10. Breathing systems and airway devices.pdf

BREATHING SYSTEMS & AIRWAY DEVICES MIMV 3rd year – 1st semester 09 September 2024 Ricardo Felisberto, DVM, Dipl. ECVAA, MRCVS  INTRODUCTION Breathing systems deliver oxygen and inhalant anaesthetics, and eliminates expired CO2 by: One-wa...

BREATHING SYSTEMS & AIRWAY DEVICES MIMV 3rd year – 1st semester 09 September 2024 Ricardo Felisberto, DVM, Dipl. ECVAA, MRCVS  INTRODUCTION Breathing systems deliver oxygen and inhalant anaesthetics, and eliminates expired CO2 by: One-way valves (in rebreathing systems) Scavenging (in non-rebreathing systems, which are fresh gas flow (FGF) dependent to push the previously expired gases into the scavenging system – FGF must meet the minute ventilation of the patient) CO2 absorbents (in rebreathing system) Two main types of breathing systems: Non-rebreathing systems Rebreathing systems  INTRODUCTION What is meant by rebreathing? Strict definition is: when inspired gas reaching the alveoli contain more CO2 than can be accounted for by mere re-inhalation from the patient's anatomical dead-space gas. When applied to the breathing system, being a rebreathing system, doesn’t mean the patient will rebreathe CO2, it means rebreathing of previously exhaled gases without the carbon dioxide (which should be absorbed) What is fresh gas flow (FGF)? Gases administered to the patient. Contain O2 +/- Air, +/- nitrous oxide, and inhalant anaesthetic. Why do we need to administer Oxygen? A high fraction inspired oxygen (FiO2) is essential to administer in patients under general anaesthesia to prevent hypoxaemia. Under GA, patients hypoventilate, which increases the PaCO2 leading to O2 displacement in the alveoli, thus reducing O2 availability to oxygenate blood. In addition, during GA intra-pulmonary shunts develop, due to atelectasis (areas of lung not ventilated or collapsed, but still perfused); the blood perfusing these areas will NOT be oxygenated and will contribute to a drop in the PaO2 (hypoxaemia) Usually there is always some degree of shunt (10 – 15% of the total cardiac output = shunt; in horses under GA this can be 20 – 30% due to dome shape of diaphragm and enlarged abdominal viscera splinting the diaphragm against lungs) Thus, supplementation of Oxygen, improves the absorption of oxygen in those still functional areas of the lung. Minute ventilation (MV): INTRODUCTION MV = tidal volume x respiratory rate MV is usually 100 – 400 mL/kg/min (average is 200 mL/kg/min), because the tidal volume is usually 10 – 20 mL/kg, and the respiratory rate at rest is 10 – 20 breaths per minute. Remembering that 1/3 of this MV will occupy the anatomical dead-space (waste ventilation), and the remaining 2/3 will reach the alveoli to participate in the alveolar ventilation  Prof. Emeritus William Wellesley NON-REBREATHING SYSTEMS Mapleson Classified according to Mapleson A to F according to their efficiency regarding the fresh gas flow required to prevent rebreathing of CO2. (A – more efficient to F less efficient for spontaneous breathing) In veterinary medicine the most commonly used are A, D, E and F: Mapleson A: Magill; Lack Mapleson D: Bain Mapleson E: T-Piece FGF required to prevent FGF required to prevent CO2 Mapleson F: T-Piece with a bag Non-rebreathing CO2 rebreathing during rebreathing during positive system spontaneous ventilation pressure ventilation T-Piece (E) 2 – 4 x MV 1 – 2 x MV * Not recommended for prolonged PPV Bain (D) 2 – 4 x MV 1 – 2 x MV because the FGF to prevent CO2 Magill (A) 1 – 2 x MV 2 – 4 x MV* rebreathing would have to be very high Lack (A) 1 – 2 x MV 2 – 4 x MV*  NON-REBREATHING SYSTEMS Example for a 30kg dog: Bain during spontaneous breathing (2 – 4 x the minute ventilation) MV = 10 – 20 mL/kg x 10 – 20 breaths per minute MV = 300 – 600 mL x 10 – 20 bpm MV = 3000 to 12000 mL/min (3 to 12 L/min) Thus, to avoid rebreathing with the Bain = 6 to 48 L/min Bain during PPV (1 – 2 x the minute ventilation) Halves the fresh gas flow to 3 to 24 L/min (more efficient) This process is inverse for the Lack and Magill systems. Problems of using very high FGF: Excessive consumption of oxygen and other agents (++ environmental contamination) Excessive cooling of the patient and patient dehydration (even making the heat and moisture devices less efficient) Vaporisers output may not be accurate (because excessive FGF cools down the vaporising chamber, decreasing vaporisation process) Desiccation of carbon dioxide absorbents  NON-REBREATHING SYSTEMS Magill (Mapleson A): Tube volume should be greater than the patient’s tidal volume Tube length usually 1.1 to 1.5 m The pop-off valve adds resistance to the system, because patient has to generate enough volume to generate enough pressure