Working Principles of ICU ventilatorsDr. Ananya Click to edit Master subtitle style

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Contents Classification History Introduction Indications Key terms- compliance , ventilatory work Components Control mechanism Variables Triggering Factors to consider in mechanical ventilation5/30/12

ClassificationAccording to Robert chatburn Broadly classified into

Negative pressure ventilators Andmanner in which according to the

ventilators

Positive pressure

they support ventilation

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Negative pressure ventilators

Exert a negative pressure on the external chest during inspiration allows air to flow into the lung, filling its volume ventilation is similar to spontaneous ventilation

Decreasing the intrathoracic pressure

Physiologically, this type of assissted

It is used mainly in chronic respiratory

failure associated with neuromascular conditions such as poliomyleitis, muscular dystrophy, a myotrophic lateral sclerosis, and 5/30/12 mysthenia gravis.

The iron lung, often

referred to in the early days as the "Drinker respirator", was invented byPhillip Drinker(1894 1972) andLouis Agassiz Shaw Junior,professors ofindustrial hygiene at theHarvard School of Public Health .The machine was

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powered by an electric motor with air pumps from two vacuum

Biphasic cuirass ventilation

Biphasic cuirass ventilation(BCV) is a

method ofventilation which requires the patient to wear an upper body shell orcuirass, so named after the body armour worn by medieval soldiers. cuirass is attached to a pump which actively controls both theinspiratory andexpiratory phases of the respiratory cycle .

The ventilation is biphasic because the

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DisadvantagesComplex and Cumbersome Difficult for transporting Difficult to access the patient in emergency claustrophobic

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Positive pressure ventilators

Inflate the lungs by exerting positive

pressure on the airway, similar to a bellows mechanism, forcing the alveoli to expand during inspirationExpiration occurs passively. modern ventilators are mainly PPV s and

are classified based on related features, principles and engineering.

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HistoryAndreas Vesalius (1555) Vesalius is credited with the first description

of positive-pressure ventilation, but it took 400 years to apply his concept to patient care. The occasion was the polio epidemic of 1955, when the demand for assisted ventilation outgrew the supply of negative-pressure tank ventilators (known as iron lungs). In Sweden, all medical schools shut down and

medical students worked in 8-hour shifts as human ventilators, manually inflating the lungs of afflicted patients.5/30/12 Invasive ventilation first used at

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INTRODUCTION TO MECHANICAL VENTILATION: CONVENTIONAL MECHANICALVENTILATIONMechanical ventilation is a useful modality for

patients who are unable to sustain the level of ventilation necessary to maintain the gas exchange functions-oxygenation and carbon dioxide eliminationThe first positive-pressure ventilators were

designed to inflate the lungs until a preset pressure was reached.5/30/12 In contrast, volume-cycled ventilation, which

INDICATIONS FOR MECHANICAL VENTILATIONRespiratory Failure Cardiac Insufficiency Neurologic dysfunction Rule 1. The indication for intubation and mechanical

ventilation is thinking of it.Rule 2. Endotracheal tubes are not a disease, and

ventilators are not an addiction5/30/12

Key termsVentilatory workDuring inspiration , the size of the thoracic cage

increases overcoming the elastic forces of the lungs and the thorax and resistance of the airways. As the volume of the thoracic cage increases, intrapleural pressure becomes more negative, resulting in lung expansion.Gas flows from the atmosphere into the lungs as a result

of transairway pressure gradient.During expiration, the elastic forces of the lung and

thorax cause the chest to decrease in volume and exhalation occurs as a result of greater pressure at the alveolus compared to atm. Press.5/30/12

This ventilatory work is proportional to the pressure

required for inspiration times the tidal volume.

LOAD The pressure required to deliver the tidal volume is

referred to as the load that the muscles or ventilator must work against. elastic ( volume & inv. Prop t0 compliance)5/30/12resistance

load

( Raw & inspiratory flow)

Equation of motion for respiratoryventilator pressure = system Muscle pressure +(volume / compliance)+ (resistance x flow)Flow- its the unit of volume by unit of time. Resistance- it is the force that must be

overcome to move the gas through the conducting airways.It is described by the poiseulles law.5/30/12

Lung compliance Lung compliance: Is the change in volume per

unit change in pressure COMPLIANCE = Volume / Pressure

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TypesStatic compliance- is measured when there

is no air flow.

Reflects the elastic properties of the lung and

the chest wall

Dynamic compliance is measured when air

flow is present

Reflects the airway resistance (non elastic

resistance) and elastic properties of lung and chest wall

Static compliance=Corrected tidal volume5/30/12 PEEP

Plateau pressure-

What is a mechanical ventilator?substitute for the ventilatory work accomplished by the patients muscles. Components INPUT POWER DRIVE MECHANISM CONTROL CIRCUIT OUTPUT WAVEFORMS ALARMS5/30/12

A machine or a device that fully or partially

q

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INPUT POWERIt can be Pneumatically powered(uses compressed

gases)

Electrically powered(uses 120 Volts

AC/12Volts DC) Here the electric motor drives pistons and compressors to generate gas flows .Microprocessor controlled- combined.

Also called as 3rd generation ventilators.5/30/12

Source of Gas SupplyAir - Central compressed air, compressor,

turbine flow generator, etc concentrator, O2 cylinder

Oxygen Central oxygen source, O2 Gas mixing unit O2 blender

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DRIVE MECHANISMIts the system used by the ventilator to

transmit or convert the input power to useful ventilatory work. pressure patterns produced by the ventilator. pistons bellows reducing valves

This determines the characteristic flow and

It includes

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pneumatic circuits

Piston mechanismBellows mechanism Pneumatic mechanism

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Pneumatic circuits- uses pressurized gas as

power source. these are microprocessor controlled with

solenoid valves. use programmed algorithms in

microprocessor to open and close solenoid valves to mimic any flow or pressure wave pattern.

