Vaporizers in Anaesthesia

1. Definition and Purpose

2. Relevant Physics

Saturated Vapour Pressure (SVP)

• The pressure exerted by vapour molecules above a liquid at equilibrium at a given temperature.
• Is temperature-dependent: higher temperature = higher SVP = more molecules in gas phase.
• Determines the maximum concentration achievable in the vaporizing chamber.

Latent Heat of Vaporization

• Energy needed to convert liquid to vapour is absorbed from surroundings (the liquid itself if no external heat source).
• This causes the liquid to cool as vaporization progresses, reducing SVP and output.
• Modern vaporizers compensate for this via temperature-compensating valves or external heating elements.

Ideal Gas Law (PV = nRT)

• Provides the framework for understanding gas behaviour inside vaporizers and alveoli.

3. Classification of Vaporizers

By Circuit Position

By Design/Mechanism

1. Variable bypass vaporizer
2. Dual-circuit (desflurane) vaporizer - Tec 6/Tec 6 Plus
3. Cassette vaporizer - GE Aladin cassette system
4. Injection-type vaporizer

4. Variable Bypass Vaporizer (Most Common)

Principle

• Bypass stream: passes directly to the vaporizer outlet without contacting liquid agent.
• Vaporizing chamber stream: flows through wicks/baffles over liquid agent, becoming saturated with vapour.

Key Components

• Inlet and outlet ports
• Concentration control dial (sets the bypass-to-vaporizing chamber ratio = "splitting ratio")
• Bypass chamber
• Vaporizing chamber with wicks and baffles (increase surface area for vaporization)
• Temperature-compensating valve (bimetallic strip or expansion element)
• Filling assembly (agent-specific, color-coded)

Splitting Ratio Example (Sevoflurane at 20°C)

• SVP of sevoflurane = 160 mmHg at 20°C → saturated vapor concentration = 160/760 = 21%
• To deliver 1% sevoflurane, 100 mL/min exits the vaporizing chamber (21 mL sevo + 79 mL carrier gas)
• Bypass flow required = 2,000 mL/min (21 mL sevo in 2,100 mL total = 1%)
• Bypass:vaporizing chamber ratio = 20:1
• For isoflurane (SVP = 238 mmHg → 31% saturated), bypass:vaporizing ratio for 1% = 30:1

Temperature Compensation

• At higher temperatures, SVP rises → more agent would be delivered without correction.
• Bimetallic strip or expansion rod deflects to divert more flow through bypass and less through vaporizing chamber, maintaining constant output.
• Example: GE Tec-type vaporizers use a bimetallic strip that opens the bypass more as temperature rises.

Formula for Liquid Agent Consumption

3 × FGF (L/min) × vol% = mL liquid volatile anaesthetic/hour

5. Desflurane Vaporizer (Tec 6 / Tec 6 Plus)

• SVP at 20°C is ~669 mmHg (nearly atmospheric pressure)
• Boiling point = 23.5°C - desflurane boils at room temperature
• Unpredictable output from a standard vaporizer

Special Design Features

• An electrically heated sump maintains desflurane at 39°C (~1,550 mmHg - approximately 2 atm), completely vaporizing all agent.
• The desflurane vapour is metered as a pure gas and then blended into the FGF stream - making the Tec 6 more accurately a gas blender than a vaporizer.
• Requires electrical power to operate; alarmed if power fails or temperature not reached.
• Outputs a constant volume percent (not constant partial pressure) regardless of altitude - contrast with variable bypass vaporizers.

Altitude Effect on Tec 6 (Clinically Important)

• Tec 6 still delivers the same volume percent but the partial pressure is reduced proportionally.
• Required dial setting adjustment:

Required dial setting = normal dial × (760 mmHg ÷ ambient pressure mmHg)

• Variable bypass vaporizers, by contrast, are essentially ambient pressure-compensated because proportioning occurs as gas exits the vaporizing chamber.

6. Cassette Vaporizer - GE Aladin System

• Used in GE Aisys and Avance Carestations.
• A single electronically controlled vaporizer unit inside the machine accepts interchangeable agent-specific cassettes (halothane, isoflurane, enflurane, sevoflurane, desflurane).
• Cassettes are color-coded and magnetically coded so the workstation identifies which agent is loaded.
• Contains a bypass chamber (fixed restrictor) and a vaporizing chamber with an electronically controlled flow control valve at the outlet.
• A CPU receives input from: concentration dial, pressure sensor, temperature sensor, bypass flow measurement, vaporizing chamber flow measurement, and carrier gas composition.
• Advantage: one permanent vaporizer module handles all agents; quick cassette swap.

