2017A Question 12
Discuss the physiological consequences of total spinal anaesthesia caused by intrathecal administration of 20mL 2% lignocaine at L3/4 level. (Do not include management)
Examiner Report
29.2 % of candidates achieved a pass in this question.
This question was best addressed with a systems-based approach. Marks were given for discussion of the onset, duration and offset of these side effects. Most candidates gave a description of the cardiovascular and respiratory consequences from sympathetic blockade, and effects at the thoracic, cervical and brainstem level. Frequently omitted effects included the consequences of specific blockade of motor and sensory functions of cranial and peripheral nervous systems. Marks were also awarded for CNS effects such as about pupil changes, thermoregulation, and changes to airway reflexes.
Discussion about management, systemic toxicity or other complications related to spinal anaesthesia did not score marks.
Model Answer
Structure:
- Introduction
- Nervous
- CVS
- Resp
- Other
Introduction
| Factor | Detail |
|---|---|
| Dose | - 20mg/mL x 20mL = 400mg = 5.7mg.kg-1 (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) |
Nervous System
| Region | Detail |
|---|---|
| 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 - Buttock and thigh pain without neuro deficit) |
Cardiovascular
| Region | Detail |
|---|---|
| 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 |
Respiratory
| Region | Detail |
|---|---|
| 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 |
Other
| System | Detail |
|---|---|
| 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 |