4. mohamedrefaateltahan@yahoo.com

5.  Understand the components & the operating principles of the anaesthesia machine, namely, gas supply, O 2 safety devices, and flowmeters.  Know how to deal with problems arising from these components.

6.  An anaesthesia system consists of the various components that communicate with each other during the administration of inhalation anaesthesia.  A thorough understanding of these parts is essential to the safe practice of anaesthesia.

9. 1.O 2 & N 2 O Sources: consisting of a pipeline supply source and a cylinder supply source. The cylinder source is regulated from 2,200 to approximately 45 Psig, and the N 2 O cylinder source is regulated from 745 to approximately 45 Psig. 2.Fail-safe valve: is a safety device located downstream from the N 2 O supply source. This valve shuts off or proportionally decreases the supply of N 2 O if the O 2 supply pressure decreases. Recent machines have an alarm device to monitor the O 2 supply pressure.

10. 3.Second-stage O 2 regulator: in most Ohmeda machines have located downstream from the O 2 supply source. It is adjusted to a precise pressure level, such as 14 Psig. 4.Flow control valves: are an important anatomic landmark because they divide the anaesthesia machine into two parts. 5.Machine outlet check valve: in many Ohmeda machines have located between the vaporizers and the common gas outlet.

11. 1.The high–pressure circuit: is the part of the machine that is confined to the cylinders and their primary pressure regulators. 2.The intermediate–pressure circuit: is the part of the machine that begins at the regulated cylinder supply sources downstream the flow control valve. 3.The low–pressure circuit: is the part of the machine that extends from the flow control valves to the common gas outlet. 1.The high–pressure circuit: is the part of the machine that is confined to the cylinders and their primary pressure regulators. 2.The intermediate–pressure circuit: is the part of the machine that begins at the regulated cylinder supply sources downstream the flow control valve. 3.The low–pressure circuit: is the part of the machine that extends from the flow control valves to the common gas outlet.

14. 1.The high-pressure circuit: consists of those parts which receive gas at cylinder pressure: Hanger yoke (including filter and unidirectional valve). Yoke block. Cylinder pressure gauge. Cylinder pressure regulators.

15. 2.The intermediate – pressure circuit: receives gases at low, relatively constant pressures (37–55 psig, which is pipeline pressure). Ventilator power inlet. Oxygen pressure–failure device (fail-safe) and alarm. Flowmeter valves. Oxygen second-stage regulator. Oxygen flush valve.

16. 3.The low-pressure circuit: includes components distal to the flowmeter needle: Valves. Flowmeter tubes. Vaporizers. Check valves (if present). Common gas outlet.

18. Most hospitals today have a central piping system to deliver medical gases such as O 2, N 2 O, & air to the operating room. % In a survey of approximately 200 hospitals in 1976, 31 % reported difficulties with pipeline systems.  Inadequate oxygen pressure.  Excessive pipeline pressures.  Accidental crossing of oxygen and nitrous oxide pipelines.

19.  The pipeline inlet fittings are gas-specific Diameter Index Safety System (D.I.S.S.).

20.  A pipeline pressure gauge must be located on the pipeline side rather than on the machine side of the check valve. A value of 50 psig, however, does not guarantee that the pipeline is supplying the machine.

21. Anaesthesia machines have reserve E cylinders if a pipeline supply source is not available or if the pipeline fails.

22. Color–coded cylinders are attached to the anaesthesia machine through the hanger yoke assembly, which orients and supports the cylinder, provides a gas – tight seal, and ensures a unidirectional flow of gases into the machine.

23. Each hanger yoke is equipped with the Pin Index Safety System. A check valve is located downstream from each cylinder if a double-yoke assembly is used.

24. The check valve has several functions: 1.It minimizes gas transfer from a cylinder at high pressure to one with lower pressure. 2.It allows an empty cylinder to be exchanged for a full one while gas flow continues from the other cylinder into the machine with minimal loss of gas. 3.It minimizes leakage from an open cylinder to the atmosphere if one cylinder is absent.

