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Mechanical Ventilation

HistoryDefinitionsParameters/Variables.ModesTrouble Shooting

Mechanical Ventilator

A mechanical ventilator is a machine that generates a controlled flow of gas into the patient’s airway, to support the ventilatory function of the respiratory system and improves oxygenation, through application of high oxygen content and a positive pressure.

Classification

Robert Chatburn:Negative Pressure Ventilators.Positive Pressure Ventilators.

(according to the manner in which they support ventilation)

Negative Pressure Ventilators

Exert a negative pressure on the external chestDecreasing the intrathoracic pressure during

inspiration allows air to flow into the lung, filling its volume

Physiologically, this type of assisted ventilation is similar to spontaneous ventilation

It is used mainly in chronic respiratory failure associated with neuromascular conditions such as poliomyleitis, muscular dystrophy, amyotrophic lateral sclerosis, and mysthenia gravis.

The Iron Lung

The iron lung, often referred to in the early days as the "Drinker respirator", was invented by Phillip Drinker(1894 – 1972) and Louis Agassiz Shaw Junior, professors of industrial hygiene at the Harvard 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

Emerson Iron Lung.

Biphasic cuirass ventilation

Biphasic cuirass ventilation (BCV) is a method of ventilation  which requires the patient to wear an upper body shell or cuirass, 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 the inspiratory and expiratory phases of the respiratory cycle .

Disadvantages

Complex and CumbersomeDifficult for transportingDifficult to access the patient in emergencyClaustrophobic

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).

Important Terms:

Peak Inspiratory PressureTotal Inspiratory TimeEnd Expiratory PressureMean Airway PressurePeak Airway PressurePlateau PressureBasic Equation for Gas Flow

ComplianceResistanceTime ConstantCritical Volume

Important Terms:

Peak Inspiratory Pressure: Highest pressure during the inspiratory phase.

It depends upon: Lung- Thoracic wall compliance.Airway Resistance.Delivered Tidal VolumeInspiratory Flow RateEnd Expiratory Pressure (PEEP)Work of Breathing

Total Inspiratory Time= Inspiratory time + Inspiratory Pause Time Inspiratory Time: Time during which Tidal

Volume is delivered. Inspiratory Pause time: Time during which

Gas is allowed to distribute from Upper to Lower Airways.

End Expiratory Pressure: Pressure maintained in the lungs during the Expiratory Phase.

If allowed to drop to atmospheric pressure: it is known as ZEEP (Zero End Expiratory Pressure)

Negative: NEEP ( Not used any more)Positive: PEEP ( Used Commonly)Mean Airway Pressure: Pressure during one

Respiratory Cycle.

Peak Airway Pressure: Maximal Airway Pressure during any time during Inflation.

Plateau Pressure: Measured when the tidal inflation volume is held in the lungs after the end of inflation, preventing the lungs from deflating.

End expiratory Pressure is set by the presence or absence of special devices.

Basic Equation for Gas flow during Positive Pressure Ventilation:

Total Pressure Gradient=(Elastance x Volume) + (Resistance x Flow)= (Positive Pressure Generated by the

Ventilator) – (Negative pleural Pressure generated by inspiratory muscles)

Work of Breathing

Work is performed whenever an applied force causes displacement of any mass.

Work = Mean Pressure x Volume Change (Area under the Pressure-Volume curve)

Work is done by overcoming:Compliance of Lung and Thoracic Cage.Resistance of airways.Frictional Resistance of TissuesInertia.

Important Terms:

Compliance: “Ease of Distension of Alveoli”Compliance = Change in Volume Change in Pressure

Elastance: Ability of the lung parenchyma to return to its original form after being stretched.

Elastance = Change in Pressure Change in Volume

Compliance

Static Compliance : Tidal Volume Plateau Pressure –

PEEPValue is affected by lung and chest wall

compliance.

Dynamic Compliance : Tidal Volume Peak Pressure –

PEEP

Value is affected by Airway Resistance and Static Compliance.

Static Compliance: measured when the flow of air has ceased, as during Breath holding or during Apnoea.