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Control circuitIts the system that governs the ventilator

drive mechanism or output control valve.

Classified as Open circuits- desired output is selected and

venti. achieves it without any further input from clinician. Closed circuits- desired output is selected

and venti. Measures a specific parameter (flow/vol/press) continuously and input is constantly adjusted to match desired output.

a.k.a SERVO controlled.

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Control parameters Pressure Volume Flow Time

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Ventilators deliver gas to the lungs using

positive pressure at a certain rate. The amount of gas delivered can be limited by time, pressure or volume. The duration can be cycled by time, pressure or flow. If volume is set, pressure varies..if pressure is set, volume varies.. .according to the compliance...5/30/12

Mechanical- employs levers or pulleys to

control drive mechanism.

Pneumatic Fluidic- applies gas flows and pressure to

control direction of other gas flows and to perform logic functions based on the COANDA effect. Electronic- uses resistors and diodes and

integrated circuits to provide control over the drive mechanism.

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Pressure controllerVentilator controls the trans-respiratory

system pressure .

This trans-respiratory system gradient

determines the depth or volume of respiration.Based on this a ventilator can be positive or

negative pressure ventilator.

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Volume controllerVolume cycled ventilation delivers a: set volume; with a variable Pressure - determined by

resistance, compliance and inspiratory effort

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Flow controllerAllows pressure to vary with changes in

patient s compliance and resistance while controlling flow. venturi pnemotachometers.

This flow is measured by vortex sensors or

Time controllermeasures and controls inspiratory and expiratory time.

These ventilators are used in newborns and infants 5/30/12

Normal inspiratory time of a spontaneously breathing healthy adult is approximately 0. 8- 1.2 seconds, with an inspiratory expiratory (I: E) ratio of 1:1.5 to 1:2 2. Its advantageous to extend the inspiratory time in order to: improve oxygenation - through the addition of an inspiratory pause; or to increase tidal volume - in pressure controlled ventilation Adverse effects of excessively long inspiratory times are haemodynamic compromise, patient ventilator dysynchrony, and the development of autoPEEP.5/30/12

Phase variablesA. Trigger . What causes the breath to begin?

B A

C

B. Limit What regulates gas flow during the breath?

C. Cycle . What causes the breath to end?

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Phases of ventilator supported breath

inspiration change from inspiration to expiration expiration change from expiration to inspiration Types of ventilator breathsMandatory breath5/30/12 Assisted breath

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Trigger variableIts the variable that determines start of

inspiration

Triggering refers to the mechanism through

which the ventilator senses inspiratory effort and delivers gas flow or a machine breath in concert with the patients inspiratory effort.Can use pressure or volume or time or flow as

a trigger.

In modern ventilators the demand valve is

triggered by either a fall in pressure (pressure 5/30/12 triggered) or a change in flow (flow triggered).

Time triggering

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Pressure Triggeringspontaneous inspiratory effort.

Breath is delivered when ventilator senses patients sensitivity refers to the amount of negative pressure the

patient must generate to receive a breath/gas flow.If the sensitivity is set at 1 cm then the patient must

generate 1 cm H2O of negative pressure for the machine to sense the patient's effort and deliver a breath.Acceptable range - -1 to -5 cm H2O below patient s

baseline pressureIf the sensitivity is too high the patient's work of breathing

will be unnecessarily increased. It is not a reasonable course of action to increase the sensitivity to reduce the 5/30/12 patient's respiratory rate as it only increases their work of

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Flow Triggering

The flow triggered system has two preset

variables for triggering, the base flow and flow sensitivity.The base flow consists of fresh gas that

flows continuously through the circuit. The patients earliest demand for flow is satisfied by the base flow. The flow sensitivity is computed as the

difference between the base flow and the exhaled flow flow

Here delivered flow= base flow- returned Hence the flow sensitivity is the magnitude of5/30/12

the flow diverted from the exhalation circuit

Flow trigger Advantages -The time taken for the onset of inspiratory effort to the onset of inspiratory flow is considerably less. -decreases the work involved in initiating a breath.

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Limit variable

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Cycle variableDefined as the length of one complete

breathing cycle. is reached.

Inspiration ends when a specific cycle variable This variable is used as a feedback signal to

end inspiratory flow delivery which then allows exhalation to start. as a feedback signal. time

Most new ventilators measure flow and use it So volume becomes a function of flow and 5/30/12

Baseline variableThe variable controlled during expiration

phase.

Mostly its pressure

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Basic definitionsAirway Pressures Peak Inspiratory Pressure (PIP) Plateau pressures Positive End Expiratory Pressure (PEEP) Continuous Positive Airway Pressure (CPAP)

Inspiratory Time or I:E ratio Tidal Volume: amount of gas delivered with each

breath

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Pressures Mechanical ventilation delivers flow and

volume to the patients as a result of the development of a positive pressure gradient between the ventilator circuit and the patients gas exchange units as illustrated in the diagram above. There are four pressures to be aware of in regards to mechanical ventilation. These are the:

Peak Plateau5/30/12 Mean; and

Peak Inspiratory Pressure (PIP)-

The peak pressure is the maximum pressure obtainable during active gas delivery. This pressure a function of the compliance of the lung and thorax and the airway resistance including the contribution made by the tracheal tube and the ventilator circuit.

Maintained at