26. Each cylinder supply source has a pressure-reducing valve known as the cylinder pressure regulator. The O 2 cylinder pressure regulator reduces the O 2 cylinder pressure from a high of 2,200 psig to approximately 45 psig. The N 2 O cylinder pressure regulator receives pressure of up to 745 psig and reduces it to approximately 45 psig.

27. The cylinders should be turned off except: 1.During the preoperative machine-checking period. 2.When a pipeline source is unavailable.

28.  O 2 & N 2 O supply sources existed as independent entities in older models of anaesthesia machines, and they were not pneumatically or mechanically interfaced.  Therefore, abrupt or insidious oxygen pressure failure had the potential to lead to the delivery of a hypoxic mixture.

29.  Contemporary anaesthesia machines have a number of safety devices that act together in a cascade manner to minimize the risk of hypoxia as oxygen pressure decreases.

30. Many older anaesthesia machines have a pneumatic alarm device that sounds a warning when the O 2 supply pressure decreases to a predetermined threshold value, such as 30 psig.

31. Electronic alarm devices have both audible and visual alarms. The O 2 pressure threshold value for the Ohmeda Modulus II Plus & the Ohmeda CD is 27 psig, and for the N.A.Dräger Narkomed 2B, 3, & 4, the value is 30 ± 3 psig.

33. It is present in the gas line supplying each of the flowmeters except that for oxygen. These valves, controlled by oxygen pressure, shut off or proportionally decrease the supply pressure of all other gases (N 2 O, air, CO 2, helium, and N 2 ) as the O 2 supply pressure decreases.

34. The misnomer "fail-safe" has led to the misconception that the device prevents administration of a hypoxic mixture. This is not the case.

35.  They are equipped with the pressure-sensor shutoff valve. It is either open or closed with a threshold pressure of 25 psig.

36.  An O 2 pressure > the threshold value is exerted upon the mobile diaphragm.  This pressure moves piston, pin, and valve off the valve seat.  N 2 O flow passes freely to the N 2 O flow control valve.

37.  They uses the Oxygen Failure Protection Device (OFPD), which interfaces the O 2 pressure with that of other gases, such as N 2 O, air, CO 2, helium, and N 2.  The OFPD is based on a proportioning principle rather than a threshold principle.  The pressure of all gases controlled by the OFPD will decrease proportionally with the oxygen pressure.

38.  The OFPD consists of a seat- nozzle assembly connected to a spring-loaded piston. Nozzle Valve seat Spring

39.  When the O 2 supply pressure is 50 psig,it pushes the piston upward, forcing the nozzle away from the valve seat.  N 2 O advances toward the flow control valve at 50 psig. 50 Psig N2ON2O O2O2

40.  When the O 2 pressure is zero psig, the spring is expanded and forces the nozzle against the seat, preventing N 2 O flow through the device. 0 Psig 50 Psig N2ON2O O2O2 0 Psig

41.  When the O 2 pressure is intermediate at 25 psig. The force of the spring partially closes the valve.  The N 2 O pressure delivered to the flow control valve is 25 Psig. 25 Psig 50 Psig N2ON2O O2O2 25 Psig

42.  There is a continuum of intermed.configurations between the extremes (0 – 50 Psig) of O 2 supply pressure. 25 Psig 50 Psig N2ON2O O2O2 25 Psig

43.  These intermediate valve configurations are responsible for the proportional nature of the OFPD. 25 Psig 50 Psig N2ON2O O2O2 25 Psig

44.  Most contemporary Ohmeda machines have a second- stage oxygen pressure regulator set at a specific value ranging from 12 to 19 Psig.  O 2 flowmeter output is constant when the O 2 supply pressure exceeds the set value.

45.  Ohmeda pressure-sensor shutoff valves are set at a higher threshold value (20 to 30 Psig).  This ensures that O 2 is the last gas flow to decrease if O 2 pressure fails, and act together in a cascade manner to minimize the risk of hypoxia as oxygen pressure decreases.