Static Compliance= ∆Volume in Litres∆ Pressure in cm of H20

(kPascals)Compliance of lungs: 0.2 litres/cm of H20 (2.0

L/kPa)Compliance of Thoracic Wall: 0.2 Litres/cm of

H20.

Therefore, a Volume change of 0.2 litres in the thorax is obtained by a pressure of 1 cm of H20 exerted against the lungs in conjunction with a pressure of 1 cm of H20 against the Thoracic wall, giving a total Thoracic Compliance of 0.1 litre/cm of H20.

= 0.2 = 0.1 litre/cm of H20

1+1

Resistance:

It is a measure of opposition to flow, which must be overcome to move a volume of air through the patient’s conducting airways in a unit of time.

Flow Rate = Tidal Volume ( L/sec) Inspiratory time

Resistance = P Peak – P Plateau (cmH20/L/sec) Flow RateNormal: 0.6-2.4 cm H2o/L/sec.

Time Constant: it is a phenomenon, whereby a given percentage of a passively exhaled breath of air, will require a constant amount of time, to be exhaled, regardless of the starting volume. (with constant lung mechanics)

Time Constant = Resistance x Compliance

Critical Volume

The alveolus behaves like an elastic semi sphere and obeys the Laplace Equation:

Distending intra alveolar pressure = 2 x Surface tension of air liquid interfaceRadius of alveolus

Mechanical Ventilation

Aims:Restore OxygenationRestore VentilationDecrease work of BreathingReplace inadequate respiratory drive due to

central nervous system dysfunction.

Power : Mechanical Ventilators are powered either by electrical energy or potential energy in compressed air/oxygen.

Oxygen Blender: Oxygen blenders need an input of air and oxygen to blend it to deliver the mixture at the set FiO2.

Ventilation Parameters:

1) Control Variables2) Phase Variables3) Conditional Variables.

Ventilation Parameters:

1. Control Variables: a) Pressure : regulates the airway pressure

to be maintained during inhalation. b) Volume and Flow: Volume= Flow x Inspiratory time c) Time. Used to control set pressure for a

prescribed time.

2) Phase Variables: Control the events taking place in a

Ventilatory Cycle: a) Initiate Inflation b) Inflation Phase c) Cycle to Exhalation d) Exhalation.

4 factors can be monitored and controlled during any of these phases:

a) Pressureb) Flowc) Volumed) Time

a) Initiate Inflation

The ventilator can be cycled to the inspiratory phase by:

1) Patient Effort: Patient Triggering2) After a predetermined time: Time

Triggering.3) Manually : Manual Triggering.

A) Patient effort:PressureWhen the patient attempts to inhale, there

occurs a drop in baseline pressure. The ventilator senses the drop in pressure.Flow: The patient’s inspiratory effort causes a

decrease in the baseline flow through the circuit.

B) Time:A timing mechanism decides the changeover,

and is independent of the patient.It is used in:- Fully controlled ventilation- back up safety system in case the patient

fails to trigger inspiration.

b) Inflation Phase:

These may be pressure, volume or flow.The limit may be reached before inspiration

ends and are used as limiting variables.The time period between the end of

inspiration and the start of exhalation is the inspiratory pause time.

C) Stop Inflation(inflation exhalation changeover

Determines the termination of the inhalation phase, can be based on:

VolumePressureTimeFlow.

Volume Cycling: Inspiratory Phase ends when a pre-set volume is delivered.

Pressure Cycling: Inspiratory Phase ends when the pressure reaches a pre-set value.

Time Cycling: Cycling occurs when the Pre-set inspiratory time is completed.

D) Exhalation Phase

The only function of the ventilator in the exhalation phase is to allow the lungs to empty.

Exhaled volumes may be measured by the lungs during this phase.

Exhalation can be to:- Atmospheric Pressure ( ZEEP)- Sub Atmospheric Pressure (NEEP)- Above Atmospheric Pressure (PEEP)

Positive End Expiratory Pressure(PEEP)

PEEP increases the end- expiratory or baseline airway pressure to a value greater than atmospheric (0 cm H2o on the ventilatory manometer)

It is often used to improve the patient’s oxygenation status, especially in hypoxemia that is refractory to high level of Fio2.

