Self Assessment1. You are asked to assess a 42-year-old woman who requires an emergency extensor tendon repair to her left hand. There is no neurovascular deficit.Sheis158cmtallandweighs102kg.She last ate 2 hours ago. What factors will you especially need to consider in her pre- assessment?

2.A55-year-oldmanwithaBodyMassIndex(BMI) of 37 is having an anterior resection for colon cancer. Twenty minutes into the case you notice his oxygen saturations are falling and are now 88%despiteFiO20.5.Whatactionscanyoutake to improve his oxygenation?

3.A65-year-oldwomanwithaBMIof41is24 hours post an elective total knee replacement. Shehasbeengivenintramuscularmorphine2 hourlyovernight.SheishypoxicwithaSpO2 of 87%onroomair.Herrespiratoryrateis8breaths per minute. What is your diagnosis and action plan?

morbidity and mortality. Excessive abdominal fat,“centralobesity”isparticularlypredictiveforNIDDM,dyslipidaemia and cardiovascular disease. Waist circumferences need to be sex and race specific.The table below is specific for Caucasian waistcircumferences.

Risk of obesity-associated metabolic problems

Increased Substantially increased

Men ≥94cm ≥102cm (37inches) (40inches)

Women ≥80cm ≥88cm (31.5inches) (34.5inches)

Since obesity is a multisystem disease affecting all organs, there are a number of implications relevant to the conduct of anaesthesia.

Respiratory systemObstructive Sleep Apnoea (OSA)At least 5% of morbidly obese patients will haveOSAparticularly if theyhaveassociatedriskfactorssuchas largecollarsize (over16.5 inches),eveningalcohol consumption andpharyngeal abnormalities.The disease is cause by passive collapse of thepharyngeal airway during deeper planes of sleep,resultinginsnoringandintermittentairwayobstruction.Resultant hypoxaemia and hypercapnia results in arousal and disruption of quality sleep thus causing the characteristic daytime somnolence. Pulmonary andsystemicvasoconstriction,polycythaemia, rightventricular failure and cor pulmonale can all occur. Indeed the relative hypoventilation can cause aprogressive desensitisation of the respiratory centres to hypercapnia with resultant Type 2 respiratory failure. Formaldiagnosis isby sleep studiesand treatmentincludes removal of precipitants, weight loss andnocturnalCPAP.

ObeSIty & AnAeStheSIA

DrKDRooney,SHOinCriticalCare,RoyalDevon&ExeterHospital,Exeter,UK. DrGCWerrett,AnaesthesiaFellow,ChristchurchHospital,NZ.E-mail:kierourooney@hotmail.com

Key Points• CalculateaBMIforallpatients.

• BMI>30isobese,BMI>35ismorbidlyobese.

• Obesityisamultiorgandisease.

• Significantcardiorespiratorydiseaseis particularly common.

• Perioperativemortalityandmorbidity increaseswithBMI.

IntroductionApproximately7%oftheworldwideadultpopulationisobese.Obesityisaglobalhealthproblemandtheprevalence varies with socio-economic status. Inaffluentcultures,thepoorhavethehighestprevalence(27% of the US population and 17% of the UKpopulation are obese). In thedevelopingworld it istheaffluentthatareatthehighestrisk.Thereisalsoarecenttrendtoanincreasingprevalenceofobesityinadolescentsandchildren. Importantly60-85%ofobeseschoolchildrenwillremainobeseasadults.

The difference between normality and obesity isarbitrarybuttheBodyMass Index (BMI) isnormallyusedtodefineobesity.Itcanbecalculatedbydividingthe patient’s weight in kilograms by their height inmetressquared(kg/m2).

