l 1 14-yr-old girl presented with many large, deep ulcers with unnatural shapes on her left f orearm, leading to a provisional diagnosis of dermatitis artefacta. Psychiatric examination revealed that she had dissocia- tive identity disorder and that mutilations only occurred when one of her identities was assumed. 224 612 9 Anesthesia Pre-operative assessment The history 613 The examination 614 Tests 614 ASA classi \201 cation (American Society of Anesthesiologists) 615 Premedication 616 Equipment and anesthetic agents Inhalational agents 618 Commonly used IV anesthetic agents 620 Other IV anesthetic agents 6 21 The ideal IV anesthetic agent 621 The ideal inhaled anesthetic agent 621 Neuromuscular b lockers 622 Practical conduct of anesthesia Fig 1. Immuno \202 uorescent labeled image of the motor unit including the neuromuscular j unction (NMJ). Axons and nerve terminals (in green) are marked with anti bodies against neu ro \201 lament and synap- tophysin respectively. Acetylcho- line receptors (in red) are la beled with rhodamine- \200 -bungarotoxin. Blockade at the NMJ is just one arm in the triad of: Induction 624 Airway control 624 Mechanical ventilation 625 Intubation 626 Rapid sequence i nduction (RSI) 626 Maintenance of anesthesia 626 End of anesthesia 626 Intubation technique 627 Monitoring during anesthesia 628 Some complications of anesthesia 628 Sedation and rec overy 630 Local and regional anesthesia 632 \200\2235 Pain management 636 Muscle relaxation Hypnosis and Analgesia that is required for eff ective an- aesthesia. See p622 for how we can interpose at this ju nction in the safest possible manner. Image courtesy of Prof. Mendel Rimer, School of Biological Sciences, University of Texas, A ustin. Other relevant pages Pain relief in labor (p66); resuscitation after delivery (p107); neona tal ITU (p108); ventilating neonates (p110); major injury (p724); choosing the correct dose of lidocaine according to body weight (p729); pain relief in children (p719); Bier \200 \231s block (p744). Online resources www.frca.co.uk Introduction Anesthesia evolved from humble origins in 1842 when CW Long gave ether, but he failed to report this landmark in pain relief. Then, in 1844, Horace Wells used nitrous ox ide for tooth extraction (p617), and in 1846 WTG Morton gave the \201 rst surgical anesth etic with ether. It is now a highly sophisticated spe- cialty in its own right. While the t riad of anesthesia (hypnosis, analgesia, and muscle relaxation) remains the fundamental pri nciple behind general anaes- thesia for surgery, the role of the anesthetist has expanded t o encompass not only the provision of ideal operating conditions for surgery, but also inte nsive care, resuscitation, alleviation of acute and chronic pain, obstetric anesthesia and anesthesia for diagnostic procedures. A detailed knowledge of general medicine, physiology, pharmacology, the physical properties of gasses and the workings of the vast array of anes thetic equipment are essential in order to practice well. \200\207We emphasize that this short chapter is no substitute for a specialist text or for experience on the ward and in theater or clinic. The main aim is to en- able understanding of the issues anesthetists face, and to prepare one \200\231s mind for intelligent discuss ions on anesthetic issues. We thank Dr Mike Nathanson \200\224our Specialist Reader for thi s chapter. \200 613 Pre-operative care Aims To ensure that patients: \200 Get the right sur- gery \200 Are happy and pain free \200 Are as \201 t as possible \200 Have individualized decisions on type of anaesthe- sia/analgesia taking into account risks, bene \201 ts, and wishes. T he pre-operative visit Have the symptoms, signs or patient \200\231s wishes changed? If so, inform the surgeon. Assess cardiovascular and respira- tory systems, exercise tolerance, e xisting illnesses, drug therapy, and allergies. Assess past history \200\224see MINIBOX. Fa mily history Ask about malignant hyperthermia (p628); dystrophia myotonica (OHCM p514); por phyria; cholinesterase problems; sickle-cell disease (test if needed). Does the patient hav e any speci \201 c worries? Drugs Ask about allergy to any drug, antiseptic, adhesive band age, & latex. \200 ACE inhibitors: No special action is required if BP & u&E are OK. \200 Antibiotics: Aminoglycosides, colistin, and tetracycline prolong neuromus- Past history screen: \200 MI or IHD \200 