Beneficial Effects of PEEP:

1. Improves Oxygenation.2. Improves Lung Compliance.3. Reduces Dead Space.4. Helps to keep alveoli open in those with

High Closing volumes.5. Decreases Work of Breathing.6. Effect on Cardiovascular Function.

Indications for PEEP

1)Refractory Hypoxemia caused by intra-pulmonary shunting.

Refractory Hypoxemia is defined as hypoxemia that responds poorly to moderate to high levels of oxygen.

May be caused by reduction of Functional Residual Capacity, Atelectasis, or Low V/Q mismatch.

2) Decreased FRC and Lung Compliance.• A severely decreased FRC and lung

compliance can increase the Alveolar Opening Pressure.

• This increases the work of breathing, and can lead to fatigue of respiratory muscles.

• PEEP increases the FRC.

Physiology of PEEP

PEEP reinflates collapsed alveoli and maintains alveolar inflation during exhalation.

Once recruitment of these alveoli occcurs, PEEP lowers the alveolar distending pressure and facilitates gas diffusion and oxygenation.

Normally, the alveolar end-expiratory pressure equilibrates with atmospheric pressure (zero pressure)

The average pleural pressure is approximately -5 cm of H20.

The alveolar distending pressure is 5 cm of H20.

This distending pressure is sufficient to maintain a normal end-expiratory alveolar volume to overcome the elastic recoil of the alveolar wall.

PEEP

INCREASE IN ALVEOLAR DISTENDING PRESSURE

•

INCREASES FRC BY ALVEOLAR RECRUITMENT

Improves Ventilation

Increases V/QImproves OxygenationDecreases work of breathing.

Disadvantages of PEEP:

1) Barotrauma2) Increase in total lung water: Due to increase in

pulmonary venous and lymphatic pressures.3) Reduction in Cardiac Output and O2 Delivery: Due to

reduced Venous Return, Raised Pulmonary Vascular Resistance and RV dilatation.

4) Organ Perfusion:PEEP increases Cerebral Venous Pressures: Increases

ICPRenal: ADH levels increase. Reduced Urine Output.Hepatic: Elevated Intra thoracic pressure, can lead to

decreased Hepatic Venous return > Hepatic Congestion.

Auto PEEP

The Physiological Manifestation of Hyperinflation from air trapping is called Intrinsic PEEP.

IN this condition, the end expiratory pressure remains positive even in the absence of external PEEP.

It can occur due to:Abnormal respiratory mechanics of the

patient.Inappropriate ventilatory settings.

Effects of Auto- PEEP

Reduced Venous Return: (reduced cardiac output, diminished

perfusion of respiratory muscles, and respiratory muscle fatigue)

Increases the work of breathingIncorrect interpretation of CVP and Wedge

Pressures.

C/F of Auto PEEP

HypotensionTachycardiaReduced Breath Sounds.Increased work of Breathing.

ON the Ventilator:High Peak- Plateau Difference of Airway

Pressure.Increasing Plateau Pressure.

3) Conditional Variables ( Modes of Ventilation)This refers to the pattern of interaction

between the patient and the machine.Terms:Spontaneous Breath: one which is initiated and

terminated by the patient.Mandatory Breath: one in which the ventilator

determines the start or end of inspirationA mandatory breath which is patient triggered

is called Assisted Breath.

Controlling influence:

Pacing Mechanism: Deciding the Respiratory Rate.

Powering Mechanism: Determining the tidal Volume.

Spontaneous Ventilation: Both rate and tidal volume are controlled by the patient.

CPAP: Spontaneous Ventilation with the addition of PEEP.

Modes of Ventilation

A ventilator mode can be defined as a set of operating characteristics that control how the ventilator functions.

Modes:

SpontaneousControlled VentilationPressure Controlled VentilationAssisted VentilationIntermittent Mandatory Ventilation.Synchronised Intermittent Mandatory

VentilationPressure Support VentilationPressure Controlled Inverse Ratio VentilationVolume Guaranteed Pressure Support

Ventilation

Closed Loop VentilationAirway Pressure Release VentilationBiPAPDuoPAP and APRVPressure Regulated Volume Control.Mandatory Minute Ventilation.Volume Assured Pressure Support

Spontaneous

Rate and tidal volume are determined by the patient.

