anesthesia for obese pt./bariatric surgery Flashcards
what is obesity?
- abnormally high percentage of body fat
- BMI > 30 accepted by clinicians
- waist circumference (WC), waist: height ratio (WHR), and waist: stature ratio (WSR) correlate better with mortality and obesity related diseases
what groups are obesity seen more with?
- women (33%), men (27%)
- minorities: Hispanics/blacks > whites
how does obesity affect health risk?
- decreased life expectancy
- 50-100% greater risk for death
- increased depression, OSA, gall bladder disease, reflux, cancer
- independent risk factor for: ischemic heart disease, HF
- obesity related: HTN, Type II DM, CAD, stroke, malignant tumors
- 1 kg wt. gain per year x 10 years increases risk to health
describe anatomic distribution of body fat
- indicator of increased health risks
- associated with certain pathophysiological characteristics
- central android (apple)
- peripheral gynecoid or gluteal (pear)
describe central android fat distribution
- “apple” shaped
- truncal, visceral (abdominal) fat
- encouraged with increased ETOH
- increased O2 consumption, CV disease, ventricular dysfunction
- metabolically active adipose tissue: increased free fatty acids (FFA), LDL, insulin resistance
describe peripheral gynecoid or gluteal fat distribution
- “pear” shaped
- hips, butt, thigh fat
- less CV risks since metabolically static adipose
describe waist circumference measurement
- new standard for determining abdominal obesity
- represents abdominal fat
- independent predictor of disease
- greater than 102 cm (40 in.) for men and 88 cm (35 in.) for women increases risk of IHD, HTN, DM, and death
describe waist : height ratio
if > 0.9 in women and > 1.0 in men increased mortality and morbidity
*ex: man height of 5’ 6’’ and WC 66 in. has WHR of 1.0
describe waist : stature ratio
recommended that WC no exceed 1/2 the stature (or height)
what are obesity effects on the respiratory system?
- increased fat intra-abdominally, chest wall, and diaphragm
- decreased chest wall compliance (esp. lying supine)
- impaired respiratory muscle strength
- decreased lung volumes/capacities, functional residual capacity (FRC), expiratory reserve volume (ERV), vital capacity (VC), total lung capacity (TLC)
- decreased FRC d/t markedly reduced ERV; residual volume (RV) is UNCHANGED (anesthesia can further reduce FRC up to 50%)
- diaphragm moves cephalad when supine
- *obese pt. will desat quickly even after preoxygenation
what is the most sensitive indicator of effects of obesity on pulmonary function testing?
- expiratory reserve volume (ERV)
* when it drops, FRC is reduced
how does obesity affect O2 demand and supply?
- increased demand: increased O2 consumption, increased CO2 production, increased alveolar ventilation
- decreased supply: decreased chest wall compliance, decreased lung volume, decreased FRC, premature air closure, V/Q mismatch, arterial hypoxemia
both: increased work of breathing, decreased respiratory muscle efficiency
describe obesity effects on FRC
- decreased FRC causes closing capacity (CC) to exceed normal tidal volume
- supine decreases FRC even more d/t cephalad diaphragm
- airway closure, atelectasis
- VQ mismatch, intrapulmonary shunt
- rapid desaturation during apnea time on induction regardless of preoxygenation
- must preoxygenate longer and intubate quick
describe obesity effects on VO2 and CO2
- increased VO2 (O2 consumption) and increased CO2 production
- increased metabolic activity of excess fat increases demand
- stress on supporting respiratory muscles also increases demand
how does the respiratory system attempt to compensate for respiratory issues of obesity?
- extra work of breathing to maintain augmented ventilation
* *VO2 used for respiratory muscle work instead of vital functions so instead increases demand further
what effects occur d/t increased pulmonary blood volume?
- further decreases compliance
- chronic hypoxia causes polycythemia (dusty/ashy skin)
- increased pulmonary blood volume causes pulm. HTN, cor pulmonale
what promotes airway closure in obesity?
increased tissue within bony enclosures can only grow towards inside the airway, creating a more narrow airway
describe obesity hypoventilation syndrome
- Pickwickian
- 10% of morbidly obese have OHS
- clinically similar to OSA, hypoxemia
- hypercapnia while awake is cardinal sign (more CO2 is produced than can be eliminated)
- respiratory center is desensitized and ventilation becomes dependent on hypoxic drive
- ventilation inefficient r/t decreased TV and inspiratory strength
- polycythemia, cyanosis, hypoxemia
- right CHF, cor pulmonale
- greater sensitivity to respiratory depressant effects of GA (don’t pre-op with versed!)
what are effects of obesity on the CV system?
- IHD, HTN (2x >), CHF
- extra blood vessels (25 mi/ 13 kg fat) and volume (CHF)
- increased CO (100ml/min for every 1 kg of fat; 2x IBW pts.)
- increased renin-angiotensin leads to increased intravascular fluid volume
- increased blood viscosity and catecholamines lead to arrhythmias
- decreased activity accelerates CAD
what causes increased CO and what happens as a result of it?
increased SV, NOT HR
- increased preload: cardiomegaly, atrial and biventricular dilation
- LVH, CHF, HTN develops
- decreased ventricle compliance leads to increased LVEDP which leads to pulmonary edema
what are CV implications?
- may display angina w/o evidence of CAD
- less cardiac reserve d/t increased workload (assess functional capacity: can you walk up 2 flights of stairs w/o difficulty? this is about equal to stimulation of laryngoscopy)
- asymptomatic/undetected CAD d/t decreased activity (increased risk of MI with increased BMI)
- greater degree of EKG changes
- arrhythmias by multiple causes
- H/O MI, HTN, angina, PVD common
- assess exercise tolerance, orthopnea may indicate LV dysfunction
- drug list will point to co morbidities
- EKG can show ventricular hypertrophy, CAD, ischemia, old infarct, etc.
