lowpressuresysteminanaesthesiamachine-130521144917-phpapp02 (1).pptx
- 1. Hassam Zulfiqar Anesthesiology Resident Low Pressure system
- 2. Anaesthesia machine:- Types Function is to deliver a precisely-known but variable gas mixture, including anesthetizing and life-sustaining gases at a controlled & known pressure. Intermittent( gas flows only during inspiration) Ex. – Entonox apparatus Continuous(Gas flows both during inspiration and expiration) Ex.- Boyle`s machine ,.
- 3. ⦁ Boyle`s anaesthetic machine is a continuous-flow type machine used for administration of anaesthetic gases. ⦁ was pioneered by Henry Gaskin Boyle (1875-1941). ⦁ the original machine from 1917 was carried around in a wooden box and used ether and nitrous oxide. ⦁ It has undergone various modifications with time to increase its safety & utility.
- 4. SYSTEM COMPONENTS ELECTRICAL SYSTEM PNEUMATIC SYSTEM - Master switch - Battery back up - Battery recharge - Electric outlet for in built monitor - Circuit breaker - High pressure system - Intermediate pressure system - Low pressure system
- 5. ⦁ Extends from the flow control valves to the common gas outlet. ⦁ Consists of : - Flowmeters - Hypoxia prevention safety devices - Unidirectional check valve - Pressure relief valves - Common gas outlet - Vaporiser & their mounting devices
- 6. TUBE: made of glass (THORPE tube). ⦁ Tubes are contained in a chromium plated metal casing protected by a plastic window.Backplate is luminous & detachable. ⦁ Anti-static coating. INDICATOR: Float / Bobbin made usually of aluminium. ⦁ It is free moving device & must not stick to tube wall. ⦁ If it moves erratically, readings may be inaccurate. TYPES Rotating type Slanted grooves Non-rotating H type Ball type STOP: Prevents float from plugging the outlet & prevents rising to a point it cannot be seen.
- 7. Types of flowmeters 1. Variable orifice flowmeters (fixed pressure difference) 2. Fixed orifice flowmeters (variable pressure difference) •The area between the outside of the bobbin and the inside of the tapered glass tube represents a orifice / annulus. It can be considered an equivalent to a circular channel of the same cross-sectional area Variable orifice flowmeters type used mostly today in modern machines. (Synonym- Rotameters) • The glass tube, slightly smaller on cross-section at bottom than at top (tapered tube) •Can be single or double taper. Single taper have gradual increase in diameter from bottom to top, used when there are different tubes for high & low flows. Dual taper have two different taper inside the same tube .one for fine flows (200mL/min to 1L/min) & one for coarse flows. -used when only one tube is used for a gas.
- 9. ⦁ Flowmeters adjust the proportions of medical gases controlled by the anesthesia machine as well as the total gas flows delivered to the patient circuit . ⦁ Flowmeters measure the drop in pressure that occurs when a gas passes through a resistance and correlates this pressure drop to flow . ⦁ When the flow control valve is opened the gas enters at the bottom and flows up the tube elevating the indicator. ⦁ The indicator floats freely at a point where the downward force on it (gravity) equals the upward force caused by gas molecules hitting the bottom of the float. ⦁ As the bobbin rises with increased flow, the size of the annulus between it and the glass tube increases. In other words, there is a variable orifice around the bobbin which depends on the gas flow.
