1. Dr. Ananya

2. Contents
 Classification
 History
 Introduction
 Indications
 Key terms- compliance , ventilatory work
 Components
 Control mechanism
 Variables
 Triggering
 Factors to consider in mechanical ventilation
 Wave-forms

3. Classification
 According to Robert chatburn
 Broadly classified into
Negative pressure ventilators
And according to the
manner in which
Positive pressure ventilators they support ventilation

4. 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 assissted ventilation is
similar to spontaneous ventilation
 It is used mainly in chronic respiratory failure
associated with neuromascular conditions such as
poliomyleitis, muscular dystrophy, a myotrophic
lateral sclerosis, and mysthenia gravis.

5.  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

6. 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 .

7. Disadvantages
 Complex and Cumbersome
 Difficult for transporting
 Difficult to access the patient in emergency
 claustrophobic

8. Positive pressure ventilators
 Inflate the lungs by exerting positive pressure on
the airway, similar to a bellows mechanism, forcing
the alveoli to expand during inspiration
 Expiration occurs passively.
 modern ventilators are mainly PPV s and are
classified based on related features, principles and
engineering.

9. 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).

11. INTRODUCTION TO MECHANICAL
VENTILATION:
 CONVENTIONAL MECHANICAL VENTILATION
 Mechanical 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 elimination
 The 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.

12. INDICATIONS FOR MECHANICAL
VENTILATION
 Respiratory 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 addiction

13. Key terms
 Ventilatory 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.

14.  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 ( α volume & inv. Prop t0 compliance)
resistance (α Raw & inspiratory flow)

15. Equation of motion for respiratory
system
 Muscle pressure + ventilator pressure =
(volume / compliance)+ (resistance x flow)
 Flow- it’s 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 poiseulle’s law.

16. Lung compliance
 Lung compliance: Is the change in volume per unit
change in pressure
COMPLIANCE =
 Volume /  Pressure

17. Types
 Static 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 volume
Plateau pressure-PEEP
 Dynamic compliance
corrected tidal volume
Peak inspiratory pressure-PEEP

18. 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 CIRCUIT
OUTPUT WAVEFORMS
ALARMS

20. INPUT POWER
 It 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.

21. Source of Gas Supply
 Air - Central compressed air, compressor, turbine flow
generator, etc
 Oxygen – Central oxygen source, O2 concentrator, O2
cylinder
 Gas mixing unit – O2 blender

22. DRIVE MECHANISM
 It’s 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 pistons
bellows
reducing valves
pneumatic circuits

23. Piston mechanism
Bellows mechanism
Pneumatic mechanism

24.  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.

25. Control circuit
 Its 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.

26. Control parameters
 Pressure
 Volume
 Flow
 Time

27.  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…...

28.  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.

29. Pressure controller
 Ventilator 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.

30. Volume controller
 Volume cycled ventilation delivers a:
 set volume;
 with a variable Pressure - determined by resistance,
compliance and inspiratory effort

31. Flow controller
 Allows 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 controller
measures and controls inspiratory and expiratory time.
These ventilators are used in newborns and infants
 Inspiratory 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.

32. 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.

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

34.  Phases of ventilator supported breath
inspiration
change from inspiration to expiration
expiration
change from expiration to inspiration
Types of ventilator breaths-
 Mandatory breath
 Assisted breath
 Spontaneous breath

36. Trigger variable
 It’s 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 patient’s 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.

37. Time triggering

38. 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
pressure
 If 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.

40. 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 patient’s 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
 Here delivered flow= base flow- returned flow
 Hence the flow sensitivity is the magnitude of the
flow diverted from the exhalation circuit into the
patient’s 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.

41. •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.

42. Limit variable

43. Cycle variable
 Defined 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 time
 Volume= flow x inspiratory time

44. Baseline variable
 The variable controlled during expiration phase.
 Mostly its pressure

45. Basic definitions
 Airway 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

46. Pressures
 Mechanical ventilation delivers flow and volume to the
patient’s as a result of the development of a positive
pressure gradient between the ventilator circuit and
the patient’s 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.

