تكمله. .. ايمن دBreathing Circuits In Anaesthesia

The Pethick Test for the Bain Circuit

A unique hazard of the use of the Bain circuit is occult disconnection or kinking of the inner, fresh gas delivery hose. If this occurs, the entire corrugated limb becomes dead space.

This results in respiratory acidosis which is unresponsive to increased minute ventilation.

To perform the Pethick test, use the following steps:

1. Occlude the patient's end of the circuit (at the elbow).

2. Close the APL valve.

3. Fill the circuit, using the oxygen flush valve.

4. Release the occlusion at the elbow and flush.

A Venturi effect flattens the reservoir bag if the inner tube is patent.

Pethick Test

Bain system (Mapleson D)

Spontaneous respiration:

The breathing system should be filled with FG before connecting to the patient. When the patient takes an inspiration, the FG from the machine, the reservoir bag and the corrugated tube flow to the patient .

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During expiration, there is a continuous FGF into the system at the patient 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 atmosphere through the valve situated at the end of the corrugated tube near the reservoir bag. 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.

During the next inspiration, the patient breaths FG as well as the mixed gas from the corrugated tube .

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Many factors influence the composition of the inspired mixture.

They are FGF, respiratory rate, expiratory pause, tidal volume and CO2 production in the body.

Factors other than FGF cannot be manipulated in a spontaneously breathing patient.

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 minimise rebreathing .

Controlled ventilation

To facilitate intermittent positive pressure ventilation, the expiratory valve 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 end. During the expiratory pause the FG continues to enter the system and pushes the mixed gas towards the reservoir .

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When the next inspiration is initiated, the patient gets ventilated with the gas in the corrugated tube i.e., a mixture of FG, alveolar gas and dead space gas.As the pressure in the system increases, the expiratory valve opens and the contents of the reservoir bag are discharged into the atmosphere.

Mapleson E and F

Valveless breathing system used for children upto 30 kg

Suitable for spontaneous and controlled ventilation

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Components

T shaped tubing with 3 ports

FGF delivered to one port

2nd port goes to patient

3rd port leads to reservoir tubing

Mapleson F

The most commonly used T-piece system is the Jackson-Rees' modification of the Ayre's T-piece (sometimes known as the Mapleson F system). This system connects a two-ended bag to the expiratory limb of the circuit; gas escapes via the `tail' of the bag.

It comprise of –

Plastic angle mount Plastic Ayre’s T-piece. Corrugated rubber hose. Reservoir bag of 0.5-1 lit.capacity. Green PVC 1.5 meter long tube with plug that fits into the fresh

gas outlet of the Boyle’s apparatus.

Gas flows required – 2-3 times MV.

Dead space – 1ml/lb( 1kg = 2.2lbs.) Tidal volume – 3 times dead space. FGF flushes expiratory limb during the pause. Expiratory limb should be more than TV to prevent air dilution &

rebreathing in spon. Breathing child.

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This allows respiratory movements to be more easily seen and permits intermittent positive ventilation if necessary. The bag is, however, not essential to the functioning of the circuit.

Intermittent positive pressure ventilation (IPPV) may be performed by occluding the tail of the bag between a finger and thumb and squeezing the bag.

Alternatively, a `bag-tail valve', which employs an adjustable resistance to gas flow, may be attached to the bag tail. This causes the bag to remain partially inflated and so facilitates one-handed performance of IPPV.

Another aid to IPPV is the Kuhn bag, which has the gas outlet on the side of the bag, rather than the tail. This allows the outlet to be occluded with the thumb during IPPV, but leads to difficulties in scavenging the waste gases.

A number of different designs of T-piece are available, which function in essentially the same way.

Modern T-pieces incorporate 15 mm fittings for the reservoir tube and endotracheal adapter.

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The volume of the reservoir tubing should approximate the patient's tidal volume. If the volume is too large rebreathing may occur and if too small, ambient air may be entrained.

To prevent rebreathing, the system requires a minimal flow of 4 litre/minute, with a fresh gas flow of 2.5 to 3 times the patient's minute volume.