- 10. As the bobbin rises from A to B, the clearance(annulus) increases (from X toY)
- 11. Laminar flow- ⦁ Flow (Q) through a tube is laminar. ⦁In order to drive fluid through a tube, a pressure difference (P = P1-P2) must be present across the ends •There is a linear relationship so that flow is directly proportional to pressure under conditions of laminar flow (Q@P) •Reducing the diameter (d) in half reduces the flow to 1/16 of its original value if the pressure drop along the tube remains the same, i.e. flow is proportional to the 4th power of the diameter (Q@d4) • Reducing the length by half, doubles the flow (Q@1/L)
- 12. Summary: Q @ P Q @ D4 Q @1/L Q @1/n Q = flow through tube P = pressure across tube D = diameter of tube L = length of tube n= viscosity of fluid All these factors are incorporated in the Hagen-Poiseuille equation: •Viscosity (n) of fluid affects resistance to laminar flow such that the higher the viscosity, the slower the flow (Q@1/n) Substituting radius (r) for diameter
- 13. ⦁ Turbulent flow is often present where there is an orifice, a sharp bend or some other irregularity which may cause local increase in velocity. ⦁ Turbulence is also affected by other factors such as viscosity and density of the fluid and diameter of the tube. ⦁ The effect of density on onset of turbulent flow can be illustrated by use of helium in respiratory disorders ⦁ Helium reduces the density of the gas inhaled and so reduces the incidence of turbulent flow, therefore lower resistance to breathing ⦁ These factors combine to give an index called Reynolds number V = linear velocity of fluid P = density D = diameter of tube U = viscosity • Reynolds number > 2000 means turbulent flow likely • Reynolds number < 2000 means flow likely to be laminar •For a fixed set of conditions, there is a critical velocity at which Reynolds number has the value of 2000 When the velocity exceeds this critical value, flow is likely to change from laminar to turbulent .
- 14. As the bobbin rises increase in the area of the annular orifice flow resistance decreases flow rate increase The rate of flow through the flowmeter tube depends on: - Pressure drop across the constriction Weight Of Float/Cross-sectionalArea - Size of annular orifice - Physical properties of the gas
- 15. At low flows: Tube = Length > Diameter -gas flow around the bobbin approximates to tubular flow (diameter of channel less than length) - gas flow is laminar so viscosity is important At high flows: - gas is flowing around the bobbin through an orifice (diameter of channel greater than length) - gas flow is turbulent so density is important Orifice = Diameter > Length
- 16. ⦁ Flowmeter are calibrated in litres per min. For <1 L/min expressed in ml or decimal fractions of a litre per minute with a zero before the decimal point. ⦁Are calibrated at atmospheric pressure (760 torr) and room temperature(200C) based on physical properties of individual gases. ⦁Changes in temperature & pressure affect density & viscosity of gas and affect flowmeter accuracy. ⦁As flow changes from laminar to turbulent within the flowmeter the flow changes from being directly proportional to pressure to proportional to the square root of pressure and hence the graduations on the flowmeters are not uniform.
- 17. ⦁ The O2 flowmeter is positioned on the right side (most distally) of the rotameter bank, downstream from the other flowmeters and closest to the common gas outlet ⦁ In the event of a leak in one of the other flowmeter tubes, this position is the one least likely to result in a hypoxic mixture. In A and B a hypoxic mixture can result because a substantial portion of oxygen flow passes through the leak, and all nitrous oxide is directed to the common gas outlet. C and D, The safest configuration exists when oxygen is located in the downstream position
- 18. Temperature and Pressure Effects ⦁ Changes in temperature and pressure alter both viscosity and density of gases, thereby affecting accuracy of the indicator on the flowmeters. ⦁ Temperature effects are slight and do not cause significant changes ⦁ At high altitude, barometric pressure decreases resulting in increased flow. ⦁ At low flow rates, flow is laminar and dependent on gas viscosity, a property not affected by altitude. ⦁ At high flow rates flow becomes turbulent, and flow becomes a function of density, a property that is influenced by altitude. ⦁ The resulting decreases in density will increase the actual flow rate so the flowmeter will read lower than the actual flow rate. ⦁ At increased pressure, as in a hyperbaric chamber, the reverse is seen; the delivered flow rate is slightly less than the actual flow rate.
