Mechanical Ventilation 1

DR. DEEPAK SOLANKI

M.D (ANAESTHESIA)[email protected]

Mechanical Ventilation

Primary Function

To facilitate the movement of gas into the lungs.

Goals: To maintain adequate

Oxygenation To maintain optimum Co2

elimination To reduce the load of work of

breathing To regulate the rate of alveolar Ventilation


Non Invasive Invasive

Non Invasive: Ventilatory support that is given without establishing endo- tracheal intubation or tracheostomy is called Non invasive mechanical ventilation

Invasive: Ventilatory support that is given through endo-tracheal intubation or tracheostomy is called as Invasive mechanical ventilation

Indications for Mechanical Ventilation

• “….An opening must be attempted in the trunk of the trachea, into which a tube or cane should be put; You will then blow into this so that lung may rise again….And the heart becomes strong….”

-Andreas Vesalius (1555)


1. Problems with ventilation

2. Problems with oxygenation

3. Airway protection



• Ventilation abnormalities– Respiratory muscle dysfunction

• Respiratory muscle fatigue• Chest wall abnormalities• Neuromuscular disease• Cardiac insufficiency

– Decreased ventilatory drive

• Acute Ventilatory Failure/Arrest- A sudden increase in the PaCO2 to greater than 50mmHg with an accompanying respiratory acidosis (pH<7.30)

• Impending Ventilatory Failure- Occurs when a patient can maintain marginally normal blood gases, but only at the expense of significantly increased WOB.

– Increased airway resistance and/or obstruction

• Ventilation abnormalities– Respiratory muscle dysfunction

• Respiratory muscle fatigue• Chest wall abnormalities• Neuromuscular disease• Cardiac insufficiency

– Decreased ventilatory drive

• Acute Ventilatory Failure/Arrest- A sudden increase in the PaCO2 to greater than 50mmHg with an accompanying respiratory acidosis (pH<7.30)

• Impending Ventilatory Failure- Occurs when a patient can maintain marginally normal blood gases, but only at the expense of significantly increased WOB.

– Increased airway resistance and/or obstruction


• Oxygenation abnormalities

– Refractory hypoxemia -present when a patient has a PaO2 less than 60mmHg on ≥ 50% oxygen or less than 40 mmHg on any FiO2.

– Need for positive end-expiratory pressure (PEEP)

– Prophylactic Ventilatory Support- Provided in clinical conditions in which the risk of pulmonary complications, ventilatory failure or oxygenation failure is high

– Excessive work of breathing

• Airway protection

-Reduce intracranial pressure through controlled hyperventilation

-Neurologic impairment (central hypoventilation/ frequent apnea, patient comatose, GCS <=8, inability to protect airway)

-airway obstruction• Postoperatively

– Pain Control– Proper Immobilization

• Oxygenation abnormalities

– Refractory hypoxemia -present when a patient has a PaO2 less than 60mmHg on ≥ 50% oxygen or less than 40 mmHg on any FiO2.

– Need for positive end-expiratory pressure (PEEP)

– Prophylactic Ventilatory Support- Provided in clinical conditions in which the risk of pulmonary complications, ventilatory failure or oxygenation failure is high

– Excessive work of breathing

• Airway protection

-Reduce intracranial pressure through controlled hyperventilation

-Neurologic impairment (central hypoventilation/ frequent apnea, patient comatose, GCS <=8, inability to protect airway)

-airway obstruction• Postoperatively

– Pain Control– Proper Immobilization

Key Terms in Mechanical Ventilation

• Tidal Volume (Vt)– The volume of air inhaled and exhaled from the

lungs.

• Breaths per Minute (RR, f)– Also known as frequency.

• Positive End Expiratory Pressure (PEEP)– Defined as the pressure in the alveoli at end

expiration.

Key Terms in Mechanical Ventilation

• Minute Ventilation (VE)– The total amount of volume moving in and out

of the lung in one minute.

• Fractional Inspired Oxygen (FiO2)– Correctly written with decimal place (21%-

0.21; 100%-1.0)

• Inspiratory:Expiratory Ratio (I:E ratio)– Normal I:E ratio 1:2-3

I:E RATIO

• Defined: Inspiratory time: expiratory time

• Normal physiologic I:E Ratio is close to 1:2. (take a regular breath, then exhale…notice it takes longer to passively exhale that same breath than to inhale.)