Interestingly, the regional distribution of excessfat is thought to be more predictive than BMI for

bMI (kg/m2)

<25 Normal

25-29 Overweight

30-35 Obese

>35 MorbidlyObese

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Specific Implications for Anaesthesia:Takeaverycareful preoperative history looking particularly for evidence of the characteristic increasing snoring andsubsequentapnoea(askarelative)anddaytimesomnolence. Avoid sedative premedication. Maintenanceoftheairwaymightbedifficult.Airwayobstructionisverylikelytooccurinthepostoperativeperiod–nurseinanHDU/ICUsetting,situpifatallpossible, give oxygen and apply CPAP if required.Regional techniques and short acting anaesthetic agents are ideal to reduce postoperative drowsiness. OSA occurs most frequently during rapid eyemovement(REM)sleep,whichpredominantlyoccursonthesecondnightpostsurgery.Considernocturnaloxygen for up to 5 days followingmajor surgery ifavailable.

AirwayObesepatientstendtohaveshort,fatnecksmakingboth mask ventilation and direct laryngoscopytechnically more challenging. A BMI of 46 isassociatedwitha13%riskofdifficultintubation.Theincreased bulk of soft tissues in the upper airwaymakethempronetopartialobstructionwiththelossof consciousness.

Specific Implications for Anaesthesia: Alwaysassesstheairwaywiththesimple,quickbedsidetestssuchasMallampati,thyromentaldistance,incisorgapandtheabilitytosubluxthemandible.Combinationsoftestsimprovethepositivepredictorvalue.Difficultmask ventilation can sometimesbe transformedbyplacementofanoralairway.Obesewomenaremorelikelytohavelargebreasts,whichcaninterferewitheasy placement of the laryngoscope, therefore aimfor adegreeof head-up tilt, avoid folding the armsacrossthechestand,ifnecessary,applytractiononthebreaststoallowplacementofthelaryngoscope.Given the increased risk of aspiration (see later)and difficult intubation, a rapid sequence inductionwill often be the safest form of induction. Haveall available intubation aids such as bougies and avarietyoflaryngoscopebladesclosetohand.Ensurethereareadequatenumbersofstaffshouldthepatientrequireturning. Ifafibrescope isavailable,considerawakeintubationbutbewaryofusinganyadditionalsedation.

VentilationThe increased body mass and metabolically activeadipose tissue leads to increased oxygen consumption and carbon dioxide production. Minute ventilationis thus increased to achieve normocapnia. There is reduced chestwall compliance (of up to 30%) duetotheheavychestwall, increasedpulmonarybloodvolume and splinted diaphragm. This reduction in compliance, together with increased respiratorydemandresultsinanincreasedworkofbreathing.

In addition, the functional residual capacity (FRC)declines exponentially with increasing BMI. Theclosing capacity in these patients can encroach on

the FRC evenwhen conscious; therefore the onsetofanaesthesia,asupinepositionandtheabnormallyhigh elevation of the diaphragm (due to increasedvisceralandabdominalwallfat)allcombinetocauseventilation-perfusionmismatch,right-to-leftshuntingand arterial hypoxaemia.

Specific Implications for Anaesthesia: Thesepatients are prone to hypoxia even when conscious and will desaturate particularly rapidly once apnoeic as their oxygen reserve is reduced (reduced FRC),andoxygenutilisation increased, thusnecessitatingmeticulous pre-oxygenation. Ideally this should bedone with the patient semi erect to increase the time to desaturation.

Due to the reduced chest compliance and sheermass of the chest wall, higher inflation pressuresare required to ventilate such patients. Such highpressures preclude the use of the laryngeal mask airway (LMA) for ventilation. Hypoventilation willoften occur when breathing spontaneously viaan LMA/facemask and thus these techniques arenot recommended. Application of PEEP via anendotracheal tube isparticularlyuseful in improvingoxygenationbyreducingsmallairwayscollapse.

Extubationisusuallybestperformedwiththepatientinthesittingpositionasawakeaspossibletoallowmaximaldiaphragmaticexcursion.Otherwisetheleftlateral position is very safe initially but abdominalsplintingmightsubsequentlyleadtohypoxia.Situponce awake.