7. Vaporizer Mount and Interlock System

• Removable mounts allow rapid vaporizer exchange (e.g., for MH risk - malignant hyperthermia - the vaporizer can be removed).
• All anaesthesia machines must prevent fresh gas from flowing through more than one vaporizer simultaneously (interlock system).
• After mounting/changing a vaporizer, the operator must confirm it is seated properly and perform a leak test if required.
• Interlock failures have been reported - potential for anaesthetic overdose.

8. Factors That Influence Vaporizer Output

9. Hazards and Special Situations

Misfilling

• Filling a vaporizer with the wrong agent changes the SVP and splitting ratio, causing overdose or underdose.
• If an isoflurane vaporizer is misfilled with desflurane (SVP much higher), substantial overdose can occur.
• Agent-specific filling devices (Quik-Fil, Selectatec) reduce but do not eliminate misfilling.
• Breathing circuit gas analysis is the key safety check.

Tipping

• Tilting a variable bypass vaporizer can allow liquid agent to enter the bypass chamber.
• Results in extremely high output when the vaporizer is turned on.
• Vaporizers should be flushed at high flow before clinical use after tipping.
• Some vaporizers have a transport ("T") dial setting that isolates the vaporizing from bypass chamber during transport.

Overfilling

• Liquid entering the bypass chamber can cause dangerous vapour delivery.
• Side-fill vaporizers largely prevent overfilling; avoid filling in the "on" position or while rocked/tilted.

Leaks

• Sources: loose filler caps, drain valves, vaporizer-machine interface, internal mechanical failure.
• Present as lower-than-expected inhaled agent concentration, awareness, or odour of agent.

Contamination

• Rare but reported: bacterial growth (e.g., S. epidermidis) in sevoflurane vaporizers with water accumulation, producing toxic volatiles.

10. Anesthetic Delivery - Wash-In

Summary Table: Variable Bypass vs. Desflurane (Tec 6) vs. Aladin Cassette

• Miller's Anesthesia, 10e - Chapter 20 (Anesthesia Workstation), pp. 2261-2298
• Barash's Clinical Anesthesia, 9e - Chapter 25, pp. 1972-1995

Laparoscopic Surgeries in MD Anaesthesia

1. Introduction

1. Physiological effects of CO2 pneumoperitoneum
2. Positioning-related effects (Trendelenburg, reverse Trendelenburg, lateral decubitus)
3. Limited access to the patient during the procedure

2. Why CO2 is Used for Insufflation

• High diffusion coefficient - rapidly absorbed and excreted via lungs
• Lower risk of gas embolism compared to air or O2 (CO2 is highly soluble)
• Non-combustible
• Readily available and inexpensive

3. Physiology of Pneumoperitoneum

3.1 Cardiovascular Effects

3.2 Respiratory Effects

• Cephalad diaphragm displacement due to raised IAP causes compression atelectasis.
• CO2 absorption via lymphatic and venous plexuses raises PaCO2 - requires increased minute ventilation (10-25% increase typically needed) to maintain normocarbia.

3.3 Hypercapnia Effects

3.4 CNS / Intracranial Effects

• Both pneumoperitoneum and Trendelenburg position independently ↑ ICP.
• Elevated CO2 → cerebral vasodilation → ↑ ICP.
• Optic nerve sheath diameter (ONSD) measured by ultrasound: non-invasive surrogate for ICP monitoring during laparoscopy. ONSD increases with both pneumoperitoneum and Trendelenburg.
• Intraocular pressure (IOP) also increases - relevant for patients with poorly controlled glaucoma.
• Prolonged steep Trendelenburg can cause facial, periorbital, and occasionally laryngeal oedema, though rarely sufficient to threaten airway patency.

3.5 Renal Effects

• ↓ Renal blood flow from ↓ CO + direct compression + renin-angiotensin activation.
• IAP >15 mmHg is associated with postoperative AKI.
• Maintain IAP below 12 mmHg when renal protection is a priority.
• Post-laparoscopic donor nephrectomy oliguria: usually self-limited; "renal-dose" dopamine, mannitol, furosemide, and fenoldopam have no proven benefit.

3.6 Other Effects

• Gastro-oesophageal regurgitation (raised IAP).
• ↑ Risk of DVT (venous stasis from raised IAP + lithotomy position).
• Trocar placement: ~0.5% visceral/vascular injury.
• Renin-angiotensin system activation.
• Neuropraxia (brachial plexus most common).
• Endotracheal tube displacement (cephalad shift of carina).

