Pain & Analgesics

Question 1

Diamorphine

Answer 1

Is a semisynthetic derivative of morphine. It is a prodrug, rapidly hydrolysed in the liver and plasma to mono-acetyl morphine. Both diamorphine and monoacetyl morphine are more lipid soluble than morphine and therefore crosses the blood brain barrier rapidly.
Quicker onset of action compared to morphine & can be used intrathecally. Also has greater receptor affinity than morphine.

Question 2

LAs are more rapidly absorbed after intercostal block or after caudal administration?

Answer 2

Intercostal block

Question 3

LAs absorption more rapid from vascular tissues. Give example of which blocks would produce higher plasma levels than others

Answer 3

More vascular locations would have more rapid absorption. E.g.:
Intercostal block (peak conc.: 1.5mcg/ml) > caudal (1.2mcg/ml) > epidural (1mcg/ml) > brachial plexus (0.6mcg/ml).

Question 4

Plasma toxicity threshold for lidocaine is?

Answer 4

5 mcg/ml.

Question 5

pKa of lidocaine and bupivacaine, and at what % are they unionised?

Answer 5

pKa: lidocaine= 7.9, bupivacaine= 8.1 (@ pH of 7.4).
Unionised: lidocaine= 25% (75% ionised); bupivacaine= 17%