The postoperativemortality of the obese patient isdouble thatof thenonobese.Aspreviouslystated,these patients are prone to hypoxia due to small airwayscollapseandshunt.Thismaybeexacerbatedif analgesia if inadequate. However, over-sedatedornarcotisedobesepatientsareevenmorelikelytodevelop partial airway obstruction. For this reasonobese patients should be maintained on oxygen,humidified if possible, on the ward postoperativelywith continuous pulse oximetry.

Postoperative physiotherapy/incentive spirometryand use of regional techniques such as epidural analgesia should reduce atelectasis and postoperative respiratoryfailure.Earlypostoperativemobilisationisvital.

Cardiovascular systemObesity isassociatedwithanumberofcardiac riskfactors.Theseincludehypertension,ischaemicheartdisease (IHD), cardiomyopathies, cardiac failure,arrhythmias,suddencardiacdeathanddyslipidaemias.Venousinsufficiency,cerebrovascularandperipheralvascular disease exacerbated by atheroscleroticprocessesmayalsobepresent.IncreasedvisceralfatisacardiovascularriskfactorevenwhentheBMIisnormal.

Hypertension is particularly common in obesity.These patients also have an increased absolute

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bloodvolumeandincreasedcardiacoutput.Thusleftventricular stroke work is increased and left ventricular hypertrophy can result. Left and right cardiacfailurecanbothoccur.Giventhehighprevalenceofassociatedcoronaryarterydisease,thetendencytohypoxia,tachycardia(increasesinlinewithincreasingcardiacoutput)andbiventricularstrain,theaetiologyfor ischaemic coronary events is strikingly apparent.

Venous return is also reduced. An obese abdomenwill directly compress venous return from the legs (also increasing the risk of deep vein thrombosis(DVT) and pulmonary embolism). Once ventilated,higher inflation pressures and application of PEEPfurtherreducesvenousreturn,whichmayresultinafall in cardiac output.

TheriskofpulmonaryembolusandDVT isdoubledintheobese.Othercausativefactorsforthisincreaseinclude hypoxia-induced polycythaemia, cardiacfailure,decreasedfibrinolysisandimmobilisation.

Specific Implications for Anaesthesia: Performa thorough preoperative assessment looking for evidence of IHD and cardiac failure on history,examination and ECG. Chest X-ray andechocardiography may be technically difficult butpotentially useful tests. Measure non-invasiveblood pressure with the correct sized cuff. Thesphygmomanometer cuff should be 20% greaterthanthediameteroftheupperarm(remember,ifthecuffistoosmall,theBPwillbeover-estimated).Inthemorbidlyobese,invasiveBPmonitoringisadvisable.Continuecardiacdrugsthroughouttheperioperativeperiod. Heparin prophylaxis, TED stockings andearly mobilisation are some measures to reducethe incidence of DVT. Postoperative oxygen mayparticularly reduce nocturnal ischaemic events.

Gastrointestinal, endocrine and other systemsThere is an increased incidence of hiatus hernia in the obese.Thevolumeandacidityofgastriccontentsisoftenincreasedandasstatedearlier,intubationmightbedifficult.Thustheriskofaspirationisparticularlyincreased. Non-insulin dependent diabetes mellitus(anditsassociatedmicrovascularandmacrovascularchanges) is much more common in the obese,causedby insulin resistanceand inadequate insulinproduction. Hypercholesterolaemia, hypothyroidism,gout, osteoarthritis, back pain, hepatic impairment,gallstones,abdominalherniae,breastandendometrialmalignanciesareallmorecommonintheobese.

Specific Implications for Anaesthesia: Prescribeoral H2 receptor antagonists (e.g. ranitidine 150mg)or proton pump inhibitors (e.g. omeprazole 20-40mg)routinely1-2hourspreoperatively,andifindoubt,performrapidsequenceinductionwithcricoidpressureatinductionandextubatewhenfullyawake.Perform a random blood sugar test on all obesepatients. Ensure good perioperative sugar controlto reduce infection and risk of myocardial events.