I:E Ratio

• This ratio is usually changed as follows:– With obstructive type disease→ I:E from 1:2

up to 1:2.5 or 1:3. This creates more time for those patients to exhale what is being obstructed by their disease process.

PEEP

• DEFINED Positive End Expiratory Pressure.

• Presumed that PEEP will increase arterial oxygenation, pulmonary compliance and FRC by expanding previously collapsed but perfused alveoli. YES, in some cases.

PEEP

• Institution of PEEP recommended when the PaO2 cannot be maintained higher than 60mmHg on FiO2 50%.

• Initially added in 2.5-5 cmH2O until improvement is seen. Monitor PAP.

• Addition of PEEP can also cause CV compromise due to decreased CO.

PEEP

• HAZARDS OF PEEP:– Decreased CO (↓ venous return)– Pulmonary Barotrauma (pneumothorax,

subcut. emphysema)– Increased Extravascular Lung Water (due to

obstruction in pulmonary lymph flow)– Redistribution of Pulmonary Blood Flow (VQ

Mismatching stemming from overly distended alveoli that are more difficult to perfuse)

Non invasive

Negative pressure

Producing Neg. pressure intermittently in the pleural space/ around the thoracic cage

Positive pressure

Delivering air/gas with positive pressure to the airway

e.g.: Iron Lung BiPAP & CPAP

Origins of mechanical ventilationOrigins of mechanical ventilation

•Negative-pressure ventilators (“iron lungs”)

•Non-invasive ventilation first used in Boston Children’s Hospital in 1928

•Used extensively during polio outbreaks in 1940s – 1950s

•Positive-pressure ventilators

• Invasive ventilation first used at Massachusetts General Hospital in 1955

•Now the modern standard of mechanical ventilation

The era of intensive care medicine began with positive-pressure ventilation

The iron lung created negative pressure in abdomen as well as the chest, decreasing cardiac output.

Iron lung polio ward at Rancho Los Amigos Hospital in 1953.

Positive Pressure

Pressure cycle Volume cycle Time cycle

Pressure Cycle: A pre determined and preset pressure terminates inspiration. Pressure is constant and volume is variable.

Volume Cycle: A pre determined and preset volume -on completion of its delivery , terminates the inspiration. Pressure is variable and volume is constant.

Time Cycle: Delivers air/gas over a set time (Insp. Time) after which the inspiration ends.

Example: Pressure Controlled ventilation

Invasive

MODES

Intermittent Mandatory Ventilation (IMV)

Synchronised intermittant mandatory ventilation (SIMV)

Assist control ventilation (ACV)

CPAP Controlled (CMV)

Tp

Rate

Psup

PinspTi

Te @

FiO2

PEEP

Vt

CMV

SIMV

PCV

PSVModesModes

ControlsControls

ConventionalConventional

ACV

CPAP

Invasive

Non Invasive


If volume is set, pressure varies…..if pressure is set, volume varies…..

….according to the compliance…...

COMPLIANCE =

Volume / Pressure

Compliance

Burton SL & Hubmayr RD: Determinants of Patient-Ventilator Interactions: Bedside Waveform Analysis, in Tobin MJ (ed): Principles & Practice of Intensive Care Monitoring

Modes of Mechanical VentilationPoint of Reference:

Spontaneous Ventilation

Modes of Mechanical VentilationPoint of Reference:

Spontaneous VentilationP

ress

ure

Assist-control, volume

Ingento EP & Drazen J: Mechanical Ventilators, in Hall JB, Scmidt GA, & Wood LDH(eds.): Principles of Critical Care

IMV, volume-limited


SIMV, volume-limited


Control vs. SIMVControl Modes• Every breath is

supported regardless of “trigger”

• Can’t wean by decreasing rate

• Patient may hyperventilate if agitated

• Patient / vent asynchrony possible and may need sedation +/- paralysis

SIMV Modes• Vent tries to synchronize

with pt’s effort

• Patient takes “own” breaths in between (+/- PS)

• Potential increased work of breathing

• Can have patient / vent asynchrony

Trigger• How does the vent know when to give a

breath? - “Trigger”–patient effort–elapsed time

• The patient’s effort can be “sensed” as a

change in pressure or a change in flow (in

the circuit)

Need a hand??