47.  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 <45cm H2O to minimize barotrauma
 Plateau Pressure-
The plateau pressure is defined as the end inspiratory
pressure during a period of no gas flow. The plateau
pressure reflects lung and chest wall compliance.

48.  As the plateau pressure is the pressure when there is
no flow within the circuit and patient airways it most
closely represents the alveolar pressure and thus is of
considerable significance as it desirable to limit the
pressure that the alveoli are subjected to.
 Excessive pressure may result in extrapulmonary air
(eg pneumothorax) and acute lung injury.
 An increase in airways resistance (including ETT
resistance) will result in an increase in PIP.
 An increase in resistance will result in a widening of
the difference between PIP and plateau pressure.
 A fall in compliance will elevate both PIP and plateau
pressure.

49.  It is generally believed that end inspiratory occlusion
pressure (ie plateau pressure) is the best clinically
applicable estimate of average peak alveolar pressure.
Although controversial it has been generally
recommended that the plateau pressure should be
limited to 35 cms H2O.

50.  Mean Airway Pressure-
The mean airway pressure is an average of the system
pressure over the entire ventilatory period.
 End Expiratory Pressure-
End expiratory pressure is the airway pressure at the
termination of the expiratory phase and is normally
equal to atmospheric or the applied PEEP level.

52. PEEP
 Positive end expiratory pressure (PEEP) refers to the
application of a fixed amount of positive pressure
applied during mechanical ventilation cycle
 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.
 PEEP and CPAP are not separate modes of ventilation
as they do not provide ventilation. Rather they are
used together with other modes of ventilation or
during spontaneous breathing to improve
oxygenation, recruit alveoli, and / or decrease the work
of breathing

53. Advantages
 ability to increase functional residual capacity (FRC)
and keep FRC above Closing Capacity.
 The increase in FRC is accomplished by increasing
alveolar volume and through the recruitment of
alveoli that would not otherwise contribute to gas
exchange. Thus increasing oxygenation and lung
compliance
 The potential ability of PEEP and CPAP to open closed
lung units increases lung compliance and tends to
make regional impedances to ventilation more
homogenous.

54.  Airway Pressures (Paw)
 For gas to flow to occur there must be a positive
pressure gradient. In spontaneous respiration gas flow
occurs due to the generation of a negative pressure in
the alveoli relative to atmospheric or circuit pressure
(as in CPAP) (refer to following diagram).

56. Physiology of PEEP
 Reinflates collapsed alveoli and maintains alveolar
inflation during exhalation
PEEP
Decreases alveolar distending pressure
Increases FRC by alveolar recruitment
Improves ventilation
Increases V/Q, improves oxygenation, decreases work of
breathing

57. Physiological Responses to CPAP /
PEEP

58. Dangers of PEEP
 High intrathoracic pressures can cause decreased
venous return and decreased cardiac output
 May produce pulmonary barotrauma
 May worsen air-trapping in obstructive pulmonary
disease
 Increases intracranial pressure
 Alterations of renal functions and water metabolism

59. AutoPEEP
 During expiration alveolar pressure is greater than circuit
pressure until expiratory flow ceases. If expiratory flow
does not cease prior to the initiation of the next breath gas
trapping may occur. Gas trapping increases the pressure in
the alveoli at the end of expiration and has been termed:
 dynamic hyperinflation;
 autoPEEP;
 inadvertent PEEP;
 intrinsic PEEP; and
 occult PEEP

60.  effects of autoPEEP can predispose the patient to:
 an increased risk of barotrauma;
 fall in cardiac output;
 hypotension;
 fluid retention; and
 an increased work of breathing

61. I:E ratio
This defines the inspiration to expiration ratio.
I:E ratios are normally set as 1:2 as expiration requires a longer
time .
In severe obstructive disease such as status asthamaticus it can
be set as 1:4
Factors affecting I:E Ratio-
1. Tidal volume
2. Respiratory rate
3. Flow rate
• Increasing inspiration time will increase TV, but may lead to
auto-PEEP