Continuestatinsovertheperioperativeperiodastheymightimprovecoronaryplaquestability.

Drug handling in obesityIntheobesepatient,volumesofdistribution,bindingand elimination of drugs are unpredictable. Thisuncertainty necessitates that the anaesthetist pay more attention to the clinical end points of drug actionsuchaslossofverbalcontact,tachycardiaetc.ratherthanfocusingspecificallyonwhethertodoseonideal,leanoractualbodyweight.

Some pharmacological certainties are a reductionintotalbodywater,higherfatmass,relativelyhigherlean mass, higher GFR, increased renal clearanceand normal hepatic clearance. The apparent volume of distribution for a fat-soluble drug suchasthiopentoneis increasedbecauseof its lipophilicnatureandthereforethedoseshouldbeincreasedbutaraisedvolumeofdistributionalsoresultsinreducedelimination resulting in prolonged effects. Recent worksuggeststhatsuxamethoniumshouldbegivenatadoseof1mg/kgactualbodyweight.

Slowemergenceafteruseoffat-solublevolatileagentsmaybeduetocentralsensitivityasmuchasduetodelayed release from adipose stores. If available,userelativelyinsolubleagentsasmuchforspeedofreversal as to reduce postoperative drowsiness. The risk of halothane hepatitis may be higher in obesepatients,althoughoverallitisstillverylow.

Regional anaesthesiaGood regional anaesthesia may reduce opioid andinhalational requirements intraoperatively in thoracic andabdominalsurgeryandmayalsobeusedasthesole technique in peripheral surgery. However it istechnicallyharderbecauseofthelossoflandmarks,increased movement of the skin and the need for long

DRUG DOSInG GUIDeLIne

Propofol Dosebetweenleanandactual bodyweight

Thiopentone Dosebetweenleanandactual bodyweight

Suxamethonium Upto1mg/kgactualbody weight

Atracurium Doseaccordingtoactual bodyweight

Vecuronium Doseaccordingtolean bodyweight

Fentanyl Doseaccordingtoactual bodyweight

Morphine Doseaccordingtoleanbody weight. Titrate to effect

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needles.Initialfailurerateishigherintheobese.Thesitting position is usually easier for spinal and epidural placement.Intheabsenceofclearbonylandmarksthe7th cervical prominence and gluteal cleft will indicate themidlineandpatientsalsocanassistbyverballyredirectingtheneedlewhenitstrikesthelamina.Itisrelativelyuncommonfortheepiduralspacetobemorethan8cmdeep.Leaveextracatheterinthespaceasitmaybesubjecttodragastheflexedpatientrelaxes.

Due to the engorged extradural veins and extra fatconstrictingthepotentialspace,lesslocalanaestheticisneededforepidurals.75-80%ofthenormaldosemaywellbesufficient.

Venousaccess,asaroutinepartofanyanaesthetictechniqueisalsotechnicallymoredifficultintheobese,especiallycentralvenousaccess,whereultrasoundisparticularlyusefulifavailable.

Surgical and mechanical issuesSurgery is technicallymoredifficult due to reducedsurgical access, difficult visualisation of underlyingstructuresandexcessbleeding.Thisleadstolongeroperating times, with subsequent exacerbationof many of the factors already mentioned. There is a higher risk of infection. Thepoor blood supplyto the fatty tissues increases the chance of bothwound infection and wound dehiscence. There may also be impaired immune system function due toneurohumeral factors.

Special equipment may need to be ordered forthe very obese patient.Most theatre tables have aweightlimitofapproximately130kgandcanoftenbetoo narrow for these potentially very wide patients. “Overflow” from the side of the table increases theriskofpressuresoresornervedamage,asthepatientis “wedged” inplace to ensure theydonot fall off.Thismayalsointerferewiththetipping/tiltingfunctionofsometables.Thesheermassofthepatientmeanstheyarehardertoposition,andpresentanincreasedrisktotheatrestaffduringhandling/lifting.Givensuchproblems it is preferable to induce anaesthesia intheatre to avoid such transferring.

