Beach Chair Position Anaesthesia

Quick Answer

Beach chair position (BCP) involves elevating the head and torso 30-70 degrees from supine with legs lowered, primarily used for shoulder surgery. Physiological effects: Gravitational effects on cerebral perfusion with MAP referenced at the brain (typically 20-30 mmHg lower than at the heart), reduced venous return, and increased risk of venous air embolism from open veins above heart level. Critical concern: Cerebral desaturation events (CDE) occur in 4-56% of patients under general anaesthesia, strongly associated with devastating neurological complications including stroke, visual impairment, and cognitive dysfunction. Cerebral monitoring: Near-infrared spectroscopy (NIRS) monitors regional cerebral oxygen saturation (rScO2) with critical thresholds <55% absolute or >20% drop from baseline requiring immediate intervention. Management strategy: Cerebral autoregulation is often impaired in BCP (COx >0.3), making cerebral blood flow pressure-dependent; maintain MAP >65-70 mmHg at the level of the brain, staged positioning changes, and immediate correction of desaturation events with blood pressure augmentation and positioning adjustments. Stroke risk: Rare but devastating (0.0004-0.4% incidence); prevention focuses on maintaining cerebral perfusion pressure and avoiding hypotension. [1-25]

Anatomy and Positioning

Position Description

Standard Beach Chair Position:

• Head elevation: 30-70 degrees from horizontal (typically 45-60 degrees)
• Hip flexion: 45-60 degrees at the hip hinge
• Knee flexion: 30 degrees to prevent patient sliding
• Foot support: Secure footrest to prevent foot drop
• Lateral positioning: Operative shoulder up (typically 30-45 degrees roll)
• Head stabilization: May use headrest or Mayfield fixation

Variations:

• Low beach chair: 30-45 degrees elevation (some procedures)
• High beach chair: 60-70 degrees elevation (improved surgical access)
• Modified lateral decubitus: Some surgeons prefer full lateral
• Sitting position: Near 90 degrees (rarely used, higher risks)

Surgical Advantages

Shoulder Surgery Benefits:

• Better visualization: Anterior shoulder anatomy clearly visible
• Reduced bleeding: Lower venous pressure reduces surgical bleeding
• Easier conversion: Can extend to deltopectoral approach if needed
• Anatomical orientation: Surgeon works in familiar anatomical position
• Bony landmarks: Easily palpable for portal placement

Common Procedures:

• Arthroscopic rotator cuff repair
• Bankart repair/shoulder stabilization
• Total shoulder arthroplasty
• Reverse total shoulder arthroplasty
• Subacromial decompression
• Acromioclavicular joint reconstruction
• Proximal humerus fracture fixation

Positioning Technique

Preparation:

• General anaesthesia with secure airway (ETT or LMA)
• Arterial line for continuous blood pressure monitoring
• NIRS monitoring (bilateral frontal lobes)
• Large-bore IV access
• Defibrillator pads placed before positioning (anterior-posterior or anterior-lateral)

Staged Positioning Protocol:

1. Baseline measurements: Record rScO2, MAP, HR in supine position
2. Initial elevation: Raise head to 30 degrees
3. Wait period: 3 minutes for hemodynamic stabilization
4. Final elevation: Raise to target angle (45-70 degrees)
5. Continuous monitoring: rScO2, MAP, processed EEG
6. Documentation: Record all values at each stage

Safety Measures:

• Secure footplate to prevent sliding
• Padding at all pressure points (occiput, scapulae, sacrum, heels)
• Drape to allow access to airway and lines
• Ensure emergency lowering capability
• Avoid extreme neck rotation (vertebral artery risk)

Cerebral Physiology in Beach Chair Position

Gravitational Effects on Cerebral Perfusion

Hydrostatic Pressure Gradient:

• Blood pressure measured at heart level (standard)
• Brain positioned 20-30 cm above heart in BCP
• Hydrostatic pressure loss: ~0.77 mmHg per cm height difference
• MAP at brain = MAP at heart - (height difference x 0.77 mmHg/cm)
• Example: MAP 80 mmHg at heart, brain 25 cm higher = MAP ~60 mmHg at brain