Advantages

 Compact ,lightweight,no drag to ETT.

Inexpensive ,easy to use & sterilize.

No valves

Minimal dead space

 Minimal resistance to breathing

Economical for controlled ventilation Disadvantages

 The bag may get twisted and impede breathing.

 High gas flow requirement .

Lack of humidification.

Uses

 Children under 20 kg weight .

Enclosed afferent reservoir system

Mapleson A within a box Efficient for both SV & IPPV Complicated Not often mentioned

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Humphrey ADE circuit

Combines the Mapleson A, D & E systems with a switch to select the mode

Efficient in both SV & IPPV Suitable for both adult & paeds Increased complexity

The Mapleson A circuit is inefficient for controlled ventilation as is the Mapleson D circuit for spontaneous ventilation.

David Humphrey has designed a single circuit that can be changed from a Mapleson A system to a Mapleson D by moving a lever on the metallic block which connects the circuit to the fresh gas outlet on the anaesthetic machine.

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The reservoir bag is situated at the fresh gas inlet end of the circuit, and gas is conducted to and from the patient down the inspiratory and expiratory limbs of the circuit.

BREATHING SYSTEMS WITH CO2 ABSORPTION

Systems so far described have relied on FGF for effective elimination of CO2.

Any desire to economize on FGF by allowing a total rebreathing, should be accompanied by removal of the expired CO2 by chemical absorption using sodalime or baralyme.

The systems designed for these purpose are again classified as:

Unidirectional flow.

-Circle system.

Bi-directional flow.

-To and fro system.

Circle System

The essential components of the circle system are,

(1) a sodalime canister,

(2) Two unidirectional valves,

(3) Fresh gas entry,

(4) Y-piece to connect to the patient,

(5) Reservoir bag

(6) a relief valve and

(7) low resistance interconnecting tubing.

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Components of circle system

For efficient functioning of the system the following criteria should fulfilled.

(1) There should be two unidirectional valves on either side of the reservoir bag,

(2) Relief valve should be positioned in the expiratory limb only,

(3) The FGF should enter the system proximal to the inspiratory unidirectional valve.

Functional analysis

During inspiration the FG along with the CO2 free gas in the reservoir bag flow through the inspiratory limb and inspiratory unidirectional valve to the patient.

No flow takes place in the expiratory limb as the expiratory unidirectional valve is closed by back pressure transmitted to the valve.

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During expiration the inspiratory unidirectional valve closes and the expired gas flows through the expiratory unidirectional valve in the expiratory limb to the sodalime canister and to the reservoir bag.

The CO2 is absorbed in the canister. The FGF from the machine continues to fill the reservoir bag. When the reservoir is full the relief valve opens and the excess gas is vented to atmosphere.

By selecting a suitable position for the relief valve, the expired gas can be selectively vented when the FGF is more than the alveolar ventilation.

To facilitate controlled ventilation the relief valve has to be partly closed and the excess gas is vented during inspiration.

Unidirectional valves

The unidirectional inspiratory and expiratory valves in most circle absorbers are of the turret type, in which the pressure generated by the patient's breathing causes the disc to rise and allow gas to pass in one direction only. Most have a transparent dome so that the operation of the valve may be observed

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The disc material may be mica, ceramic or plastic. Plastic is less expensive, but tends to warp and allow the valve to become incompetent.

Incompetence may also be caused by the valve sticking in the open position, owing to condensation of water vapour.

Incompetent inspiratory or expiratory valves will reduce the efficiency of gas circulation and result in rebreathing and consequent carbon dioxide retention.

Some machines are equipped with valves made of deformable rubber:

As the rubber ages, these discs tend to harden in a semi-open position, again allowing the valve to become incompetent.

Connecting tubing

The body of the absorber is connected to the patient by means of inspiratory and expiratory tubes and a Y-piece. This may be constructed of corrugated black rubber, neoprene or, more recently, plastic.