to open up the valve for gas scavenging Ideally used in animals >10kg < 70-80kg FGF: 1 – 2 x MV for spontaneous breathing and 2 – 4 x MV for PPV Has some apparatus dead-space (from patient end to division between inspired and expired gases) Modest drag, because Pop-off valve is near the patient (can be cumbersome for dental or facial surgery) Easy to use, to clean, and to scavenge gases and vapours Not efficient for PPV. Apparatus dead-space  NON-REBREATHING SYSTEMS Magill spontaneous breathing:  NON-REBREATHING SYSTEMS Magill positive pressure ventilation:  NON-REBREATHING SYSTEMS Lack (Mapleson A): It exists in 2 forms (coaxial and parallel systems) FGF for spontaneous breathing 0.8 – 2 x MV Slightly more efficient than Magill as it offers less resistance to breathing Has less apparatus dead-space than the Magill For animals >10kg and < 70-80kg Less circuit drag (pop-off valve is away from the patient end, facilitating scavenging too) Not efficient for PPV, we would need to allow long expiratory pause to scavenge previously exhaled gases (must be used with capnography to evaluate rebreathing over time)  NON-REBREATHING SYSTEMS Mini-Lack (Mapleson A): A mini-parallel lack is available to be used in patients 10kg, 15 L/min = vaporiser output may be inaccurate; and may cause pneumothorax Must scavenge large volumes of gas Differences in PPV vs spontaneous ventilation efficiencies No rebreathable humidified warmed gases, which may lead to hypothermia and dehydration (which decreases the respiratory mucociliary escalator and high airway obstruction with dry secretions) If malignant hyperthermia develops, not enough increase in FGF is possible to avoid CO2 rebreathing.  REBREATHING SYSTEMS Have CO2 absorbent canister allow re-inhalation of previously exhaled gases (but without CO2). For the first 20 minutes, FGF > MV to allow denitrogenation (replacement of the nitrogen in the airways with administered gas mixture. After 20 minutes, FGF can be equal or lower than MV. Allows for different flows: Low flow: O2 FGF = O2 demand (4 – 10 mL/kg/min) Medium flow: O2 FGF > O2 demand < MV High flow: O2 FGF > O2 demand > MV O2 demand may be different from patient to patient, thus we consider low flow anaesthesia 10 mL/kg/min (e.g., 30 kg dog, low flow = 300 mL/kg/min) If O2 : Air mixture is used: 250 mL/min O2 + 250 mL/min Air → this gives a total of 500 mL/min, but of which 300 mL/min is O2 (the remaining 200mL/min is nitrogen).  REBREATHING SYSTEMS Low flow anaesthesia pros: Less utilization of oxygen, only what the body requires. Less waste of inhalant anaesthetics Less environmental impact Less loss of heat and moisture from the breathing system More cost-effective Low flow anaesthesia cons: Require gas analysers to evaluate the inspired and end-tidal agent’s concentration Vaporisers may not be as accurate at low flows Slower increase or decrease of inspired inhalant agent concentration Risk of overheating the breathing system and cause fire (with sevoflurane +++) Sidestream capnography removes gases from the system for analysis at a rate of 200 mL/min, which must be returned to the breathing system to maintain volume of gases, otherwise a negative pressure generates.  REBREATHING SYSTEMS To and Fro system Included for historical reasons; First built using Waters Canister to hold the CO2 absorbent, which is placed horizontally, allowing granules to settle, creating a small channel on top (path of least resistance) allowing gas to flow through there: This reduces the surface area of contact of the exhaled gases with the CO2 absorbent, reducing CO2 absorption efficiency and CO2 rebreathing → Channelling. Even if we prevent Channelling: The CO2 absorbent becomes exhausted first closer to the patient end; this increases apparatus dead-space. Pop-off valve is near the patient end Movement of gas in and out (reversing gas flow) creates inertia and high resistance to breathing, thus not suitable for smaller animals (mainly used in large animals) Resistance to breathing is created by: Reversing gas CO2 absorbent position Pop-off valve For animals > 15kg Rebreathing bag 2 – 6 x TV Has a large bulk and drag, and dust from the granules (airway irritant) may be inhaled  REBREATHING SYSTEMS Circle: Contains a vertical CO2 absorbent canister, which reduces chances of channelling The cannister should not be fully filled, so that exhaled gases enter at the top and slow down, this causes: ↓ resistance to gas flow. Allows more time for the exhaled gases to contact with the CO2 absorbent. Contain one-way valves, which allow unidirectional gas flow (into the patient’s lungs and outside of the patient’s lungs) These add resistance to the system as the animal’s respiratory efforts must be enough to move the valves. To use in animals > 10-15kg (there are new circle systems that can be used for animals > 7kg) Can be used in large animals, for which a larger canister and rebreathing bag should be used.  