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
Negative Pressure Ventilators.
Positive Pressure Ventilators.
(according to the manner in which they support
Negative Pressure Ventilators
Exert a negative pressure on the external chest
Decreasing the intrathoracic pressure during inspiration
allows air to flow into the lung, filling its volume
Physiologically, this type of assisted ventilation is similar to
It is used mainly in chronic respiratory failure associated
with neuromascular conditions such as poliomyleitis,
muscular dystrophy, amyotrophic lateral sclerosis, and
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
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 .
Complex and Cumbersome
Difficult for transporting
Difficult to access the patient in emergency
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).
Peak Inspiratory Pressure
Total Inspiratory Time
End Expiratory Pressure
Mean Airway Pressure
Peak Airway Pressure
Basic Equation for Gas Flow
Peak Inspiratory Pressure: Highest pressure during
the inspiratory phase.
It depends upon:
Lung- Thoracic wall compliance.
Delivered Tidal Volume
Inspiratory Flow Rate
End Expiratory Pressure (PEEP)
Work of Breathing
Total Inspiratory Time=
Inspiratory time + Inspiratory Pause Time
Inspiratory Time: Time during which Tidal Volume
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
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
Total Pressure Gradient
=(Elastance x Volume) + (Resistance x Flow)
= (Positive Pressure Generated by the Ventilator) –
(Negative pleural Pressure generated by inspiratory
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 Tissues
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
Static Compliance : Tidal Volume
Plateau Pressure – PEEP
Value is affected by lung and chest wall compliance.
Dynamic Compliance : Tidal Volume
Peak Pressure – PEEP
Value is affected by Airway Resistance and Static
Static Compliance: measured when the flow of air
has ceased, as during Breath holding or during
Static Compliance= ∆Volume in Litres
∆ Pressure in cm of H20 (kPascals)
Compliance of lungs: 0.2 litres/cm of H20 (2.0
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
= 0.2 = 0.1 litre/cm of H20
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)
Resistance = P Peak – P Plateau (cmH20/L/sec)
Normal: 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
Time Constant = Resistance x Compliance
The alveolus behaves like an elastic semi sphere and
obeys the Laplace Equation:
Distending intra alveolar pressure =
2 x Surface tension of air liquid interface
Radius of alveolus
Decrease work of Breathing
Replace inadequate respiratory drive due to central
nervous system dysfunction.
Power : Mechanical Ventilators are powered either
by electrical energy or potential energy in
Oxygen Blender: Oxygen blenders need an input of
air and oxygen to blend it to deliver the mixture at
the set FiO2.
1. Control Variables:
a) Pressure : regulates the airway pressure to be
maintained during inhalation.
b) Volume and Flow:
Volume= Flow x Inspiratory time
Used to control set pressure for a prescribed time.
2) Phase Variables:
Control the events taking place in a Ventilatory
a) Initiate Inflation
b) Inflation Phase
c) Cycle to Exhalation
4 factors can be monitored and controlled during
any of these phases:
a) Initiate Inflation
The ventilator can be cycled to the inspiratory phase
1) Patient Effort: Patient Triggering
2) After a predetermined time: Time Triggering.
3) Manually : Manual Triggering.
A) Patient effort:
When the patient attempts to inhale, there occurs a
drop in baseline pressure.
The ventilator senses the drop in pressure.
The patient’s inspiratory effort causes a decrease in
the baseline flow through the circuit.
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
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:
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
5. Decreases Work of Breathing.
6. Effect on Cardiovascular Function.
Indications for PEEP
1)Refractory Hypoxemia caused by intra-pulmonary
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
The average pleural pressure is approximately -5 cm
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.
INCREASE IN ALVEOLAR DISTENDING PRESSURE
INCREASES FRC BY ALVEOLAR RECRUITMENT
Decreases work of breathing.
Disadvantages of PEEP:
2) 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
Renal: ADH levels increase. Reduced Urine Output.
Hepatic: Elevated Intra thoracic pressure, can lead to
decreased Hepatic Venous return > Hepatic Congestion.
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
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 breathing
Incorrect interpretation of CVP and Wedge
C/F of Auto PEEP
Reduced 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.
Spontaneous Breath: one which is initiated and
terminated by the patient.
Mandatory Breath: one in which the ventilator
determines the start or end of inspiration
A mandatory breath which is patient triggered is
called Assisted Breath.
Pacing Mechanism: Deciding the Respiratory Rate.
Powering Mechanism: Determining the tidal
Spontaneous Ventilation: Both rate and tidal volume
are controlled by the patient.
CPAP: Spontaneous Ventilation with the addition of
Modes of Ventilation
A ventilator mode can be defined as a set of
operating characteristics that control how the
Rate and tidal volume are determined by the patient.
Role of the Ventilator:
1) Provide inspiratory flow to the patient in a timely
2) Provide flow adequate to fulfill a patient’s
inspiratory demand (tidal volume or inspiratory
3) Provide adjunctive modes such as PEEP to
complement a patient’s spontaneous breathing
Controlled Ventilation (CMV):
Rate and tidal volume is controlled by the machine.
Can be given as VOLUME CONTROL or PRESSURE
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
They have a higher incidence of barotrauma.
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
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
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
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
3) Inspiration lasts only for as long as the patient actively
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
PS is used in SIMV mode in order to wean the
1) Increasing the patient’s spontaneous tidal volume.
2) Decreases the patients spontaneous respiratory
3) Decreases the work of breathing.
PRESSURE CONTROLLED INVERSE RATIO
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
Volume Guaranteed Pressure Support- VSV
Volume Support Ventilation
The 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
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
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
Knowledge based weaning System (KBW)
Adjustments are made real time to adjust pressure
support ventilation in order to maintain
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
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
5. Trigger sensitivity.
The ventilator uses test breaths to measure the
system compliance, airway resistance, and any
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
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.
Of value in preventing intubation of the end stage
Patients with restrictive chest wall disease.
If the patient is breathing spontaneously, the IPAP
and EPAP may be set at 8 cm H20 and 4 cm H2o
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
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 ventilation
Combines features of volume & pressure ventilation
It is used primarily to achieve volume support while
keeping the peak inspiratory pressure (PIP) at lowest level
This is achieved by altering the peak flow and inspiratory
time in response to changing airway or compliance
First test breath is delivered to calculate compliance
Mandatory Minute Ventilation (MMV)
A.k.a Minimum minute ventilation.
Provides a predetermined minute ventilation when
the patient’s spontaneous breathing effort becomes
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
In VAPS, the therapist must preset the desired
minimum tidal volume and the pressure support
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
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
F) Alarm Settings:
Settings for High Pressure, Low Pressure
(Disconnection), Apnoea and other alarms must be
Too many alarms cause “Alarm Fatigue” in ICU
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
B) Airway: ETT/ Tracheostomy Tube.
Displaced tube- Bronchial intubation.
Kinked ET tube.
Obstructed Tube- Clot, mucus, foreign body
Biting ET tube.
Pathology in airway (bronchospasm), Parenchyma
Ascites, elevated intra abdominal pressure.
Increased muscle tension- tetanus, seizures.
Monitoring the ventilated Patient.
Continuous rhythm with ECG
Continuous BP with arterial line
Venous pressure with CVP line
Hourly u/o & daily fluid balance
12 hourly ABG
Daily X ray chest
Daily haematology & biochemistry
Clinical Application of Mechanical Ventilation. 3rd
edition. David W. Chang.
Critical Care Manual. Christian Medical College,
Wylie and Churchill-Davidson’s: A practice of
Anaesthesia. 5th Edition
Wikipaedia. (world wide web)