- echo can show cardiomyopathy, tricuspid regurg, abnormal wall motion
- chemical stress test may be indicated
- CXR may identify cardiomegaly and cephalad diaphragm
what are some EKG changes seen in obesity?
- low QRS voltage (has to travel through more fat)
- left axis shift consistent with LVH
- atrial enlargement
- left shift of the P wave, lengthened QT interval
- flattened T waves
how does obesity affect the GI system?
- after 8 hrs. of fasting, gastric volume > 25 ml in 80-90%
- gastric volume up to 75% larger
- gastric acidity (ph
how does obesity affect the liver?
- liver function abnormalities
- fatty liver
what are endocrine effects of obesity?
increased possibility of type 2 DM
- risk increases linearly with BMI
- 80% are obese
- rarely thyroid or pituitary problem
what effects does obesity have on metabolic state?
metabolic insulin resistance syndrome
- insulin resistance
- impaired glucose tolerance
- HTN
- dyslipidemia
how does obesity affect lipid soluble drugs?
- increased volume of distribution d/t increased fat stores
- propofol, benzos, opioids
- clearance may be delayed
- base on total body weight BUT give less frequent maintenance doses
how does obesity affect water soluble drugs?
- limited volume of distribution
- NMB agents
- based on lean mass or ideal body weight (if based on TBW can lead to OD)
how does obesity affect inhalation agents?
- longer to reach equilibrium
- emergence delayed
- Desflurane faster on and off than Sevo or Iso
- low blood solubility volatile inhalation agents have faster uptake and distribution and elimination
what drug route is most reliable in obesity patients?
IV over IM or SQ
how does obesity affect protein binding?
-increased alpha1 acid glycoprotein binding causes reduction in free drug concentration
how does obesity affect renally excreted drugs?
drugs dependent on renal elimination have higher clearance rates
how does obesity affect succinylcholine dosing?
- increased dose needed
- base in TBW
what are airway management implications for obese patients?
- HOB increased or reverse trendelenburg (less aspiration)
- fat rolls restrict neck ROM
- larger tongue
- use extra airway adjuncts: boogie, LMA, blades, nasal airway
- increased risk of airway obstruction post extubation
- longer preoxygenation/denitrogenation
- PEEP
- ETT over LMA even when LMA is normally utilized
- use Succs
how does PEEP help respiratory function in obese patients?
only method shown to improve respiratory function
- reduces venous return
- CO2, O2 delivery
why is an ETT ideal over a LMA in obese pts.?
increased PiPs make LMA contraindicated
describe proper airway management of the obese patient
- increase HOB: increased ventilation, comfort, FRC, decreased reflux
- porper “ramped” position: elevate head, neck, upper body and shoulder to point that an imaginary horizontal line from sternal notch to external auditory meatus can be drawn (can be difficult to place in this position once failed intubation has occurred)
what is the single best predictor of problematic intubation?
neck circumference
what are skin considerations with obese patients?
- use EKG electrodes outside of folds (irritation and sweat)
- expect reduced EKG voltage on monitor
- correct BP cuff size; forearm as reliable as upper arm
- skin folds may have yeast, excess sweat and moisture leading to broken areas and infection
- excess skin/fat may overgang table; place armboards to support or may cause skin necrosis or nerve damage
how does obesity affect IV placement?
may require CVL or PICC
what are positioning considerations with obese patients?
- increased nerve damage d/t excess weight on structures
- less flexion and extension of limbs d/t less ROM
- extra padding needed
- optimize CV and respiratory function with position
- minimize fluid shifts with position
- excess skin can become ischemic
how does obesity affect regional anesthesia?
- anatomical landmarks are difficult to locate even under ultrasound
- unable to position to palpate landmarks
- fat in axilla complicates brachial plexus block
- inability to sit up and lean over for neuraxial anesthesia; longer needles may be needed
- undesirable cephalad spread of local
- redundant fat rolls compromise sterile field and need angle positioning (femoral block)
- may not be able to locate precise nerve with nerve stimulator probe
- tap blocks very difficult and may not work
when should CPAP be used with diagnoses OSA pts.?
- preop: as early as possible to improve physical status and reduce perioperative risk
- post op: immediately to prevent hypoxemia; does not increase surgical complications
what are optimal conditions for anesthetic induction for the obese patient?
- PEEP application approx. 10 cm H2O
- upper-body “ramped”
- 30 degree reverse trendelenburg
- possibility of airway management difficulties at emergence and in the PACU should be considered
how can intra op hypoxemia be treated with the obese patient?
-10-15 cm H2O of PEEP combined with recruitment maneuvers, carefully balancing hemodynamics
how long should smoking cessation counseling endure pre op?
8 weeks pre surgery
what NMB agents need to be dosed on IBW with obese patients?
- vecuronium
- rocuronium
- cisatracurium
what anesthetic agents should be dosed on TBW with obese patients?
- mivacurium
- propofol
- succinylcholine (1mg/kg TBW)
how should fentanyl and remifentanil be dosed in the obese patient?
using lean body mass (LBM)
how should sufentanil infusions be dosed in the obese patient?
TBW
what position is beneficial to the obese patient in the OR?
- upper body elevated and lateral decubitus
- reduces airway collapse and hypoxemia
what type of analgesia plan should be used in the obese patient?
- opioid sparing analgesia, including NSAIDS
- local anesthetic wound infiltration
- use unless contraindicated