REBREATHING SYSTEMS Circle system functioning:  REBREATHING SYSTEMS Circle system leak test  REBREATHING SYSTEMS Universal F-Circuit: It works as a circle system It has a coaxial arrangement (inspiration limb inside expiratory limb) Decreases breathing system drag Helps conservation of heat in the system, due to heat transfer from previously exhaled gases into the inhaled gases.  REBREATHING SYSTEMS F-circuit leak test:  HYBRID SYSTEM Humphrey ADE: It combines the advantages of the Mapleson A for spontaneous ventilation; of the Mapleson D and E for positive pressure ventilation. It has also the possibility to behave as a circle system with CO2 absorbent canister, which is efficient for both spontaneous and mechanical ventilations. Has parallel limbs With narrow and smooth tubes and very low resistance pop-off valve Can be used in patients from 30g to 100 kg ADE system: 7-10kg  REBREATHING SYSTEMS CONSIDERATIONS Inhalant anaesthetic concentration Time constant Carbon dioxide absorbents  INHALANT ANAESTHETIC CONCENTRATION FGF is diluted by exhaled gases, which makes it difficult to know what are the inspired concentrations of oxygen, nitrous oxide and inhalant anaesthetics. Therefore, gases analyser is essential when using a circle breathing system  TIME CONSTANT Rate of change of inhalant anaesthetic concentration is inversely proportional to the FGF and proportional to the volume of the system. 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 Time constant = 𝐹𝐺𝐹 If the volume of the system is large = large time constant. If the FGF is high = time constant is low. After 1 time constant, the change in inhalant anaesthetic concentration is 63% complete; after 2-time constants, the change is 86.7% completed; after 3-time constants it is 95% completed, and after 5-time constants it is 99.3%. If we want a faster change in the IA concentration: Increase the FGF Increase the vaporiser setting If we want to decrease the IA concentration, empty the reservoir bag several times  CARBON DIOXIDE ABSORBENTS Contain the following components: Calcium hydroxide (80%) Activators: sodium hydroxide and/or potassium hydroxide React with carbonic acid to continue a series of reactions that allow absorption of CO2. Hardeners: silica (to reduce dust formation) The dust increases resistance to gas flow in the canister May also lead to caking of the granules (↑ channelling) Dust may enter the breathing system leading to malfunction of one-way valves and airway irritation and burns Water (14 – 20%) CO2 + H2O → H2CO3 Helps maintaining reactions going H2CO3 + 2NaOH → Na2CO3 + 2H2O + Heat pH indicator dye (changes colour when pH drops): Na2CO3 + Ca(OH)2 → CaCO3 + 2NaOH Ethyl violet: from white to purple once exhausted Clayton yellow: from pink to white As the granules absorb CO2 their pH decreases; at the end of an anaesthetic the granules may regenerate (changing to its original colour). This is due to the movement of the active chemicals (hydroxide ions) from the centre of the granules to the surface, thus increasing the pH and changing the colour to its original state. There are different absorbents in the market, being the difference the % of water content and type of activators (soda lime [most common], Baralyme) 1kg of soda lime absorbs 250L of CO2; 1kg of baralyme absorbs 270L of CO2.  CARBON DIOXIDE ABSORBENTS Signs of absorbent exhaustion: Colour change (most granules change colour) Caister doesn’t feel warm when used (depends on the FGF) Rebreathing of CO2 on capnography Hypercapnia (tachycardia, increased TV, increased respiratory rate, increased blood pressure, flushed mucous membranes)  REBREATHING SYSTEMS Advantages: Rebreathing of exhaled gases (except CO2), allowing conservation of heat and moisture in the system Economical (less use of oxygen and inhalant anaesthetics) Less waste and less pollution (allow low flow anaesthesia) Efficient spontaneous and positive pressure ventilation Disadvantages: Higher resistance (valves, canister) Bulky “Y” piece (drag) Higher apparatus dead-space High costs of the CO2 absorbent Inhalation of CO2 absorbent dust is possible Channelling is possible, which may allow CO2 rebreathing Complex, harder to clean Require time for denitrogenation There may occur Nitrous oxide build up which may create hypoxic gas mixtures Requires Inhale and exhaled gases analyser Does not make good use of high precision vaporisers Has high Time constant, thus the change of inhalant anaesthetic concentration is slow Leaks are likely to develop  