Dose Adjustment for Obese patients

From Ingrade, et al. Dose Adjustment of anesthetics in the morbidly obese, Br. J. Anaesth. (2010) 105 (suppl 1): i16-i23.doi: 10.1093/bja/aeq312 

Drug	Dosing scalar	Comments
Thiopental	Induction: LBW Maintenance: TBW	Simulations showed a 60% decrease in peak plasma concentration in MO subjects compared with lean subjects after a 250 mg dose.26 Induction dose adjusted to LBW results in same peak plasma concentration as dose adjusted to CO.26 Volumes and clearances increase proportionally with TBW.25
Propofol	Induction: LBW Maintenance: TBW	MO subjects given an induction dose based on LBW required similar amounts of propofol and similar times to loss of consciousness compared with lean subjects given propofol based on TBW.29 Volume of distribution and clearance at steady state increases with increasing TBW.28
Fentanyl	LBW	Clearance increases linearly with ‘PK mass’, an arbitrary scalar highly correlated to LBW.46
Remifentanil	LBW	An infusion based on LBW results in similar plasma concentrations as normal weight subjects were given an infusion based on TBW.51
Succinylcholine	TBW	Administration of 1 mg kg−1 based on TBW resulted in a more profound block and better intubating conditions compared with doses based on IBW or LBW.67
Vecuronium	IBW	Doses based on TBW result in a prolonged duration of action in obese vs non-obese subjects.69 70
Rocuronium	IBW	There is an increased duration of action when the drug is given based on TBW vs IBW.71
Atracurium, Cisatracurium	IBW	The duration of action is prolonged in obese subjects when given on the basis of TBW vs IBW.73 74

 

 

Parkinson’s Disease and Anesthesia

From Stoelting’s Anesthesia and Co-existing disease

Neurodegenerative disorder –> loss of dopaminergic fibers in basal ganglia, regional dopamine depletion –> decreased inhibition of neurons in extrapyramidal system –> unopposed stimulation by Ach

Sx and Symptoms: Triad of tremor, rigidity, akinesia; dementia and depression.

Medical Tx: goal to increase dopamine in basal ganglia or decrease neuronal effects of Ach

• Levodopa + decarboxylase inhibitor (side effects: dyskinesia, psych disturbances)
• Amantadine – antiviral helps symptoms
• Selegiline – type B MOA inhibitor, inhibits catabolism of dopamine in CNS. Not associated with tyramine-associated hypertensive crisis.

Surgical Tx: Deep Brain stimulator may help tremor; Pallidotomy improves levodopa induced dyskinesias

Anesthesia Management: Levodopa should be continued on day of surgery and peri-op, and repeated intra-op via NG/OG if needed as muscle rigidity can interfere with respiration. Butyrophenonenes (droperidol, haloperidol) antagonize dopamine in basal ganglia. Alfentanil can cause acute dystonic rx 2/2 opioid-induced decreases in central dopaminergic transmission. Ketamine can exageratte sympathetic responses.

Anesthesia for Deep Brain Stimulator Placement: Some patients may hold levodopa, usually done under sedation, make sure to avoid propofol and benzodiazepines as they can alter microelectrode monitoring, opioids and precedex are good alternatives. Sitting position is usual, so air embolism is risk.

Neuroanesthesia Notes

Cerebral Salt Wasting Syndrome
triad: hypoNa, volume contraction (low CVP), and high urine Na
a/w SAH –>bnp –>salt waste

SIADH is your differential, but SIADH is 2/2 renal retention of free water, so 24hr urine Na is normal in SIADH, normal CVP

DI and primary hyperaldosteronism –> elevated Na

Ref. Miller 8th ed p 2177

Intracranial HTN >15mmHg

SSEPs
-stimulation of peripheral nerves
-ascends ipsilateral dorsal column
-recorded on c/l somatosensory cortex

Hyperventilation lowers ICP
-reduces CBV and CBF
-duration wanes in 6-10hrs
-CBF decreases 2% for every mmHg decrease in PaCO2
-1ml/100g/min CBF increase per 1mmHg increase in Paco2 so since normal CBF is 50ml/100g/min then this is a 2% change

 

 

Anesthesia for patients with opioid abuse and tolerance

From Anesthesiology News: http://www.anesthesiologynews.com/download/Opioid_AN0613_WM.pdf

Eval

-try to ask most use in a day, rather than average, more objective
-time of last dose, who is prescribing
-single questions alternative to CAGE-AID: How many times have you used illegal drugs in the past year?

Long term opioid therapy

Methadone
-opioid agonists, with some NMDA antagonism
-typically 30-40mg qd

Buprenorphine
-parital opioid agonist
-no high

Suboxone
-combo of buprenorphine and naloxone
-when injected, only naloxone works

Vivitrol
-extended release naltrexone
-opioid antagonist
-for etoh and opioid abuse
-monthly injection

I/O management

3 areas: manage intox, prevent/tx withdrawal, achieve effective analgesia

Antagonist reserved for life-threatening resp depression, as precipitating w/d can make management harder
If not receiving agonist therapy:
-take long-standing Rx as directed before surgery
-clonidine to tx withdrawal sx, 0.1mg bid, also tx specific sx as they arise

Multimodal pain relief
-iv tylenol, exparel
-ketamine, a2 agonists
-nsaids, regional
-pregalbin and gabapentin good for neuropathic pain

P/o

comfort and safety

opioid tolerant pts requires 2-3x more than naive patients
but still at risk from resp depression

Withdrawal happens 2/2 bolus nature of pain relief
combine long acting and pca for pain
make sure to restart methadone, divide into 3x/d dose to take advantage of analgesic properties

Post-op for Suboxone
-more complicated, buprenorphine has a celing effect (partial u agonist)
-so increasing dose intial helps, but tops out so side effects also top out, making it safer for long term management, but tought post-op
-Buprenorphine has 1000x affininty for u than morphine
–>longer half life, so persists on receptor
–>only partial agonist, so analgesia not complete
4 recs:
1. maintenance therapy continue, short-acting opioids titrate to effect
2. divide bupre into TID (if surgery not that invasive/painful)
3. d/c bupre and treat with full opioids, remember bupre can be present for up to 5 days after d/c. Be aware of withdrawal sx from bupre.
4. convert to methadone 30-40mg/day, must be written as rx for pain not withdrawal (illegal).

Post-Op for Naltrexone once monthly
-naltrex is competetive antagonist
-opioid admin is tough to predict
-depends if pt missed a dose, at end of dosing cycle etc.
-postpone elective surgery if possible until tx complete
-otherwise, multimodal with opioids