Vaporizers — Principles, Classification & Safety Features

1. Physical Principles Underlying Vaporization

Vapor Pressure (SVP)

• Dependent only on temperature and the physical properties of the liquid
• Independent of atmospheric pressure
• Rises with increasing temperature

Desflurane's near-room-temperature boiling point and very high SVP make it unsuitable for conventional variable bypass vaporizers.

Latent Heat of Vaporization

Specific Heat

1. Determines how much heat must be supplied to the anesthetic liquid to maintain temperature during vaporization
2. Vaporizer components (metals) are chosen for high specific heat to buffer temperature swings

Thermal Conductivity

2. Classification of Vaporizers

2a. Variable Bypass Vaporizers (Most Common)

1. Bypass chamber — gas flows directly to the outlet without contacting liquid agent
2. Vaporizing chamber — gas flows over a wick system saturated with liquid agent and becomes saturated with vapor

2b. Desflurane Vaporizer — Tec 6 / D-Vapor (Dual-Circuit Blender)

1. Its SVP (669 mmHg at 20°C) is so high that prohibitive amounts of bypass gas (>12 L/min for 1 MAC) would be required to dilute the output
2. Its high MAC means large quantities must be vaporized, causing extreme evaporative cooling that conventional design cannot offset
3. Its boiling point (22.8°C) is near room temperature — if it boiled inside the vaporizer, output would be uncontrollable

• The sump is electrically heated to 39°C, creating a desflurane vapor reservoir at ~1300 mmHg
• Two fully independent gas circuits run in parallel:
  • Fresh gas circuit — carrier gas passes through a fixed restrictor (R1)
  • Vapor circuit — desflurane vapor from the heated sump passes through a pressure-regulating valve (equalises pressures) and a variable restrictor (concentration control valve, R2)
• The circuits combine at the outlet. The operator adjusts R2 to set the output concentration
• The Tec 6 is therefore more accurately described as a dual-gas blender than a traditional vaporizer

2c. GE Aladin Cassette Vaporizer

• An agent-specific cassette (containing only the liquid agent and a sump) is inserted into a permanent electronic bypass module that stays in the workstation
• The bypass chamber physically resides in the workstation and is separated from the cassette
• Electronic flow sensing and computer control regulate output concentration
• Allows easy agent changeover by swapping the cassette

2d. Injection Vaporizers

2e. Measured Flow Vaporizers (Obsolete)

3. Factors Influencing Vaporizer Output

4. Safety Features

4a. Agent-Specific Filling Systems (Key Lock / Selectatec)

• Vaporizers use keyed, indexed filling ports specific to each agent bottle — prevents filling with the wrong agent
• Selectatec / Select-a-Tec system: Agent bottles have agent-specific adaptors; the bottle cannot be inverted into the wrong vaporizer
• Most modern vaporizers are side-fill rather than top-fill, minimising overfilling risk

4b. Vaporizer Mount and Interlock System

• On multi-vaporizer manifolds, an interlock prevents simultaneous activation of more than one vaporizer — eliminates accidental administration of two volatile agents at once
• GE Selectatec and Dräger Vapor manifolds each have proprietary interlock mechanisms

4c. Temperature Compensation

4d. Pressure Compensation (Anti-Pumping Effect)

• Smaller vaporizing chamber volume — limits the amount of vapor that can be discharged retrograde
• Long spiral inlet tube (labyrinth) — retrograde vapor must traverse a long path before reaching the bypass; pressure dissipates within it
• Extensive baffle systems in the vaporizing chamber
• One-way check valves downstream of the vaporizer

4e. Transport Lock (Dräger Vapor 2000/3000)

• A "T" (transport) dial position isolates the sump from the bypass chamber before removal
• Prevents liquid agent from spilling into the bypass during transport or tilting
• The vaporizer cannot be removed from the workstation unless the dial is set to "T"

4f. Tipping Protocol

• If a variable bypass vaporizer has been tipped (liquid enters bypass), it must be purged for 20–30 minutes at high fresh gas flows before clinical use
• 1 mL of liquid anesthetic produces ~200 mL of vapor — even small spillage into the bypass can produce a massive overdose
• Aladin cassette and Tec 6 / D-Vapor designs are essentially immune to tipping hazards