Cerebral Perfusion Pressure (CPP):

• CPP = MAP (at brain level) - Intracranial Pressure (ICP)
• ICP typically near zero or negative in upright position
• CPP primarily dependent on MAP at brain level
• Autoregulation normally maintains CBF across wide CPP range (50-150 mmHg)
• In BCP: Autoregulation may be impaired, making CBF pressure-dependent

Measurement Corrections:

• Arterial transducer should be zeroed at brain level (external auditory meatus)
• Provides true cerebral perfusion pressure
• If zeroed at heart: Add 20-30 mmHg to target MAP for brain-level equivalent

Cerebral Blood Flow Autoregulation

Normal Autoregulation:

• Cerebral blood flow maintained constant across MAP 60-150 mmHg
• Myogenic response of cerebral arterioles
• Metabolic coupling to neural activity
• Neurogenic regulation

Impaired Autoregulation in BCP:

• Multiple studies show impaired autoregulation in BCP under anaesthesia
• Cerebral Oximetry Index (COx): Pearson correlation coefficient between MAP and rScO2
• COx approaching 0: Intact autoregulation (no correlation between MAP and rScO2)
• COx >0.3: Impaired autoregulation (CBF pressure-dependent)
• Studies consistently show COx >0.3 in BCP patients under GA

Clinical Significance:

• Without autoregulation, cerebral blood flow directly proportional to MAP
• Hypotension causes proportional reduction in CBF
• Cerebral ischemia risk if MAP insufficient for height difference
• Individual variation in autoregulatory capacity
• Chronic hypertension shifts autoregulatory curve rightward

Cerebral Oxygenation Monitoring

Near-Infrared Spectroscopy (NIRS):

• Non-invasive monitoring of regional cerebral oxygen saturation (rScO2)
• Measures oxyhemoglobin and deoxyhemoglobin in frontal cortex
• Provides continuous trend monitoring
• Bilateral sensors placed on forehead

Normal Values and Thresholds:

• Normal rScO2: 60-80% (baseline varies between individuals)
• Critical threshold: rScO2 <55% absolute value
• Relative threshold: >20% drop from baseline
• Either threshold requires immediate intervention

Cerebral Desaturation Event (CDE):

• Definition: rScO2 <55% or >20% drop from baseline
• Duration typically measured (e.g., >2 minutes)
• Incidence: 4-56% depending on monitoring and management
• Associated with adverse neurological outcomes

Factors Affecting rScO2:

• Arterial oxygen saturation (SpO2)
• Hemoglobin concentration
• Cerebral blood flow (MAP, autoregulation)
• Cerebral metabolic rate (anaesthetic depth, temperature)
• Carbon dioxide (PaCO2 vasoreactivity)

Cerebral Desaturation: Recognition and Management

Monitoring Strategies

Mandatory Monitoring:

• NIRS (cerebral oximetry): Bilateral frontal monitoring
• Arterial line: Continuous BP, zeroed at brain level
• Processed EEG: BIS or similar (target 40-60)
• Standard monitoring: ECG, SpO2, EtCO2, temperature

Optional/Advanced Monitoring:

• Transcranial Doppler: Cerebral blood flow velocity
• Jugular venous bulb oximetry: Global cerebral oxygenation
• Arterial blood gas: PaCO2, PaO2, hemoglobin, lactate
• Cerebral autoregulation monitoring: COx calculation

Baseline Establishment:

• Record rScO2 in supine position before any elevation
• Establish patient-specific baseline (normal range 60-80%)
• Calculate critical thresholds (baseline - 20%)
• Document MAP at each position change

Interventions for Cerebral Desaturation

Immediate Response to CDE:

1. Increase blood pressure:

   • Phenylephrine bolus (50-100 mcg) or infusion
   • Noradrenaline if cardiac output issue
   • Target MAP >80 mmHg at brain level (>100 mmHg at heart level)

2. Optimize oxygenation:

   • Increase FiO2 to 100%
   • Check SpO2 and arterial line function
   • Consider PaO2 if ABG available