Role of the Ventilator:1) Provide inspiratory flow to the patient in a

timely manner.2) Provide flow adequate to fulfill a patient’s

inspiratory demand (tidal volume or inspiratory flow)

3) Provide adjunctive modes such as PEEP to complement a patient’s spontaneous breathing effort.

Controlled Ventilation (CMV):

Rate and tidal volume is controlled by the machine.

Can be given as VOLUME CONTROL or PRESSURE CONTROL.

Continuous positive airway pressure (CPAP) refers to the addition of a fixed amount of positive airway pressure to spontaneous respirations, in the presence or absence of an endotracheal tube.

Pressure Control Ventilation

In PCV The pressure- controlled breaths are time triggered by a preset respiratory rate.

Once inspiration begins, a pressure plateau is created and maintained for a preset inspiratory time.

Pressure-controlled breaths are therefore time triggered, pressure limited and time cycled.

In pressure support, the plateau pressure is maintained as long as the patient maintains a spontaneous inspiratory flow.

In pressure control, the pressure plateau is maintained for a preset inspiratory time.

PC, is usually indicated for patients with severe ARDS who require extremely high peak inspiratory pressures during mechanical ventilation in a volume cycled mode.

They have a higher incidence of barotrauma.

Assisted Ventilation

Rate is controlled by the patient but the tidal volume is delivered by the machine.

ASSIST CONTROL VENTILATION: The patient can breathe at his own rate assisted by the machine but in addition the machine delivers a minimum set number of controlled breaths at the rate set on the machines.

Intermittent Mandatory Ventilation:

The patient is allowed to breathe spontaneously with no machine assistance but the machine delivers a minimum set rate and tidal volume. Breath Stacking occurs.

SIMV: Similar to IMV, but breath stacking is avoided by synchronising the mandatory breath delivered by the machine to the patient’s inspiratory effort. i.e. the mandatory breath is triggered by the patient.

Synchronised intermittent mandatory ventilation (SIMV)

Advantages:Maintains respiratory muscle strength/

avoids muscle atrophy.Reduces ventilation to perfusion mismatch.Decreases mean airway pressure.Facilitates weaning.

Pressure Support Ventilation

Pressure Support Ventilation is used to lower the work of spontaneous breathing and augment a patient’s spontaneous tidal volume.

PSV applies a preset pressure plateau to the patient’s airway for the duration of a spontaneous breath.

PSV is considered spontaneous because:1) They are patient triggered.2) The tidal volume varies with the patient’s inspiratory

flow demand.3) Inspiration lasts only for as long as the patient actively

inspires.4) Inspiration is terminated when the patient’s inspiratory

flow demand decreases to a preset minimum value.

PSV is patient triggered, pressure limited and flow cycled.

PS is used in SIMV mode in order to wean the patient by:

1) Increasing the patient’s spontaneous tidal volume.

2) Decreases the patients spontaneous respiratory rate.

3) Decreases the work of breathing.

PRESSURE CONTROLLED INVERSE RATIO VENTILATION

It is a version of pressure controlled – CMV in which all breaths are pressure limited and time cycled and the patient cannot trigger a breath.

Net effects of inverse I:E ratio are:1. Increase in Mean airway pressure (MAP) without

increasing peak pressure despite a constant tidal volume and PEEP.

2. Improved ventilation of alveoli which have longer time constants for expansion.

3. Build up of intrinsic PEEP, as the tidal volume cannot be exhaled before the inspiration begins thus avoiding end expiratory collapse of the alveoli with long time constants.

Volume Guaranteed Pressure Support- VSV

Volume Support VentilationThe pressure support is continuously

adjusted to deliver a preset tidal volume.

Closed Loop Ventilation

The Goal is to automatically adjust the ventilator support according to the patients ventilator demand and changes in respiratory mechanics.

