Low pressure system

1. Low pressure system Presented by : Dr Rashmi Moderator: Dr GianChauhan

2. Starts downstream of flow control devices • Pressures: Slightly above atmospheric pressure (Max upto 6-8psig) & variable • Components: • Flowmeters • Hypoxia prevention safety devices • Unidirectional valves • Pressure relief devices • Common gas outlet • Vaporizers & Mounting devices including backbar

4. Flowmeters: (aka flow indicators, flow tubes, rotameters) • Types: Mechanical & Eletronic • Mechanical flowmeters: Based on principle that flow past a resistance is proportional to pressure • Variable orifice type tubes (Thorpe tube), internally tapered narrowest at bottom • Indicator: free to move inside the tube • Gas passes through an annular opening b/w the indicator and the tube towards the outlet • Wt. of bobbin balances the upward force by the gas pressure

6. Rate of flow depends on: • Pressure drop across the constriction (frictional resistance caused to the gas, equals wt. of float/ area), remains constant throughout (Constant pressure flowmeters) • Size of the annular opening • Physical properties of the gas • (for Low flows (Laminar flow) flow is dependent on viscosity; Hagen Poiseuille equation (Tubular orifice) • (High flows(Turbulent flow) flow is dependent on density; Graham’s law (orificial orifice)

7. Calibrated at atmospheric pressure (760 mmHg) & room temperature (20 Degree) • If pressure decrease as in high altitude, more gas will flow that what the indicator specified and in hyperbaric chamber, flow will be less than what is indicated. • Temperature may alter viscosity and density of gas but effect on gas flows is less significant.

8. Flowmeter Assembly: • Tube , Scale , Indicator, Stop at the top • Lights • Coding (color, touch, name) • Plastic shield (for protection)

9. Tubes: • Glass tubes • Single taper (different tubes for low and high flows) • Double taper (single tube with gradations for fine and coarse flows)

11. Stop (at the top): • Prevents bobbin from plugging the outlet (Damage, hidden bobbin) • If falls into the tube on the bobbin will increase the wt. & so show falsely low reading • Scale: • Flow indicator (marked on or immediately adjacent to tube) • L/ min (lower than 1 L as ml/min or Decimals of L) • Lights/ fluorescent background: helps in darkened rooms

12. Flowmeter Tube Arrangement: • Tubes for various gases arranged side by side, meet at the top into a common manifold (mixing chamber) • Arranged in series or tandem • One flow control valve for one gas (single/ double tube) • Normal direction of flow: bottom to top, then left to right

13. Flowmeter arrangement

14. Auxiliary Oxygen Flowmeters: • Self contained flowmeter with its own flow control valve, flow indicator, outlet • Short tube, max of 10 L/min flow • Usually mounted on the left side of the machine • Can be used to supply oxygen even without turning the machine on

15. Problems with Flowmeters: • Inaccuracy: • Inseparable unit (mixing of components not allowed) • Calibrated for a particular gas (other gas cannot be used) • Greatest accuracy in the middle half of the tube (single taper provides better accuracy) • Indicator problems: (worn, distorted, sudden burst of gas pushing bobbin to top, stop may dislodge- rest on bobbin)

16. Leaks: • leak upstream of the bobbin will result in low FGF but will not be indicated by the bobbin • If cylinder/ yoke plug not in place, open flow control valve will l/t leakage • Using wrong flowmeter: • Changing the position of the gases (arrangement) l/t errors as anaesthetists are used to a particular arrangement • Usually with Nitrous and air

17. Electronic Flowmeters: • Use electronic flow sensors and conventional flow control valves • Slow measured is represented digitally or by a simulated flowmeter • Clockwise movement opens the gas flow • Advantage: Information is available in a form which can be directly sent to data management system

18. Hypoxia Prevention Safety Devices Mandatory Minimum Oxygen Flow: (50 to 250 mL/minute), preset by the manufacturer (altered on customer specification), activated when master switch is turned on • Minimum Oxygen Ratio: Prevents delivery of mixture of oxygen and nitrous oxide having an oxygen concentration below 21% oxygen • Mechanical Linkage: 14-tooth sprocket on the nitrous oxide flow control valve and a 29-tooth sprocket on the oxygen flow control valve (maintain a min. of 25% oxygen), if we increase nitrous beyond this it engages the Oxygen valve. • aka ‘Link 25’ system, drawback: links only 2 gases

19. Mechanical linkage

20. Electronic linkage: electronic proportioning valve controls the oxygen concentration in the fresh gas, supply a minimum of 25% oxygen • Alarms: To alert the operator that % of Oxygen has fallen in the gas mixture

