Anesthesia Breathing Systems Anesthesia Breathing Systems Purpose To deliver anesthetic gases and oxygen Offer a means to deliver anesthesia without significant increase in airway resistance To offer a convenient and safe method of delivering inhaled anesthetic agents Anesthesia Breathing Systems Basic Principles All anesthesia breathing systems have 2 fundamental purposes Delivery of O2/Anesthetic gases Elimination of CO2 All breathing circuits create some degree of resistance to flow Anesthesia Breathing Systems Resistance to flow can be minimized by: Reducing the circuit’s length Increasing the diameter (who’s law is that??) Hagen-Poiseuille P = (L)(v)(V) r4 P is pressure gradient. L is length. v is viscosity. V is flow rate RESISTANCE IS INDIRECTLY PROPORTIONAL TO FLOW RATE WITH LAMINAR FLOW Flow = P1-P1/R where P1 is pressure at one end of a tube and P2 is pressure at the other end of the tube FOR TURBULENT FLOW, GAS DENSITY IS MORE IMPORTANT THAN VISCOSITY RESISTANCE IS PROPORTIONAL TO THE “SQUARE” OF FLOW RATE (TURBULENT FLOW) IN CLINICAL PRACTICE, FLOW IS USUALLY A MIXTURE OF LAMINAR & TURBULENT FLOW Avoiding the use of sharp bends (turbulent flow) Eliminating unnecessary valves Maintaining laminar flow Another look at Poiseuille’s Law Laminar flow: orderly movement of gas inside a “hose” (gas in the center of the tube moves faster than gas closer to walls) Turbulent flow: resistance is increased (seen with sudden narrowing or branching of tube) Poiseuille’s Law follows Laminar flow R = 8 n l (R: resistance, n: viscosity, l: length, r: radius) r4 Example: doubling the radius of the tube will decrease the resistance 16 times (2)4=16 Anesthesia Breathing Systems Classifications (controversial) Traditional attempts to classify circuits combine functional aspects (eg, extent of rebreathing) with physical characteristics (eg, presence of valves) Based A on the presence or absence of gas reservoir bag provides gas for the moments during inspiration where flow in the trachea is greater than fresh gas flow (FGF) Rebreathing Means of exhaled gases to chemically neutralize CO2 Unidirectional valves Anesthesia Breathing Systems Classifications Open Semiopen Semiclosed Closed Anesthesia Breathing Systems Anesthesia Breathing Systems Classifications Open NO reservoir NO rebreathing No neutralization of CO2 No unidirectional valves Examples include Nasal Cannula Open drop ether Anesthesia Breathing Systems Classifications Semiopen Gas reservoir bag present NO rebreathing No neutralization of CO2 No unidirectional valves Fresh gas flow needed exceeds minute ventilation (two to three times minute ventilation to prevent rebreathing). Minimum FGF 5L/min Examples include Mapleson A, B, C, D Bain Jackson-Rees Anesthesia Breathing Systems Classifications Semiclosed A type of “circle system” Always has a gas reservoir bag Allows for PARTIAL rebreathing of exhaled gases Always provides for chemical neutralization of CO2 Always contains 3 unidirectional valves (insp, exp, APL) Fresh gas flow is less than minute ventilation Examples – The machine we use everyday! Anesthesia Breathing Systems Classifications Closed Always has a gas reservoir bag Allows for TOTAL rebreathing of exhaled gases Always provides for chemical neutralization of CO2 Always contains unidirectional valves We don’t use these….Suffice to say you can do this with the machines we have now if you keep your fresh gas flow to metabolic requirements around 150ml/min (supply of O2, N2O and VAA just matches pt’s requirements) If pt spontaneously ventilating, APL valve should totally closed (no scavenging since no waste total rebreathing) Anesthesia Breathing Systems Non-rebreathing circuits Mapleson Classification – 1954 Mapleson D still commonly used Modified Mapleson D is also called Bain. Arrangement of components (entry point of fresh gas, reservoir gas, APL valve) is similar in both. The main difference is that the Bain has the fresh gas hose inside the expiratory corrugated limb (tube within a tube). Unrecognized kinking of inner inspiratory hose will convert the expiratory outer hose into dead space. Mapleson F is better known as Jackson-Rees modification of Ayre’s T-piece Used almost exclusively in children Very low resistance to breathing The degree of rebreathing is influenced by method of ventilation Adjustable overflow valve Delivery of FGF should be at least 2x the minute volume Anesthesia Breathing Systems (APL) APL Insp Exp Non-rebreathing Circuits All non-rebreathing (NRB) circuits lack unidirectional valves (insp & exp) and soda lime CO2 absorption Amount of rebreathing is highly