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Recently, the so-called Universal F circuit has become popular. This is a co-axial system, the inspiratory tube running inside the expiratory limb:

This arrangement aids warming and humidification of the inspired gases, albeit at the expense of an increase in inspiratory resistance to breathing.

About dead space

Most circle absorbers are satisfactory for use in patients weighing up to around 100 kg.

The major problem with using standard circle absorbers in smaller patients is that of dead space. 

Patients with very small tidal volumes may not generate enough pressure to open the valves effectively. The effective dead space of the Y-piece is larger than it appears.

Inevitably, some portion of the expired gas is directed down the inspiratory limb of the circuit, and some portion of the inspired gas comes from the expiratory limb, and some mixing of inspired and expired gases occurs.

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These difficulties may be reduced by the use of purpose-built infant absorbers, which are smaller than the standard models. Paediatric tubing and Y-pieces, which are simply smaller in diameter than the standard type, may be helpful.

Double-canister absorbers

Many absorbers designed for use employ two canisters placed in series:

The top canister is exposed to the expired gases first and removes most of the carbon dioxide.

Any remaining carbon dioxide is then removed by the bottom canister.

When the top canister is exhausted, the absorbent is discarded, the bottom canister is placed in the top position and a canister with fresh absorbent is inserted underneath it.

This arrangement provides optimal efficiency and economy in carbon dioxide absorption. However, these absorbers are bulkier, heavier and more expensive than single-canister models.

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Canisters

Sodalime – Brian Sword 1926

Barylime.

Sterilization of canister :

Washing with soap water, Immersion in Cidex for 2 hours.

Sodalime

Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia , submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning.

It is made by treating slaked lime with concentrated sodium hydroxide solution.

Sodalime

Chemical components

The main components of soda lime are

Calcium hydroxide, Ca(OH)2 (about 75%),

Water, H2O (about 20%),

Sodium hydroxide, NaOH (about 3%), and

Potassium hydroxide, KOH (about 1%).

Chemical composition of absorbents

Sodium/ Potasium:

Small quantities of sodium hydroxide (1.5-5%) are usually added to enhance the reactivity & the hygroscopic property of the mixture.Hence the reason that absorbents are often referred to as ‘soda lime’.

Some manufacturers have added potassium hydroxide for similar reason.

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Water content

The optimal moisture content of the absorbent mixture is between 14 – 16 % .

This is essential for the chain reactions

Chemical reaction

The overall reaction is:

CO2 + Ca(OH)2 → CaCO3 + H2O + heat (in the presence of water)

The reaction can be considered as a strong base catalysed, water facilitated reaction.

steps:

1) CO2 + H2O → CO2 (aq) (CO2 dissolves in water - slow and rate determining)

2) CO2 (aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)

3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH (NaOH recycled to step 2) - hence a catalyst)

Each mole of CO2 (44g) reacted produces one mole of water (18g)

The reaction is interesting in that

It produces heat energy ( an exothermic reaction).

It changes the pH of the soda lime, which allows the use of indicator dye to show the soda lime is exhausted.

It produces more water than that used up in the reaction.

The exothermic reaction

The heat & water produced by the reaction of soda lime on carbon dioxide is considered beneficial in that they warm & partialy humidify the inspiratory gas.

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The temperature & humidity of inspired gas is related to a number of factors:

If the FGF is high the dry gas entering the system reduces both the humidity & the temperature.

At low FGFs, the gas circulation time is high, the humidity & the temperature rises.

The longer the system is in use at low FGFs, the greater are the humidity & the temperature of circulating gas.

The heat produced can increase the chemical reaction between volatile anaesthetic agents & absorbents.

Trichloroethylene can be decomposed to dichloroacetylene which is neurotoxic & further to phosgene if temp. exceeds 60°C.

Anaesthetic agents like desflurane,enflurane& isoflurane react with dry, warm soda lime or barium lime to produce carbon monoxide.

Dry barium lime which contains potassium hydroxide has greater tendency to produce CO.

Other constituents

Zeolite:

Zeolites are three dimensional,microporous crystalline solids that contains aluminium,silica & oxygen.