This is done by utilizing appropriate software.The following closed loop systems are

commercially available:1) Proportional Assist Ventilation (PAV)2) Neurally Adapted Ventilatory Assistance

(NAVA)3) Knowledge based weaning systems (KBW) 4) Adaptive Support Ventilation (ASV)

Proportional Assist Ventilation (PAV)

This is a pressure regulated ventilator mode in which the inspiratory airway pressure with each breath is titrated by the ventilator in proportion to the patient’s inspiratory airflow.

In PAV, there is no target flow, volume or pressure during mechanical ventilation.

The pressure used to provide the pressure support is variable and is in proportion to the patient’s pulmonary characteristics (elastance and airflow resistance) and demand (volume or flow)

Unlike traditional modes, PAV changes with the patient’s breathing effort.

Advantage: It can track changes in breathing effort over time.

By varying the pressure to augment flow and volume, a more uniform breathing pattern becomes possible.

When PAV is used with CPAP, reduction of inspiratory work occurs and reaches values close to those in normal subjects.

Neurally Adapted Ventilatory Assistance (NAVA)

New mode of Mechanical Ventilation that delivers ventilatory assist in proportion to the electrical activity of the diaphragm.

An Oesophageal catheter is inserted to obtain a diaphragmatic EMG in order to achieve better patient-ventilator synchrony.

Knowledge based weaning System (KBW)

Adjustments are made real time to adjust pressure support ventilation in order to maintain

respiratory rate, tidal volume and end tidal CO2 within a predefined range.At a minimal level of pressure support, a trial

of spontaneous breathing is analysed and if successful, a suggestion is made to discontinue ventilation.

Adaptive Support Ventilation (ASV)

Similar to Mandatory Minute Ventilation (MMV),where a set proportion of the minute ventilation is delivered by the machine and the rest is by the patient.

It switches automatically from a PCV-like behaviour to an SIMV-like or PSV like behaviour, according to the patient’s status.

ASV always maintains control of ventilation volume and guarantees:

A minimum minute ventilation set by the user.

An effective tidal volume, well above the theoretical dead space of the patient.

A minimal breath rate.

Basic Principles of ASV:

Pressure controlled SIMV is employed, but with automatically adjusted pressure levels and SIMV rate based on measured lung mechanics. Adjustments are made with each breath. User sets:

1. Ideal body weight (IBW)2. Minimum Minute Ventilation 3. FiO24. PEEP5. Trigger sensitivity.

The ventilator uses test breaths to measure the system compliance, airway resistance, and any intrinsic PEEP.

Following determination of these variables, the ventilator selects and provides the frequency, inspiratory time, I:E ratio, and high pressure limit for mandatory and spontaneous breaths.

Airway Pressure Release Ventilation (APRV)

In this mode, there are two levels of positive airway pressure applied for set time periods and spontaneous breathing is possible at both levels.

Mandatory breaths can be set and occurs when the pressure increases from the lower to the higher pressure.

It can be conceptualized as two levels of CPAP.

Also called BiPAP.

Airway Pressure Release Ventilation(APRV)

It is similar to CPAP in that the patient is allowed to breathe spontaneously without restriction.

During spontaneous exhalation, the PEEP is dropped.(released) to a lower level and this action simulates an effective exhalation manouvre.

BiPAP

Of value in preventing intubation of the end stage COPD patient.

Other indications:Patients with restrictive chest wall disease.Neuromuscular disease.Nocturnal hypoventilation.

BiPAP

If the patient is breathing spontaneously, the IPAP and EPAP may be set at 8 cm H20 and 4 cm H2o respectively.

Spontaneous/Timed mode: Breaths per min. is set 2-5 breaths below the patient’s spontaneous rate.

Timed mode: BPM is set slightly higher than the patient’s spontaneous rate.

The spontaneous breaths are triggered, limited, controlled and cycled by pressure.

Mandatory breathes are pressure controlled and pressure limited but time triggered and time cycled.

DuoPAP and APRV

Two modes of pressure ventilation.These modes support spontaneous breathing at two

operator selected levels of positive airway pressure.They can combine spontaneous and mandatory

breaths.The spontaneous breaths can be pressure

supported.Cycling between the two levels can be triggered by

time or patient effort.The two modes differ in the operator settings

determining the pattern of breaths.

