The Local Anesthetics action correlates with lipid solubility. Cm is the minimum concentration of local anesthetic that will block nerve impulse conduction.
The Local Anesthetics action depends on number of factors like:
i. Dose and concentration:
The dose as well as the concentration of the drug is an important determinant of Local Anesthetics Action.
It is the pH at which a local anaesthetic is 50% ionized and 50% non ionized. Since local anaesthetics are weak bases, agents with pKa closer to physiologic pH will have more drug in non-ionized form which can diffuse through axonal membrane and so onset will be rapid. That is why lignocaine with pKa of 7.8 is fast acting than bupivacaine (pKa 8.1).
This is the rationale of adding soda bicarbonate to local anaesthetic which increases the pH and so more drug is available in non-ionized form. This potentiates the Local Anesthetics Action.
Likewise in ischemic tissue, onset of block will be delayed (in acidic pH more drug will be in ionized form).
iii. Type of nerve fibre
The nerve fibres are classified based on the diameter of their fibres into A, B and C fibres. The smaller the diameter the quicker will the nerve fibre effected by the local anesthetic. Another important factor is the mylenation of the nerve fibre.
iv. Frequency of nerve stimulation: A stimulated nerve will be blocked early.
These were a few factors on which the Local Anesthetics Action depends.