46.  Opening the flow control valve allows gas to travel through the space between the float and the flowtube [the annular space].  The indicator float hovers freely in an equilibrium position, where the upward force resulting from gas flow equals the downward force on the float resulting from gravity at a given flow rate.

47. These flowmeters are commonly referred to as constant – pressure flowmeters because the pressure decrease across the float remains constant for all positions in the tube.

48. It is composed of a flow control knob, a needle valve, a valve seat, and a pair of valve stops. It can receive its pneumatic input either directly from the pipeline source (50 Psig) or from a second-stage pressure regulator.

49. The location of the needle valve in the valve seat changes to establish different orifices when the flow control valve is adjusted. Gas flow increases when the flow control valve is turned counter- clockwise, and vice versa.

50. Extreme clockwise rotation results in damage to the needle valve and valve seat.

51. Therefore, flow control valves are equipped with valve stops to prevent this occurrence. The stops come into contact with each other at zero flow on most flow control valves.

52. The Ohmeda Modulus I, Modulus II, Modulus II Plus, and Ohmeda 8000 are set at an O 2 flow rate of approximately 200 ml.min -1.

53. On recent Ohmeda machines, minimum O 2 flow results from incomplete closure of the O 2 flow control valve. N.A.Dräger machines, the O 2 flow control valve does close completely.

54. 1.The oxygen flow control knob is physically distinguishable [fluted, projects beyond the control knobs of the other gases, and is larger in diameter] from other gas knobs.

55. 2.All knobs are color-coded for the appropriate gas & the name of the gas is permanently marked on each.

56. 3.The knobs are recessed or protected with a shield or barrier to minimize inadvertent change from a preset position.

57. The flowmeter subassembly consists of: 1.The flow tube. 2.The indicator scale. 3.The indicator float with float stops.

58. They are made of glass, have a single taper in which the inner diameter of the flow tube increases uniformly from bottom to top.

59. Manufacturers provide double flow tubes for O 2 & N 2 O to provide better visual discrimination at low flow rates.

60. A fine flow tube indicates flow approximately 1-200 ml.min -1. A coarse flow tube indicates flow approximately 1-10 L.min -1.

61. The two tubes are connected in series & supplied by a single flow control valve. The total gas flow is that shown on the higher flowmeter.

62. Some older machines have two flow tubes for a single gas arranged in parallel. Each of the tubes has a flow control valve. The total flow is the sum of the individual flows.

63. There are different types of bobbins or floats, including plumb-bob floats, rotating skirted floats & ball floats.

64. Flow is read at the top of plumb-bob and skirted floats and at the center of the ball on the ball-type floats. Flow tubes are equipped with float stops at the top & bottom of the tube.

65. The upper stop prevents the float from ascending to the top of the tube, plugging the outlet & ensures that the float will be visible at maximum flows instead of being hidden in the manifold.

66. The bottom float stop provides a central foundation for the indicator when the flow control valve is turned off.

67.  The flowmeter scale can be marked directly on the flow tube or can be located to the right of the tube.

68.  Gradations are closer together at the top of the scale because the annular space increases more rapidly than does the internal diameter from the bottom to the top of the tube.

69.  Rib guides are used in N.A.Dräger flow tubes with ball-type indicators to minimize the compression effect.  They are tapered glass ridges that run the length of the tube.

70.  There are usually 3 rib guides, which are equally spaced around the inner circumference of the tube.

71.  The annular space from the bottom to the top of the tube increases almost proportionally with the internal diameter, resulting in a nearly linear scale.

72. 1.The flowmeter subassembly for each gas on the Ohmeda Modulus I, Modulus II, Modulus II Plus, and CD is housed in an independent, color-coded, pin-specific module. 2.The flow scale and the chemical formula or name of the gas is permanently etched on the backing to the right of the flow tube. 3.Flowmeter scales are individually hand-calibrated by use of the specific float to provide a high degree of accuracy. 4.The tube, float, and scale make an inseparable unit. The entire set must be replaced if any component is damaged.