62. Tidal Volume
 Tidal volume refers to the size of the breath that is
delivered to the patient.
 Normal physiologic tidal volumes are approximately 5-7
ml / kg whereas the traditional aim for tidal volumes has
been approximately 10 - 15 ml / kg.
 The rationale for increasing the size of the tidal volume in
ventilated patients has been to prevent atelectasis and
overcome the deadspace of the ventilator circuitry and
endotracheal tube.
 Inspired and expired tidal volumes are plotted on the y
axis against time as depicted in the following diagram.

64.  The inspired and expired tidal volumes should generally
correlate.
 Expired tidal volumes may be less than inspired tidal
volumes if:
 there is a leak in the ventilator circuit - causing some of
the gas delivered to the patient to leak into the atmosphere
 there is a leak around the endotracheal / tracheostomy
tube - due to tube position, inadequate seal or cuff leak
 there is a leak from the patient, such as a bronchopleural
fistula
 Expired tidal volumes may be larger than inspired
tidal volumes due to:
 the addition of water vapour in the ventilator circuitry from
a hot water bath humidifier.

65. Flow (V)
 Flow rate refers to the speed at which a volume of gas
is delivered, or exhaled, per unit of time. Flow is
described in litres per minute .
 The peak (inspiratory) flow rate is therefore the
maximum flow delivered to a patient per ventilator
breath.
 Flow is plotted on the y axis of the ventilator graphics
against time on the x axis .
 In the following diagram that inspiratory flow is
plotted above the zero flow line, whereas expiratory
flow is plotted as a negative deflection. When the
graph depicting flow is at zero there is no gas flow
going into or out of the patient.

66. Flow

67. primary factors to consider when
applying mechanical ventilation
 the components of each individual breath, specifically
whether pressure, flow, volume and time are set by the
operator, variable or dependent on other parameters
 the method of triggering the mechanical ventilator
breath/gas flow,
 how the ventilator breath is terminated:
 potential complications of mechanical ventilation.
 methods to improve patient ventilator synchrony; and
 the nursing observations required to provide a safe and
effective level of care for the patient receiving mechanical
ventilation

68. Time (Ti)
 Time in mechanical ventilation is divided between
inspiratory and expiratory time.
 Inspiratory Time
 In most volume cycled ventilators used in the intensive
care environment it is not possible to set the inspiratory
time.
 The inspiratory time is determined by the peak inspiratory
flow rate, flow waveform and inspiratory pause. Where
inspiratory time is able to be set, flow becomes dependent
on inspiratory time and tidal volume.

69.  The following example illustrate how these parameters effect
inspiratory time.
 Ventilator settings
 · Tidal volume 1000mls
 · Peak Flow 60 lpm
 · Flow Waveform square / constant
 · Insp. Pause 0 secs
 The inspiratory time for this patient would be 1 second because
gas is constantly being delivered at a flow rate of 60 lpm,
which equals 1 litre per second. If an inspiratory pause of 0.5
seconds were applied then the inspiratory time would be
increased to 1.5 seconds.
 Changing the patients flow waveform from a square to a
decelerating flow waveform, without changing the flow rate,
will result in an increase in inspiratory time, because the flow
of gas is only initially set at 60 lpm and decreases throughout
inspiration

70. Output waveforms
 Graphical representation of the control or phase
variables in relation to time.
 presented as pressure
flow waveforms
volume
 The ventilator determines the shape of control variable
whereas the other two depend on the patient
compliance and resistance.
 Conventionally flow above X-axis is inspiration.

71. Advantages
• Allows user to interpret, evaluate, and troubleshoot
the ventilator and the patient’s response to
ventilator.
• Monitors the patient’s disease status (C and Raw).
• Assesses patient’s response to therapy.
• Monitors ventilator function
• Allows fine tuning of ventilator to decrease WOB,
optimize ventilation, and maximize patient comfort.