HOW TO CHOOSE A BREATHING SYSTEM Patient size (consider resistance offered by the system; respiratory capability) Mode of ventilation (will positive pressure ventilation (PPV) be required or only spontaneous) Economy (do you want to use less oxygen and inhalant agents?) If you want low flow anaesthesia, are your flowmeters and vaporisers calibrated to be used at such low flows? Do you want to administer nitrous oxide – be sure to have a monitor that measures inhalant agents inhaled and exhaled concentrations particularly if using a rebreathing system, because nitrous oxide is administered at high concentrations and there is the risk of administering too little oxygen (hypoxic mixture) – gases measurement avoids this. Expected length of the procedures Do you require heat and moisture preservation (especially important in small animals) Do you need a sterile breathing system (consider disposable systems) Where is the surgery (excessive system drag may make surgery difficult) Ease of scavenging (if you are administering inhalant anaesthesia and don’t want to contaminate the operating theatre with the inhalant anaesthetics, be sure to have easy access to the pop-off valve to facilitate scavenging  SCAVENGING Occupational exposure standards (OES) in 1996 defined that for an 8h time weighted average (TWA) the limits are (from UK): N2O: 100 ppm Halothane: 10 ppm Isoflurane: 50 ppm Sevoflurane: 60 ppm To avoid reaching this limits, the use of efficient scavenging system is essential Two main types: Passive Active  SCAVENGING Passive: Waste gas is vented from the pop-off valve of the breathing system to: A ventilation shaft and out of the building Into an activated charcoal canister It has a positive pressure valve which opens at 10 cmH2O (if excessive pressure builds in the system – opens to room atmosphere) It has a negative pressure valve which opens at -0.5 cmH2O (if excessive suction pressure is generated in the scavenging system – opens to suction room air) If using activated charcoal canister it only absorbs inhalant anaesthetics, does not absorb N2O; N2O must be scavenged outside the building.  SCAVENGING Active: Requires an extractor fan / vacuum pump A connector is attached to the pop-off valve → corrugated tube goes to air brake → rigid hose connects to air brake and is attached to wall mount for scavenging → from wall mount to the vacuum pump → directs waste gas outside the building. It contains positive and negative pressure relief valves in cases of high suction demand (allows waste gas escape to the room) or very low demand (allows room air to be suctioned, to avoid excessive scavenge of the patient’s gases). Air-Brake: With Perspex window that allows visualisation of bobbin, which should be floating, indicating a scavenge flow rate of 120 mL/min. With filter and grill at the bottom that allows gas escape if a very large gas volume needs to be scavenged; or allows room air to enter in case of excessive scavenge (avoids suctioning of patient’s gas).  REDUCING IA AT WORKPLACE No induction or maintenance of anaesthesia with inhalant agents (use injectable agents) Use cuffed endotracheal tubes Low flow anaesthesia Avoid leaks Fill vaporisers in fume cupboard Active / passive scavenging systems Connect the breathing system to the endotracheal tube ad then turn on the inhalant agents Bung the breathing system at the end of the anaesthesia Well-ventilated room (15 air changes per hour) Monitor exposure to Inhalant agents regularly  AIRWAY EQUIPMENT Can be of rubber; silicone; plastic; PVC Can be armoured (reinforced with spiral metal to prevent kinking or compression) Can be cuffed or uncuffed or of the Cole pattern Cole tubes: with 6 flexible silicone baffles that create airway seal but allow gas to escape from the lungs if there is excessive pressure build-up. The number of the endotracheal tubes (ETT) indicate their internal diameter: The type of material and if they are or not cuff, determine their outer diameter The outer diameter is actually what determines if the tube fits in the trachea or not Recommendation → use the largest possible ETT without causing trauma, this is because: Larger diameter = allows higher gas flows and less resistance to breathing (remember Hagen-Poiseuille equation) Larger diameter = avoids the need to inflate the cuff too much to create a seal (this avoids tracheal wall damage due to excessive cuff distension) High flow if: High Pressure difference Hagen Poiseuille equation: High radius (r) 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑥 𝛑 𝑥 𝑟4 Flow = Low viscosity (𝛈) 8𝑥𝛈𝑥𝑙 Low length (l)  Tracheal bronchus AIRWAY EQUIPMENT The ETT positioning: Should not be advanced beyond the carina (otherwise = one lung