Anesthesia Update 2009

Educational Objectives

The goals of this program are to increase knowledge of memory and awareness in anesthesia and to provide effective anesthetic care for the burn patient. After hearing and assimilating this program, the clinician will be better able to:

1.   Discuss the incidence of conscious recall after general anesthesia.

2.   Describe effective measures for prevention of awareness during anesthesia.

3.   Manage treatment and follow-up of awareness cases.

4.   Identify the pathophysiologic effects of burn injury on the pulmonary, cardiovascular, renal, and metabolic sys­tems.

5.   Provide effective anesthetic management of the burn patient in the acute and intraoperative periods.

Faculty Disclosure

In adherence to ACCME Standards for Commercial Support, Audio-Digest requires all faculty and members of the planning committee to disclose relevant financial relationships within the past 12 months that might create any personal conflicts of in­terest. Any identified conflicts were resolved to ensure that this educational activity promotes quality in health care and not a proprietary business or commercial interest. For this program, the following has been disclosed: Dr. Sebel is a paid consultant for Aspect Medical Systems. The planning committee reported nothing to disclose.

Acknowledgments

Dr. Sebel spoke in Vail, CO, at New Horizons in Anesthesiology, held August 31 to September 5, 2008, and sponsored by the Department of Anesthesiology, Emory University School of Medicine, Atlanta, GA; Dr. Kovac, spoke in Kansas City, MO, at the 59th Annual Postgraduate Symposium on Anesthesiology, held March 27-29, 2009, and sponsored by the Uni­versity of Kansas Medical Center Departments of Anesthesiology and Continuing Education. The Audio-Digest Foundation thanks the speakers and the sponsors for their cooperation in the production of this program.

Memory and Awareness in Anesthesia

Peter S. Sebel, MB, BS, PhD, Professor and Vice Chair, Department of Anesthesiology, Emory University School of Medicine, Atlanta, GA

Introduction: most anesthesia providers report no experience with cases of awareness; speaker attributes this to fail­ure of anesthesia providers to look for awareness; patients aware during anesthesia describe feeling �entombed in a corpse�; neuromuscular blocking agents substantial issue in awareness during anesthesia

Depth of anesthesia: other than effective neuromonitoring techniques, no way to determine which patients at risk for slow emergence and which are at risk for awareness during anesthesia; providers tend to relatively overdose all pa­tients; depressant effects of general anesthetics need to be matched by stimulating effects of surgery

Levels of awareness: 1) conscious with no amnesia, 2) conscious awareness with amnesia, 3) subconscious aware­ness with amnesia, and 4) no awareness; divided into levels for convenience, but not separate or discrete; should be considered continuum; awareness in anesthesia cause for concern for  humanitarian as well as medicolegal reasons, and to limit potential for posttraumatic stress disorder (PTSD)

Sandin et al: interviewed >1100 patients; found 0.18% incidence of awareness with neuromuscular blocking agent (NMBA) and 0.1% without NMBA; performed interviews in postanesthesia care unit (PACU) 1 to 3 days postoper­atively, and 7 to 14 days postoperatively; 11 cases identified in PACU; 17 cases identified at final interview; no anx­iety or PTSD in nonparalyzed patients who reported awareness; 11 of 14 paralyzed had pain, anxiety, or PTSD; follow-up 2 yr later showed 4 patients still severely disabled

Incidence of awareness: 0.13% in United States; possible awareness 0.2%; dreaming �6%

Awareness descriptors: auditory (appears relatively well-retained during anesthesia); unable to move or breathe (48% incidence); pain (relatively low incidence in study)

Pollard et al:  reported incidence of awareness 0.0068% in study of >200,000 patients; used continuous quality im­provement (CQI) database; speaker does not trust CQI database (does not record all data, and not dependable for incidence data); wording of some questions inadequate to elicit information about awareness

Davidson et al: looked at incidence of awareness in pediatric patients; 4 independent adjudicators had to agree on ep­isode of awareness; 1000 patients aged 5 to 12 yr; incidence of awareness �approaching 1 in 100�

Moerman et al: patients report having experienced auditory and pain sensations and feelings of paralysis during an­esthesia; also describe feeling anxiety, powerlessness, and helplessness; 70% report unpleasant after effects (eg, el­ements of PTSD); majority do not tell anesthesia provider about episodes of awareness; blood pressure (BP) and heart rate (HR) do not increase during awareness; case-matched controls analyzed for hypertension and tachycar­dia; no reliable correlation found between these complications and awareness during anesthesia

