tailieunhanh - The Anaesthesia Science Viva Book - part 6

- Basilic tĩnh mạch: Tĩnh mạch basilic cống phía bên trụ của cẳng tay và tăng dọc theo biên giới trung gian của bắp tay xuyên qua các fascia sâu ở giữa trên cánh tay trước khi đi để hình thành tĩnh mạch nách. - Trung bình cubital tĩnh mạch: Tĩnh mạch cubital trung bình có nguồn gốc từ tĩnh mạch xoay thai xa epicondyle bên, và sau đó chạy lên trên medially qua hố antecubital để tham gia các tĩnh mạch basilic trên khuỷu tay | CHAPTER 4 The anaesthesia science viva book Alkalisation of local anaesthetics Commentary This technique is of clinical interest because it is used to shorten the latency of onset of effective anaesthesia and is particularly useful in the context of extending an epidural block for urgent operative delivery. It is of interest to FRCA examiners because it allows candidates to demonstrate their understanding of the basic mechanisms of local anaesthetic action. The viva You will be asked why it might be useful to add alkali to a local anaesthetic. Basic chemistry All local anaesthetics are chemical descendants of cocaine and comprise a lipophilic aromatic portion which is joined via an ester or amide linkage to a hydrophilic tertiary amine chain. The presence of the amino group means that they are weak bases existing in solution partly as the free base and partly as the cation as the conjugate acid. When the acid HA dissociates to H and A the anion A is a base because it serves as a proton receptor in the reverse reaction. The special relationship of base A to the acid HA is acknowledged by calling it the conjugate base of the acid. Drug action The axoplasmic part of the sodium channel is blocked by the ionised part of the local anaesthetic molecule but a charged moiety will not traverse the lipid and connective tissue membranes. It is only when existing in the uncharged form that the drug can gain access to the axoplasm. Equilibrium Drugs exist in rapid equilibrium between the non-ionised N and the ionised species NH . Both ionised and non-ionised drug forms can inhibit Na channels but access to the axoplasm is via the uncharged species. Once within the axoplasm the local anaesthetic becomes protonated. The ratio of the two forms is given by the Henderson-Hasselbalch equation which in this context can be written as pKa pH log base conjugate acid . The Ka is the dissociation constant which governs the position of equilibrium between the base and acid. By analogy to pH