Asthma/COPD \200 Hypertension \200 Rheumat ic fever \200 Epilepsy \200 Liver/renal disease \200 Dental problems \200 Neck prob lems \200 GI re \202 ux or vomiting (needing rapid sequence induction \200\224see p626) a i s e h t s e a n A Past anesthesia/ problems (eg intubation diffi culty/PONV) Recent GA? cular blockade, even depolarizing neuromuscular blockers. Anticoagulants: Know the indication. Check the INR and switch warfarin to heparin pre-opera tively, leaving suffi cient time for the INR to drop to <1.5 before surgery. Admit early, a nd discuss the plan of action so that all goes smoothly. Avoid epidural/spinal blocks. Bewa re regional anesthesia. Anticonvulsants: Give the usual dose up to 1h before surgery. Give drugs IV (or by NGT) pos t-op, until able to take oral drugs. Sodium valproate: an IV form is available (give the pa tient \200\231s usual dose). Phenytoin: give IV slowly (<50mg/min). IM phenytoin absorption is unreliable. Beta-blockers: Continue up to and including the day of surgery as this re- duces the likelihood of a labile cardiovascular response. Bronchodilators: Continue and consider supplementing with nebulizers. \200 Contraceptive Pill: See BNF. Stop 4 weeks before major (or leg) surgery, restarting at 2wks post-op if mobile. Digoxin: Continue up to and including morning of surgery. Check for toxic- ity and check pl asma K+. Suxamethonium \200\200serum K+ by 1mmol/L, and can lead to ventricular arrhythmi as in the fully digitalized. Diuretics: Beware hypokalaemia. Check U&E. \200 HRT: Stop before hip surgery; use heparin thromboprophylaxis + stockings. \200 Insulin: Stop insulin on day of surgery and start a GKI infusion (glucose, potassium, and insulin). See OHCM p591. Levodopa: Possible arrhythmias when the patient is under GA. \200 Lithium: Check for a re cent serum level; non-depolarizing muscle relaxants may be potentiated. Beware post-op toxi city U&Es imbalance: p354. \200 Monoamine oxidase inhibitors: Get expert help as inter action with narcot- ics and anesthetics may lead to hypotensive/hypertensive crisis. Newer selective MAOIs are safer. Ophthalmic drugs: Anticholinesterases used to treat glaucoma may cause sensitivity to, and prolong duration of, drugs metabolized by cholinesteras- es, eg suxamethonium. Beta-blocker eye drops may cause systemic symp- toms of bronchospasm/hypotension. Steroids: If the patient is on or has recently taken steroids at an equivalent of >10mg pre dnisolone per day give extra cover for the peri-operative pe- riod (p616). See BNF section 6.3.2 for steroid equivalence doses. Tricyclic antidepressants: These enhance the eff ects of adrenaline in ad- dition to exerti ng anticholinergic eff ects causing \200\200HR, arrhythmias and \200\201BP. Pre-operative examination & tests It is the anesthetist \200\231s duty to assess suitabilit y for anesthesia. It requires an appreciation of the patient s wishes and desires, as well as their pre-morbid state. It requires an understanding of the proposed surgery and the par ticular anesthetic techniques to suit both the patient and the surgeon. It requires this to be done for a range of situations within a range of urgency, eg dedicat- ed pre-operative assessment clinics by day, trauma theaters by night. Planning and preparation with the pati ent are invaluable (see BOX), though when neces- sary, the ability to bring a patient safel y into theater for life-saving surgery as soon as possible comes with the con \201 dence g ained through experience. The ward doctor assists with a good history and examination, but should not be responsible for consent (OHCM p570), though can discuss post-operative com- p lications \200\224both general (OHCM p578) and speci \201 c (OHCM p582). DoH guidelines ar e that the consenting doctor must be capable of performing the procedure or have been speci ally trained in taking consent.1 \200\207Be alert to risk factors (see BOX). Communicate y our concerns to the anesthetist. Other issues: 614 \200 DVT prophylaxis (eg p706) \200 Frozen section (tell pathology) \200 Perioperative antib iotics (OHCM p572) Bowel prep (OHCM p572) \200 Post-op physio \200 On-table x-rays. a i s e h t s e a n A Tests Be guided by age, history, examination and proposed surgery, \201 nding the safe ba lance between too many investigations and too few. Be guided by NICE, which takes into acco unt the grade of surgery, from grade 1 (minor) to grade 4 (complex+) and beyond: cardiovasc ular and neurosurgery. \200 1 NB: tests may not be needed for young \201 t adults havin g intermediate surgery, eg knee arthroscopy. \200 FBC, ward test for blood glucose, and U &E \200\224see MINIBOX for when to check . If Hb <10g/dL tell anesthetist. Investigate/treat as appropriate. Group & save for all major surgery; crossmatch according to local transfu- sion guidelines, depending on pre-op Hb and type of surgery. Consider using autotransfusion devices, such a s specialized suction and drains. LFTs in jaundice, malignancy, or alcohol abuse. Amylase if needed (eg trauma). \200 Blood glucose in diabetic patients (OHCM p576). \200 Drug levels as appropriate (eg digoxin, l ithium). \200 Clotting studies in liver disease, DIC, massive Check U&Es if: \200 On diuretics \200 Diabetes \200 Burns victim \200 Major trauma \200 Hepatic/renal disease \200 Intestinal obstruction/ blood loss, already on warfarin or heparin. Virology: HIV, HBsAg in high-risk patients \200\224after appropriate counseling. Sickle-cell test in those from Africa, West In- dies, or Mediterranean area \200\224and oth ers whose origins are in malarial areas (including most of In- dia). Take consent before pe rforming the test, and off er genetic counseling. ileus Parenteral nutrition. Thyroid function tests in those with thyroid disease. \200 CXR: If known cardiorespirator y disease, pathology or symptoms. \200 ECG: In those with cardiorespiratory disease \200 \224ie poor exercise tolerance, an- gina etc, BP \200\200, past rheumatic fever/Kawasaki \200\231s (p646) \200\224and age >60 (high incidence of \200\230silent \200\231 ischaemia) . Lateral c-spine x-ray: Consider in rheumatoid arthritis/anklyosing spondyli- tis/Down \200\231s syndrome to check for atlanto-axial instability. \200 Urinalysis is no t speci \201 cally indicated, though may be useful. If the patient needs referral to anoth er medical team for optimization or in- vestigation, ensure that both the patient and surge on are involved and aware. 1 Use only words the patient understands. Ensure he believes your facts and can retain pros and cons long enough to inform his decision. Make sure his choice is free from pressure fr om others. A patient may complain if: \200 He is unaware of what will happen \200 He ha s not been off ered all options \200 He was sedated at the time of consent \200 He chan ged his mind \200 He was not told a treatment was experimental \200 A 2nd opinion has b een denied \200 Details of prognosis were glossed over. 615 Risk factors in the perioperative period2 \200 Age: The risk of dying doubles every 7 yea rs from the age of 10. Such that the mortality risk at 90 is 5000 \211\210 greater than the risk at age 10. \200 Sex: Men are 1.7 \211\210 more likely to die than women of the same age. \200 Socioeconomic status : The impoverished are 2 \211\210 as likely to die as the rich. \200 Aerobic \201 tness : Ask about a patient s functional capacity \200\224this can be measured in metabolic equiv alents (METs), where 1 MET equals the basal metabolic rate. Ask the patient if they can wal k indoors or 100m on lev- el ground (2 \200\2233 METs), or climb 2 \202 ights of stairs ( 4 METs) or participate in strenuous sport (eg singles tennis; skiing 10METs). When functio nal capac- ity is high, the prognosis is excellent, even in the presence of other risk fac- tors. A low functional capacity has been associated with poorer outcomes in thoracic surge ry, although it has less predictive power with non-cardiac surgery. 3 In the pre-assessment clinic aerobic \201 tness can be assessed by a shuttle walk, or cardio-pulmonary exercis e testing. a i s e h t s e a n A Diagnosed myocardial infarction (MI), heart failure, stroke, kidney fail- ure (creatinine > 150 mol/L), peripheral arterial disease: Multiply long- term mortality risk by 1.5. Angina (without MI) and transient ischaemic at- tacks increase risk to a lesser degree. Other: Ask about COPD/asthma, smoking, diabetes, hypertension and hyper- cholesterolaemia a s these may contribute to perioperative risk. \200\207Post-op chest infections are \211 \2106 more likely in smokers. Stopping smoking 6 \200\2238 weeks pre-op is best but stoppin g smoking even 1 day pre-op is of bene \201 t. 4 Pre-op fasting Assoc. of Anesthetists of \200 Fast the patient: For elective surgery, if there is no GI co-morbidity, allow clear \202 uids (including black tea or coff ee) \211 2h