Domino et al: 80 cases in closed-claims database; hypertension and tachycardia relatively infrequent in cases of awareness; cues for light anesthesia absent in most cases; these findings unexplained

Conclusions: awareness often occurs without hypertension or tachycardia; uncommon in absence of NMBA (�tend to move before becoming aware�); if awareness occurs, follow up appropriately; patients report sympathetic re­sponse and reassurance from provider most important; prescribe early counseling and psychiatric therapy; at 1-wk follow-up, determine whether patient experiencing sleep disturbance or nightmares; patients with history of aware­ness during surgery avoid hospitals and physicians; those who develop PTSD can be referred for medical, psycho­logic, and behavioral therapies

Bispectral (BIS) index monitoring to reduce incidence of awareness: actions of anesthetics include consciousness, hypnosis, analgesia, and muscle relaxation; should be considered in balance together; measure hypnotic component with neuromonitoring; speaker does not believe muscle relaxation necessary or sufficient part of anesthetic state; many monitors commercially available to guide anesthetic, but BIS best studied

Flaishon et al: focused on return of consciousness after single dose of propofol or thiopental in paralyzed patient with isolated forearm; induction with 4 mg/kg thiopental produces BIS between 60 and 20; recovery occurred be­tween 2 and 11 min, with BIS �60; BP and HR did not change consistently on return of consciousness

Kerssens study: propofol target controlled infusions, with BIS values of 60 to 70; isolated forearm technique at mean BIS value of 64; 81% of commands resulted in no response, 5% had inadequate response, and 13% were able to squeeze fingers twice; 9 patients reported conscious recall

Review of published literature: Mychaskiw reported explicit intraoperative recall at BIS of 47; further analysis of data shows BIS value recorded did not correlate with actual time of awareness (BIS �60 at that time); Ekman et al found 77% reduction in incidence of awareness using BIS monitoring

Myles et al: B-Aware trial; expected high incidence of awareness (1%); prospectively randomized to BIS-monitored and -guided or no BIS; found 11 aware patients in no BIS group, but 82% reduction in awareness when BIS moni­toring used

Avidan study: compared BIS to end-tidal anesthetic gas (ETAG) monitoring; study had many flaws; concluded that findings do not support routine BIS monitoring as part of standard anesthetic practice; speaker concludes that find­ings of Avidan et al duplicate findings of Myles and demonstrate measurement of ETAG and BIS monitoring equally effective

Awareness related to excessive use of NMBA: speaker recommends �maintaining 1 or 2 twitches�; cautions that most cases of awareness related to inadequate volatile or intravenous anesthetic, rather than NMBA; reduce inci­dence of awareness by reducing NMBA and by anesthetic agent and BIS monitoring; may need to repeat induction agent during difficult intubations

Indirect or implicit memory: explicit memory requires conscious recollection of event; implicit memory revealed by change in task performance that occurs without direct recall of learning episode

Trauma study: 96 patients received etomidate, isoflurane, or fentanyl, and NMBA, but no benzodiazepines or sco­polamine; anesthesia team blinded to BIS values; played words while patients anesthetized and asked them to com­plete word stem after surgery; words had basic completion rate of �30%; automatically tagged word with BIS value at time of word presentation; hit rate for target word gradually increased as BIS value increased; study indicates ac­curacy of implicit memory related to depth of anesthesia

Anesthesia for Burn Patients

Anthony L. Kovac, MD, Kasumi Arakawa Professor of Anesthesiology, University of Kansas School of Medi­cine, Kansas City

Burn incidence and treatment per year in United States: thermal injuries, 1.1 million; hospitalized thermal inju­ries, 45,000; burn center admission, 22,000; large percentage of patients have total burn surface area (TBSA) of 10% to 60%

Major thermal injury: admit to burn center if 2nd degree partial thickness (TBSA >25%; bullae on skin), 1st degree full thickness (TBSA >10%; patient may not have sensation), smoke inhalation, electrical burn, or for extremes of age (ie, pediatric or geriatric patients)

Pathophysiologic Effects

Inhalation injury: occurs in 20% of burn patients and 60% of burn patients who die; 80% of fatalities due to inhala­tion injury; increased mortality from major thermal injury correlated with age of patient (age >60 yr associated with poor prognosis), burn size, and development of pneumonia, as well as with inhalation injury (increases mortal­ity 20% to 60%)

Studies of survival estimates for burn patients: with no risk factors, chance of death 0.3%; in older patient with large burn area and inhalation injury, chance of death �90%; smoke inhalation may lead to pneumonia, respiratory dysfunction, and sepsis; factors additive

