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prior to induction followed byPEEP with the face mask increasedthe PaO2 and apnea time.Obesity and the cardiovascularsystem - The cardiovascularimplications of obesity areenumerated in Table 1.l Hypertension- There is 3-4mm increase in systolic bloodpressure and 2mm of Hg increasein diastolic blood pressurewith every 10kg increasein weight. An expansion ofextracellular volume resultingin hypervolemia and an increasein cardiac output arecharacteristic of obesity inducedhypertension. Hypertensionleads to concentricLVH and progressively noncompliantleft ventricle whichwhen added to increasedblood volume increases therisk of cardiac failure [Fig 2].l Ischaemic heart disease -Obesity is an independentrisk factor for IHD and ismore common in patientswith central obesity.40% ofobese patients with angina donot have coronary arterydisease, implying that anginamay be a direct symptom ofobesity.l Cardiac function – obesepatients demonstrate anincrease in cardiac output andleft ventricular end diastolicpressure [LVEDP] and leftventricular hypertrophy[LVH]. Left ventricularsystolic function is alsoderanged. An entity called‘obesity cardiomyopathy’ isseen in morbidly obesepatients which is associatedwith right sided failure,hypertension and ischaemicheart disease [Table 4].Anaesthetic implications-Preoperative electrocardiogrammay underestimate the severityof ventricular hypertrophybecause of the low voltagecomplexes.Axis deviations andatrial tachyarrhytmias may becommonly present.Transesophageal echocardiographymay provide better images ,especially of the left heart.Thesepatients are prone tointraoperative ventricular failure[ fluid imbalance, negativeinotropy of anesthetic agents,pulmonary hypertensionprecipitated by hypoxia orhypercapnia]. Appropriate sizedblood pressure cuffs may not beavailable , anesthetist should havea low threshold for invasivearterial blood pressuremonitoring.Obesity and gastrointestinaldisorders-Recent studies haveFig 4: Obesity caardiomyopathyshown that obese patientswithout gastroesophageal reflux75% greater volume than normalsubjects but gastric emptying isfaster. Residual volume mayhowever be larger hence it issensible to precautions againstaspiration.Anesthetic implications -Antiaspiration prophylaxis withH2 antagonists, prokinetics andantacids is adviced. Rapidsequence induction with use ofsuccinylcholine and cricoidpressure is indicated.Awakeextubation should be planned.Drug pharmacokinetics inobesity-Obese patients haveboth an increase in lean bodymass and increased fat mass, theincrease in fat mass is greater thanthe lean body mass. Blood flowto the fat compartment is low[5%].Blood volume increasesdirectly with body weight andobese individuals have increasedblood volume and perfusion tovessel rich group. These changeslead to alteration in drug62Journal of Postgraduate Medical Education, Training & ResearchVol. II, No. 5, September-October 2007
distribution, binding andelimination.Oral absorptionremains unchanged.Volume ofdistribution [V D] – Increased inall lipophilic drugs and therebyreducing the elimination half life.The increased concentrationsof triglycerides, lipoproteins,cholesterol and free fatty acidsmay inhibit protein binding andincrease free fraction of thedrugs. The increase in á 1acidglycoprotein may increase degreeof protein binding of localanesthetics and reduce the freefraction.Elimination – Hepaticclearance is not normally reducedin the obese. Phase I reactions[oxidation, reduction andhydrolysis] are normal orincreased and metabolism byPhase II reactions is consistentlyincreased. Increased reductivemetabolism may be an importantfactor in development ofhalothane induced hepatotoxicityin obese patients at risk fromhypoxemia and reduced liverblood flow. Renal clearanceincreases in obesity because ofthe increased renal blood flowand glomerular filtration rate.Thetable below highlights the effectof obesity on common anestheticagents.Table 7: Effect of obesity on anesthetic agents.Drug Altered pharmacokinetics ImplicationsThiopentone V D/ prolonged T 1/2Increased absolute dose/ reduceddose by body wt/ prolonged actionPropofol Little known Dose calculated by Servin’s formulafor corrected body weight [ IBW+ 0.4 X excess body wight.Midazolam/ ! V D/ prolonged T 1/2Increased absolute dose/ reduceddiazepamdose by body wt/ prolonged actionSuccinylcholine ! plasma cholinesterase Increased absolute dose [ Total bodyactivityweight to calculate dose]Atracurium Unchanged Dose based on TBWVecuronium ! V D/ prolonged T 1/2Dose based on lean body weightPancuronium Low lipid solubility Unchanged doseRocuronium ! V D/ prolonged T 1/2Dose based on Ideal body weightFentanyl Unchanged Doses based on TBW delayed recoveryAlfentanil Elimination prolonged Dose based on LBWRemifentanil Elimination prolonged Dose based on LBWLocal Increased epidural fat 75% of dose cal culated by TBWanestheticscontent/ venous engorgementHalothane Deposition in adipose Delayed recoverytissueIncreased risk of reductive Increased risk of halothane hepatitismetabolismIsoflurane Unchanged safeSevoflurane Flouride concentration Rapid recoveryunchangedDesflurane Low solubilityblood and fat Faster recovery than Isoflurane andsolubilitysevofluraneJournal of Postgraduate Medical Education, Training & Research63
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