Description of Mechanical ventilator

1. ASSESSMENT OF RESPIRATORY SYSTEM
MECHANICS AT BEDSIDE IN A
MECHANICALLY VENTILATED PATIENT
Presenter : Dr Srikanth
Moderator : Dr Shanker

2. Introduction
◦ Respiratory mechanics refers to the expression of lung function through
measures of pressure and flow.
◦ From these measurements, a variety of derived indices can be determined,
such as volume, compliance, resistance, and work of breathing

3. OUTLINE
◦ Anatomy and physiology of RS
◦ Trigger, limit, cycle
◦ Scalars
◦ Loops
◦ Modes
◦ Patient ventilator Asynchrony

6. COMPLIANCE
1. Elastic recoil of lungs
• Collagen and elastin fibres in lung parenchyma
2. Surface tension
• When alveoli are filled with air, there is interface of air
and fluid lining alveoli
• This molecules of fluid tend to attract causing contraction
of alveoli→ collapse of alveoli
• Pressure inside an alveolus depends on surface tension and
alveolar radius
• According to LAPLACE LAW P= 2T /r (T –surface
tension )
• Extent to which lungs expand for each unit increase in
transpulmonary pressure (ΔV/ΔP)
• Expressed as ml/cm H2O or l/k.Pa
• Normal compliance of lungs in adult is 200 ml air/ 1 cm H20
• Compliance determined by two elastic forces
1. Elastic recoil of lungs -1/3rd
2. Surface tension of alveoli -2/3rd
COMPLIANCE OF LUNGS :-

7. COMPLIANCE OF RESPIRATORY SYSTEM (THORAX AND LUNGS TOGETHER)
• Measured by expanding lungs of totally paralyzed person
• Twice as much pressure is required to inflate lungs removed from chest cage
• Compliance of lungs and thorax is 110 ml air / cm H20 –half of that for lungs alone
Static compliance → pulmonary
compliance during periods without gas
flow, such as during an inspiratory pause
Dynamic compliance → pulmonary compliance
during periods of gas flow, such as during
active inspiration
• Low compliance –
stiff lung – ARDS
• High compliance
soft lung – COPD

8. Gas distribution in lung depends on
✓ Pressure volume relationship
✓ Pleural pressure gradient across lung
✓ Flow rate of gas
1.Pressure volume relationship
• Transpulmonary pressure is more in upper
part of lung and decreases in lower part of
lung
• For a given change in pressure , increase in
volume is more in lower part of lung than
upper part
Therefore gas distribution is more in lower
parts of lung
2.Pleural pressure gradient
• Pleural pressure increases from apex to base of
lung
• This is due to gravitational effect
• In upright position pleural pressure is more in
base of lung
• This vertical pleural pressure gradient is less in
supine position as weight of heart compresses
dependent portions of lung ,hence
nondependent portions expand.

9. RESISTANCE
• Two types
➢ Elastic resistance
➢ Nonelastic resistance –airway resistance & tissue resistance
◦ Tissue resistance
• Caused by deformation of tissues during inspiration and expiration
◦ Airway resistance (RAW)
• Results from frictional resistance to airways
• RAW = PEAK PRESSURE – PLATEAU PRESSURE / FLOW
• Normal value – 0.5- 3.0 cm H2O/ L /sec
• Obstructive lung disease -↑ to > 5 cm H2O/L /sec
• Higher during expiration than inspiration
◦ Determinants of airway resistance
1. Diameter of airways
(Volume related airway collapse)
2. Flow – laminar or turbulent
Hagen–Poiseuille Equation
Laminar flow versus Turbulent flow
(Flow related airway collapse)

10. WORK OF BREATHING
◦ Under resting conditions there is work during inspiration only
1.Compliance or elastic work – to overcome elastic forces - 65% total work
2. Tissue resistance work –to overcome tissue resistance of lungs - 7 % of total work
3.Airway resistance work- 28% of total work
• Twice as much energy is required for total lung –thoracic cage system as compliance is
reduced to almost half that of lungs alone
• total work of quiet breathing range from 0.3 up to 0.8 kg-m/min.
◦ WOB is increased with
◦ decreased chest wall compliance
◦ Decreased Lung compliance
◦ increased Raw

