2017A12 Discuss the physiological consequences of total spinal anaesthesia
caused by intrathecal administration of 20mL 2% lignocaine at L3/4 level.
(Do not include management)

Dose

·  20mg/mL x 20mL = 400mg = 5.7mg/kg

(exceeds toxic subcut dose)

Effects

·  CSF effects >> systemic effects

·  Blockade of lumbar cord -> thoracic -> cervical -> brainstem

Time course

·  Total spinal within ~10 mins

·  Duration ~ 2 hours (shorter than many operations)

Lower body

·  Sensory and motor loss

·  Muscle relaxation facilitates surgical access

Upper body

Head and neck

·  Sensory > motor loss

Cranial nerves

·  CNX (pharynx and larynx for intubation)

·  CNIII EW (mydriasis)

Medulla

·  Block ascending reticular activating system -> coma

Delayed complications

·  Risk of transient neurological symptoms

o  Buttock and thigh pain without neuro deficit)

Lumbar and low thoracic level

·  Venodilatation -> ↓preload (exacerbated by hypovolaemia)

·  Vasodilatation -> ↓SVR

·  ↓mAP

·  Reflex ↑HR may be significant (baroreceptor response)

High thoracic and cervical level

·  ↓HR and ↓contractility (acceleromotor T1-4)

·  ↓↓mAP

·  Absent baroreceptor response

Brainstem level

·  Zero SNS output (RVLM), zero PSNS output (dorsal motor nucleus and nucleus ambiguus)

·  ↓↓mAP

·  HR at intrinsic SA node rate 110bpm, absent baroreceptor response

(this is from first principles – I have no evidence for it)

Thoracic level

·  Intercostal paralysis

Upper cervical level

·  C3-5: diaphragm paralysis

·  (Note phrenic nerve is large and resistant to blockade)

Brainstem level

·  CNIX, X: ↓↓airway reflexes, glottic opening, risk aspiration if unprotected

·  CNIII: dilated pupils

·  Medulla: paralysed resp centre, apnoea

Musculoskeletal

·  ↓↓Muscle tone -> improved surgical access

GIT

·  ↓PSNS output -> ↓GIT activity

Temp

·  Vasodilatation, venodilatation, -> peripheral heat distribution -> ↑↑heat loss

Systemic effect

·  Risk of CVS toxicity: conduction delay, re-entrant tachyarrhythmias, arrest