21. Vaporizer Mounting Systems: (back bar) Permanent mounting (Cage mount): • tools are required to remove or install a vaporizer on the anesthesia machine, back bar is a component of vaporizer • Adv: Less physical damage to vaporizers and fewer leaks • Disadv: May not have enough mounting locations to accommodate all of the vaporizers, malfunctioning vaporizer cannot easily be exchanged • Penlon “off-line” system: A modified cage mount arrangement, Flexible back bar hoses with connections which can be separated and connected to a vaporizer (which will then be included in the backbar)

22. Penlon “Back entry” system: • Vaporizer is attached to the back bar by a fixing bolt protruding through the middle of the rear panel of the machine • Protruding pegs and seals on the back bar automatically locate and seal the inlet and outlet connections to vaporizer • Fixing bolt is tightened using a cylinder key • 3 entry systems can be mounted in series • Each system has a cover (blanking plate),should be fitted properly when a vaporizer is not connected (or else leak)

24. Detachable mounting: Are standard on most new anesthesia machines, allow the vaporizer to be mounted & removed without use of tools. • Select-a-tecsystem: A pair of port valves for each vaporizer, Vaporizer has special mounting brackets with 2 plungers (spindles) which fit over the port valves. Seal created by wt. of vaporizer and O-ring around the post valve. Locking lever on the back of vaporizer.

25. Before mounting the vaporizer, control dial should be in off position • Vaporizer is fitted onto the mounting system and locked in position • When the vaporizer is turned on, the two plungers move downward, opening the valve ports and connecting the vaporizer into the fresh gas stream • When the vaporizer is turned off, the gases may still flow through the head of the vaporizer (even when not locked properly)

26. Drager Mounting System: • Vapor 2000 vaporizer must be in the “T” (travel) position before it can be unlocked from the machine • Teats to be performed before operation of vaporizers: • Looking at the level of each vaporizer (at same level) • Attempt to lift each off the manifold without unlocking it (shouldn’t be able to) • It should be possible to turn ON only one vaporizer at a time • Perform leak tests

27. Advantages of detachable mounting: • Machine can have fewer mounting locations, allowing a more compact machine • Vaporizers can be easily removed and replaced, even during a case • If malignant hyperthermia is a potential problem, the vaporizers can be removed: better results than if the vaporizers remain in the off position

28. Disadvantages: • Partial or complete obstruction to gas flow from problems with the mounting system • Leaks may occur • an absent or damaged O-ring • leaving the locking lever in the unlocked position • something is pushed under the vaporizer enough to lift it slightly off of the O-ring • Differences among vaporizers and interlocks from different manufacturers can pose problems of compatibility

29. Interlocking Devices: • Prevent more than one vaporizer from being turned on at a time • Checking the interlock device should be done • For Datex-Ohmeda vaporizers, operating the dial release activates two extension rods that prevent operation of any other vaporizer installed on the manifold • Older versions of the Select-a-tec system: provide mounting positions for three vaporizers, if only two vaporizers are fitted, the center position must be occupied. If the center position is not occupied, the interlock is ineffective • Newer ones: interlocking irrespective of center position occupancy

30. Unidirectional valves (check valves): • Backpressure from: • Breathing system during IPPV • Oxygen flush • Partial switching from open to closed circuit • Can affect flowmeter readings and conc. of volatile agents • Located b/w the vaporizer and common gas outlet, upstream of where the oxygen flush joins the fresh gas flow • Disadvantage: while checking for leaks we cannot detect a leak upstream of the check valve

31. Pressure relief device: • Near common gas outlet to protect the machine from high pressures • Opens to atmosphere • Disadv: May limit the ability of anaesthesia machine to provide adequate pressure for jet ventilation (separate ports recommended for jet ventilation) • Low pressure piping: • Has large number of connections so prone for breakages and leaks

32. Common Gas Outlet: • Delivers the gas mixture to the breathing system • Common site for disconnections so machine standards mandate that it should be difficult to accidentally disengage the system • Have a 15mm female slip joint fitting (accepts a tracheal tube connector) and a co-axial 22mm male connector • Fresh gas supply tube of the breathing system connects to this outlet

33. Some machines may have 2 such outlets (may l/t confusion, gas may be directed to the wrong outlet) According to standards there should be only one CGO. • Some may not have even one (have internal breathing system connections) • This outlet should not be used to administer supplemental oxygen (delays use of circuit if emergency arises, also if accidentally vaporizer remains on inhalational agent will be delivered unknowingly)- prefer auxiliary Oxygen flowmeter

34. Tests for Low pressure system: • Check vaporizer: Level of agent, cap to be tight, should not be lifted off. • Leak test: performed without the basal flow so machine master switch to be turned off. To be performed with vaporizer off first and then on. • Positive pressure test • Oxygen flush test • Pressure gauge test • Negative pressure test

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