VT131 Anesthetic Equipment

EVOLUTION AND FUNDAMENTALS OF ANESTHETIC EQUIPMENT

• Historical Administration Methods:
      * Before modern machines, inhalant administration was hazardous.
      * Open systems used up until the early 20th century: "open cone," "open drop," or chamber induction.
      * Agents used: ether and chloroform.
      * Limitations: Crude control of anesthetic depth, inability to protect the airway, lack of supplemental $O_2$, and impossible to assist ventilation.
• Modern Equipment Benefits:
      * Allows precise delivery of concentrations of anesthetic and $O_2$.
      * Increases safety and efficacy during controlled conditions.
      * Essential knowledge for technicians includes function, maintenance, and prevention of malfunctions.

ENDOTRACHEAL TUBES AND ASSOCIATED EQUIPMENT

• Definition & Purpose:
      * An endotracheal tube (ET tube) is a flexible tube placed in the trachea to transfer gases directly from the machine to the lungs.
      * Functions: Bypassing oral/nasal cavities, pharynx, and larynx; maintaining an open airway; decreasing anatomic dead space; preventing pulmonary aspiration (stomach contents, blood); allowing precise administration of $O_2$ and gas; enabling emergency respiratory response; and permitting accurate monitoring.
• Tube Types:
      * Murphy Tubes: Feature a beveled end and a side hole called the "Murphy eye." May be cuffed or uncuffed.
      * Cole Tubes: No cuff or side hole. Designed with an abrupt decrease in diameter near the patient end. Used for species with complete tracheal rings (birds, some reptiles) to prevent tracheal damage.
• Construction Materials:
      * Polyvinyl Chloride (PVC): Transparent and stiff. Stiffness minimizes collapse risk but increases risk of tracheal mucosa trauma during intubation or patient movement.
      * Red Rubber: Flexible and less traumatic. Disadvantages: Prone to kinking/collapse, absorbs disinfectant solutions (irritating tissues), and tends to crack over time.
      * Silicone: Expensive but combines strength with pliability. Resistant to collapse and less irritating than rubber or vinyl.
      * Spiral/Anode Tubes: Contain a metal or nylon coil embedded in the wall to resist kinking from external pressure.
• Sizing and Scaling:
      * Tube size is expressed as Internal Diameter (ID) in millimeters ($mm$). Range: $1.0\,mm$ (exotics) to $30.0\,mm$ (large animals).
      * Standard lengths feature a scale in centimeters ($cm$) from the patient end.
• Endotracheal Tube Parts:
      * Patient End: Beveled to facilitate placement.
      * Murphy Eye: Minimizes asphyxiation risk if the end hole is blocked by mucus.
      * Machine End: Connects to the breathing circuit via a connector.
      * Cuff: Balloon-like structure inflated to create a tracheal seal.
      * Pilot Line & Pilot Balloon: Connect the cuff to a valve used for inflation; allows the anesthetist to monitor cuff inflation status.
• Cuff Types:
      * High Volume/Low Pressure: Distribute pressure evenly along a longer length of the cuff. Preferred for minimizing tissue damage.
      * Low Volume/High Pressure: Exert high pressure on a small area; higher risk of tracheal mucosa necrosis or rupture.
• Safety Guidelines for Intubation:
      * Dead Space: Tube should be no longer than the distance from the most rostral aspect of the mouth to the thoracic inlet.
      * Risks of Excess Length: If too deep, it may enter a single mainstem bronchus leading to hypoventilation/hypoxemia. If too long outside the mouth, it increases mechanical dead space.
      * Tolerance: Tube should not be more than $2.5\,cm$ longer than the mouth-to-thoracic-inlet distance.
      * Laser Surgery: Requires special laser-resistant tubes or wrapping with FDA-approved materials. Cuffs may be filled with saline instead of air to prevent ignition.

THE ANESTHETIC MACHINE SYSTEMS

• Four Primary Systems:
      1. Compressed Gas Supply: Supplies carrier gases ($O_2$, sometimes $N_2O$).
      2. Anesthetic Vaporizer: Vaporizes liquid agent and mixes it with carrier gas.
      3. Breathing Circuit: Conveys gases to the patient and removes $CO_2$.
      4. Scavenging System: Disposes of waste anesthetic gases.
• Principles of Operation:
      * Carrier gas delivered at a controlled rate via flowmeters.
      * Precise concentration of liquid anesthetic (Isoflurane or Sevoflurane) vaporized.
      * $CO_2$ removed via absorption or non-rebreathing venting.

