Local anaesthetic fibre order — FRCA Primary MCQ

Correct answer: E — Small myelinated fibres block relatively early

Small myelinated fibres tend to be blocked earlier than large motor fibres. Central chemoreceptors respond to CSF hydrogen ion concentration is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. The loop of Henle creates the medullary osmotic gradient is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. Citrate binds ionised calcium is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. Renin rises with reduced afferent arteriolar pressure is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. In Primary FRCA physiology, the correct answer is usually the variable that best explains the observed change in the anaesthetised patient.

Reference: West's Respiratory Physiology; Ganong Review of Medical Physiology; Primary FRCA textbooks