Daycasesurgeryisnotcontraindicatedintheobese.RatherthanhavingarigidcutoffbasedonBMI,itispreferable tohaveapolicybasedupon the typeofsurgerytobeperformed.Ithasbeenshownthatwithcareful selection patients with a BMI over 35 havesimilaroutcomesto“normal”patients.

Bariatricsurgeryisdefinedassurgeryspecificallyforseverelyobesepatients.Itisincreasinglyconsideredforthetreatmentofmorbidlyobesepatientswhohaveseriouscomorbidityorinwhommedicalorbehaviouralweightreductiontherapiesareineffective.Operationsincludegastricbanding,gastricbypass,biliopancreaticdiversion, liposuction and jaw wiring. However, thechoice of the optimal therapeutic strategy in these patientsdependsonarisk/benefitratio,whichneedstobeassessedindividually.Itcanachievelong-term

weight reduction and is increasingly being used inmanagement of the severely obese. Economic andpractical issues and significant morbidity limit it tothe most extreme cases. The surgery itself is usually laparoscopic,and leads to lessopioidconsumptionandmorerapidrecoveryandmobilisation.

AnSWeRS tO SeLF-ASSeSSMent

no. 1 You are asked to assess a 42-year-old woman who requires an emergency extensor tendon repair to her left hand. There is no neurovascular deficit. She is 158cm tall and weighs 102kg. She last ate 2 hours ago. What factors will you especially need to consider in her pre-assessment?

Her calculated BMI is 40.85; therefore she isdefinedasmorbidlyobese.Sheisnotadequatelystarved, and is not an emergency, therefore sheshould be delayed for at least another 4 hours,if possible overnight. A careful history shouldbe taken, specifically considering symptomsof sleep apnoea, gastro-oesophageal reflux,diabetes, ischaemic chest pain and her normalexercise tolerance should be established. Pre-operative investigations would include a bloodsugar, anECG,pulseoximetryandnon-invasiveblood pressure with an appropriately sized cuff.Discussionshouldtakeplacewiththepatientandsurgeon as to the option of a regional technique. IfsherequiresaGAthenshewillneedintubationso a careful assessment of her airway is required additionally considering factors such as breastsize to facilitate laryngoscope placement. Pre- medications would ideally include a proton pump inhibitor or H2 antagonist. Sedation of any sortwouldideallybeavoided.

no. 2 A 55-year-old man with a BMI of 37 is having an anterior resection for colon cancer. Twenty minutes into the case you notice his oxygen saturations are falling and are now 88% despite FiO2 0.5. What actions can you take to improve his oxygenation?

Increase the FiO2 to 100% immediately with anappropriate increase in volatile agent if previously using nitrous oxide. Check the position of theendotracheal tubeandconfirmbilateral air entrywithauscultationandthepresenceofaCO2 trace on the capnograph. Ensure adequate musclerelaxation. Try a recruitment manoeuvre such as increasing the tidal volume with hand ventilation or a sustained increase in airway pressure of 30-40cmH2O for up to 40 seconds. Beware ofcardiovascularcompromise.Ifnotalreadyapplied,addPEEPtokeepanyrecruitedalveolipatent.Ifthesurgeryallows,thenthereverseTrendelenburgposition might help.

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no. 3 A 65-year-old woman with a BMI of 41 is 24 hours post an elective total knee replacement. She has been given intramuscular morphine 2 hourly overnight. She is hypoxic with a SpO2 of 87% on room air. Her respiratory rate is 8 breaths per minute. What is your diagnosis and action plan?

Shehasrespiratorydepressionfromexcessopioid.There is a high chance that this lady suffers sleep apnoea and is particularly sensitive to the effects of opioids.