COMPRESSED GAS SUPPLY AND SAFETY

• Gas Cylinders (Tanks):
      * Gases stored under high pressure (up to $2,200\,psi$ or $15,000\,kPa$).
      * E Tanks: Small tanks attached directly to the machine yoke or stored on carts ($660\,L$ capacity for $O_2$).
      * H Tanks: Large tanks stored remotely and connected via intermediate gas lines ($6,900\,L$ capacity for $O_2$).
      * Color Coding:
          * $O_2$: Green (US) or White (International).
          * $N_2O$: Blue.
          * Medical Air: Yellow (US) or White/Black (International).
          * $CO_2$: Gray.
• Valves and Regulators:
      * Outlet Port: Gas exits here.
      * Pressure-Reducing Valve: Reduces tank pressure to a constant safe operating pressure of $40$ to $50\,psi$ ($275$ to $345\,kPa$).
      * Tank Pressure Gauge: Indicates pressure remaining in the cylinder. Oxygen volume calculation:
          * E Tank: $psi \times 0.3 = \text{Liters}$.
          * H Tank: $psi \times 3 = \text{Liters}$.
      * Changing Threshold: Tanks should be changed when pressure drops below $500\,psi$ ($3,400\,kPa$).
• Flowmeters:
      * Vertical glass cylinders that reduce gas pressure to $15\,psi$.
      * Readings: Center of a ball indicator or top of a bobbin indicator.
      * Maintenance: Turn off clockwise gently; do not overtighten.
• Oxygen Flush Valve:
      * Delivers pure $O_2$ at $35$ to $75\,L/min$.
      * Usage: Filling reservoir bag, diluting anesthetic, or emergency oxygen.
      * Warning: Do not use with non-rebreathing systems unless directed; high flow can cause pulmonary barotrauma.
• Safety Handling:
      * Keep away from ignition sources; prevent cylinders from falling (potential "torpedo" effect); use Pin Index Safety System (PISS) for E-tanks and Diameter Index Safety System (DISS) for quick-release connectors.

ANESTHETIC VAPORIZERS

• Function: Converts high vapor pressure liquids into gases.
• Categories:
      * Precision Vaporizers: Required for high vapor pressure agents (Isoflurane, Sevoflurane, Desflurane). Deliver precise % concentrations regardless of flow, temperature, and back pressure.
      * Non-precision Vaporizers: Used previously for low vapor pressure agents like Methoxyflurane.
• Circuit Location:
      * VOC (Vaporizer-Out-Of-Circuit): Standard for precision vaporizers; located outside the breathing circle.
      * VIC (Vaporizer-In-Circuit): Located within the breathing circle where exhaled gases enter the vaporizer. Only for non-precision units with low resistance.
• Factors Affecting Output:
      * Temperature: Liquids vaporize faster at higher temps.
      * Carrier Gas Flow: High flows may decrease anesthetic % if the vaporizer is not compensated.
      * Back Pressure: Manual ventilation (bagging) can cause fluctuations in non-compensated models.
• Maintenance: Should be kept at least half full; do not overfill. If tipped, must be flushed with and $O_2$ at high flow for $15\,min$ with the dial off.

BREATHING CIRCUITS

Rebreathing (Circle) Systems

• Definition: Circular flow where exhaled gases (minus $CO_2$) are recirculated. Suitable for patients > 7.0\,kg (or > 2.5\,kg with pediatric hoses).
• Components:
      * Unidirectional Valves: Control one-way flow.
      * Pop-off (APL) Valve: Vents excess gas and prevents pressure build-up. Closed only for manual ventilation.
      * Carbon Dioxide Absorber: Contains granules (Calcium Hydroxide). Chemical reaction produces heat and water. Granules must be replaced after $6$ to $8\,hours$ of use or when colors change (usually to violet).
      * Reservoir Bag: Calculated as $50\,mL/kg$ (or $5\,times$ the tidal volume, $V_T$).
          * Guidelines: $500\,mL$ (up to $3\,kg$); $1\,L$ ($4$-$7\,kg$); $2\,L$ ($8$-$15\,kg$); $3\,L$ ($16$-$50\,kg$).
      * Pressure Manometer: Measures circuit pressure in $cm\,H_2O$.
          * Safe spontaneous breathing: $0$ to $2\,cm\,H_2O$.
          * Max manual ventilation pressure: $20\,cm\,H_2O$ (small animals), $40\,cm\,H_2O$ (large animals).
• Classification:
      * Closed System: Pop-off closed; flow meets metabolic needs ($5$ to $10\,mL/kg/min$). High risk of $CO_2$ buildup.
      * Semiclosed System: Pop-off partially open; higher flows ($20$ to $40\,mL/kg/min$ maintenance).

Non-Rebreathing Systems

• Definition: Little/no exhaled gas is returned to the patient. Required for patients < 2.5 to $3.0\,kg$ due to low resistance.
• Configurations (Mapleson Classification):
      * Magill (Mapleson A): Overflow valve at patient end.
      * Bain Coaxial (Modified D): "Tube-within-a-tube"; incoming gas warmed by exhaled gas.
      * Jackson-Rees (Mapleson F): Fresh gas inlet at patient end, bag at opposite end.
      * Ayre’s T-piece (Mapleson E): No reservoir bag.
• Pros/Cons: Fast changes in depth and lightweight, but high cost/waste and low heat/moisture conservation.