4g. Prevention of Overfilling and Underfilling

• Overfilling: Liquid enters bypass → up to 10× intended concentration deliverable. Side-fill design prevents this
• Underfilling: At low fill states (<25% for Tec 5 sevoflurane) + high FGF (>7.5 L/min) + high dial setting → vaporizer output may drop abruptly. Clinically important during inhalational inductions

4h. Leak Detection

• Loose filler cap is the most common leak source; O-ring junction leaks also occur
• Detection requires the concentration dial to be in the "on" position during leak testing
• GE machines: use a negative-pressure (suction bulb) leak test, because the outlet check valve prevents positive-pressure testing from detecting internal vaporizer leaks
• Dräger machines: both positive- and negative-pressure methods can detect leaks (no outlet check valve)
• Modern workstation self-tests may not detect internal vaporizer leaks — each vaporizer's dial must be turned on individually during the self-test sequence

4i. Desflurane-Specific Safety (Tec 6 / D-Vapor)

• An alarm activates when the sump is not heated to 39°C or power fails — the shut-off valve closes and desflurane delivery halts
• Pressure-equalisation valve prevents delivery of uncontrolled concentrations if back-pressure fluctuations occur
• The vaporizer requires electrical power — a power failure disables desflurane delivery

4j. Agent Identification and MRI Considerations

• In MRI suites, ferromagnetic vaporizer components are hazardous — dedicated MRI-compatible vaporizers (e.g., specific Dräger models) are available
• Colour coding and labelling of vaporizers and agent bottles provide a further layer of protection

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

Massive Blood Loss, Massive Transfusion & Blood Conservation Strategies

1. Definition of Massive Blood Loss

• Loss of one entire circulating blood volume (≈5 L in a 70 kg adult) within 24 hours, or
• Loss of 50% of the circulating blood volume within 3 hours, or
• Ongoing loss at a rate exceeding 150 mL/min

• Loss requiring transfusion of >10 units of packed red blood cells (pRBCs) within 24 hours
• In children: blood loss >40 mL/kg of anticipated blood product requirements

2. Definition of Massive Transfusion

"A common definition for massive transfusion is the transfusion of at least one circulating patient blood volume within 24 hours. An approximate alternative definition for an average-sized adult patient may be the transfusion of 10 or more pRBC units within 24 hours." — Tietz Textbook of Laboratory Medicine 7e, p. 3749

Massive Transfusion Protocol (MTP)

• 6 units pRBC + 6 units Fresh Frozen Plasma (FFP) + 1 unit apheresis platelets
• This exemplifies a 1:1:1 ratio of plasma : platelets : pRBC
• Some evidence supports a 1:1:2 strategy (favouring pRBCs) in certain populations
• Cryoprecipitate and factor concentrates may be incorporated

3. Blood Conservation Strategies During Surgery

A. Preoperative Optimisation (Foundation for Intraoperative Success)

1. Identify and treat preoperative anaemia — iron deficiency corrected with supplemental iron weeks before elective surgery; erythropoiesis-stimulating agents (EPO ± iron) for non-iron-deficiency anaemia or patients who refuse transfusion
2. Optimise coagulation status — stop antiplatelet agents (dual antiplatelet therapy), warfarin, and novel oral anticoagulants at appropriate intervals before surgery. Point-of-care testing (TEG/ROTEM) to confirm resolution of drug effect
3. Risk stratification — identify patients at high risk: advanced age, pre-existing anaemia, coagulopathy, complex redo surgery

B. Intraoperative Blood Conservation Strategies

1. Acute Normovolaemic Haemodilution (ANH)

• 1–2 units of whole blood withdrawn from the patient immediately before heparinisation / surgical start, and replaced with crystalloid or colloid to maintain normovolaemia
• Stored at room temperature in the operating theatre; re-infused at the end of surgery (post-CPB in cardiac surgery, after haemostasis in others)
• Provides fresh whole blood with functional platelets and coagulation factors, mitigating dilutional coagulopathy
• Contraindications: preoperative anaemia, severe coronary artery disease (e.g., high-grade left main), severe aortic stenosis, haemodynamic instability

2. Cell Salvage (Intraoperative Autologous Transfusion)

• Shed blood is suctioned from the surgical field → collected into a reservoir → washed and centrifuged → concentrated red cells (haematocrit up to 60–70%) re-infused to the patient
• Eliminates risks of allogeneic transfusion (infection, incompatibility, TRALI)
• Limitation: washing removes platelets and coagulation factors; supplemental FFP/platelets may still be needed
• Contraindicated when surgical field is contaminated with bowel contents, infection, or malignant cells (relative contraindication for cancer surgery; irradiation of salvaged blood has been used as a workaround)
• Widely used in cardiac, vascular, orthopaedic, and obstetric surgery