4. Positioning Effects

4.1 Trendelenburg (Head-Down) - Gynaecological, Urological Laparoscopy

• ↑ Preload (blood funnelled from lower limbs to RA).
• ↑ ICP and IOP (see above).
• Cephalad shift of diaphragm → ↓ FRC, ↑ airway pressures, V/Q mismatch.
• Risk of mainstem intubation (carina shifts cephalad) - ensure ETT cuff is just beyond vocal cords.
• Protects against venous gas embolism (↑ CVP reduces CO2 entrainment).

4.2 Reverse Trendelenburg (Head-Up) - Upper Abdominal Laparoscopy (Cholecystectomy, etc.)

• ↓ Preload, ↓ CO.
• Risk of sliding caudally on OR table.
• Better pulmonary compliance than head-down (gravity moves bowel away from diaphragm).

4.3 Lateral Decubitus (Laparoscopic Nephrectomy)

• Patient positioned laterally with various degrees of Trendelenburg + table flexion.
• Attention to padding pressure points, preventing nerve injury, maintaining neutral spine.

5. Specific Physiological Summary Table

6. Anaesthetic Management

6.1 Preoperative Assessment

• Cardiopulmonary reserve is key: patients with pre-existing cardiac or respiratory disease tolerate pneumoperitoneum poorly.
• For laparoscopic prostatectomy/hysterectomy: additional cardiac monitoring (e.g., arterial line) may be warranted for patients with compensated CCF, severe CAD.
• Identify high-risk patients: obesity (↑ atelectasis, ↓ FRC, ↑ airway pressures), COPD, pulmonary hypertension, severe glaucoma.

6.2 Airway

• General anaesthesia with endotracheal intubation is standard for all major laparoscopic procedures.
• LMA is acceptable for short procedures (diagnostic laparoscopy) in some protocols, but not for Trendelenburg - risk of aspiration and airway loss under pneumoperitoneum.
• Secure ETT carefully - ensure cuff just beyond vocal cords to minimise mainstem intubation risk when diaphragm shifts cephalad.
• Orogastric or nasogastric tube: decompresses stomach, reduces IAP and aspiration risk, improves surgical visualization.

6.3 Ventilation Strategy

• Increase minute ventilation by 10-25% to compensate for absorbed CO2.
• Monitor end-tidal CO2 (EtCO2) continuously; note that EtCO2 - PaCO2 gradient may widen during laparoscopy, especially in patients with lung disease.
• Pressure-controlled ventilation (PCV) is recommended over volume control in steep Trendelenburg: lowers peak airway pressures and improves pulmonary compliance.
• Lung-protective ventilation: low tidal volume (6-8 mL/kg IBW), appropriate PEEP, avoid excessive plateau pressures.
• Recruitment manoeuvres prevent atelectasis.
• PEEP titration: balance between improving oxygenation/compliance and avoiding haemodynamic compromise.
• Permissive hypercapnia may be acceptable in some patients but avoid in:
  • Renal impairment (respiratory acidosis → significant hyperkalaemia)
  • Raised ICP states
  • Severe pulmonary hypertension

6.4 Neuromuscular Blockade

• Deep neuromuscular blockade (NMB) is advantageous and recommended:
  • Allows lower insufflation pressures (12 mmHg vs 15 mmHg) for equivalent surgical exposure.
  • Lower IAP → less cardiovascular and renal compromise.
  • Essential for robot-assisted cases (patient movement risks tissue tearing by fixed robotic arms).
• Monitor NMB continuously (TOF); ensure reversal is complete before extubation.

6.5 Haemodynamic Management

• Fluid optimisation preoperatively: preinduction colloid boluses may improve stroke volume and urine output with pneumoperitoneum.
• Maintain IAP <12-15 mmHg to limit cardiovascular and renal effects.
• Vasopressors (phenylephrine, noradrenaline) for refractory hypotension.
• For patients with known cardiac disease: consider invasive arterial monitoring, TEE, or PA catheter.

6.6 Pain Management (Postoperative)

• Laparoscopic approach: reduced pain and shorter recovery vs open surgery.
• Multimodal analgesia: paracetamol + NSAID (avoid NSAIDs in urological cases due to nephrotoxicity) + opioid.
• Epidural anaesthesia: rarely indicated for laparoscopic cases (unlike open surgery).
• Local anaesthetic wound infiltration at port sites.
• Rectus sheath / retroperitoneal sheath catheters: continuous LA infusion has shown reduced pain, opioid requirements, PONV, and time to discharge.

