Questions and Answers
What is the formula for calculating minute ventilation (MV)?
What is the average minute ventilation (MV) value in mL/kg/min?
Which portion of minute ventilation occupies anatomical dead space?
Which Mapleson classification is most efficient for spontaneous breathing?
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What is the Fresh Gas Flow (FGF) required to prevent CO2 rebreathing in spontaneous ventilation using a Bain system?
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Which non-rebreathing system is least efficient for preventing CO2 rebreathing during positive pressure ventilation?
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For a 30kg dog, if the minute ventilation is 3000 to 12000 mL/min, what is the corresponding FGF for the Bain system during positive pressure ventilation?
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What happens to the FGF requirements for preventing CO2 rebreathing in the Lack system compared to the Bain system during spontaneous ventilation?
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What is the primary function of one-way valves in rebreathing systems?
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Which statement about fresh gas flow (FGF) is true?
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What is the relationship between the flow of gas flow (FGF) and the time constant for inhalant anesthetic concentration changes?
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What is the definition of rebreathing in the context of respiratory mechanics?
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Why is a high fraction inspired oxygen (FiO2) essential during general anaesthesia?
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What is the significance of using a gas analyser in a circle breathing system?
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Which component is NOT part of carbon dioxide absorbents?
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In which breathing system is scavenging utilized?
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How is the rate of change in inhalant anesthetic concentration described mathematically?
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What happens to PaCO2 levels under general anaesthesia?
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Which factor can contribute to intra-pulmonary shunts during general anaesthesia?
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What action would you take to decrease the inhalant anesthetic concentration in the system?
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What is a common shunt percentage in horses under general anaesthesia?
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What is one sign indicating the exhaustion of a carbon dioxide absorbent?
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Which of the following is a disadvantage of rebreathing systems?
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What is a key consideration when choosing a breathing system based on patient size?
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How does a rebreathing system conserve resources during anesthesia?
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Which condition may lead to hypoxic gas mixtures in a rebreathing system?
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What is a potential risk associated with the use of carbon dioxide absorbent in rebreathing systems?
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Which of the following describes a characteristic of rebreathing systems?
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What can happen if channelling occurs in a rebreathing system?
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What is the primary function of a scavenging system in anaesthesia?
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Which type of scavenging system does not absorb nitrous oxide (N2O)?
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What is the main advantage of using an active scavenging system?
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At what pressure does the positive pressure valve of a passive scavenging system open?
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What is the purpose of the air-brake in an active scavenging system?
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Which of the following is a feature of a passive scavenging system?
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What is the maximum allowed occupational exposure limit for halothane during an 8-hour time-weighted average?
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What occurs if excessive suction pressure is generated in a passive scavenging system?
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What is the role of silica in carbon dioxide absorbents?
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What pH indicator dye changes from pink to white as it becomes exhausted?
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How many liters of CO2 can be absorbed by 1kg of Baralyme?
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Which reaction maintains reactions going by producing carbonic acid?
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What happens to the pH of the granules as they absorb CO2?
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What is indicated by a colour change in carbon dioxide absorbents?
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Which of the following is an advantage of using a rebreathing system?
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Which factor should be considered when choosing a breathing system based on the mode of ventilation?
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Which of these factors represents a disadvantage of rebreathing systems?
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What could potentially lead to hypoxic gas mixtures in a rebreathing system?
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What is a direct consequence of channelling in a rebreathing system?
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Which sign indicates that a CO2 absorbent may be exhausted?
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Which of the following factors can complicate the use of rebreathing systems?
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What is the primary consequence of using a cuff that herniates on an endotracheal tube (ETT)?
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What characteristic of the laryngeal mask airway (LMA) poses a risk during its usage?
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How does the optimal size of an endotracheal tube (ETT) affect airflow according to the Hagen-Poiseuille equation?
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What method is commonly used to determine the appropriate size for an endotracheal tube (ETT)?
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Which feature of cuffs on endotracheal tubes is crucial for avoiding aspiration of gastric contents?
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What is a significant risk when using cuffs on endotracheal tubes in cats?
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What is the maximum peak inspiratory pressure that V-gel allows during positive pressure ventilation in cats?
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What is the primary purpose of capnography in relation to airway devices?
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What is one potential issue when using very high Fresh Gas Flow (FGF)?
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In the Magill system, what feature adds resistance during gas scavenging?
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Which of the following is a characteristic of the Lack system compared to the Magill system?
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What is a potential disadvantage of using uncuffed tubes in cats?
