Local Anaesthetics

Question 1

Order of Sensation Block

Answer 1

Pain
Temperature
Touch
Deep Pressure

Question 2

Method of Action

Answer 2

They preferentially bind voltage gated Na channels in active and inactive states.
Inhibition of subsequent activation prevents membrane depolerisation and inhibits AP’s.

Question 3

Membrane Concentration Rule

Answer 3

Any fat soluble molecule which achieves a concentration >50umol/g will produce local anaesthesia.

Question 4

Critical Volume Hypothesis

Answer 4

Some agents may penetrate the membrane sufficiently to expand it and thereby inhibit ion channels

Question 5

Cm

Answer 5

The minimum concentration of a local anaesthetic that will block nerve impulse conduction

Question 6

Factors affecting order and duration of block

Answer 6

Nerve fibre size
Myelination
Nerve fibre position
pH of environment
Firing rate
Electrolyte plasma concentration
Local blood flow

Question 7

Vasoconstrictors

Answer 7

Adrenaline, felypressin
Decrease systemic absorption
Prolong duration of action
Shorter acting agents are more affected

Question 8

CVS Effects

Answer 8

Depress cardiac automaticity
Decrease duration of refractory period
Decrease myocardial contractility and conduction velocity at high conc
Arterial smooth muscle relaxation
All of these cause decreased arterial pressure, heart block, cardiac arrest

Question 9

CNS Effects

Answer 9

Early: Circumoral numbness, tongue paraesthesia, dizziness
Sensory: Tinnitus, blurred vision
Depression: slurred speech, drowsiness, unconsciousness
Excitatory: restlessness, agitation, anxiety, paranoia
Tonic clonic seizures often follow beginning of muscle twitches
Post-ictal depression- respiratory arrest