Major causes of mortality associated with smoke inhalation: include burn shock, burn-wound sepsis, respiratory complications, and carbon monoxide (CO) poisoning

Anesthesia provider can improve outcome: with proper fluid administration, by reducing length of surgery, and by minimizing hypotension and hypothermia

Acute Management

Upper airway injury: intubate early; identify patients at risk by laryngoscopy or bronchoscopy

Pediatric airway: smaller pediatric airway can predispose to obstruction; 1-mm increase in tracheal thickness in­creases airway resistance 16 times, due to 75% decrease in cross-sectional area (only 3 times increased resistance in adults and 44% decrease in cross-sectional area); mask ventilation may easily obstruct airway; right mainstem intubation occurs more often than in adults; cricoid cartilage narrowest part of airway

Plan for securing airway: mask inhalation induction; awake intubation; fiberoptic bronchoscope; tracheostomy

Tracheostomy: indicated for prolonged endotracheal intubation; possible higher complication rate, especially if in­cision through burn area

Circumferential chest wall thickening: increased work of breathing; alters compliance, due to loss of skin elastic­ity; escharotomy may be necessary to improve expandability of chest wall

Smoke inhalation injury: direct thermal effects due to dry air (at 300�C) or steam (at 100�C); CO; toxic components of smoke (eg, nitrates, sulfides)

Direct thermal injury: hot air can cause injury to upper airway; steam (4000 times heat capacity of air) can cause in­jury to upper and lower airway

Carbon monoxide poisoning: early treatment with O₂ decreases mortality; nonsmokers have 1% to 3% carboxyhe­moglobin (COHb) levels; smokers have 1% to 8% COHb level; fire victims near 100% CO levels; frequent imme­diate cause of death; CO affinity for hemoglobin 250 times that of O₂; even small amounts of CO can produce equal levels of oxyhemoglobin and COHb; causes left shift of oxyhemoglobin dissociation curve (less O₂ released to bloodstream)

Smoke inhalation: toxic components; particles of ash suspended in mixture of gases; composition depends on type of material burned; toxicity can alter mucociliary clearance and cause mucosal tissue necrosis; most reliable early indirect indicators include enclosed space fire, COHb >10%, or carbon in sputum; admit for observation with these indicators, or with history of unconsciousness, facial burns, burned or singed nasal hairs, respiratory distress, dyspnea, hoarseness, abnormal blood gas measurement, or abnormal chest x-ray; unconsciousness pre­vents patient from protecting airway; later diagnostic tests include chest x-ray, lung scan, endoscopy, and pulmo­nary function tests; look for hypoxia and decreased cardiac output in first 36 hr, pulmonary edema between 6 and 72 hr, and bronchopneumonia at 3 to 10 days

Treatment: early endotracheal intubation indicated for upper airway obstruction and managing secretions; aerosol­ized surfactant (ExoSurf) used in past to replace lost surfactant in less severe lung injury

Electrical burns: entrance and exit points; may have few external injuries but extensive internal injuries (eg, cardiac, muscle); alternating current (AC) burn may cause ventricular fibrillation; direct current burn may cause asystole; kidneys at risk for myoglobinemia

Major metabolic changes: stages include resuscitation (0 to 36 hr), postresuscitation (days 2 to 6), inflammation (days 6 to wound closure), and rehabilitation (ongoing); hypermetabolic state (increased caloric needs) common in first 24 to 48 hr; possible tachycardia; sepsis also of concern

Cardiac effect of major burn: increased vascular permeability leads to release of variety of inflammatory sub­stances, including histamine, serotonin, and bradykinin; alter cell membrane; loss of fluids and electrolytes; may lead to anemia and red blood cell destruction; circulatory changes occur with decreased cardiac output; hyperdy­namic circulation begins on second burn day and may last 3 to 4 wk

Fluid administration: hypovolemic burn shock may lead to hypotension if fluid resuscitation inadequate; use �rule of nines� to determine total body surface area (TBSA) for calculation of amount of fluid to administer; urine output and color of urine good indicators; numerous formulas (eg, adapted from Baxter, Brooke, Parkland, and MGH) for calculating fluid volumes; many facilities give colloids (rather than crystalloids) initially; must exercise caution to avoid under- or overhydration; renal failure may occur in 0.5% to 38% of patients; urinary output (using indwelling urinary catheter) of 0.5 mL/kg per hour easiest and most valuable monitor

Intraoperative Management

Choice of anesthesia: tailor each anesthetic to each patient; inhalation anesthetics include halothane (may cause ar­rhythmia), isoflurane (most economical for long-term ventilation), sevoflurane, and desflurane

