low pressure system
DESCRIPTION
Low pressure system. Presented by : Dr Rashmi Moderator: Dr Gian Chauhan. Starts downstream of flow control devices Pressures: Slightly above atmospheric pressure (Max upto 6-8psig) & variable Components: Flowmeters Hypoxia prevention safety devices Unidirectional valves - PowerPoint PPT PresentationTRANSCRIPT
LOW PRESSURE SYSTEM
Presented by : Dr RashmiModerator: Dr Gian Chauhan
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
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
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)
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.
Flowmeter Assembly:
Tube , Scale , Indicator,
Stop at the top
Lights
Coding (color, touch,
name)
Plastic shield (for
protection)
Tubes:
Glass tubes
Single taper (different tubes for low and high
flows)
Double taper (single tube with gradations for
fine and coarse flows)
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
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
FLOWMETER ARRANGEMENT
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
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)
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
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
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
MECHANICAL LINKAGE
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
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)
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)
Detachable mounting: Are standard on most new anesthesia machines, allow the vaporizer to be mounted & removed without use of tools.
Select-a-tec system: 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.
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)
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
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
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
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
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
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
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
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
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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