Principles of ICU ventilators

Working Principles of ICU ventilatorsDr. Ananya

Contents ClassificationHistoryIntroductionIndicationsKey terms- compliance , ventilatory workComponentsControl mechanismVariablesTriggeringFactors to consider in mechanical ventilationWave-forms

Classification According to Robert chatburn

Broadly classified into Negative pressure ventilators And according to the manner in which Positive pressure ventilators they support ventilation

Negative pressure ventilatorsExert a negative pressure on the external chestDecreasing the intrathoracic pressure during inspiration allows air to flow into the lung, filling its volumePhysiologically, this type of assissted ventilation is similar to spontaneous ventilationIt is used mainly in chronic respiratory failure associated with neuromascular conditions such as poliomyleitis, muscular dystrophy, a myotrophic lateral sclerosis, and 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 powered by an electric motor with air pumps from two vacuum cleaners. The air pumps changed the pressure inside a rectangular, airtight metal box, pulling air in and out of the lungs

Biphasic cuirass ventilationBiphasic 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. The ventilation is biphasic because the cuirass is attached to a pump which actively controls both theinspiratory andexpiratory phases of the respiratory cycle .

Disadvantages Complex and CumbersomeDifficult for transportingDifficult to access the patient in emergencyclaustrophobic

Positive pressure ventilatorsInflate 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.

History

Andreas 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.Invasive ventilation first used at Massachusetts General Hospital in 1955.Thus began the era of positive-pressure mechanical ventilation (and the era of intensive care medicine).

INTRODUCTION TO MECHANICAL VENTILATION:CONVENTIONAL MECHANICAL VENTILATIONMechanical 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. In contrast, volume-cycled ventilation, which inflates the lungs to a predetermined volume, delivers a constant alveolar volume despite changes in the mechanical properties of the lungs.

INDICATIONS FOR MECHANICAL VENTILATION

Respiratory FailureCardiac 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 addiction

Key termsVentilatory work- During 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.

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. load elastic ( volume & inv. Prop t0 compliance)resistance ( Raw & inspiratory flow)

Equation of motion for respiratory systemMuscle pressure + ventilator 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.

Lung complianceLung compliance: Is the change in volume per unit change in pressureCOMPLIANCE = Volume / Pressure

Types Static compliance- is measured when there is no air flow. Reflects the elastic properties of the lung and the chest wallDynamic compliance is measured when air flow is presentReflects the airway resistance (non elastic resistance) and elastic properties of lung and chest wallStatic compliance=Corrected tidal volume Plateau pressure-PEEPDynamic compliance corrected tidal volume Peak inspiratory pressure-PEEP

What is a mechanical ventilator?A machine or a device that fully or partially substitute for the ventilatory work accomplished by the patients muscles.Components INPUT POWER DRIVE MECHANISM CONTROL CIRCUITOUTPUT WAVEFORMSALARMS

INPUT POWERIt can bePneumatically 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.

Source of Gas Supply

Air - Central compressed air, compressor, turbine flow generator, etcOxygen Central oxygen source, O2 concentrator, O2 cylinderGas mixing unit O2 blender

DRIVE MECHANISMIts the system used by the ventilator to transmit or convert the input power to useful ventilatory work.This determines the characteristic flow and pressure patterns produced by the ventilator.

It includes pistonsbellowsreducing valvespneumatic circuits

Piston mechanism

Bellows mechanismPneumatic mechanism

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.

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.

Control parameters

Pressure Volume Flow Time

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...

Mechanical- employs levers or pulleys to control drive mechanism.PneumaticFluidic- 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.

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.

Volume controllerVolume cycled ventilation delivers a: set volume; with a variable Pressure - determined by resistance, compliance and inspiratory effort

Flow controllerAllows pressure to vary with changes in patient s compliance and resistance while controlling flow.This flow is measured by vortex sensors or venturi pnemotachometers.

Time controllermeasures and controls inspiratory and expiratory time.These ventilators are used in newborns and infantsInspiratory time is a combination of the inspiratory flow period and time taken for inspiratory pause. The following diagram depicts how the addition of an inspiratory pause extends total inspiratory time.

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 toincrease tidal volume - in pressure controlled ventilationAdverse effects of excessively long inspiratory times are haemodynamic compromise,patient ventilator dysynchrony, and the development of autoPEEP.

Phase variables A. Trigger .What causes the breath to begin?B. Limit What regulates gas flow during the breath?C. Cycle .What causes the breath to end?

ABC

Phases of ventilator supported breathinspirationchange from inspiration to expirationexpiration change from expiration to inspirationTypes of ventilator breaths-Mandatory breathAssisted breathSpontaneous breath

Trigger variableIts the variable that determines start of inspirationTriggering 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 triggered) or a change in flow (flow triggered). With pressure triggered a preset pressure sensitivity has to be achieved before the ventilator delivers fresh gas into the inspiratory circuit. With flow triggered a preset flow sensitivity is employed as the trigger mechanism.

Time triggering

Pressure Triggering

Breath is delivered when ventilator senses patients spontaneous inspiratory effort.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 patient's respiratory rate as it only increases their work of breathing.

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 flowHere delivered flow= base flow- returned flow Hence the flow sensitivity is the magnitude of the flow diverted from the exhalation circuit into the patients lungs. As the subject inhales and the set flow sensitivity is reached the flow pressure control algorithm is activated, the proportional valve opens, and fresh gas is delivered.

Flow triggerAdvantages --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.

Limit variable

Cycle variableDefined as the length of one complete breathing cycle.Inspiration ends when a specific cycle variable is reached.This variable is used as a feedback signal to end inspiratory flow delivery which then allows exhalation to start.Most new ventilators measure flow and use it as a feedback signal.So volume becomes a function of flow and timeVolume= flow x inspiratory time

Baseline variableThe variable controlled during expiration phase.Mostly its pressure

Basic definitionsAirway PressuresPeak Inspiratory Pressure (PIP)Plateau pressuresPositive End Expiratory Pressure (PEEP)Continuous Positive Airway Pressure (CPAP)

Inspiratory Time or I:E ratioTidal Volume: amount of gas delivered with each breath

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 Plateau Mean; and End expiratory pressures.

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