- 19. Back Pressure: ⦁ In machines without an outlet check valve, if pressure at the common gas outlet increases, this is transmitted back to the flowmeters, compressing the gas above the float ⦁ Pressure above the indicator rises forcing the float down, causing the flowmeter to be read lower than the actual gas flow rate Static Electricity: Static electricity causes the float to stick to the side of the tube causing reading inaccuracy.These electrostatic charges are negligible as long as the float rotates freely Hidden Floats: The float may adhere to the stop at the top of the tube even if no gas is flowing The float may disappear from view if there is no stop present e.g. broken float stop
- 20. ⦁ Protection against hypoxic mixture at the flowmeter level. ⦁ Prevention of delivery of a hypoxic gas mixture is a major consideration in the design of contemporary anesthesia machines. Mandotary minimum oxygen flow: Some machines require a minimum flow (50-250ml/min) of oxygen before other gas will flow. ⦁ Some machine activate an alarm if O2 flow falls below a certain limit. Minimum oxygen ratio: ⦁ In modern anesthesia machines, N2O and O2 flow controls are physically interlinked so that a fresh gas mixture containing at least 25% O2 is delivered at the flowmeters when only N2O and O2 are used . Ohmeda = mechanical + pneumatic interlink (Link–25) North American Dräger = pneumatic interlink
- 21. ⦁ A 14-tooth sprocket is attached to the N2O flow control valve, and a 28-tooth sprocket is attached to the O2 flow control valve. A chain mechanically links the sprockets. ⦁ For every 2 revolutions of the N2O flow control spindle, an O2 flow control, set to the lowest O2 flow, rotates once because of the 14:28 ratio of the gear teeth.
- 22. ⦁ Regardless, of the O2 flow set, if the flow of N2O is increased >75%, the gear on O2 spindle will engage automatically with the O2 flow control knob causing it to rotate and thereby causing O2 flow to increase to maintain O2 Conc of 25% with a maximum N2O:O2 ratio of 3:1. ⦁ If attempt is made to increase N2O flow beyond that ratio, O2 flow is automatically increased & if O2 flow is lowered too much N2O flow reduces proportionately. The final 3:1 flow ratio results because the N2O flow control valve is supplied by approximately 26 psig, whereas the O2 flow control valve is supplied by 14 psig. Thus, the combination of the mechanical and pneumatic aspects of the system yields the final oxygen concentration.
- 23. ⦁ The ORMC also rings alarms (it has an electronic component) to prevent a hypoxic mixture delivery . ⦁ Dräger S-ORC (sensitive oxygen ratio controller), newest hypoxic guard system as found on Fabius GS guarantees a minimum FIO2 of 23%. Its fail-safe component shuts off nitrous oxide if the oxygen flow is less than 200 mL/min, or if the oxygen fresh gas valve is closed.
- 24. Machines equipped with proportioning systems can still deliver a hypoxic mixture under the following conditions ⦁ Wrong Supply Gas in oxygen pipeline or cylinder. ⦁ Defective pneumatic or mechanical components. ⦁ Leaks exist downstream of flow control valves. ⦁ Inert gas administration( He,CO2) : Proportioning systems generally link only N2O and O2. Use of an oxygen analyzer is mandatory if the operator uses a third inert gas.
- 25. ⦁ Present on some machines (Ohmeda) between the vaporizers and common gas outlet, upstream of where oxygen flush flow joins the fresh gas flow . ⦁ Positive pressure ventilation & use of O2 flush cause back flow of the gas. ⦁ This back flow can cause “pumping effect”, if not prevented, could cause increased vaporizer output concentrations. •Pressure increase can also increase leaks and cause inaccurate flow indicator readings.
- 26. ⦁ The purpose of the outlet check valve, where present, is to prevent reverse gas flow, ⦁ Newer machines (North American Dräger) are equipped with vaporizers that incorporate a baffle system and specially designed manifold to prevent pumping effect, making an outlet check valve unnecessary. *Important: Testing the breathing system for leaks will not detect a leak in the machine equipped with a check valve
- 27. ⦁ Situated on the back bar of the machine downstream of voporizers or near common gas outlet. ⦁ Prevents high pressure from transmitted in to machine & from machine to patient. ⦁ When preset pressure is exceeded valve opens & gas is vented outside. ⦁ Usually opens when pressure in the back bar exceeds 35 Kpa.
- 28. ⦁ Located in between flowmeter device and common gas outlet. ⦁ Permanent mounting. ⦁ Vapourizers and flowmeters are connected to each other and then bolted with back bar.
- 29. • Receives all gases & vapors from the machine & delivers the mixture to the breathing system. • Oxygen flush is a "straight shot" from pipeline to common gas outlet (bypassing vaporizers and flowmeters), 35-75 L/min.