3. Optimize ventilation:

   • Ensure EtCO2 35-40 mmHg (normocapnia)
   • Avoid hypocapnia (cerebral vasoconstriction)
   • Avoid hypercapnia (unless therapeutic)

4. Position adjustment:

   • Lower head of bed 10-20 degrees if possible
   • Reduce angle of elevation temporarily
   • Coordinate with surgeon

5. Check for surgical causes:

   • Air embolism (suction on veins, bone bleeding)
   • Excessive traction on shoulder
   • Position-related venous obstruction

Preventive Strategies:

• Staged positioning (allow autoregulatory adaptation)
• Maintain MAP >70 mmHg at brain level (>=90 mmHg at heart)
• Avoid deep anaesthesia (cerebral vasodilation lost)
• Maintain normocapnia
• Adequate hemoglobin (>100 g/L in high-risk)
• Avoid extreme angles (>60 degrees) when possible

Haemodynamic Management

Blood Pressure Targets

Absolute Targets (at brain level):

• Minimum MAP: 65-70 mmHg
• Optimal MAP: 70-80 mmHg
• Hypertensive patients: Maintain near baseline (MAP 80-90 mmHg)

Conversion to Heart-Level Measurements:

• If arterial line zeroed at heart: Target MAP 85-100 mmHg
• Add 20-30 mmHg correction for height difference
• Example: Brain-level MAP 70 mmHg = Heart-level MAP 90-100 mmHg

Blood Pressure Management:

• Phenylephrine: First-line for vasodilation-induced hypotension
• Ephedrine: If mild hypotension with bradycardia
• Noradrenaline: If reduced cardiac output component
• Avoid excessive beta-blockade (impairs cardiac response)

Fluid Management

Preloading:

• Standard fluid administration before positioning
• Does not reliably prevent hypotension in BCP
• May cause fluid overload without benefit

Maintenance:

• Balanced crystalloids (Plasma-Lyte, Hartmann's)
• Avoid excessive crystalloid (cerebral edema risk)
• Colloids if volume expansion needed
• Maintain urine output >0.5 mL/kg/hr

Blood Loss Management:

• Shoulder surgery typically low blood loss
• Transfuse if hemoglobin <80-100 g/L (individualize)
• Maintain hematocrit >30% in high-risk patients
• Cell salvage rarely needed

Vasopressor Use

Phenylephrine:

• Pure alpha-agonist
• Increases afterload and MAP
• May reduce cardiac output (afterload increase)
• Bolus: 50-100 mcg for acute hypotension
• Infusion: 50-200 mcg/min for maintenance

Noradrenaline:

• Alpha and beta-1 agonist
• Increases MAP with preserved or increased cardiac output
• Preferred if cardiac output monitoring shows reduction
• Infusion: 0.05-0.5 mcg/kg/min

Metaraminol:

• Indirect acting sympathomimetic
• Good for bolus administration
• Can cause tachycardia

ANZCA Exam Focus

Common Viva Topics

Physiology:

• Explain the gravitational effects on cerebral perfusion in beach chair position
• Describe cerebral autoregulation and why it fails in BCP under general anaesthesia
• How do you calculate MAP at the brain level from heart-level measurements?
• Discuss the pathophysiology of cerebral desaturation events

Clinical Management:

• How would you monitor and manage a patient in 60 degrees beach chair position?
• Compare general anaesthesia vs. regional anaesthesia for shoulder surgery in BCP
• Describe your response to a cerebral desaturation event
• What is your strategy for preventing venous air embolism?

Complications:

• What are the risk factors for stroke in beach chair position?
• How does regional anaesthesia reduce the risk of cerebral desaturation?
• Describe the recognition and management of venous air embolism
• What is the significance of COx >0.3?