pre-op; all other intake (ie food/solids) up to 6h beforehand. In emergency surgery, it may be best to restrict all oral intake to \211 6h pre-op. Involve the anesthetist in any deci- sions if the situation is unclear. Children undergoing elective surgery are allowed formula/cows \200\231 milk/solids upto 6h pre-op; breast milk up to 4h pre- op and clear \202 uids up to 2h pre-op. Chewing gum may be allowed up to 2h pre-op. Great Britain & Ireland 20105 ASA score ASA = American Society of Anesthesiologists 1 Normally healthy 2 Mild systemic disease, but with no limitation of activity 3 Severe sys temic disease that limits activity; not incapacitating 4 Incapacitating systemic disease wh ich poses a threat to life 5 Moribund. Not expected to survive 24h even with operation 6 Br ain dead patient whose organs are being removed for donor purposes This should re \202 ect the health of the patient at the time of surgery. There is a slot for ASA numbers on most anesthetic charts; the pre \201 x \200\230E \200\231 denotes emergencies. NB: in most (bu t not all) studies ASA correlates with morbidity. 6 Patient safety: use of checklists \200\207Follow local procedures. Safety must be a leadin g concern. 7 Checklists help achieve safety by reducing complex tasks to component parts, a nd by ensur- ing that nothing is omitted, eg: 8 \200 Identity \200 Procedure \200 Con sent \200 Equipment check \200 Site marked \200 Allergies? \200 Aspiration risk? \200 Anticipated blood loss >500mL (7mL/kg if a child)? \200 Have team members introduce d themselves by name & role? \200 Any patient-speci \201 c concerns? \200 Post-op: hav e equipment fail- ures been addressed? \200 Have surgeon & anesthetist liaised over recov ery? Full checklist: www.who.int/patientsafety/safesurgery Premedication 616 \200 Historically most patients would have received Premedication: 7As \200 Analgesia \200 Anxiolysis \200 Amnesia \200 Antacid \200 Anti-emesis (see BOX) \200 Antibiotics \200 Anti-autonomic. some sort of premedication to allay anxiety and contribute to a smooth induction of anesthe sia by decreasing secretions (more important when ether was used), promoting amnesia and an algesia, and decreasing vagal re \202 exes. With increasing amounts of day-surgery and gre ater awareness of "hango- ver" eff ects from premedication routine use of an- xiolytics is far less common. However, there are a variety of reasons patients may receive certain pre- operative medications which may improve patient outcome and satisfaction (see MINIBOX.) Tim ing 2h pre-op for oral drugs. Anxiolysis and amnesia Bear in mind that amnesia may add to the un- pleasantness of the experience, though it can be useful in those not wanting to kno w, and children. The most common agents used are benzodiazepines. These should be prescribe d by the anesthetist. Examples for the 70kg man: \200 Lorazepam 2mg PO. \200 Diazepam 5 mg PO. \200 Midazolam 0.05 \200\2230.1mg/kg PO. NB: relaxing music may be as eff ective a s mi- dazolam in reducing anxiety. 9 a i s e h t s e a n A Examples for children: (Always use oral premeds in children as \201 rst choice.) Midazolam 0.5mg/kg (tastes bitter so often put in paracetamol suspension). \200 Local anes thetic creams: Tetracaine 4% (Ametop ; apply 45min before inserting IVI) is more popular t han EMLA , as it does not vasoconstrict. \200\207NB: The presence of a parent at inductio n is more powerful than any premedication in reducing anxiety. 10 Speci \201 c premedications Analgesia: Pre-emptive analgesia in elective pa- tients aims t o dampen the pain pathways before the signals start to arrive, thus modulating longer term pain response. 11,12 Patients admitted for acute procedures, who have pain pre-operatively should be given adequate pain relief. Antacids: For re \202 ux either ranitidine 150mg PO or omeprazole 40mg PO/IV the night before and then 2h pre-op can be given \200\224ranitidin e reduces both gastric pH and volume. Emergency cesareans: see p78. \200\207Aspiration is a much greater risk in emergency surgery, pregnancy, diabetes, and with a hiatus hernia, ca using fatality in 1 in 70,000 of all anesthetics. Antibiotic prophylaxis: See OHCM p572. B eta-blockers can be used to reduce the risk of peri-operative ischaemia. Bronchodilators: e g salbutamol nebulizer. Nitrates: Can be applied as a patch or IVI as in cardiac surgery. S teroids: Minor opera- tions: eg 25 \200\22350mg hydrocortisone