Drugs for dressing changes: infusion with dexmedetomidine (Precedex), 1 mg/kg per hour (side effects include hy­potension); 50% of patients may not require analgesics (although they should be available); avoid etomidate (adre­nal suppression may occur with repeated administration); disadvantages of ketamine include hallucinations and psychic effects

Burn patients and NMBA: become more sensitive to depolarizing NMBA; decrease in pseudocholinesterase with succinylcholine; more resistant to nondepolarizing NMBA (may need to increase to 3 to 5 times normal dose, espe­cially if burn >25% of TBSA); avoid succinylcholine if >8 to 12 hr after burn; consider high-dose rocuronium in­stead; monitor twitch to regulate NMBA dose

Patient monitoring: individualize monitors to patient; assess response to fluid therapy; warm fluids; keep tempera­ture in operating room (OR) >28�C; pulse oximetry cannot distinguish hemoglobin types; falsely elevated SpO₂ in patient with COHb

Choice of monitors: use standard monitors; be creative in placement of monitors and electrodes; in some cases, co­axial continuous catheter necessary

Blood loss: difficult to determine; serial hemoglobin measurements necessary; vasopressor-soaked gauzes (eg, epi­nephrine) can lead to false sense of security (ie, cause increased BP); limit surgery to loss of one blood volume

Avoid sudden intraoperative hypovolemia: maintain good communication with OR team; limit excision area at each debridement to 2 hr, 4 unit blood loss, temperature > 34�C, and 20% TBSA of burn

Monitor perioperative heat loss: minimize by placing plastic around body after dressings placed

Suggested Reading

Avidan MS et al: Anesthesia awareness and the bispectral index. N Engl J Med 358:1097, 2008; Cohen MA, Guzzardi LJ: Inhala­tion of products of combustion. Ann Emerg Med 12:628, 1983; Curreri PW et al: Burn injury. Analysis of survival and hospitaliza­tion time for 937 patients. Ann Surg 192:472, 1980; Davidson AJ et al: Awareness during anesthesia in children: a prospective cohort study. Anesth Analg 100:653, 2005; Domino KB et al: Awareness during anesthesia: a closed claims analysis. Anesthesiology 90:1053, 1999; Eger EI 2nd et al: Hypothesis: inhaled anesthetics produce immobility and amnesia by different mechanisms at differ­ent sites. Anesth Analg 84:915, 1997; Ekman A et al: Reduction in the incidence of awareness using BIS monitoring. Acta Anaesthe­siol Scand 48:20, 2004; Flaishon R et al: Recovery of consciousness after thiopental or propofol. Bispectral index and isolated forearm technique. Anesthesiology 86:613, 1997; Kerssens C et al: Memory function during propofol and alfentanil anesthesia: pre­dictive value of individual differences. Anesthesiology 97:382, 2002; Liu WH et al: Incidence of awareness with recall during gen­eral anaesthesia. Anaesthesia 46:435, 1991; Lubke GH et al: Dependence of explicit and implicit memory on hypnotic state in trauma patients. Anesthesiology 90:670, 1999; Lubke GH et al: Memory formation during general anesthesia for emergency cesar­ean sections. Anesthesiology 92:1029, 2000; Moerman N et al: Awareness and recall during general anesthesia. Facts and feelings. Anesthesiology 79:454, 1993; Mychaskiw G 2nd et al: Explicit intraoperative recall at a Bispectral Index of 47. Anesth Analg 92:808, 2001; Myles et al: Bispectral index monitoring to prevent awareness during anesthesia: the B-Aware randomised controlled trial. Lancet 363:1757, 2004; O'Connor MF et al: BIS monitoring to prevent awareness during general anesthesia. Anesthesiology 94:520, 2001; Osterman JE et al: Awareness under anesthesia and the development of posttraumatic stress disorder. Gen Hosp Psy­chiatry 23:198, 2001; Pollard RJ et al: Intraoperative awareness in a regional medical system: a review of 3 years' data. Anesthesiol­ogy 106:269, 2007; Sandin RH et al: Awareness during anaesthesia: a prospective case study. Lancet 355:707, 2000; Schwender D et al: Midlatency auditory evoked potentials and explicit and implicit memory in patients undergoing cardiac surgery. Anesthesiology 80:493, 1994; Shirani KZ et al: The influence of inhalation injury and pneumonia on burn mortality. Ann Surg 205:82, 1987; Wennervirta J et al: Awareness and recall in outpatient anesthesia. Anesth Analg 95:72, 2002.