3. Antifibrinolytic Agents

• Tranexamic acid (TXA): binds plasminogen, blocking its ability to bind lysine residues on fibrinogen → prevents fibrinolysis. Associated with decreased post-CPB bleeding and transfusion requirements. Now widely used in trauma, obstetric, orthopaedic, and cardiac surgery
• Epsilon-aminocaproic acid (EACA): similar mechanism to TXA; less potent but also reduces perioperative bleeding

4. Retrograde Autologous Priming (RAP) — Cardiac Surgery

• Allows the patient's own blood to displace the crystalloid priming solution from the cardiopulmonary bypass (CPB) circuit in a retrograde manner before bypass
• Reduces CPB-related haemodilution and decreases extravascular lung water
• Can be combined with ANH for maximum effect
• Transient hypotension during RAP may require vasoconstrictors or Trendelenburg positioning

5. Minimally Invasive Extracorporeal Circulation (MiECC)

• Shorter CPB tubing and closed circuits reduce the volume of crystalloid prime required
• Decreases haemodilution and blood transfusion requirements in cardiac surgery

6. Modified Ultrafiltration (MUF)

• A semipermeable membrane is used (during or after CPB) to remove excess plasma water
• Results in haemoconcentration — increases haematocrit and concentrates clotting factors and platelets
• Also reduces circulating inflammatory mediators generated during CPB

7. Surgical Technique and Haemostasis

• Meticulous surgical technique to minimise blood loss
• Electrocautery and bipolar diathermy to reduce field bleeding
• Topical haemostatic agents: gelatin foam, oxidised cellulose, fibrin glue, thrombin-based agents
• Positioning to reduce venous pressure at the operative site (e.g., reverse Trendelenburg for head/neck surgery)
• Tourniquet use in limb surgery — reduces intraoperative blood loss dramatically

8. Controlled Hypotensive Anaesthesia

• Deliberate reduction of mean arterial pressure (MAP) to 50–65 mmHg during surgery reduces surgical field bleeding
• Reduces blood loss and transfusion requirements in spinal, ENT, orthopaedic, and plastic surgery
• Contraindicated in patients with cerebrovascular disease, ischaemic heart disease, renal impairment, or hypovolaemia

9. Haemodilution and Volume Management

• Permissive haemodilution with crystalloid/colloid maintains circulating volume while reducing the haematocrit — each mL of blood lost contains fewer red cells per mL
• Restrictive transfusion trigger: using a threshold of Hb ≤7–7.5 g/dL (rather than 10 g/dL) before transfusing pRBCs reduces unnecessary allogeneic blood use without increasing adverse outcomes

10. Point-of-Care Coagulation Monitoring (Goal-Directed Therapy)

• Thromboelastography (TEG) and Rotational Thromboelastometry (ROTEM) provide real-time, whole-blood assessment of clot formation, strength, and lysis
• Guides targeted component therapy (FFP, platelets, cryoprecipitate, fibrinogen concentrate) only when a specific defect is identified
• Avoids empirical "shotgun" transfusion of multiple components
• Associated with reduced total blood product usage and improved outcomes

11. Pharmacological Adjuncts

• Desmopressin (DDAVP): promotes release of vWF and factor VIII from endothelium; useful in patients with platelet dysfunction (uraemia, aspirin effect, type I von Willebrand disease)
• Recombinant Factor VIIa (rFVIIa): reserved for refractory surgical haemorrhage unresponsive to conventional therapy (licensed for haemophilia with inhibitors; off-label in massive surgical haemorrhage)
• Prothrombin Complex Concentrate (PCC): rapid reversal of warfarin and factor II, VII, IX, X deficiencies; faster to prepare than FFP
• Fibrinogen concentrate: when hypofibrinogenaemia is identified on TEG/ROTEM

12. Maintenance of Normothermia

• Hypothermia impairs platelet function and enzymatic coagulation cascade activity
• Warming of IV fluids, forced-air warming blankets, warm irrigation fluids, and minimising patient exposure all help prevent coagulopathy of hypothermia
• Core temperature should be maintained >36°C throughout the procedure

Summary Table: Blood Conservation Strategies