planes of sedation
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Guedel stages planes of anesthesia diagram
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Planes of Sedation / Stages of Anaesthesia
There are two major frameworks used: Guedel's Classical Stages (historical, for ether/chloroform) and the Modern Continuum defined by the ASA, alongside scoring scales used in clinical and ICU practice.
1. Guedel's Classical Stages of Anaesthesia (1937)
Arthur Guedel described four progressive stages based on clinical signs observed with ether anaesthesia. Although derived from a single-agent era, the clinical signs (somatic responsiveness, brainstem reflexes, pupil size, respiratory pattern, autonomic responses) have been updated for modern multidrug anaesthesia by Brown and remain foundational.
| Stage | Name | Clinical Features |
|---|---|---|
| Stage I | Analgesia / Induction | Conscious, amnesic, analgesic. Corneal and laryngeal reflexes intact. Regular breathing. Ends with loss of consciousness. |
| Stage II | Excitement / Delirium | Loss of consciousness but uncontrolled, irregular excitatory activity. Breath-holding, coughing, vomiting possible. Eyes move irregularly. Dangerous stage - avoid prolonged duration. |
| Stage III | Surgical Anaesthesia | Safe, controlled anaesthesia. Divided into 4 planes (see below). Most surgery is performed here. |
| Stage IV | Medullary Depression | Brainstem and respiratory centre paralysed. Fixed, dilated pupils. Cardiovascular collapse. Death if not reversed. |
Stage III Planes (Surgical Anaesthesia)
Stage III is subdivided into four planes based on eye movements, pupil size, respiratory character, and reflex responses:
| Plane | Eye Movements | Respiration | Reflexes | Pupils |
|---|---|---|---|---|
| Plane 1 | Roving eye movements cease; eyes central | Regular, deep, thoracic | Laryngeal/pharyngeal reflexes active | Normal size |
| Plane 2 | Eyes fixed and central | Regular; intercostal contribution diminishes | Laryngeal reflexes lost; corneal reflex present | Slightly dilated |
| Plane 3 | Eyes central; onset of intercostal paralysis | Abdominal breathing predominates; intercostals progressively paralysed | Corneal reflex lost | Dilated, sluggish light reflex |
| Plane 4 | Complete intercostal paralysis | Diaphragm only (about to fail) | All reflexes lost | Widely dilated, no light reflex |
Planes 1-2 are adequate for most surgical procedures. Plane 3-4 approaches Stage IV and is dangerous.
2. Modern ASA Continuum of Depth of Sedation
The ASA defines a spectrum (not discrete stages) based on responsiveness, airway, ventilation, and cardiovascular function - Barash et al., Clinical Anesthesia, 9e:
| Level | Responsiveness | Airway | Spontaneous Ventilation | CVS |
|---|---|---|---|---|
| Minimal Sedation (Anxiolysis) | Normal response to verbal stimulation | Unaffected | Unaffected | Unaffected |
| Moderate Sedation ("Conscious Sedation") | Purposeful response to verbal or tactile stimulation | No intervention required | Adequate | Usually maintained |
| Deep Sedation/Analgesia | Purposeful response to repeated or painful stimulation only | Intervention may be required | May be inadequate | Usually maintained |
| General Anaesthesia | Unarousable, even with painful stimulus | Intervention often required | Frequently inadequate | May be impaired |
Note: Reflex withdrawal from a painful stimulus is NOT considered a purposeful response. A patient can easily slide from one level to the next.
3. Clinical Sedation Scoring Scales
Used mainly in ICU and procedural sedation settings:
Ramsay Sedation Scale (RSS) - 1974
Originally 6 levels; a modified version adds 2 more (total 8):
Awake levels:
- Anxious, agitated, or restless
- Cooperative, oriented, tranquil
- Responsive to commands only
Asleep levels (response to glabellar tap or auditory stimulus):
4. Brisk response
5. Sluggish response
6. No response
Richmond Agitation-Sedation Scale (RASS) - 2002
A 10-level scale (-5 to +4) with high inter-rater reliability across physicians, nurses, and pharmacists:
| Score | Description |
|---|---|
| +4 | Combative |
| +3 | Very agitated |
| +2 | Agitated |
| +1 | Restless |
| 0 | Alert and calm |
| -1 | Drowsy (sustained eye opening >10 s) |
| -2 | Light sedation (eye opening <10 s) |
| -3 | Moderate sedation (movement to voice, no eye contact) |
| -4 | Deep sedation (movement to physical stimulation only) |
| -5 | Unarousable |
Observer's Assessment of Alertness/Sedation (OAA/S) Scale
Assesses four domains: responsiveness, speech, facial expression, and eyes. Composite score 1 (deep sleep) to 5 (fully alert). Widely used in procedural sedation research.
Key Clinical Points
- Guedel's framework was derived from ether anaesthesia and has historical primacy; modern multidrug anaesthesia makes individual signs less reliable indicators of depth.
- The ASA Continuum is the current standard framework for procedural sedation.
- RASS is the most widely validated ICU sedation tool with excellent inter-rater reliability.
- Clinical signs such as pupil size, lacrimation, sweating, blood pressure, and heart rate remain useful depth-of-anaesthesia indicators in modern practice, linked to brainstem reflex circuits.
- Neuromuscular blocking drugs abolish somatic responses, making monitoring depth more challenging; EEG-based tools like BIS (Bispectral Index) monitoring every 15 minutes are used as adjuncts.
Sources: Barash, Cullen & Stoelting's Clinical Anesthesia 9e; Miller's Anesthesia 10e
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