LA for children Flashcards
How to nerves work?
- Intracellular and extracellular fluid is separated by lipo-protein membrane
- Inside nerve is negatively charged compared to positively charged outside
- Resting potential between outer and inner surfaces of membrane that surrounds each separate nerve fibre
- Impulse passes
- Change in polarity as ions pass through and action potential is generated as voltage gate Na+ channels open (depolarisation)
- Very short cycle, resting potential is restored in milliseconds
- If subsequent stimulation to nerve receptor before equilibrium of resting potential has been regained, there can be no transmission of second impulse
How does LA affect nerves and action potentials?
- LA diffuses inside cell and enters nerve at Node of Ranvier
- Once uncharged molecules enter the cell re-equlibirates in aqueous environment to a mixture of charged and non-charged molecules
- Bind reversibly to specific receptors in the sodium channels
- Inactivates them
- Nerve membrane is stabilised
- Thereby not permitting any ion passage through the membrane
- Thus action potentials cannot be generated
- And no conduction of impulses can take place
Why does LA need to soluble in water and fat?
Soluble in fat (in non-charged state- - gain entry into cell
Soluble in water(charged state)- to bind to receptor on sodium channel
How is LA able to be in a charged state and lipid soluble?
Weak base when LA is in solution.
What 2 factors govern the proportion of charged and uncharged molecules following injection?
- pH of region
- Dissociation constant of LA molecule (pKa)
What are the implications of a low pH region of injection?
Less uncharged LA molecules present in solution
What is the implication of a lower pKa of LA solution?
more uncharged molecules exist
Why is lidocaine the more effective LA solution?
Low pKa than older LA
Apart from pKa what other reason would a LA differ in effectiveness?
Their differential effect on blood vessels.
Define analgesia
Loss of pain sensation unaccompanied by loss of other forms of sensibility.
Define anaesthesia
Loss of all forms of sensation including pain, touch, temperature and pressure perception and may be accompanied by impairment of motor function.
Why should we avoid injecting LA when an acute infection is present?
- Can result in the spread of infection
- The acid condition prevalent in infected area reduces efficacy of LA
What LA should be used if the patient is allergic to latex?
Citanest (Prilocaine with felypressin)
What are the drug interactions with adrenaline?
- Beta Blockers (potential increase in BP)
- Tricyclic anti-depressents (Potential increase in BP)
- Thiazude Diuretic (exacerbates lowered potassium level in blood)
What LA agent does not contain a vasoconstrictor?
Mepivacaine 3% (Scandonest)