Pressure Support• “Triggering” vent requires certain amount of

work by patient• Can decrease work of breathing by providing

flow during inspiration for patient triggered breaths

• Can be given with spontaneous breaths in IMV modes or as stand alone mode without set rate

• Flow-cycled

Continuous Positive Airway Pressure (CPAP)

Given through air tight mask/ ET/ Tracheostomy tube

Applies continuous positive pressure to the air way.

Tidal volume and Resp. Rate are patient dependent.

FiO2 & PEEP are to be set in the equipment.

No machine breaths delivered

Allows spontaneous breathing at elevated baseline pressure

Assist Controlled Ventilation (ACV):

• Delivers a preset tidal volume for every breath initiated by the machine or triggered through the patient’s effort.

• Volume or time-cycled breaths + minimal ventilator rate

• Order: AC Vt 500, RR12, 100% FiO2, 5 PEEP

• Advantages: reduced work of breathing; allows patient to modify minute ventilation

• Disadvantages: potential adverse hemodynamic effects or inappropriate hyperventilation

Controlled Mandatory Ventilation (CMV)

• Delivers a preset tidal volume / pressure at a preset rate, ignoring the patients own ventilatory effort.

• Preset rate with volume or time-cycled breaths• No patient interaction with ventilator• Advantages: rests muscles of respiration• Disadvantages: requires sedation/neuro-

muscular blockade, potential adverse hemodynamic effects

Intermittent Mandatory Ventilation (IMV):

• Delivers a preset tidal volume at a preset rate while allowing the patient to breathe at his own rate and tidal volume in between.

• Can cause “Breath stacking” – because preset frequency of the machine may not occur in the same phase as the patient’s own efforts.

Pressure-Support VentilationPressure-Support Ventilation• Pressure assist during spontaneous inspiration with flow-

cycled breath• Pressure assist continues until inspiratory effort decreases• Delivered tidal volume dependent on inspiratory effort and

resistance/compliance of lung/thorax

Order: PS 10, PEEP 0, 50% FiO2• Advantages

– Patient comfort and decreased work of breathing– May enhance patient-ventilator synchrony– Used with SIMV to support spontaneous breaths

• Disadvantages– Variable tidal volume if pulmonary resistance/compliance

changes rapidly– If sole mode of ventilation, apnea alarm mode may be only

backup– Gas leak from circuit may interfere with cycling

• Pressure assist during spontaneous inspiration with flow-cycled breath

• Pressure assist continues until inspiratory effort decreases• Delivered tidal volume dependent on inspiratory effort and

resistance/compliance of lung/thorax

Order: PS 10, PEEP 0, 50% FiO2• Advantages

– Patient comfort and decreased work of breathing– May enhance patient-ventilator synchrony– Used with SIMV to support spontaneous breaths

• Disadvantages– Variable tidal volume if pulmonary resistance/compliance

changes rapidly– If sole mode of ventilation, apnea alarm mode may be only

backup– Gas leak from circuit may interfere with cycling

Synchronised intermittent Mandatory Ventilation (SIMV):-

Delivers a preset, mandatory tidal volume synchronised to the patient’s respiratory effort.

Volume or time-cycled breaths at a preset rate

Additional spontaneous breaths at tidal volume and rate determined by patient

Used with pressure support

SIMV, Vt 500, PS 5, RR 8, 50% FiO2, 0 PEEP

Potential advantages– More comfortable for some patients– Less hemodynamic effects

Potential disadvantages – Increased work of breathing

Summary: Ventilatory Modes• Modes

– Continuous Positive Airway Pressure (CPAP)– Controlled Mechanical Ventilation (CMV)– Assist Control Ventilation (ACV)– Pressure Support Ventilation (PSV)– Synchronized Intermittent Mandatory

Ventilation (SIMV)• Choice of mode is based on patient characteristics

including level of consciousness, need for oxygenation and ventilation.

Advanced Modes

• Pressure-regulated volume control (PRVC)

• Volume support• Inverse ratio (IRV) or airway-pressure

release ventilation (APRV)• Bilevel• High-frequency

Advanced Modes

PRVC

A control mode, which delivers a set tidal volume with each breath at the lowest possible peak pressure. Delivers the breath with a decelerating flow pattern that is thought to be less injurious to the lung…… “the guided hand”.

Advanced Modes

Volume Support– equivalent to smart pressure support

– set a “goal” tidal volume

– the machine watches the delivered volumes and adjusts the pressure support to meet desired “goal” within limits set by you.