dependent on fresh gas flow (FGF) Work of breathing is low (no unidirectional valves or soda lime granules to create resistance) How do NRB’s work? During expiration, fresh gas flow (FGF) pushes exhaled gas down the expiratory limb, where it collects in the reservoir (breathing) bag and opens the pop-off (APL) valve. The next inspiration draws on the gas in the expiratory limb. The expiratory limb will have less carbon dioxide (less rebreathing) if FGF inflow is high, tidal volume (VT) is low, and the duration of the expiratory pause is long (a long expiratory pause is desirable as exhaled gas will be flushed out more thoroughly). All NRB circuits are convenient, lightweight, easily scavenged (if using appropriate FGF). Anesthesia Breathing Systems Mapleson Advantages Used during transport of children Minimal dead space, low resistance to breathing Scavenging (variable ability, depending on FGF used) Disadvantages Scavenging (variable ability, depending on FGF used) High flows required (cools children, more costly) Lack of humidification/heat (except Bain) Possibility of high airway pressures and barotrauma Unrecognized kink of inner hose in Bain Pollution and higher cost Difficult to assemble The classification is according to the relative positions of the APL valve, reservoir bag and FGF. Mapleson systems need significantly higher FGF to prevent rebreathing compared to the circle breathing system and therefore the expensive use of volatile agents. Their use in modern anaesthesia is very limited with the wide spread of the circle breathing system. Mapleson Components Breathing Tubes Corrugated tubes connect components of Mapleson to pt Large diameter (22mm) creates low-resistance pathway for gases & potential reservoir for gases Volume of breathing circuit = or > TV to minimize FGF requirements Fresh Gas Inlet (position will determine type of Mapleson performance and classification) Mapleson Components Pressure-Relief Valve (Pop-Off Valve, APL) If gas inflow > pt’s uptake & circuit uptake = press buildup opens APL (gas out via scavenger) APL fully open during spontaneous ventilation APL partial closure while squeezing breathing bag (assisted ventilation) Breathing Bag Reservoir Bag of gases Method of generating positive pressure ventilation Mapleson A ( Magill vs Lack ) Since No gas is vented during expiration, high unpredictable FGF (> 3 times minute ventilation) needed to prevent rebreathing during mechanical ventilation (Poor choice) Most efficient design during spontAneous ventilation since a FGF = minute ventilation will be enough to prevent rebreathing) In practice, a higher FGF is selected to compensate for leaks; the rate selected is usually equal to the patient’s total minute volume. Dead space ? The major disadvantage of the Magill attachment during surgery is that the spill valve is attached close to the mask. This makes the system heavy, particularly when a scavenging system is used, and it is inconvenient during head or neck surgery. The Lack system is a modification of the Mapleson A system with a coaxial arrangement of tubing. This permits positioning of the spill valve at the proximal end of the system. The inner tube must be of sufficient wide bore to allow the patient to exhale with minimal resistance. The Lack system is not quite as efficient as the Magill attachment Mapleson D • FGF forces alveolar gas away from pt toward APL valve • Efficient during ControlleD Ventilation • has a swivel mount at the patient end. This ensures that the internal tube cannot kink, thereby ensuring delivery of fresh gas to the patient. Mapleson Mapleson D Mapleson C (Jackson-Rees) Mapleson F Anesthesia Breathing Systems Bain system Coaxial (tube within a tube) version of Mapleson D Fresh gas enters through narrow inner tube Exhaled gas exits through corrugated outer tube FGF required to prevent rebreathing : 200-300ml/kg/min with spontaneous breathing (2 times VE) 70ml/kg/min with controlled ventilation Increasing the length of the tubing does not affect the physical properties of the breathing system. Bain at work (spontaneous) Spontaneous: The breathing system should be filled with FG before connecting to pt. During inspiration, the FG from the machine, the reservoir bag and the corrugated tube flow to the pt. During expiration, there is a continuous FGF into the system at the pt’s end. The expired gas gets continuously mixed with the FG as it flows back into the corrugated tube and the reservoir bag. Once the system is full, the excess gas is vented to the scavenger. During the expiratory pause the FG continues to flow and fill the proximal