73. 5.N.A.Dräger does not use a modular system for the flowmeter subassembly. The flow scale, the chemical symbol, and the gas- specific color codes are etched directly onto the flow tube. The scale in use is obvious when two flow tubes for the same gas are used.

74. 1.Leaks:  It is a substantial hazard because the flowmeters are located downstream from all machine safety devices except the O 2 analyzer.  It can occur at the O rings between the glass flow tube & the metal manifold and even in the glass flow tubes.  Gross damage to glass flow tubes is usually apparent, but subtle cracks & chips may be overlooked, resulting in errors of delivered flows.

75. 1.Leaks:  Eger et al (1963) demonstrated that in the presence of a flowmeter leak, a hypoxic mixture is less likely to occur if the oxygen flowmeter is located downstream from all other flowmeters.  O 2 escapes through the leak & N 2 O flows toward the common outlet, particularly at high N 2 O/ O 2 flow ratios.

76. 1.Leaks:

77. Ohmeda Mod II Anaesthesia Machine

78. 2.Inaccuracy: Flow error can occur even when flowmeters are assembled properly with appropriate components. 1.Dirt or static electricity can cause a float to stick, and the actual flow may be higher or lower than that indicated. 2.Sticking is more common in the low flow range because the annular space is smaller.

79. 2.Inaccuracy: 3.A damaged float can cause inaccurate readings because the precise relationship between the float and the flow tube is altered. 4.Back-pressure from the breathing circuit can cause a float to drop so that it reads less than the actual flow.

80. 2.Inaccuracy: 5.Finally, if flowmeters are not aligned properly in the vertical position, readings can be inaccurate because tilting distorts the annular space.

81. 3.Ambiguous Scale: Before the standardization of flowmeter scales and the widespread use of oxygen analyzers, at least two deaths resulted from confusion created by ambiguous scales. The operator read the float position beside an adjacent but erroneous scale in both cases.

82. 3.Ambiguous Scale: Today this error is less likely to occur because contemporary flowmeter scales are marked either directly onto or to the right of the appropriate flow tube, therefore confusion is minimized.

83. 3.Proportioning System:  Manufacturers have equipped newer machines with proportioning systems in an attempt to prevent delivery of a hypoxic mixture. %  N 2 O & O 2 are interfaced either mechanically or pneumatically so that the minimum O 2 concentration at the common outlet is 25%.

84. 3.Proportioning System: 1.Ohmeda Link-25 Proportion Limiting Control System. % It allows independent adjustment of either valve, yet automatically intercedes to maintain a min.25% O 2 concentration with a maximum N 2 O/O 2 flow ratio of 3:1. The N 2 O & O 2 flow control valves are identical.

85. A 14-tooth sprocket is attached to the N 2 O flow control valve, and a 28- tooth sprocket is attached to the O 2 flow control valve. 3.Proportioning System: 1.Ohmeda Link-25 Proportion Limiting Control System. A chain physically links the sprockets.

86. When N 2 O flow control valve is turned through two revolutions, O 2 flow control valve will revolve once because of the 2:1 gear ratio. 3.Proportioning System: 1.Ohmeda Link-25 Proportion Limiting Control System.

87. The final 3:1 ratio results because N 2 O flow control valve is supplied by 26 Psig, whereas O 2 flow control valve is supplied by 14 Psig. Thus, the combination of the mechanical & pneumatic aspects yields the final O 2 concentration. 3.Proportioning System: 1.Ohmeda Link-25 Proportion Limiting Control System.

88. It is used on the N.A.Dräger Narkomed 2A, 2B, 3, and 4. ±% it is a pneumatic N 2 O/O 2 interlock system designed to maintain a FG O 2 conc. of at least 25 ± 3%. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC).