Assessment Content

SAQ 1: Cerebral Physiology and Monitoring (20 marks)

A 68-year-old male is scheduled for arthroscopic rotator cuff repair in the beach chair position at 60 degrees head-up tilt. His height is 175 cm. Arterial line transducer is zeroed at the heart.

a) Calculate the approximate difference in mean arterial pressure between the heart and the brain in this position. (4 marks)

b) If the arterial line shows MAP 75 mmHg (zeroed at heart), what is the approximate MAP at the brain level? Is this adequate for cerebral perfusion? (4 marks)

c) Explain why cerebral autoregulation is often impaired in the beach chair position under general anaesthesia and what this means for haemodynamic management. (6 marks)

d) Describe the monitoring you would use and your management if regional cerebral oxygen saturation (rScO2) drops from a baseline of 72% to 54%. (6 marks)

Model Answer:

a) MAP difference calculation:

• Height difference between heart and brain: approximately 25-30 cm (accept 20-30 cm) (1 mark)
• Hydrostatic pressure gradient: 0.77 mmHg per cm (1 mark)
• Pressure difference: 25 cm x 0.77 mmHg/cm = 19.25 mmHg (accept 15-25 mmHg) (2 marks)

b) Brain-level MAP:

• Heart-level MAP: 75 mmHg (1 mark)
• Brain-level MAP: 75 - 19.25 = 55.75 mmHg (accept 50-60 mmHg) (1 mark)
• This is inadequate (1 mark)
• Minimum adequate MAP at brain level: 65-70 mmHg (1 mark)

c) Impaired autoregulation:

• Normal autoregulation maintains CBF constant across MAP 60-150 mmHg (1 mark)
• In BCP under GA, COx (correlation coefficient between MAP and rScO2) >0.3 indicates failure (1 mark)
• Anaesthetic agents impair autoregulation (1 mark)
• Gravitational pooling affects cerebral circulation (1 mark)
• CBF becomes pressure-dependent (linear relationship with MAP) (1 mark)
• Requires higher MAP targets and careful BP control (1 mark)

d) Monitoring and management of CDE: Monitoring:

• NIRS (cerebral oximetry) essential (1 mark)
• Arterial line zeroed at brain level (external auditory meatus) (1 mark)
• BIS monitoring (avoid deep anaesthesia) (0.5 marks)

Management of CDE:

1. Increase blood pressure immediately (phenylephrine bolus 50-100 mcg or infusion) (1 mark)
2. Increase FiO2 to 100% (1 mark)
3. Ensure normocapnia (avoid hypocapnia) (0.5 marks)
4. Lower head of bed 10-20 degrees if possible (1 mark)

SAQ 2: Complications and Air Embolism (20 marks)

During a beach chair position shoulder replacement, the surgeon reports dark blood returning from the surgical field and the patient becomes hypotensive (MAP 50 mmHg). EtCO2 drops from 38 to 24 mmHg.

a) What is your differential diagnosis? (4 marks)

b) What is the most likely diagnosis given the clinical picture, and what is the mechanism? (4 marks)

c) Describe your immediate management of this situation. (8 marks)

d) What steps could have been taken to prevent this complication? (4 marks)

Model Answer:

a) Differential diagnosis:

• Venous air embolism (1 mark)
• Severe hemorrhage/hypovolemia (1 mark)
• Anaphylaxis (1 mark)
• Cardiac event (arrhythmia, ischemia) (1 mark)
• Pneumothorax (1 mark)

b) Most likely diagnosis:

• Venous air embolism (VAE) (2 marks)
• Mechanism: Negative pressure in open veins above heart level allows air entrainment (1 mark)
• Large air volume causes RV outflow obstruction, reduced cardiac output (1 mark)

c) Immediate management:

1. Notify surgeon immediately - flood field with saline (2 marks)
2. Ask surgeon to occlude open veins with bone wax or pressure (1 mark)
3. Place patient supine or left lateral decubitus (if possible) (1 mark)
4. Increase FiO2 to 100% (1 mark)
5. Support cardiovascular system (fluids, vasopressors) (1 mark)
6. Aspiration of air from RA via central line (if available - rarely effective) (1 mark)
7. CPR if cardiovascular collapse (1 mark)

d) Prevention:

1. Position with head not excessively elevated (1 mark)
2. Maintain euvolemia (avoid hypovolemia) (1 mark)
3. Surgical field irrigation to reduce negative pressure (1 mark)
4. Prompt occlusion of open venous channels by surgeon (1 mark)

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