portion of the corrugated tube while the mixed gas is vented through the valve. Bain at work (spontaneous) During the next inspiration, the pt breathes in FG as well as the mixed gas from the corrugated tube. Many factors influence the composition of the inspired mixture (FGF, resp rate, expiratory pause, TV and CO2 production in the body). Factors other than FGF cannot be manipulated in a spontaneously breathing pt. It has been mathematically calculated and clinically proved that the FGF should be at least 1.5 to 2 times the patient’s minute ventilation in order to minimize rebreathing to acceptable levels. Bain at work (controlled) Controlled: To facilitate intermittent positive pressure ventilation, the APL has to be partly closed so that it opens only after sufficient pressure has developed in the system. When the system is filled with fresh gas, the patient gets ventilated with the FGF from the machine, the corrugated tube and the reservoir bag. During expiration, the expired gas continuously gets mixed with the fresh gas that is flowing into the system at the patient’s end. During the expiratory pause the FG continues to enter the system and pushes the mixed gas towards the reservoir. Bain at work (controlled) When the next inspiration is initiated, the patient gets ventilated with the gas in the corrugated tube (a mixture of FG, alveolar gas and dead space gas). As the pressure in the system increases, the APL valve opens and the contents of the reservoir bag are discharged into the scavenger (gas follows the path of least resistance) Anesthesia Breathing Systems Bain Advantages Warming of fresh gas inflow by surrounding exhaled gases (countercurrent exchange) Improved Ease humidification with partial rebreathing of scavenging waste gases Overflow/pressure Disposable/sterile valve (APL valve) Anesthesia Breathing Systems Bain Disadvantages Unrecognized Kinking disconnection of inner fresh gas flow tubing Requires high flows Not easily converted to portable when commercially used anesthesia machine adapter Bain circuit used Look at the Bain and identify what makes it modified from the standard Mapleson D Pethick’s Test for the Bain Circuit A unique hazard of the use of the Bain circuit is occult disconnection or kinking of the inner hose (fresh gas delivery hose). To perform the Pethick’s test, use the following steps: Occlude the patient's end of the circuit (at the elbow). Close the APL valve. Fill the circuit, using the oxygen flush valve (like pressurizing the circuit when you are doing a leak test) Release the occlusion at the elbow and flush. A Venturi effect flattens the reservoir bag if the inner tube is patent. T-piece system (Mapleson E and F) This is a valveless breathing system used in anaesthesia for children up to 25–30 kg body weight . It is suitable for both spontaneous and controlled ventilation. 1. The system requires an FGF of 2.5–3 times the minute volume to prevent rebreathing with a minimal flow of 4 L/min. 2. The double-ended bag acts as a visual monitor during spontaneous ventilation. In addition, the bag can be used for assisted or controlled ventilation. 3. The bag can provide a degree of continuous positive airway pressure (CPAP) during spontaneous ventilation. 4. Controlled ventilation is performed either by manual squeezing of the double-ended bag (intermittent occlusion of the reservoir tubing in the Mapleson E) or by removing the bag and connecting the reservoir tubing to a ventilator such as the Penlon Nuffield 200. 5. The volume of the reservoir tubing determines the degree of rebreathing (too large a tube) or entrainment of ambient air (too small a tube). The volume of the reservoir tubing should approximate to the patient’s tidal volume. Circle System CAN: Canister of CO2 absorber RB: Reservoir bag Optimization of Circle Design Unidirectional Valves Placed in close proximity to pt to prevent backflow into inspiratory limb if circuit leak develops. Fresh Gas Inlet Placed b/w absorber & inspiratory valve. If placed downstream from insp valve, it would allow FG to bypass pt during exhalation and be wasted. If FG were placed b/w expiration valve and absorber, FG would be diluted by recirculating gas Optimization of Circle Design APL valve Placed immediately before absorber to conserve absorption capacity and to minimize venting of FG Breathing Bag Placed in expiratory limb to decrease resistance to exhalation. Bag compression during controlled ventilation will vent alveolar gas thru APL valve, conserving absorbent Circle system can be: closed (fresh gas inflow exactly equal to patient uptake, complete rebreathing