6.7 TIVA vs Volatile

• Both are acceptable for laparoscopic anaesthesia.
• TIVA (propofol-based): reduces PONV (important as laparoscopy has high PONV risk), has been studied for lower ICP effects during steep Trendelenburg (inconclusive evidence).
• Volatile agents: acceptable with appropriate ventilation management.

7. Complications

7.1 Venous Gas Embolism (VGE / CO2 Embolism)

• Most catastrophic complication of laparoscopy.
• CO2 enters venous system (via Veress needle misplacement, transected vessels, deep dorsal vein complex, round/broad ligament).
• Subclinical CO2 emboli detected by TEE in ~20% of laparoscopic radical prostatectomies and ~100% of laparoscopic total hysterectomies.
• Significant emboli are rare.

• Acute tachycardia, arrhythmias, QRS widening
• Hypotension, hypoxia
• Low EtCO2 (sudden drop)
• Cyanosis
• "Mill wheel" murmur (churning of gas + blood in right heart)
• TEE: "near white-out" of right heart chambers - right ventricular air lock

1. Immediate cessation of CO2 insufflation + abdominal decompression
2. 100% O2 and hyperventilation (accelerates CO2 elimination)
3. Left lateral decubitus + Trendelenburg (Durant's manoeuvre) - minimises RV outflow obstruction
4. Rapid IV fluids for hypotension
5. ACLS if cardiac arrest

7.2 Subcutaneous Emphysema

• CO2 tracks along fascial planes.
• Signs: crepitus, sudden rise in EtCO2.
• Management: check needle placement; lower IAP; hyperventilate.

7.3 Pneumothorax / Capnothorax

• CO2 enters pleural space (diaphragm defect or tracking).
• Consider if sudden fall in SpO2 or ↑ airway pressures.
• Management: decompress; may need chest drain in severe cases.

7.4 Endobronchial Intubation

• Cephalad diaphragm shift causes carina to move cephalad.
• ETT tip may advance into right main bronchus.
• Check breath sounds after insufflation and any position change.

7.5 Haemodynamic Instability

• Vagal-mediated bradycardia/asystole during peritoneal insufflation (reflex from peritoneal stretch).
• Treat: stop insufflation, atropine, reassess.

7.6 Trocar Complications

• Visceral or major vascular injury at Veress needle or trocar insertion: ~0.5%.
• May require immediate conversion to laparotomy.

7.7 Positioning Complications

• Brachial plexus neuropraxia (arm position in Trendelenburg).
• Compartment syndrome (lithotomy position, prolonged procedures).
• Facial and corneal injury (robotic arms).
• Patient sliding/falls (extreme Trendelenburg).

8. Special Patient Populations

Obesity

• ↑ IAP at baseline → exacerbated by pneumoperitoneum.
• ↑ Atelectasis, ↑ airway pressures, ↓ FRC.
• 22% higher respiratory rate, 8% lower tidal volumes, 38% higher peak inspiratory pressures compared to non-obese.
• Increased risk of GORD and aspiration.
• RSI preferred; higher PEEP needed.

Cardiac Disease

• 30% ↓ CO + ↑ SVR poorly tolerated in compensated CCF/severe IHD.
• Additional monitoring (arterial line, TEE) recommended.
• Keep IAP <12 mmHg; permissive hypercapnia avoided.

Raised ICP / Cerebrovascular Disease

• Transcranial Doppler or cerebral oximetry monitoring recommended.
• Avoid steep Trendelenburg when possible; maintain normocarbia.

Renal Impairment

• Avoid IAP >15 mmHg (AKI risk).
• Permissive hypercapnia contraindicated (respiratory acidosis → hyperkalaemia).
• Avoid NSAIDs postoperatively.

Pulmonary Hypertension

• Hypercarbia → ↑ pulmonary vascular resistance → RV failure.
• Careful ventilation; avoid hypoxia, acidosis, high airway pressures.

9. Specific Procedures and Anaesthetic Considerations

10. Monitoring

11. Absolute/Relative Contraindications to Laparoscopy

• Miller's Anesthesia, 10e - Chapters 12, 27, 67 (pp. 1323-1324, 9851-9870)
• Barash's Clinical Anesthesia, 9e - Chapters 44, 50 (pp. 3730-3748, 4289-4295)