ventilation, therefore, high risk of hypoxaemia and of lung injury due to overinflation) Should not be too much beyond the incisor arcade to avoid excessive apparatus dead-space Some species have a tracheal bronchus (pigs and cattle): This bronchus leaves from the trachea soon after the larynx, and may be covered by the ETT cuff ETT should have a Murphy eye, which is an opening at the tip of the ETT that avoids complete obstruction of the ETT if the centre of the tip of the tube gets obstructed with secretions or a clot. Silicone rubber cuffed tube Red rubber Magill cuffed tube PVC cuffed tube with Murphy eye PVC uncuffed tube Cole pattern tube  AIRWAY EQUIPMENT How to choose the correct ETT size?: The ETT size is important as according to the Hagen-Poiseuille equation, halving the radius decreases air flow by 16-fold! If too small can also be an issue of not being able to reach a seal (environmental pollution; inadequate provision of ventilation) Methods: Body mass + length of fourth digital pad Digital palpation of the trachea (width) + assess nasal septum width But most common is individual experience!  AIRWAY EQUIPMENT Supraglottic Airway Devices (SGADs): Laryngeal mask (LMA) is a tube connected to a soft inflatable cushion that provides peri-pharyngeal seal May however lead to laryngospasm, so laryngeal desensitisation is essential Designed specifically for rabbits and cats (V-gel®) V-gel: Soft, anatomically-shaped silicone tube that seals the airway and blocks the proximal oesophagus In cats, there is a cuffed version which improves the airway seal Come in 6 sizes for cats Also allow positive pressure ventilation up to a peak inspiratory pressure of 15cmH2O Capnography is essential and is the gold standard to confirm correct placement (for both ETT and SGADs)  AIRWAY EQUIPMENT Cuffs: Inflatable sleeves at the distal part of the ETT end Should inflate symmetrically Should not herniate Provide air-tight seal between the tube and the tracheal wall (avoid aspiration of gastric contents, and avoid gas leaks) Helps maintaining the tube in place at the centre of the trachea Cats: Cuffs may damage tracheas and cause pneumoediatinum; becay-use they have a delicate dorsal tracheal ligament If moving the intubated cat, ALWAYS disconnect the breathing system from the ETT before moving. Uncuffed tubes may be better in cats (as we can possible use a larger internal diameter tube, but leaks may exist ETT has been shown to increase mortality in cats (CEPSAF 2008) – possibly due to tracheal damage / laryngeal spasm  AIRWAY EQUIPMENT Types of ETT cuffs: High volume / low pressure (plastic) Low volume /high pressure (rubber) Medium volume / medium pressure (silicone) Low volume / high pressure (Low profile cuff) High volume / low pressure (high profile cuff) Less bulky (less damage to larynx on intubation) Bulky (more damage to the larynx on intubation) Smooth inflation, but may have asymmetrical Smooth symmetrical inflation but may form wrinkles inflation Distort tracheal contour Conform to the tracheal contour Small area of contact with the trachea (focal Large area of contact with the trachea (less damage necrosis) to trachea) No Wrinkles development (good seal) Inflation with wrinkles (not so good seal) Intra-cuff pressure measurement – does not represent Intra-cuff pressure measurement – represents the the pressure against tracheal wall pressure against tracheal wall  AIRWAY EQUIPMENT Laryngoscopes: Macintosh Miller Macintosh modified (McCoy) Polio Macintosh  PROBLEMS WITH AIRWAY DEVICES Malposition (in oesophagus, in main bronchus) Injury to the larynx / trachea (e.g., cuff pressure too high) Laryngospasm (cats = pigs > Llamas = Alpacas > small ruminants) Autonomic nervous system stimulation (tachycardia, hypertension or vagal reflex) → have ECG and NIBP monitoring during endotracheal intubation Increased intraocular pressure and intracranial pressure (gagging, coughing) Leak / disconnection of the ETT Extubation / tube kinking Epiglottic retroversion on blind ET intubation in horses Bitten ETT / Broken ETT that may end up in the airway / stomach Occlusion with secretions / blood clots Collapse of the ETT if too much intracuff pressure Tracheal rupture if moving animal without disconnecting ETT from breathing system (cats ++++) Supraglottic airway device displacement (airway obstruction) Laryngeal mask with cuff may cause tongue compression and cyanosis Neuropraxia (lingual nerve; glossopharyngeal nerve; recurrent laryngeal nerve) due to mechanical trauma / high cuff pressures Spring load mouth gags in cats lead to wide jaw opening, and if left for too long may cause transient or permanent blindness due to compression of the maxillary artery V-gels in rabbits may get displaced