IV at induction. Major opera tions: 50mg hydrocortisone IV at induction, then repeat 3 times (8-hourly before restarting oral); ditto if adrenal insuffi ciency or adrenal surgery, or steroid therapy within last 3 months with over 10mg of prednisolone per day. Common reasons for cancelation \200 Curr ent or still symptomatic respiratory tract infection. \200 Patient not in optimum conditi on, eg poor control of drug therapy (insulin for diabetic patients, digoxin, thyroxine, phe nytoin); exacerbation of illness. Recent myocardial infarction (eg within last 3 months). \200 U&E imbalance (particularly K+); anemia. \200 Inadequate preparation (results not available, not crossmatched/fasted) . \200 Undiagnosed or untreated hypertension or uncontrolled atrial \201 brillation (heart rate > 100/min). Insuffi cient ITU beds, staff , or theater time or other logistical problems. 617 Post-operative nausea and vomiting (PONV) This remains one of the more unpleasant side eff ects imposed by anesthesia, and is expe- rienced by at least 25% of patients. Control is de sirable not least for comfort, but also for minimization of post-op complications such as: \200 Dehydration \200 Electrolyte imbalance \200 Metabolic imbalance (metabolic alkal osis) \200 Pulmonary aspiration \200 Hernia formation \200 Damage to site of surgery (direct, eg ENT, or PONV risk factors: \200 Patient factors: Female \200 Previous history \200 Obesity \200 Motion sickness \200 Pre-operative anxie ty NB: smoking is protective Anesthetic agents: Opioids \200 Nitrous oxide (N2O) \200 Etomidate/ketamine \200 Volatile agents a i s e h t s e a n A indirect, eg neurosurgery) Inability to take oral medication \200 Delayed discharge. NB: TIVA total IV anaethesia with propofol \200\201PONV \200 Surgery type: The exact mechanisms of action for all the factors involved (see MINIBOX) are unknown, thou gh the central mechanisms behind the vomiting re \202 ex are somewhat better under- stood. GI \200 GU/Gynae \200 Neurosurgery \200 Middle ear \200 Ophthalmic \200 Post-op facto rs: \200 Dehydration \200 Hypotension \200 Hypoxia \200 Early oral intake 13 It is initiated in the vomiting center of the me- dulla (see \201 g 1), which itself rece ives input from higher centers, the chemoreceptor trigger zone (CTZ), aff erent somatic and visceral \201 bres, and the vestibular apparatus of the middle & inner ear. Of these, th e CTZ in the area postrema (lo- cated in the \202 oor of the fourth ventricle) is prob- a bly the most important. For speci \201 c anti-emetics and their mode of action see OHCM p241, and in this book on p630. Neurokinin 1 (NK1) receptor antagonists, eg aprepitant (80mg PO, given 3h prior to an esthesia), are a new class of antiemetics which appear more eff ective than 5-HT3 antagonis ts (e.g. ondansetron). 14 Non-pharmacolog- ical approaches such as P6 acupuncture point sti mulation are eff ective. 15 Sensory (taste, smell, sight) Personality, memory, anxiety Higher centers 2, M D 1, H 1, 5-HT 3 receptors Sedatives (benzodiazepines) Chemoreceptor trigger zone (area postrema) Vomiting center (medulla) Drugs Cranial nerves 7, 9, 11, 12, and spinal nerve afferents 2 and 5-HT D 3 receptors Somatic and visceral afferents Nausea and vomiting Middle ear/ labyrinth 1 and M H 1 receptors Motion, pressure (eg N2O) Surgery, trauma Main afferent pathway Risk factor Principal receptor sites Final common pathway 5-HT 1, histamine receptor; M H 3, serotonin receptors; D 2, dopamine receptor; 1, muscarinic receptor Fig 1. The neural pathways, receptor sites and triggers for PONV. Reproduced from Postoperative nausea and vomiting: Barry Miller, Anesthesia and Intensive C are Medicine: 7 : 12; 453 \200\223455. 2006, with permission from Elsevier Equipment See Assoc. of Anesthetists of Great Britain & Ireland (16,17) Careful checking of equipment is vital before any anesthetic or sedative pro- cedure. Check anesthetic machine (plugged in and on) plus: \200 Tilting bed or trolley (in case of vomiting). \200 High -volume suction with rigid Yankauer/long suction catheters. \200 Reliable oxygen supply, capable of delivering 15L/min. \200 Self-in \202 ating bag with oxygen reservoir, non-re breathing valve, and compat- 618 \200 ible mask (a \200\230bag-valve-mask \200\231 system). Oropharyngeal, nasopharyngeal, and laryngeal mask airways. \200 A range of anatomical fac emask sizes. \200 Anesthetic circuit: make sure that there