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What is a risk associated with the Mini-Lack system?
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Which type of endotracheal tube cuff is characterized by less bulk and reduced risk of laryngeal damage during intubation?
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Why is the Magill system considered easy to use?
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What can occur if cuff pressure is too high during the use of airway devices?
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What complication can arise from using very high FGF related to vaporizers?
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What is a potential consequence of using spring load mouth gags in cats if left for too long?
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What is one parameter that indicates the efficiency of gas scavenging in the Lack system during positive pressure ventilation?
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Which symptom may indicate stimulation of the autonomic nervous system during endotracheal intubation?
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Which size of animal is the Lack non-rebreathing system ideally used for?
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What is the primary risk of using a laryngeal mask with a cuff during intubation?
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Which complication can arise from movement of an animal while the endotracheal tube is still connected to the breathing system?
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How do high volume / low pressure cuffs behave during inflation compared to low volume / high pressure cuffs?
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Study Notes
Introduction to Breathing Systems
- Breathing systems supply oxygen and inhalant anesthetics, while removing expired CO2.
- Mechanisms include one-way valves in rebreathing systems, scavenging in non-rebreathing systems, and CO2 absorbents.
- Two main types of breathing systems: Non-rebreathing and Rebreathing systems.
- Rebreathing: Defined as inspired gas with higher CO2 than can be explained by anatomical dead-space; does not imply CO2 re-inhalation.
- Fresh Gas Flow (FGF): Gases delivered to the patient, essential for preventing hypoxia during general anesthesia.
Importance of Oxygen Supplementation
- High fraction inspired oxygen (FiO2) prevents hypoxemia; patients under general anesthesia may hypoventilate, leading to increased PaCO2 and reduced oxygen availability.
- Intra-pulmonary shunts increase during anesthesia; 10-15% shunting is normal, but can reach 20-30% in horses due to diaphragm shape.
Minute Ventilation (MV)
- MV is calculated as tidal volume multiplied by respiratory rate; typical range is 100-400 mL/kg/min, with an average of 200 mL/kg/min.
- One-third of MV occupies anatomical dead-space, while two-thirds contributes to alveolar ventilation.
Non-Rebreathing Systems
- Classified as Mapleson A to F based on efficiency of FGF in preventing CO2 rebreathing (A being most efficient, F least).
- Common systems in veterinary medicine include Mapleson A (Magill, Lack), D (Bain), E (T-Piece), F (T-Piece with bag).
- Fresh gas flow requirements vary:
- Bain and T-Piece need 2-4 x MV for spontaneous ventilation and 1-2 x MV for positive pressure ventilation.
- Magill and Lack require 1-2 x MV for spontaneous and 2-4 x MV for positive pressure ventilation.
Rebreathing System Considerations
- Factors include inhalant anesthetic concentration, time constant, and carbon dioxide absorbents.
- A gas analyzer is critical for verifying inspired concentrations of gases in circle systems.
Time Constant
- Time constant inversely related to FGF; a larger system volume results in a higher time constant.
- After certain time constants (1-3), inhalant anesthetic changes approach completion (63%-95%).
- Faster concentration changes can be achieved by increasing FGF or vaporizer settings.
Carbon Dioxide Absorbents
- Composed mainly of calcium hydroxide (80%) with sodium or potassium hydroxide as activators.
- Exhaustion signs: color change of granules, cool canister, rebreathing CO2 on capnography, symptoms of hypercapnia.
Advantages and Disadvantages of Rebreathing Systems
- Advantages: Conservation of heat and moisture, cost-effective, reduced waste, efficient ventilation.
- Disadvantages: Higher resistance due to valves, complex system, risk of CO2 rebreathing, requires time for denitrogenation, potential for nitrous oxide buildup.
Factors in Choosing a Breathing System
- Consider patient size, mode of ventilation needed, economic factors, length of procedures, and necessity for heat/moisture preservation.
- Must ensure calibration of flowmeters and vaporizers for low-flow anesthesia and possess monitoring for inhalant agents.
Scavenging Systems
- Occupational Exposure Standards define limits for anesthetic gases; efficient scavenging systems are essential.
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Types of scavenging:
- Passive: Vents waste gases through the system to the environment or activated charcoal; utilizes pressure valves for safety.
- Active: Requires extractor fan or vacuum pump; connects to the breathing system and directs waste gas outside; features pressure relief valves.
Active Scavenging System Components
- Air-brake with visual indicators to ensure appropriate scavenge flow; includes filters and protective mechanisms to prevent excessive suction of patient gases.