Sheisinimminentdangerofrespiratoryarrest.Give100% oxygen and assist ventilation with a bag-valve-mask.GiveIVnaloxone100-200mcginitiallyandupto400mcgifrequired.Givennaloxonehas

ashorterhalf-lifethanmorphine;closemonitoringof respiratory rate, sedation score and oxygensaturationisvitalforthesubsequent2hours.Itishighlylikelytheladywillrequireafurtherbolusorpossiblyaninfusionofnaloxone.

Atrial fibrillation (AF) is one of the commonestarrhythmias.Itmaybeparoxysmal(suddenepisodes),persistent or permanent. Atrial depolarization is very rapid,irregularanddisorganized.Thiscausesirregularand rapid ventricular conduction. AF may be seenin patients presenting for anaesthesia or may occur during anaesthesia.1

Haemodynamicdeteriorationoccursduetothelossof atrial mechanical function, irregular ventricularresponse and a rapid heart rate. The loss of atrial contractiondecreasescardiacoutputbyupto30%,particularly in patients with impaired ventricular diastolic filling. This is of importance in patientswith hypertension, left ventricular hypertrophy,mitral stenosis and hypertrophic and restrictive cardiomyopathy.2 The ventricular response rate depends on electrophysiological factors in the atrioventricular (AV) node, drugs and sympatheticand vagal tone. A rapid ventricular response results in a reduced cardiac output due to inadequate time forpassivefillingoftheventricles.3 A persistent rapid ventricular response may result in a dilated ventricular cardiomyopathy.2

Decreasedbloodflowintheleftatrium(LA)andleftatrialappendage (LAA) isassociatedwith thrombusformation. Embolism from the LA may result in astroke or other arterial occlusion. The pathogenesis of thromboembolismiscomplexandaffectedbyotherfactors including intrinsic cerebrovascular disease,hypertension, atheroma in the proximal aorta andcarotid artery stenosis.2

Pre-operative assessment and atrial fibrillationAskaboutthe first detected episode.AFisrecurrentafter 2 or more attacks. Distinguish betweenparoxysmal,persistentandpermanentAF.

Paroxysmal AF is self terminating. The patient may havenosymptomsandbeunawareoftheepisode(s)of atrial fibrillation. Symptoms include palpitations,chestpain,dyspnoea,fatigue,light-headednessandsyncope. Symptoms may vary depending on theduration,theventricularrateandthefunctionalstatusof the patient.2

Persistent AF is sustained and requires electrical orchemicalcardioversiontoestablishsinusrhythm.Inquireaboutmodesof terminationanddrugsusedon a regular or as needed basis: the “Pill-in-the-Pocket”approach.4

Permanent AFislong-standingpersistentAFwherecardioversion was not successful or has not beenattempted.2

Althoughatrialfibrillationmayoccurwithoutassociateddisease in younger patients, it may be associatedwith underlying disease. Treatment of the associated conditionswhilemanagingtheAFnormallyresolvesthe arrhythmia.2

examinationExpect a completely irregular pulse and jugularvenous pulsation. The loudness of the first heartsound may be variable. A rate of between 60 and80bpm at rest and between 90 an 115bpm during

AtRIAL FIbRILLAtIOn AnD AnAeStheSIA

DrADTheron,SHOinAnaesthesia,RoyalDevonandExeterHospital,Exeter,UK. E-mail:drabrietheron@yahoo.co.uk

References and further reading

ShankmanZ,ShirY,BrodskyJ.Perioperativemanagementoftheobesepatient.BrJAnaesth1993,70:349-59

AdamsJP,MurphyPG.Obesityinanaesthesiaandintensivecare.BrJAnaesth2000,85:91-108

CheahMH, Kam PCA. Obesity: basic science andmedical aspectsrelevanttoanaesthetists.Anaesthesia2005,60:1009-25.

Saravanakumar K, Rao SG, Cooper GM. Obesity and obstetricanaesthesia.Anaesthesia2006,61:36-48

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