anesthesiologists

Peri-operative Anesthesia Management of Obese Patients and the
Current Practice in USA
Pei-Shan Zhao, MD, PhD.
Dept. of Anesthesia, Brockton Hospital, Brockton, MA, USA
Obesity, defined as a Body Mass Index (BMI) of over 30 kg/m2, is a serious
health problem worldwide. Obesity rates have raised more than 3 folds since 1980 in
some areas of the world, including North America and China (1). According to data from
the National Health and Nutrition Examination Survey (NHANES) 2003 to 2004, 32.2%
adults in the United States are obese (2). Recent data from Chinese Education Ministry
shows that 8% of 10 to 12 year-olds in cities are obese and an additional 15% are
considered overweight (3) although WHO data indicates that the overall obesity rate in
China is less than 5% (1). Medical implications of obesity, including increased risks for
coronary artery disease, hypertension, dyslipidemia, diabetes mellitus, obstructive sleep
apnea, gallbladder disease, degenerative joint disease and psychosocial impairment, have
been established (4). Obesity also imposes unique challenges on both surgeons and
anesthesiologists.
American Society of Anesthesiologists (ASA) fasting guideline recommended
that healthy patients could safely drink clear liquids until 2 hours before elective surgery.
This may need to be modified for patients with diseases that affect gastric empty or fluid
volume (5). Obese patients have been considered to be at an increased risk of pulmonary
aspiration. But recent data shown that obesity per se is not a risk factor for pulmonary
aspiration and otherwise healthy obese adults (6) and children (7) can follow the same
fasting protocols as non-obese patients. Obese patients have also been considered to be
more difficult for intubation. This has not been our experience in a busy community
teaching hospital where we operate on many obese patients. Actually, our experience is
consistent with a recent study. Brodsky and colleagues (8) performed study on 100 （科教范文网 fw.nseac.com编辑发布）
morbidly obese patients (BMI>40 kg/m2) and found no association between increasing
body weight or BMI and difficult intubation. They identified that large neck
circumference at the level of thyroid cartilage and high Mallampati score were the only
predictors of potential problematic intubation. With a neck circumference of 40 cm, the
probability of a problematic intubation was about 5%, and at 60 cm the probability of a
problematic intubation was approximately 35%. Although 3 patients qualified as having
difficult intubation (number of direct laryngoscopy attempts plus the grade of
laryngoscopy view>4), 99 patients were successfully intubated with three or fewer direct
laryngoscopy attempts. Increased BMI per se is also not a predictor of difficult
laryngoscopy defined as grade III or IV laryngoscopy view (9, 10). Obese patients do
have increased risk of hypoxia during the induction period. Increased fat in chest wall
and abdominal wall reduces lung and chest wall compliance, and increases airway
resistance. Closure of airways results in alveoli collapse, atelectasis, V/Q mismatch, and
hypoxia induced by shunting. Impairment of diaphragmatic descent reduces Functional
Residual Capacity (FRC) and oxygen reserve. Arterial oxygen saturation and FRC are
further reduced with supine position and induction of general anesthesia. In the meantime,
obese patients have increased metabolic rate and greater oxygen consumption. Pre-
oxygenation in head-up (11) or sitting position (12) effectively increased patient's
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tolerance to apnea. In "difficult mask" patients, laryngeal mask airway (LMA) is an
effective temporary ventilatory device (13).
Previously, large tidal volume of up to 15-20 ml/kg was recommended to improve
FRC. But oxygenation has not been improved significantly in the presence of increased
FRC. Furthermore, excessive lung expansion from large tidal volumes may cause lung （科教作文网http://zw.ΝsΕac.cOM编辑）
injury (volutrauma). Currently, most anesthesiologists use tidal volumes of 10-12 ml/kg
ideal body weight to avoid volutrauma and 12-14 breaths/minute to maintain
normocapnia. Moderate level of PEEP has been shown to improve respiratory functions
and oxygenation in obese patients (14). Reverse Trendelenburg Position, by increasing
pulmonary compliance and FRC, improves oxygenation (15). Pressure control ventilation
can also be used as long as the minute ventilation will maintain adequate oxygenation and
normocapnia. Pressure of up to 50 cm H2O has been used without adverse pulmonary
effects.
The relationship between a measured plasma concentration and a known
administered dosage defines an apparent volume of distribution (Vd). Vd=total
dosage/plasma concentration. In obese patient, the Vd for lipophilic drugs, such as
barbiturates, benzodiazepines and some opioids, is greatly increased. Therefore, it
requires a larger loading dose to achieve a given plasma level. Exceptions to this rule are
propofol and remifentanil, which are highly lipophilic. Several studies have demonstrated
that the best determinants of the induction dose for propofol include age and ideal body
weight. In addition, an excessive induction dose based on total body weight for obese
patient might cause severe cardiovascular depression. Study showed that propofol
elimination half life was not prolonged in obese patients and its clearance was correlated
to body weight. There is no evidence of propofol accumulation in the obese patients, or
of any prolongation of the duration of action (16). Pharmacokinetic study showed that Vd
for remifentanil is relatively consistent in both obese and lean people (17). Less-
lipophilic drugs, such as muscle relaxants, have little or no changes in Vd in the obesity.
These drugs can be dosed according to the ideal body weight (IBW), or more accurately,
lean body weight (LBW), because 20%-40% of the increased total body weight in obese （转载自http://zw.ＮＳＥaＣ.com科教作文网）
patient is an increase in LBW. LBW equals to 1.2 times of the estimated IBW. Because
plasma cholinesterase activity increases in obese patients, succinylcholine should be
dosed based on TBW. Atracurium can also be given according to TBW since their
duration of action is the same in obese and non-obese patients. See table below.
Drug Dosing Remarks
Propofol Induction: IBW
Infusion: TBW
Deleterious hemodynamic effects by dosage based on
TBW. Hepatic metabolism directly relates with TBW.
Thiopental TBW Increased blood volume. Increased Vd. Increased absolute
dose and prolonged duration of action.
Midazolam TBW Increased Vd. Increased absolute dose and prolonged
duration of action.
Succinylcholine TBW Plasma cholinesterase activity increases in proportion to
TBW
Vecuronium IBW Recovery may be delayed if given according to TBW
Rocuronium IBW Duration of action is prolonged when dosed according to
TBW
Cisatracurium IBW Duration of action is prolonged if dosed according to TBW
Atracurium TBW Unchanged dose per unit body weight without
prolongation of duration due to organ-independent
elimination
Fentanyl
Sufentanil
Bolus: TBW
Infusion: IBW
Increased Vd and elimination half-life, which correlate
linearly with the degree of obesity
Remifentnil IBW Pharmacokinetics are similar in obese and lean patients
Desflurane has been shown to be an optimal inhalational agent in morbidly obese
patients because of its very low lipid solubility.
Same as the induction, additional experienced assistance is important during the
emergence of anesthesia. Take the time to wake up patient and do not rush to extubate
patient. Avoidance of agitation from emergence will prevent patient

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