are no obstructions to the \202 ow. Check the vaporizers, \202 owmeters, and ventilator. Laryngoscope with range of blade sizes, spare bulbs, and batteries. \200 Tracheal tubes ( range) and catheter mount. \200 Intravenous infusion cannulae and \202 uids (warmed if necessary). \200 Anesthetic and resuscitation drugs and anesthetic gasses. \200 De \201 brillator. \200 Monitoring equipment (eg pulse oximeter end-tidal CO2 monitor, p6 28). a i s e h t s e a n A Inhalational agents These are volatile liquids which readily vaporize, permitting administr ation by inhalation in O2-enriched air or an O2/N2O mix. They help maintain anesthesia and decrease awareness (by an unclear mechanism). Halothane ( \201 g 1) This is a colorless, p leasant- smelling gas (which, unlike the other agents, is not an ether) \201 rst used as an anesthetic agent in the 1950s. It has little analgesic eff ect, but decreases cardiac ou tput (vagal tone \200\200, leading to bradycardia, vasodilation, and hypotension). It sens itizes the myo- cardium to catecholamines (beware in patients with arrhythmias; surgical in \201 ltration with local anesthetic and adrenaline). Halo- thane is no longer used in the UK \200\224it has been replaced by safer inhalational agents (below), due to the rare but serious complication of postoperative hepatitis (up to 1 in 4,000 for multiple exposures; less frequently in children). 18 Iso \202 urane ( \201 g 2) This is a halogenated ether. The- oretically induction should be quick, but iso \202 urane is irritant, so coughing, la ryngospasm, or breath- holding may complicate the onset of anesthesia. Sevo \202 urane ( \201 g 3) This is a halogenated ether which is well tolerated. It is the agent of choice fo r inhalation induction of general anesthesia due to its combination of being low irritant, and having relatively fast onset \200\224off set. Des \202 urane ( \201 g 4) is another h alogenated ether with a rapid onset of anesthesia, and quick recov- ery. Having perhaps the lowest absorption into fat, des \202 urane is often chosen for surgery in the mor- bidly obese as it provides for the quickest recovery post surgery. 19 However, it is more of a re spiratory irritant than sevo \202 urane, so is sometimes only used for maintenance rather than induction of anesthesia. It may also have greater depressant eff ects of the cardiovas cular system than sevo \202 urane. Stopping inhalation reverses all the above eff ects \200\224except for hepatitis resulting from drug metabolism. \200 All can cause malignant hyper- thermia (p628). Cl F = Br F F Fig 1. Halothane. F Cl F = F O F F Fig 2. Iso \202 urane. F3C = O F CF3 Fig 3. Sevo \202 urane. F F = F3C O F Fig 4. Des \202 urane. 619 Inhalational agents, volatility, vapor pressures and potency The ease with which the agent vaporizes in the chamber (=volatility) is a function of its saturation vapor pressure (SVP) and the temperature of the liquid. Agents with a higher SVP (eg iso \202 urane) need less gas to \202 ow over them to reach a given concentration. A vaporizer should ideally deliver a predictable and controllable concen- tration of anesth etic agent to the patient for inhalation. There are 2 main types of device that can be used to create the inhaled vapor: \200 High-resistance devices: eg the plenum vaporizer. Inha lational agents are added to the fresh gas \202 ow by passing a fraction of the carrier g as (N2O/O2 or O2 \200\223enriched air) through a chamber \200\224the plenum ( \211\210 \200\230full space \200\231). a i s e h t s e a n A More gas Carrier gas Vaporizer To patient Volatile agent Fig 1. Schematic diagram of a high-resistance device. This requires a positive pressure to drive it and it is therefore a high- resistance device . It works as a precision vaporizer, which delivers more accurate concentrations by overcom ing the eff ect of temperature variation with a heat sink or a valve to vary gas \202 ow. The plenum cannot be used inside the patient circuit as its internal resistance is so high . Low-resistance draw-over devices: eg Oxford miniature vaporizer, Goldman vaporizer. The pat ient \200\231s breathing generates enough \202 ow to drive the gas \202 ow. With the in crease in the use of low- \202 ow anesthesia, low- resistance vaporizers have been develop ed that \201 t directly into the breath- ing circuit. 