They may be added to soda lime to increase:

Porosity of the mixture

Hardness

Water content.

Silica

Small quantities of sodium & potassium hydroxide (1.5-5%) are usually added to enhance the reactivity & the hygroscopic property of the mixture.

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However ,they are the main cause of degradation by soda lime of isoflurane,desflurane to CO & sevoflurane to compound A,formaldehyde & methanol.

The granules require the addition of silica ( LoFloSorb) to overcome this.

The new absorbents have been shown to minimize the production of unwanted compounds even when dry

Calcium chloride

The addition of 3% calcium chloride (Amsorb Plus) in place of sodium or potassium prevent the formation of CO with isoflurane & desflurane and compound A with sevoflurane.

The product also contain 1% calcium sulphate to improve the hardness of the granule.

Indicators for Absorbents

Indicator Colour when fresh Colour when Exhausted

Phenolphthalein White Pink

Ethyl violet White Purple

Clayton yellow Red Yellow

Ethyl orange Orange Yellow

Mimoza Z Red White

So colour chane of indicator is one of the sign of exhaustion of sodalime.

Other signs of exhaustion of sodalime,

Tachycardia

Hypertension

Increased oozing from wound site

Increased end tidal CO2 on capnography.

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Now a days a good quality sodalime with prolonged life ,Durasorb is used which is pink when fresh & white when exhausted.

Hardness of granules should be > 75.

Moisture 14- 19% needed for CO2 absorption.

Size of granules 4 – 8 mesh.

The canister should have 53% of air space.

CO2 absorption is a heat generating process 13,700 calories are produced for 1 mole of CO2 absorbed.

1lb canister lasts for 2 hrs if used continuously.

100 gms of sodalime can absorb 24 to 26 liters of CO2.

Barylime

Composition of Barylime

Ca(OH)2 : 80%

Ba (OH)2 : 20%

It does not require silica.

Barylime is 15% less efficient than sodalime.

Barylime perform well in dry climate.

Indicator – ethyl violet or mimoza Z.

Disadvantage – Chemical skin burn,

- Excessive heat due to exhaustion,

- No regeneration.

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Factors affecting CO2 absorption in closed circuit

Freshness of sodalime.

Tidal volume of patient.

High flows.

Dead space.

Ill filled sodalime.

Resistance to outflow.

Operational requirements

 The volume of the breathing bag must be greater than the patient's inspiratory capacity. This is usually estimated at 30 ml/kg body weight.

  Since soda lime contains 50% - 70% air around the granules, the volume of the absorber canister should be at least double that of the tidal volume of the patient for optimal efficiency.

Fresh gas flow requirements

Closed systems

In truly closed systems, the patient consumes oxygen and expires carbon dioxide, which is removed from the system by absorption. The volume of oxygen flowing into the system must, therefore, equal the patient's oxygen consumption.

Resting oxygen consumption is approximated by the formula:

Oxygen consumption (ml/min) = 10 x BW 0.75.

Body weight (kg) Oxygen consumption (ml /min)

5 33 10 56 20 95 40 160

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The use of nitrous oxide in closed systems presents the difficulty that, after equilibration, nitrous oxide will accumulate in the circuit and result in a hypoxic breathing mixture. If it is desired to use nitrous oxide in a closed system, it is mandatory to employ an inspired oxygen concentration monitor.

Semi-closed

When using a semi-closed system, the oxygen flow rate must exceed the patient's oxygen consumption. Any excess is simply lost via the pressure relief valve.

Initially, it is necessary to use both a high flow rate and high vaporiser setting to raise the concentration of anaesthetic in the circuit. For maintenance, both the vaporiser setting and fresh gas flow rate may be reduced.

As a general rule, a flow rate of 2 to 3 litres per minute initially, and 500 ml to 1 litre per minute during maintenance of anaesthesia, will usually prove satisfactory.

Advantages of the circle system

Economical.

Warming and humidification of the inspired gases.

Reduced atmospheric pollution.

