2019A08 20mL of 1% ropivacaine is inadvertently administered intravenously over
15 seconds to a 60 year old, 60kg woman. Describe the potential complications
and mechanisms of these. Do not discuss treatment.

 

List:

·      Ropivacaine’s action

·      Calculations

·      CNS

·      CVS

 

Ropivacaine’s action:

Class

·  Amide local anaesthetic

Target

·  Voltage-dependent Na+ channel (VDNaC)

·  (less active at VDKC, VDCaC)

Entry

·  From ICF: unionised form diffuses into axoplasm, ionised form enters active receptor

·  From ECF: ionised form enters active receptor (less common)

Binding

·  Rate ∝ time spent in active state ∝ rate of cycling ∝ nerve activity

·  Affinity ∝ Inactive > resting > activated

Effect

·  ↓Rate of cycling -> no action potential propagation

·  Initial: incomplete blockade, tonic

·  Repeated stimulation: complete blockade, phasic

·  No change to resting potential

Toxic thresholds

·  Subcut dose: 3mg/kg

·  Plasma concentration: 4mcg/mL

·  CC:CNS ratio 5:1 (c.f. lignocaine 7:1); hence ‘warning shot’ is late

 

Calculations:

Amount given

·  10mg/mL x 20mL = 200mg

Cardiac output

·  70mL/kg/min x 60kg = 4200mL

·  In 15 seconds: 1050mL

Estimated plasma concentration

·  200mg / 4200mL ≈ 50mcg/mL

·  Way above toxic threshold of 4mcg/mL

Consequences

·  Coma and cardiac arrest almost certain

·  Rx intralipid

·  Likely prolonged CPR +/- CPB; long duration of action, t1/2β 120 mins

 

CNS toxicity:

Mechanism

·  Inhibition of neuronal VDNaC

·  Highly lipid soluble (300x procaine), easy passage across BBB

Effects

·  Initial excitatory effects

o Inhibition of inhibitory interneurons

o Peri-oral tingling, tinnitus, vertigo, slurred speech, inattention, twitching, seizures

·  Then inhibitory effects

o Inhibition of excitatory interneurons and others

o Coma, apnoea

 

CVS toxicity:

Mechanism

·  Inhibition of cardiac VDNaC (very high affinity)

·  Inhibition of VDKC and VDCC (lower affinity)

·  ?↓cAMP

Effects

·  First phase: ↑HR, ↑mAP

·  Second phase: myocardial depression, ↓mAP

·  Terminal phase: peripheral vasodilatation, ↓↓mAP, arrhythmia, arrest

ECG

·  “As if being pulled apart” ↑QTC, ↑QRS duration

·  Arrhythmias: sinus bradycardia, AV block, ventricular tachyarrhythmia, asystole