89. It limits N 2 O flow to prevent delivery of a hypoxic mixture. This is unlike the Ohmeda Link-25, which actively increases O 2 flow. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC).

90. It is composed of an O 2 chamber, a N 2 O chamber & a N 2 O slave control valve; all are inter- connected by a mobile horizontal shaft. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC). O 2 chamberN 2 O chamber

91. The pneumatic input into the device is from the O 2 & N 2 O flowmeters. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC).

92. These flowmeters are unique because they have specific resistors, located downstream from the flow control valves, which create back-pressures directed to the O 2 & N 2 O chambers. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC). Resistor

93. The value of the O 2 flow tube’s resistor is 3–4 times that of the N 2 O flow tube’ s resistor& the relative value of these resistors determines the value of the controlled FG O 2 conc. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC). Resistor The backpressure in the O 2 and N 2 O chambers pushes against rubber diaphragms attached to the mobile horizontal shaft. Diaphragms Movement of the shaft regulates the N 2 O slave control valve, which feeds the N 2 O flow control valve. N 2 O slave control valve

95. If the O 2 pressure is proportionally > the N 2 O pressure, the N 2 O slave control valve opens more widely, allowing more N 2 O to flow. As the N 2 O flow is increased manually, the N 2 O pressure forces the shaft toward the O 2 chamber. The valve opening becomes more restrictive and limits the N 2 O flow to the flowmeter. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC).

96. The N 2 O slave control valve is closed because of inadequate O 2 back-pressure. 3.Proportioning System: 2.N.A. Dr ä ger Oxygen Ratio Monitor Controller (ORMC).

98. 1.Wrong Supply Gas:  In the Link-25 system, the N 2 O and O 2 flow control valves will continue to be mechanically linked, and a hypoxic mixture will proceed to the common outlet. 1.Wrong Supply Gas:  The oxygen rubber diaphragm of the ORMC will recognize adequate " O 2 " pressure, and flow of both the wrong gas plus N 2 O will result. 1.Wrong Supply Gas:  The oxygen analyzer is the only machine monitor that will detect this condition in both systems.

99. 2.Defective Pneumatics Or Mechanics: Pneumatic integrity in the Ohmeda system depends on properly functioning second–stage pressure regulators. A N 2 O/O 2 ratio other than 3:1 will result if the regulators are not precise. The chain connecting the two sprockets must be intact; a 97% N 2 O concentration can occur if it is cut or broken.

100. 2.Defective Pneumatics Or Mechanics: In the N.A.Dräger system, a functional OFPD is necessary to supply appropriate pressure to the ORMC. The mechanical aspects of the ORMC [e.g. rubber diaphragms, flow tube resistors & N 2 O slave control valve] must likewise be intact.

101. 3.Leaks Downstream: The oxygen analyzer is the only machine safety device that can detect the problem. N.A.Dräger system recommends a preoperative +ve pressure leak test to detect such a leak. Ohmeda recommends a preoperative –ve pressure leak test because of the check valve located at the common outlet.

102. 3.Leaks Downstream:

104. 4.Inert Gas Administration:  Administration of a third inert gas, such as helium, N 2, or CO 2, can cause a hypoxic mixture because contemporary proportioning systems link only N 2 O & O 2.  The oxygen analyzer is the only machine safety device that can detect the problem.

106.  Understanding of the components & the operating principles of the gas supply system, safety devices, and flowmeters, is a part from the corner stone in anaesthesia practice.

107. 1.Andrews JJ: Inhaled Anesthetic Delivery Systems. In: Miller RD, ed., Anesthesia, 6 th ed., Philadelphia, Churchill Livingston, pp. 273 – 317, 2005. 2.Andrews JJ: Delivery system for inhaled anesthetics. In: Barash PG, Cullen BF, Stoelting RK, eds., Clinical Anesthesia, 4 th ed., Philadelphia, Pennsylvania, Lippincott Williams and Wilkins, pp 567 – 594, 2001.

109. mohamedrefaateltahan@yahoo.com