For condition in which there is very high production of CO2 only closed circuit can eliminate such high CO2.(malignant hyperthermia).

Disadvantages

 Heavy weight cuicuits.

Chances of accidental extubation & disconnections are high.

If sodolime is exhausted can produce dangerous hypercarbia.

Cross infection from apparatus.

Breathing resistance & dead space high.

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Some inhalational agents can produce toxic products when used with sodalime.

Apparatus dead space ( closed Circuit)

3 corrugated tubes – 1000 ml.

Canister (Intergranular space) – 500 ml.

Reservoir bag – 1500 ml.

Anatomical dead space – 150 ml.

LOW FLOW ANESTHESIA

Why Low Flow ?

Low waste

Low expense

Safe

Effective

Planet-friendly

Warm patient

Moist CO2 absorbent

A FEW DEFINITIONS

Low Flow Anesthesia (LFA) has been variously defined as an inhalation technique in which a circle system with absorbent is used with a fresh gas inflow of :

- less than the patient’s alveolar minute volume- less than 1-1.5 l/min- 3 l/min or less- 0.5 – 2 l/min- less than 4 l/min- 500 – 1000 ml/min

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Closed System Anesthesia is a form of LFA in which the FGF = uptake of anesthetic gases and oxygen by the patient and gas sampling.

No gas is vented by the APL valve.

Dorsch and Dorsch

Understanding Anesthesia Equipment

Definition of low-flow

Metabolic flow = 250ml/minMinimal flow = 250-500 ml/minLow flow = 500-1000 ml/minMedium flow = 1-2 L/minHigh flow = 2-4 L/minSuper-high flow = >4 L/min

Low-flow anesthesia can be defined as a technique which, using a rebreathing system, results in at least 50% of the exhaled air being returned to the lungs after CO2 absorption.

(Low-flow anaesthesia, Anaesthesia, 1995, Volume 50 (supplement), pages 37-44)

THE REBREATHING SYSTEMS

REBREATHING describes a technique in which non-consumed gases, contained in the exhaled air are partially or completely re-routed back to the patient during the following inspiration, purified from CO2 AND admixed with a certain amount of fresh gas.

Basic concepts

The recognition of the functional residual capacity as deadspace or extension of the circuit.

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The recognition of the alveolar membrane as a barrier of anesthetic uptake.

Via the observation that anaesthetic vapours are excreted unchanged with the expired air and serial experiments, John Snow concluded In 1850 that:

“ It follows as a necessary consequence of this mode of excretion of a vapour that, if its exhalation by the breath could in any way be stopped, its narcotic effects ought to be much prolonged.”

TECHNICAL ASPECTS

In LFA there are a few technical requirements.

1. Circle rebreathing system with CO2 absorption.

2. Accurate flow meters for adjustments of FGF below 1L/min.

3. Gas tight breathing system.

4. Ascending bellows.

5. Continuous gas monitoring MUST be employed. From the clinical standpoint the measurement of expiratory gas concentrations close to the Y-piece is of crucial importance. That information is essential in controlling the patient’s alveolar gas concentrations.

ADVANTAGES OF THE LFA

1. QUALITY OF PATIENT CARE2.

2.ECONOMIC BENEFITS

Over 80% of anesthetic gases are wasted when flows of 5L/min are used.

3. ENVIRONMENTAL BENEFITS

THE OTHER SIDE OF THE COIN

Disadvantages of LFA:

1.Limitations of currently used vaporizers.

2.Accumulation of unwanted gases into the breathing system:

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HOW TO ADJUST FGF AT DIFFERENT PHASES OF LFA

Premedication, pre-oxygenation and induction of sleep are performed according to the usual practice. Concerning adjustment of FGF anesthesia can be divided into 3 phases:

1. Initial HIGH flow

2. Low flow

3. Recovery

Water’s Canister

To-and-fro system. By Prof.Ralph Water 1922.

Since gases travel through canister during controlled ventilation and return through canister to reservoir bag,it is called to & fro canister.

Mapleson C with a soda lime metalic canister between the APL valve and the reservoir.