Problems of Using Very High Fresh Gas Flow (FGF)
- High FGF leads to excessive oxygen and agent consumption, increasing environmental contamination.
- Patients may experience excessive cooling and dehydration, diminishing the efficiency of heat and moisture devices.
- Vaporiser output accuracy decreases with high FGF due to cooling of the vaporising chamber.
- Carbon dioxide absorbents can become desiccated, affecting efficacy.
Non-Rebreathing Systems
Magill (Mapleson A)
- Tube volume must exceed the patient’s tidal volume, with an ideal length between 1.1 to 1.5 m.
- The pop-off valve causes resistance, requiring patients to generate adequate volume and pressure for gas scavenging.
- Suitable for animals weighing between 10 kg and 70-80 kg.
- FGF should be 1-2 times minute volume (MV) for spontaneous breathing and 2-4 times MV for positive pressure ventilation (PPV).
- Features added dead-space between inspired and expired gases, adding modest drag due to valve proximity to the patient.
- Simple to use and clean, effective for gas scavenging, but not optimal for PPV.
Lack (Mapleson A)
- Available in coaxial and parallel systems, with FGF for spontaneous breathing at 0.8-2 times MV.
- Slightly more efficient than Magill due to reduced breathing resistance and less dead-space.
- Also meant for animals between 10 kg and 70-80 kg, with reduced circuit drag.
- Not effective for PPV; requires prolonged expiratory pauses and capnography to monitor rebreathing.
Mini-Lack (Mapleson A)
- Designed for patients weighing up to 10 kg, requires high FGF and can risk pneumothorax.
- Low availability of humidified warmed gases can lead to hypothermia and dehydration, impairing airway function.
- In cases of malignant hyperthermia, high FGF may not suffice to prevent CO2 rebreathing.
Carbon Dioxide Absorbents
- Common absorbents include soda lime (250 L CO2 absorbed per kg) and Baralyme (270 L CO2 absorbed per kg).
- Signs of absorbent exhaustion include color change, inadequate warmth from the canister, and hypercapnia symptoms like tachycardia and increased respiratory rate.
- Chemical reactions occur during CO2 absorption, changing the absorbent’s pH and color.
Rebreathing Systems
- Advantages include conserving heat and moisture, reducing oxygen and anesthetic usage, and providing efficient ventilation.
- Disadvantages involve increased resistance, expense of CO2 absorbents, potential inhalation of absorbent dust, high dead-space, and the need for analyzer equipment.
Choosing a Breathing System
- Consider patient size, mode of ventilation (spontaneous or PPV), and economy regarding oxygen and anesthetic use.
- Ensure flowmeters and vaporisers can be calibrated for low flow during anesthesia.
- Monitoring inhalant gas concentrations becomes crucial, particularly with nitrous oxide use.
Airway Equipment
Endotracheal Tube (ETT) Sizing
- Selecting the correct ETT size is vital; undersized tubes can limit airflow and seal integrity.
- Sizing methods include assessing body mass and digital palpation of trachea width.
Supraglottic Airway Devices (SGADs)
- Laryngeal mask (LMA) provides a pharyngeal seal, may induce laryngospasm requiring desensitization.
- V-gel® designed for rabbits and cats, offering anatomical sealing and ventilation capabilities.
- Capnography is essential to confirm correct device placement.
ETT Cuffs
- Cuffs must inflate symmetrically, provide an airtight seal, and prevent aspiration.
- They maintain the tube position in the trachea; in cats, uncuffed tubes may be preferable to minimize trauma.
Types of ETT Cuffs
- High volume/low pressure cuffs are less bulky but risk laryngeal damage; low volume/high pressure cuffs may offer a better seal but can damage trachea due to higher pressures.
Issues with Airway Devices
- Common problems include misplacement, injury from cuff pressure, laryngospasm risk, and increased intraocular and intracranial pressures.
- Potential complications involve leakages, occlusion, and tracheal rupture during movement.
- Careful monitoring (NIBP and ECG) is necessary to manage autonomic reactions during intubation.
Laryngoscopes
- Types include Macintosh, Miller, McCoy (modified Macintosh), and Polio Macintosh. Each design assists in ETT placement.
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Description
This quiz covers the fundamentals of breathing systems and airway devices, focusing on their roles in delivering oxygen and anaesthetics, along with the mechanics of gas exchange and scavenging systems. Ideal for MIMV students in their third year, this quiz will help reinforce key concepts and applications in veterinary anaesthesia.