72. Flow Waveforms
 inspiratory flow is controlled by setting the peak flow
and flow waveform.
 The peak flow rate is the maximum amount of flow
delivered to the patient during inspiration, whereas
the flow waveform determines the how quickly gas will
be delivered to the patient throughout various stages
of the inspiratory cycle.
 There are four different types of flow waveforms
available. These include the square,
decelerating (ramp),
accelerating
sine/sinusoidal waveform

74.  Square waveform-
 The square flow waveform delivers a set flow rate
throughout ventilator inspiration. If for example the peak
flow rate is set at 60 lpm then the patient will receive 60
lpm throughout ventilator inspiration.
 Decelerating waveform
 The decelerating flow waveform delivers the peak flow at
the start of ventilator inspiration and slowly decreases
until a percentage of the peak inspiratory flow rate is
attained.

76.  Accelerating waveform-
 The accelerating flow waveform initially delivers a
fraction of the peak inspiratory flow and steadily
increasing the rate of flow until the peak flow has
been reached.
 Sine / sinusoidal waveform-
 The sine waveform was designed to match the normal
flow waveform of a spontaneously breathing patient.

77. Setting the Peak Flow and Flow
Waveform
 The flow rate should be set to match the patient’s
inspiratory demand. Where the patient’s inspiratory
flow requirements exceed the preset flow rate there
will be an imposed work of breathing which may cause
the patient to fight the ventilator and become fatigued.
 Where flow rate is unable to match the patient’s
inspiratory flow requirements the pressure waveform
on the ventilator graphics screen may show a
depressed or “scooped out” pressure waveform.
 This is often referred to as flow starvation.

79.  The decelerating flow waveform is the most frequently
selected flow waveform as it produces the lowest peak
inspiratory pressures of all the flow waveforms.
 This is because of the characteristics of alveolar
expansion. Initially a high flow rate is required to open
the alveoli. Once alveolar opening has occurred a lower
flow rate is sufficient to procure alveolar expansion.
 Flow waveforms which produce a high flow rate at the
end of inspiration (ie. square and accelerating flow
waveforms) exceed the flow requirements for alveolar
expansion, resulting in elevated peak inspiratory
pressures

80. Pressure waveforms
 Rectangular
 Exponential rise
 Sine
• Can be used to monitor-
• Air trapping (auto-PEEP)
• Airway Obstruction
• Bronchodilator Response
• Respiratory Mechanics (C/Raw)
• Active Exhalation
• Breath Type (Pressure vs. Volume)
• PIP, Pplat
• CPAP, PEEP
• Asynchrony
• Triggering Effort

81. References
 Guide to mechanical ventilation- chang s
 Breathing and mechanical support- wolfgang oczenski
 Internet references

82. Thank you

83. Advantages of Volume Cycled Ventilation
 Ease of Use
 Set Volumes: One of the major advantages of volume cycled
ventilation is the ability to set a tidal volume. This is of critical
importance to patient’s requiring tight regulation of carbon dioxide
elimination. Neurosurgical patients - post surgery / head injury and
patients suffering a neurological insult (eg post cardiac arrest) often
require CO2 regulation. This is because carbon dioxide is a potent
vasodilator.
 Increased levels of carbon dioxide, in these groups of patients, may
therefore increase cerebral blood volume with a concomitant
elevation of intracranial pressure. A raised intracranial pressure may
decrease the delivery of oxygenated blood to the brain - resulting in
cerebral ischaemia. Conversely a low CO2 may cause constriction of
the cerebral vasculature also resulting in decreased oxygen delivery
and cerebral ischaemia. For these reasons volume cycled ventilation
is often the mode of choice for patients requiring CO2 regulation.

84. Disadvantages
 The major disadvantages of volume cycled ventilation
are the variable pressure and set flow rate. It is
therefore a necessary part of nursing practice to closely
monitor the patient's inspiratory pressure and observe
the patient for signs of “flow starvation”.