Cylindrical in shape,handy,can be autoclaved.

Size – 8 by 12 cms,holds 1 kg sodalime.

Has 2 buffer porous plates to hold sodalime.

Intergranular space = Tidal volume.

Resistance – 2.5 – 3 cm H2O.

Not currently widely used

Advantage:

Efficient for 1-2 hrs for controlled ventilation.

Easy sterilization by autoclaving ; can be used for infected pts like pulm.TB.

Handy , easy to carry.

Disadvantages:

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Non transparent(metallic).

Very close to pt’s respiratory tract – Risk of dust reaching the pt. Risk of inhaling soda lime

It becomes warm with heat generated during chemical reaction.

It needs frequent changing.

Channeling , caking of sodalime.

Increasing dead space as granules near the patient are exhausted first

Nonrebreathing systems

They use non rebreathing valves and there is no mixing of fresh gas and the expired gas.

The fresh gas flow (FGF) should be equal to the minute ventilation (MV) of the patient.

These systems satisfy all essential requirements, but are not very popular because of the following reasons:

1) Fresh gas flow has to be constantly adjusted and is not economical.

2) There is no humidification of inspired gas.

3) There is no conservation of heat.

4) They are not convenient as the bulk of the valve has to be positioned near the patient.

5) The valves can malfunction due to condensation of moisture and lead to complications.

Functional analysis

When the patient takes a breath, or if the reservoir bag is squeezed, the inspiratory unidirectional valve opens and the gases flow into the patient’s lungs .

The expiratory unidirectional valve closes the expiratory port during spontaneous breathing.

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The inspiratory unidirectional valve itself closes the expiratory port during controlled ventilation.

At the start of expiration, the inspiratory unidirectional valve returns back to position and expiration takes place through the expiratory port, opening the expiratory valve.

Nonrebreathing system

Non rebreathing valves

Ruben Non-Rebreathing Valve

Light transparent piece .

Has a clear plastic body with metal fitting.

One end to the patient,other end to the gas supply & perforated end to void the exalations to the atmosphere.

It works on a bobbin which moves against the tension of a light spring.

Dead space- 9ml.

Resistance to gas flow- 0.8cmH2O during inspiration &1 cmH2O during expiration when flow rate is 25 L/min.

Can be used for spontaneous & controlled ventilation.

The valve should not be sterilized by heating as made up of plastic material.

Can be effectively sterilized by soaking with 5% chlorhexidine in water for 5 min.

Ruben Non-Rebreathing Valve

Ambu Non-rebreathing Valve

Has two silicone rubber flaps instead of a bobbin.

Can be easily dismantled, cleaned & autoclaved or gas sterilized.

Can be used for spontaneous & controlled ventilation.

Three types – Ambu E

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Ambu Hesse

Ambu E2.

Ambu E

Ambu E valve has got dead space of 10 ml.

Resistance to gas flow ranges from 0.6 to 2.1 cm of H2O in inspiration & 0.6 to 2.5 cm of H2O in expiration with a flow rate 5 to 40 L/min.

In Ambu E the fresh gas pass through first nonreturn valve which opens during inspiration.

During expiration first valve closes & second valve opens to allow the expired gases to void in the atmosphere.

During inspiration the second valve remains closed so that air from outside cannot enter.

During expiratory pause excess gases pass outside as both valves open partially.This prevents excessive build up of pressure.

Ambu Hesse

Ambu Hesse was founded in 1937 in Denmark by engineer Dr. Holger Hesse, Ph.D.

Ambu Hesse valve is of low resistance to gas flow only 0.4 cm of H2O during inspiration & 0.6 cm of H2O during expiration with flow rate of 25 L/min.

Dead space – 14 ml.

More suitable valve to use with EMO apparatus.

Ambu E2 valve

The Ambu E2 valve differs from the Ambu E valve in having no expiration shuttter, thus in spontaneous respiration air is drawn in from the expiratory side.

In controlled ventilation only fresh gases are passed to the patient due to closure of exalation port.

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