after carbon dioxide absorbed, and pop-off closed) semi-closed (some rebreathing occurs, FGF and pop-off settings at intermediate values), or semi-open (no rebreathing, high fresh gas flow) Anesthesia Breathing Systems Circle systems Most commonly used Adult and child appropriate sizes Can be semiopen, semiclosed, or closed dependent solely on fresh gas flow (FGF) Uses chemical neutralization of CO2 Conservation of moisture and body heat Low FGF’s saves money Anesthesia Breathing Systems Circle systems Unidirectional valves Prevent inhalation of exhaled gases until they have passed through the CO2 absorber (enforced pattern of flow) Incompetent valve will allow rebreathing of CO2 Hypercarbia and failure of ETCO2 wave to return to baseline Pop off (APL) Valve Allows pressure control of inspiratory controlled ventilation Allows for manual and assisted ventilation with mask, LMA, or ETT (anesthetist will regulate APL valve to keep breathing bag not too deflated or inflated) Anesthesia Breathing Systems Circle system Allows for mechanical ventilation of the lungs using the attached ventilator Allows for adjustment of ventilatory pressure Allows for semiopen, semiclosed, and closed systems based solely on FGF Is easily scavenged to avoid pollution of OR environment Anesthesia Breathing Systems Advantages of rebreathing Cost reduction (use less agent and O2) Increased tracheal warmth and humidity Decreased exposure of OR personnel to waste gases Decreased pollution of the environment REMEMBER that the degree of rebreathing in an anesthesia circuit is increased as the fresh gas flow (FGF) supplied to the circuit is decreased Anesthesia Breathing Systems Anesthesia Breathing Systems Dead space Increases with the use of any anesthesia system Unlike Mapleson circuits, the length of the breathing tube of a circle system DOES NOT directly affect dead space Like Mapleson’s, length DOES affect circuit compliance (affecting amount of TV lost to the circuit during mech vent) Increasing dead space increases rebreathing of CO2 To avoid hypercarbia in the face of an acute increase in dead space, a patient must increase minute ventilation Dead space ends where the inspiratory and expiratory gas streams converge Use of a mask is associated with greater dead space than an ETT Anesthesia Breathing Systems Carbon dioxide neutralization Influenced by Size of granules Presence or absence of channeling in the canister (areas of loosely packed granules, minimized by baffle system) Tidal volume in comparison to void space of the canister TV should not exceed air space between absorbent granules (1/2 absorbent capacity) Ph sensitive dye Ethyl violet indicator turns purple when soda lime exhausted (change when 50-70% has changed color) Regeneration: Exhausted granules may revert to original color if rested, no significant recovery of absorptive capacity occurs (change canister!!) Anesthesia Breathing Systems Carbon dioxide neutralization Maximum absorbent capacity 26L of CO2/100g granules Granules designated by Mesh size (4-8 mesh) A compromise between higher absorptive surface area of small granules & the lower resistance to gas flow of larger granules Toxic byproducts The drier the soda lime, the more likely it will absorb & degrade volatile anesthetics Disadvantages of Circle System Greater size, less portability Increased complexity Increased resistance (of valves during spontaneous ventilation) Higher risk of disconnection or malfunction Dissuading use in Pediatrics (unless a circle pedi system used) Difficult prediction of inspired gas concentration during low fresh gas flow Anesthesia Breathing Systems Airway Humidity Concerns Anesthesia machine FGF dry and cold Medical gas delivery systems supply dehumidified gases at room temp. Exhaled gas is saturated with H2O at body temp High flows (5 L/min) low humidity Low flows (<0.5 L/min) allow greater H2O saturation Absorbent granules: significant source of heat/moisture (soda lime 14-19% water content) Normal upper airway humidification bypassed under General Anesthesia Passive heat and humidity (“Artificial Nose”) Active heat and humidity (electrically heated humidifier) Bacterial Contamination Slight risk of microorganism retention in Circle system that could (theoretically) lead to respiratory infections in subsequent pts Bacterial filters are incorporated into EXPIRATORY LIMB of the circuit Mode Reservoir Rebreathing Example Open No No Open drop Semi-open Yes No Nonrebreathing circuit or Circle at high FGF (>VE) Semi-closed Yes Yes, partial Circle at low FGF (<VE) Closed Yes Yes, complete Circle (if APL valve closed)
