Case 52 - brachial plexus anesthesia

Question 1

What are side effects / complications from an interscalene nerve block?

Answer 1

• phrenic nerve blockade
• nerve injury
  
  • phrenic nerve
  • long thoracic nerve
  • dorsal scapular nerve
• horner syndrome
• SA puncture/dural sleeve injection
• vertebral artery puncture (anterior to roots)
• recurrent laryngeal nerve blockade

Question 2

What are benefits of ultrasound use?

Answer 2

• decrease time to block performance
• decrease vascular punctures
• decrease needle passes
• faster onset of sensory block, increase block success
• lower volume of LA (may benefit avoiding phrenic N blockade)

Question 3

What is LAST, how is it diagnosed?

Answer 3

LAST

• intravascular accumulation of LA
• typically see CNS symptoms before CVS symptoms
• CVS: CNS toxic dose ratio

CNS

• tinnitus
• metallic taste
• circumoral nubness
• excitation/agitation
• end result –> seziures and resp arrest

CVS

• brady cardia
• ventric arrhythmias
• cardiac arrest

CVS: CNS toxicity ratio

• highest for bupivaine (1:2) = twice the CNS toxic dose to obtain CVS toxicity
  
  • lidocaine (1:7) = seven times the CNS toxic dose to obtain CVS toxicity
• bupi = lipid-soluble agent, binds to inactivated sodium channels for a long time
  
  • more resistent to tx

Question 4

Patient is experiencing cardiac arrrest from LAST. How will you treat this?

Answer 4

1) Get Help
2) Airway Managment

• ventlate with 100% oxygen
• ??hyperventilate??
• avoid hypoxia/hypercarbia –> exacerbates CVS toxicity

3) Suppress seziures

• BZD
• avoid propofol if cardiovascular instability develops

4) cardiac arrhythmias

• ACLS
• Ventricular arrhythmias - tx with amiodarone
• avoid vasopressin, CCB, BB, or LA
• reduce epinephrine dose to 10-100 mcg bolus (high doses linked to poor outcome in these pts)

5) LIPID EMULSION (20%) therapy
* lipid sink and cause redistribution of LA away from cardiac Na+ channels and into the lipid bilayer

Question 5

What are the pros and cons of hyperventilation in LAST

Answer 5

Pros:

• cerebral vasoconstrict –> dec CBF –> dec LA delivery to brain
• decrease ionized form of LA (active form required to block sodium channel receptors)
• dec PaCo2 leads to inc PAO2 via alverolar oxygenation equation
  
  • PAo2 = FIo2 (Patm-Ph2o) - (PaCo2/R)

Cons:

• hypokalemia
• more LA in non-ionised form (although not active form, this form has greater ability to cross lipid membranes)

A base in an alkaline solution will be non-ionised and have a greater ability to cross lipid membranes. However, in an acid environment, it will be trapped, as it is ionised. The result is that an alkaline drug will be concentrated in a compartment with a low pH.

Question 6

Describe dosing of lipid emulsion therapy in LAST

Answer 6

Lipid Emulsion therapy - for 70kg patient

Initial:

• Bolus 1.5 mL/kg over 1 min (approx 100mL)
• continuous infusion 0.25 ml/kg/min (approx 18 mL per min)

if remains HD instable

• repeat bolus once or twice for peristent CV collapse
• double continous infusion rate to 0.5 ml/kg/min if BP remains low

maintenance

• continue infusion for at least 10 min after attaining HD stability
• recommended upper limit: approx 10 mL/kg over first 30 min