ANCILLARY AIRWAY EQUIPMENT

• Laryngoscopes:
      * Components: Handle, blade, and light source.
      * Blade types: Miller (straight) and McIntosh (curved).
      * Sizes: $0$ (small) to $5$ (large); large animal blades up to $18\,inches$.
• Supraglottic Airway Devices (SADs):
      * Create a seal around the glottis without invading the trachea. Example: v-gel (autoclavable up to $40\,times$). Reduces laryngospasm and irritation.
• Anesthetic Masks: Cone-shaped devices for oxygen or gas; used for induction or Maintenance in very small patients (birds, exotics).
• Anesthetic Chambers: Acrylic boxes for inducing feral or aggressive patients. Hard to monitor; risk of personnel exposure.

CLINICAL PROCEDURES AND OXYGEN FLOW RATES

• Dosage Formula for Injectables:
      * $\text{Volume (mL)} = \frac{\text{Weight (kg)} \times \text{Dose (mg/kg)}}{\text{Concentration (mg/mL)}}$
• Recommended Oxygen Flow Rates (Small Animals):
      * Mask Induction: $1$ to $3\,L/min$ ($\le 10\,kg$); $3$ to $5\,L/min$ (> 10\,kg).
      * Chamber Induction: $5\,L/min$.
      * Rebreathing (Semiclosed):
          * Maintenance: $20$ to $40\,mL/kg/min$.
          * Induction/Recovery/Changes: $50$ to $100\,mL/kg/min$ ($\text{Max } 5\,L/min$).
      * Non-Rebreathing: $100$ to $300\,mL/kg/min$ (Bain) or $2$ to $3\,times\,RMV$.
• Equipment Testing:
      * High-Pressure Test: Checks cylinder-to-flowmeter lines.
      * Low-Pressure Test: Checks flowmeter-to-Y-piece. System should hold $30\,cm\,H_2O$ for $10\,seconds$.

MONITORING AND ASA CLASSIFICATION

• ASA Physical Status Classes:
      * ASA I: Normal, healthy (Elective OHE/Castration).
      * ASA II: Low risk; mild systemic disease (Geriatric, neonatal, obese, mild dehydration).
      * ASA III: Moderate risk; severe systemic disease (Anemia, compensated organ disease).
      * ASA IV: High risk; systemic disease is a constant threat to life (Ruptured bladder, pyometra).
      * ASA V: Extreme risk; moribund, not expected to survive (GDV, severe trauma).
      * ASA E: Emergency (added to any class).
• Vital Signs Monitoring:
      * Circulation: HR, rhythm, MM color, CRT (target $< 2\,seconds$), Pulse strength, Blood pressure.         * Mean Arterial Pressure (MAP) target: $> 60\,mmHg$ (Small animal), > 70\,mmHg (Equine).
      * Oxygenation: MM color (Cyanosis indicates late-stage deoxygenation), Pulse Oximetry (SpO_2 > 95\%).
      * Ventilation: RR (target $8$ to $20\,bpm$ in small animals), $V_T$, Capnography ($ETCO_2 \text{ target } 35$ to $45\,mmHg$).
• Reflexes:
      * Palpebral: Absent in surgical plane.
      * Swallowing: Must be present for safe extubation.
      * Pedal: Absent in surgical plane.

QUESTIONS & DISCUSSION

• Case 4-1 (Molly the Cat):
      * Scenario: A 3.5-kg cat on a Bain circuit shows labored breathing and gray mucous membranes despite oxygen being on. Lung sounds are normal.
      * Possible Causes: Kinked internal tube of the Bain coaxial circuit, exhausted oxygen supply undetected by lack of flowmeter check, or disconnected scavenger causing backpressure.
      * Discussion: How do you test the internal hose of a Bain circuit? Answer: Occlude the patient end and observe the oxygen flowmeter bobbin; it should drop to zero if the internal tube is intact.
• Case 9-1 (Caesar the German Shepherd):
      * Scenario: A 42-kg dog on 2.5% isoflurane suddenly moves and has an HR of 170 and RR of 40.
      * Question: What should be done first?
      * Answer: Check the ET tube for placement and the vaporizer for liquid level. If equipment is fine, check the seal of the cuff and adjust the vaporizer up to a higher setting while increasing oxygen flow to speed the change in concentration.
• Discussion on Laryngospasm (Cats/Swine):
      * Why is it dangerous? It can lead to hypoxia and cyanosis.
      * Prevention: Apply $0.1\,mL$ of $2\%$ lidocaine to the glottis; ensure adequate depth before intubation.
• ASA Monitoring Guidelines:
      * Recommendations for sedation: Ensuring oxygenation and hemodynamic stability. Brachycephalic breeds are at higher risk under heavy sedation.