20 Accuracy/effi ciency depends on the exact device used. \200\207Cross-contamination and overdose in multiple devices can be a problem, though precautions are taken against this. 21 Potency The potency of an inhaled agent can be given by its minimum alveolar concentration (MAC), which is the concentration that will produce no movement response to incision of the skin in 50% of unpremedicated patients. It is not a useful clinical measurement, but it ca n be used to com- pare potency of the diff erent inhalational agents. It is aff ected by a number of factors, eg age, thyroid status and anxiety state of the patient. Footnotes in History Inevitably, not all stories along the trail of discovery and development in medicine are joyous ones. In 1844, the dentist Horace Wells was sub- ject to the \201 rst tooth extraction under anesthesia with the \200\230Exhilirating or Laughing Gas \200 \231, nitrous oxide. Taking up the technique, he subsequently went on to spectacularly fail to anesthetize a patient in a demonstration of tooth ex- traction at the Massachusetts Gen eral Hospital, leaving the patient in agony as he performed the procedure. For Wells, this was the start of a downward spiral into chloroform addiction, and ultimately suicide in jai l after being arrested for throwing sulphuric acid at two prostitutes. Let \200\231s allow it to be a somber testa- ment to us all to make sure that we look after ourselves and our c olleagues. 22,23 This tale was sourced from The Greatest Bene \201 t to Mankind, by Roy Po rter, Fontana Press, 1999 Commonly used IV anesthetic agents Propofol (2,6-diisopropylphenol; t =30 \200\223600min)1 This lipophilic phenol deriva- tive is produced commercially as an emulsion in soybean oil and has become the most commonly used IV anesthetic agent in the developed world. Its good recovery characteristics and anti-emetic eff ect make it popular, espe- cially in day-case surgery. 24 It is fast-acting and its off set of action is due to rapid redistribution, an d not metabolism. \200 Dose examples: Induction = 2mg/kg IVI at 2 \200\2234mg/ sec. Maint enance = 4 \200\22312mg/kg/h IVI. NB: \200\207Rapid injection can cause cardiovascular dep ression ( \200\201BP), and respiratory depression can occur when combined with IV narcotics . For procedural seda- tion: 0.5 \200\2231mg/kg IVI over 1 \200\2235min. Dose needs to be r educed in the elderly, debilitated and shocked. Dose also depends partly on premed use: wit h midazolam induction dose requirements for propofol are reduced by 20 \200\22350%. This ef f ect seems to be more pronounced in the elderly. 26,27 \200 Uses: It is used in inductio n and maintenance of GA, and for sedation dur- ing regional anesthesia, short procedures, a nd as a sedative in ITU (though contraindicated in children for this last use). Once opened , use ampoules or discard, because of the risk of bacterial growth. 620 \200 \200\207Contraindications: \200 Extremes of age \200 <17yr for sedation \200 Egg or soy allergy 25 \200 Compromised airway2 a i s e h t s e a n A Problems: Pain on injection occurs in up to 40% of patients. This can be minimized by eithe r adding a small amount of lidocaine (eg 2mL of 1%) to the propofol, or by pre-injection of local anesthetic. 28 Thiopental sodium (t =11h) a barbiturate that is typically mixed wit h water to give a 2.5% solution (ie 25mg/mL). It has a rapid onset of action (arm \200\223 brain circulation time about 30sec). Eff ects last 3 \200\2238min, and awakening is due lar gely to redistribution, not metabolism. Some 30% of the injected dose is still present in t he body after 24h, giving rise to a hangover eff ect. It has been replaced by propofol as t he most popular induction agent. \200 Dose examples: 100 \200\223150mg (less if elderly/d ebilitated) IV over 10 \200\22315sec (longer if elderly/debilitated), followed by further q uantity if needed accord- ing to response after 30 \200\22360sec (or up to 4mg/kg; max 500m g). Uses: Induction of GA; it is also a potent anticonvulsant \200\224used in status epi- lepticus management when other measures have failed (p208). Problems: Anaphylaxis is rare (1 in 20,000). Is a negative inotrope so can drop cardiac out put by 20%. May also lead to bronchoconstriction. \200\207Intra- arterial injection produc es pain and blanching of the hand/limb below the level of injection due to arterial spasm, followed by ischaemic damage and gangrene \200\224following inadvertent brachial artery pun cture in the antecubital fossa. This is less of a problem now that an indwelling cannula is ob- ligatory (compared with the historical use of a needle). \200\207Contraindications: \200 Airway obstruction \200 Barbiturate allergy \200 Fix ed cardiac output states \200 Hypovolaemia/ \200\201BP \200 Porphyria \200 Compromised airway2 Treatment: 1 Leave the needle in the artery and inject a vaso- dilator, eg papaverine. 