intrinsic memory--intrinsic requiring a greater depth of sedation (22,23). Assessing 'depth' of anaesthesia with changes in autonomic nervous system clinical activity alone is the most readily used method, but remains unproven (24,25) and unreliable. Only 15% of cases of awareness have documented hypertension or tachycardia (6). Senior anaesthetists are poor discriminators between cases of awareness from control cases, using haemodynamic changes alone (8). Assessing motor response to noxious stimuli, one of the eight Guedel's signs of anaesthetic depth, is also commonly relied upon (1). However, the administration of neuromuscular blockers abolishes seven of the eight Guedel's signs. Other techniques such as the Isolated Forearm Technique (26) and oesophageal motility are also unreliable (6,27), difficult to perform and inconvenient, even more so within the retrieval environment. Measurement of the concentration of the inhalational anaesthetic agent is a useful concept in anaesthesia (28) but impractical for use in the retrieval environment and confined vehicular spaces.

The effect of anaesthesia on the electroencephalogram (EEG) was first noted in 1875 by Caton (29). Auditory evoked potentials (AEP) and the electromyography (EMG) also show predictable changes during anaesthesia. There are now commercially available monitors that use processed EEG signals (BiSpectral Index (BIS), Aspect Medical Systems), AEP (30) and EMG from the frontalis muscle (M-Entropy, Ohmeda) to monitor depth of anaesthesia. The reliability of frontalis EMG for detection of depth of anaesthesia has however been questioned". In comparison, the BIS, whilst not completely sensitive and specific, has been shown to reduce the incidence of awareness during anaesthesia by 78 to 82% (31,32), the amount of anaesthetic agent given and the agent's adverse effects (33). The BIS has proven to be less reliable in an ICU setting and correlates poorly with clinical sedation scores (34,35).

The EEG recording can be affected by electrical interference from both physiological (e.g. muscle activity) and nonphysiological (such as 50-60 Hz AC power, infusion pumps, fluorescent lighting, diathermy) (36) factors in up to 38% of all readings (34). For example, the BIS value can be reduced in ICU patients purely from the administration of NMB (37) alone. It should also be noted that BIS algorithm was not based upon ICU patients and that BIS is less reliable a measure of analgesia in comparison to hypnosis (38) of anaesthesia. The BIS has also not been validated for retrieval, and although EEG recordings can be reliably recorded during retrieva1 (39), the retrieval environment is limiting because of radiofrequency interference (40), space, lighting (41) and high and low frequency vehicular vibration.

The choice of sedation during retrieval is limited to intravenous agents, for which there is wide inter-individual variation in dosage required for a set stimulus". Propofol infusion, as used in this case, may lower blood pressure by 20 to 30% (43), while usual doses may not provide amnesia (43,44). Propofol has no analgesic properties (43) and high dosages may be required if used as the sole agent (44). Higher dosages may not be achievable in critically ill patients who have a reduced cardiorespiratory reserve. Opioids in combination with a benzodiazepine are an alternative to propofol. Most such agents have a lesser cardiorespiratory impact, and provide better analgesia and amnesia.

In ICU, sustained neuromuscular blockade using NMB drugs is used in as few as 1% of ICU admissions (34) and predominately to facilitate mechanical ventilation'. It would be expected, because of patient casemix, that in the retrieval setting the proportion of patients given NMB drugs would be closer to that of the ICU setting rather than the anaesthesic setting. In our retrieval practice, 69% of patients received repetitive doses of a NMB drug. This is significant as the use of NMB drugs increases the risk of awarenees (3). Further research is required to identify the basis for this higher use of NMB drugs in the retrieval setting.

In summary, this case report documents, possibly for the first time, the occurrence of awareness in association with retrieval.

The scale of the problem is unknown, but may be greater than that in the anaesthetic population for a variety of factors (incidence of NMB use, relative lack of sedative/amnestic agents, underreporting, lack of appropriate monitoring and so on). We recommend an increased vigilance and training for awareness risk recognition amongst doctors expected to conduct retrievals. We also suggest a more cautious approach to the use of NMB drugs during retrieval, avoiding where possible, the sole use of short-acting anaesthetic agents, aiming to optimize the patient's cardiorespiratory state prior to transportation so as to minimize its disturbance when in transit, reduce patient exposure to environmental stimuli (e.g. noise with ear protection, high and low frequency vibrations through adequate patient restraint, properly applied limb fracture splints, thermal protection, vehicle driving strategies), reduce the occurrence of equipment failure and develop strategies for when they occur. Trials of the efficacy of monitors for depth of sedation/anaesthesia in the retrieval setting should be encouraged so that future monitor development is more closely aligned to retrieval. A post retrieval patient follow-up and a registry of cases of awareness will help quantify the magnitude and better describe the nature of the problem.

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J. GIBSON *, A. FLABOURIS [dagger]

NRMA Careflight, Westmead, Sydney, New South Wales, Australia

* M.B., B.S. (Hons), F.A.N.Z.C.A., Retrieval Registrar, NRMA Careflight, Westmead, New South Wales.

[dagger] M.B., B.S., F.F.I.C.A.N.Z.C.A., Intensivist, Liverpool Hospital, Liverpool and Retrieval Specialist, NRMA Careflight, Westmead, New South Wales.

Address for reprints: Dr Jonathon Gibson, Department of Anaesthesia, Westmead Hospital, Westmead, N.S.W 2145.

Accepted for publication on June 19, 2006.
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Title Annotation:Case Report
Author:Gibson, J.; Flabouris, A.
Publication:Anaesthesia and Intensive Care
Date:Oct 1, 2006
Words:3406
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