Advanced ModesAirway Pressure Release Ventilation

– Can be thought of as giving a patient two different levels of CPAP

– Set “high” and “low” pressures with release time

– Length of time at “high” pressure generally greater than length of time at “low” pressure

– By “releasing” to lower pressure, allow lung volume to decrease to FRC

Advanced ModesInverse Ratio Ventilation

– Pressure Control Mode– I:E > 1– Can increase MAP without increasing PIP:

improve oxygenation but limit barotrauma– Significant risk for air trapping – Patient will need to be deeply sedated and

perhaps paralyzed as well

Advanced Modes

High Frequency Oscillatory Ventilation– extremely high rates (Hz = 60/min)– tidal volumes < anatomic dead space– set & titrate Mean Airway Pressure– amplitude equivalent to tidal volume– mechanism of gas exchange unclear– traditionally “rescue” therapy– active expiration

Advanced ModesHigh Frequency Oscillatory Ventilation

– patient must be paralyzed– cannot suction frequently as disconnecting

the patient from the oscillator can result in volume loss in the lung

– likewise, patient cannot be turned frequently so decubiti can be an issue

– turn and suction patient 1-2x/day if they can tolerate it

Advanced ModesNon Invasive Positive Pressure Ventilation

– Deliver PS and CPAP via tight fitting mask

(BiPAP: bi-level positive airway pressure)

– Can set “back up” rate

– May still need sedation

Types of Ventilators

• Positive Pressure Ventilators– Gas blown into lungs– All Current ICU and Theatre Ventilators– Unphysiological but practical

• Negative Pressure Ventilators– “Iron Lung”– Cuirass (breastplate) ventilators– Physiological but impractical

History

• Need arose from polio epidemics in 1950s and changes in anaesthetic techniques (muscle relaxants)

• Originally engineering challenge

• Inflexible

Classification of ventilators• Most classifications obsolete but need to be known• Based on cycling

– Pressure cycling – cycles when pressure attained in system• Compensates for leaks• Vt changes with changes in compliance

– Volume cycling – cycles when preset volume delivered• Doesn’t compensate for leaks• Will generally deliver preset volume (unless limit reached)

– Time cycling – cycles after given time• Unresponsive to leaks or compliance changes

• Based on Inspiratory flow patterns– Flow generation

• High powered ventilator can deliver constant flow through inspiration – flow rate unaffected by patient characteristics

– Pressure generationLow powered ventilator delivering decreasing flow through inspiration -

Anaesthetic Ventilators

• Need to be capable of being attached to anaesthetic machine and scavenging

• Less sophisticated / flexible than ICU ventilators

• Nowadays , generally must be usable with circle absorber.

Manley Ventilator

• Minute Volume divider

• Vt set by operator. Rate=FGF/Vt

• Driving Force = Fresh Gas Pressure

Penlon Nuffield

• Tubing from ventilator plugs into bag port on bain or circle

• Uses “Fluid Logic” (coanda effect)

• Used in paediatrics (with Newton Valve)

Ohmeda

• Bag in bottle

• Driving gas blown into bottle

compressing bellows (“bag”)

• Bellows contain anaesthetic

gas

• “Pneumatic bag squeezer”

• Controlled by electronic

management of driving gas.

Puritan Bennett 840

Servo i

Drager Divan

Advanced features of new ventilators

• High performance pneumatics and dual-microprocessor electronics allow sensitive, precise breath delivery for critically ill infants, pediatric, and adult patients.

• Touch Screens display monitored data.• More accurate Alarm Systems for early problem

detection.• Addition of “Pistons” for ability to deliver tidal

volume accurately.• Facilitate Advanced Ventilation Modes like

APRV,PRVC,IRV etc.

SETTINGS

O2 Air Power

Ventilator

Patient

Types of Ventilator BreathsTypes of Ventilator Breaths• Volume-cycled breath

– “Volume breath”– Preset tidal volume

• Time-cycled breath– “Pressure control breath”– Constant pressure for preset time

• Flow-cycled breath– “Pressure support breath”– Constant pressure during inspiration

• Volume-cycled breath– “Volume breath”– Preset tidal volume

• Time-cycled breath– “Pressure control breath”– Constant pressure for preset time

• Flow-cycled breath– “Pressure support breath”– Constant pressure during inspiration

Documents

Mechanical Ventilation 1