2 Ask an experienced colleague to perform, bra- chial plexus or stellate ganglion block. (T his should dilate vessels and \200\201ischaemia.) 3 Heparin IV to stop thrombus forming. 4 Give pain relief \200\224postpone surgery unless desperate. Extravascular injection causes severe pain and local necrosis. \200\207Get expert help. In \201 ltrate with hyaluronida se 1500IU dissolved in 1mL water through the cannula. 1 The pharmacokinetics are best modeled by a three-compartment model. 2 A compromised airwa y is a relative contraindication to any iv induction, regardless of the agent used. 621 Other IV anesthetic agents Etomidate (t =3.5h) This is a carboxylated imidazole and is use d as an induc- tion agent. Dose: 0.15 \200\2230.3mg/kg. Uses: Histamine release is not a fe ature, but rapidity of recovery and cardiovascular stability are; therefore it is often use d in emergent patients with trauma/head injuries for whom avoidance of even a brief episode of hypotension is important. Problems: Be aware it can induce involuntary muscle movements , nausea, and adrenal suppression. 29,30 Local thrombophlebitis can occur after injection. Ketamine (a phencyclidine derivative, produced as a racemic mixture; N-methyl-d-aspartate r eceptor antagonist; t =2.2h) Dose example: 2mg/kg usually gives 5 \200\22310min of surgica l anesthesia (for long procedures \200\224see BNF 15.1.1). Uses: Mainly for pediatric anest hesia and procedural sedation. Car- diac output is unchanged or increased, and so it is a g ood \200\230on site \200\231 or \200\230in the \201 eld \200\231 agent, as it can be gi ven IM, producing profound analgesia without com- pounding shock. Ketamine has potent bronc hodilatory properties, so can be considered during intubation in status asthmaticus. 31 Pro blems: Hypertonus and salivation, but there is some maintenance of laryngeal re \202 exes (but do not rely on this). Recovery is slow. Emergence phenomena are troublesome (de- liriu m, hallucinations, nightmares; all made worse if the patient is disturbed during recovery). \200\207Avoid in the hypertensive patient, those with a history of stroke or raised intra cranial pressure (further \200\200 produced), patients with a re- cent penetrating eye inj ury (risk of \200\200 intra-ocular pressure), and psychiatric patients. Avoid adrenaline i n \201 ltrations. In the UK, the Home Offi ce classi \201 es it as a class C drug as it i s prone to misuse ( \200\230Special-K \200\231). Ideal weight and drug dosages To avoid exc essive dosage in obese patients it may be best to calculate the dose on the basis of ideal body weight (IBW). a i s e h t s e a n A Ideal body weight in kg: \200 = 50kg + 2.3kg for each inch over 5 feet tall. \200 = 45. 5kg+2.3kg for each inch over 5 feet tall. See www.globalrph.com The ideal (but imaginary) IV anesthetic agent The ideal IV agent would be stable in solutio n and in the presence of light, be water-soluble and have a long shelf-life. It would be pa inless when given IV; non-irritant if injected extravascularly (with a low incidence of thr ombosis) with some pain (as a warning) if given intra-arterially. Furthermore: \200 It sh ould act rapidly within one arm \200\223brain circulation. \200 Recovery should be quick and complete with no hangover eff ect. \200 It should provoke no excitatory phenomena. \200 Analgesic properties are advantageous. \200 Respiratory and cardiovascular eff ects should be minimal. \200 It should not interact with other anesthetic agents. \200 Ther e should be no hypersensitivity reactions. \200 There should be no post-op phenomena, eg nausea or hallucinations. The ideal (but imaginary) inhaled anesthetic agent 32 Inhaled agents have advantages (eg no IV access required, more precise con- trol) and disadvantages (eg claustrophobic) over IV agents. The ideal inhaled agent should: \200 Have low solubility in blood and tissues (to allow rapid recovery). \200 Be resistant to any degradation. \200 Have no injurious ef f ects on vital tissues. \200 Be administrable in a reliable and known concentration.