In DuoPAP, the switchover between the two levels depends on the set respiratory rate and the time setting for T(high). The two pressure levels are set by P(high) and PEEP.

The baseline pressure is the PEEP pressure.

In APRV, the switchover is decided by the time settings T(high) and T(low) and pressure settings by the P(high) and P(low).

The Baseline pressure is P(low) pressure.

Pressure Regulated Volume Controlled

This mode is a form of closed loop ventilationCombines features of volume & pressure

ventilationIt is used primarily to achieve volume support

while keeping the peak inspiratory pressure (PIP) at lowest level possible.

This is achieved by altering the peak flow and inspiratory time in response to changing airway or compliance characteristics.

First test breath is delivered to calculate compliance

Increased Airflow resistance (non elastic/elastic)= Increased PIP/ Flow

Increased Airflow resistance (non elastic/elastic)= PIP/ Decresed Flow.

Mandatory Minute Ventilation (MMV)

A.k.a Minimum minute ventilation.Provides a predetermined minute ventilation

when the patient’s spontaneous breathing effort becomes inadequate.

For eg.: an apnoea episode may cause the actual minute ventilation to fall below a preset level.

When this occurs, the mandatory rate is increased automatically to compensate for the decrease in minute ventilation caused by the apnoea.

Volume Assured Pressure Support (VAPS)

Incorporates inspiratory pressure support ventilation (PSV) with conventional volume-assisted cycles.

In VAPS, the therapist must preset the desired minimum tidal volume and the pressure support level.

During VAPS, the mechanical breaths may be patient or time triggered.

Once a breath is triggered, the ventilator tries to reach the pressure support level as soon as possible.

The delivered volume is then compared with the preset volume for further action by the ventilator.

Setting the Ventilator:

A) Does the patient need Full Ventilatory Support (FVS) or Partial Ventilatory Support (PVS)

B) Adjust settings for PaO2.FiO2, start at 100% O2 and reduce to

maintain PaO2 >60 mm Hg.PEEP, start at 5 cm H2O and adjust.

C) Adjust settings for PaCO2- pH.Tidal Volume of about 6-7 ml/kg and adjust to

keep:PaCO2 30-50 mm Hg.PaCO2 28-32 mm Hg on those with raised

intracranial pressure.

IN the ARDS Network Study on Low Tidal Volume, the Tidal Volume was calculated on the basis of a predicted body weight based on the fact that normal lung volumes depend on the height and weight of the patient. The formula used were:

Men: Predicted Weight: 50 + 0.91( Ht- 152.4)Women: Predicted Weight: 45.5 + 0.91( Ht-

152.4)

D) Respiratory Rate: Depending on Patient’s age.

E) Mode Of Ventilation.Commonly used mode is SIMV with pressure

support.F) Alarm Settings:Settings for High Pressure, Low Pressure

(Disconnection), Apnoea and other alarms must be set.

Too many alarms cause “Alarm Fatigue” in ICU personnel.

Approach to High Airway Pressures:

A) Ventilator and Pneumatic Circuit.High Tidal Volumes.High Flow, Low Inspiratory Time, Inverse I:E

Ratio.Fluid in Circuit/Filter, Blocked FIlter, Kinked

Circuit.

B) Airway: ETT/ Tracheostomy Tube.Displaced tube- Bronchial intubation.Kinked ET tube.Obstructed Tube- Clot, mucus, foreign bodyCuff Herniation

C) Patient:Biting ET tube.CoughDysynchronyPathology in airway (bronchospasm),

Parenchyma (reduced compliance)Pleura (air/fluid)Ascites, elevated intra abdominal pressure.Increased muscle tension- tetanus, seizures.

Monitoring the ventilated Patient.

MandatoryContinuous rhythm with ECGContinuous BP with arterial lineVenous pressure with CVP lineHourly u/o & daily fluid balance12 hourly ABGDaily X ray chestDaily haematology & biochemistry

Troubleshooting.

References:

Clinical Application of Mechanical Ventilation. 3rd edition. David W. Chang.

Critical Care Manual. Christian Medical College, Vellore.

Wylie and Churchill-Davidson’s: A practice of Anaesthesia. 5th Edition

Wikipaedia. (world wide web)