11. Different types of ventilation :-
A. Negative pressure ventilation
Non Invasive
B. Positive pressure ventilation
a) Invasive
b) Non Invasive

12. INTERMITTENT POSITIVE PRESSURE VENTILATION (IPPV)
◦ With the IPPV principle, the patient’s respiratory system is
integrated into the ventilator system.
◦ A positive pressure is applied intermittently to the patient’s airway.
◦ When the airway pressure is temporarily higher than the alveolar pressure, fresh
gas is pushed into the lungs, the process of inspiration.
◦ When the airway pressure is lower than alveolar pressure, the gas is expelled out
of the lungs, the process of expiration.
◦ Both inspiration and expiration are regulated by the operator’s setting

13. • During inspiration, alveolar pressure rises as more and more gas
enters the lungs.

14. ◦ The lungs at their resting position (FRC)
◦ This is a static state with zero airway flow, because the inherent forces to open and
retract the lungs are equal and there is no externally applied force
◦ The lungs at their inflated position (TV Breath) :-
◦ Represents another static state with zero airway flow, here the forces (applied) to inflate the
lungs and the forces to deflate the lungs are equal

15. At FRC,
• The inherent force to keep the lungs open –
✓ by the chest wall, pleural negative pressure, & surfactant
• The inherent force to retract the lungs –
✓ mainly the elastic recoil force of the lungs and chest wall

16. ◦ INSPIRATION :-
◦ In a Pressure breath,
◦ Pao rises quickly to and stays at a preset level .
◦ The pressure gradient is the greatest at the beginning,
resulting in the maximum inspiratory flow.
◦ Over time, Palv increases as more and more gas enters the
lungs.
◦ The pressure gradient diminishes, causing a corresponding
drop in inspiratory flow.
◦ If the inspiration is sufficiently long, the lungs reach the
inflated position.
◦ 2. In a volume breath with constant inspiratory flow pattern,
◦ the applied positive Pao pushes gas into the lungs at a
constant, defined inspiratory flow.
◦ The applied Pao must increase steadily to maintain the
required pressure gradient.
◦ At the end of inspiration, the lungs do not reach their
inflated position unless an inspiratory pause is imposed.

17. EXPIRATION :-
a. Expiration is the process to decrease lung volume.
b. The applied positive Pao drops suddenly to the baseline.
c. The additional recoil force causes the lungs to retract.
d. The pressure gradient (Palv > Pao) pushes the gas out.
e. Over time, Palv and the resultant expiratory flow decrease.
f. The lungs return to their resting position if sufficient expiratory time is allowed.
g. The expiration process is the same in both pressure and volume breaths

18. INITIATION OF MECHANICAL VENTILATION :-
1. TIDAL VOLUME
2. RESPIRATORY RATE
3. PEEP
4. FLOW RATE
5. I : E Ratio
6. FIO2

19. Tidal volume
◦ Amount of air delivered each breath
◦ Appropriate initial tidal volume depends on numerous factors, most
notably the disease for which the patient requires mechanical
ventilation
◦ Starting with 6 ml/kg IBW is normal
◦ Males: IBW = 50 kg + 2.3 kg for each inch over 5 feet
◦ Females: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet
◦ In time of crisis start with 400 – 450ml

20. PEEP
◦ PEEP is the alveolar pressure above the atmospheric pressure at end-expiration
◦ It is added to keep alveoli open at the end of expiration
◦ Typical initial PEEP applied is 4 to 6 cms h2o
◦ Initial PEEP is high, 10 to 12 cms H2o in hypoxemic patients eg:- ARDS,
Pulmonary edema, pneumonia
◦ Improves oxygenation
(I) keep lungs open at the end of expiration, thus promoting alveolar
stabilization
(II) prevent opening and closing of distal small airways and alveolar units
(III) increase lymphatic flow through the thoracic duct, which may facilitate
drainage of lung edema

21. FLOW RATE
◦ Peak flow rate is the maximum flow delivered during inspiration by ventilator
◦ Peak flow rate of 6O Lts/min may be sufficient, although higher rates are
frequently necessary
◦ An insufficient peak flow rate is characterised by dyspnoea and spuriously low
peak inspiratory pressure.

22. Inspiratory Time : Expiratory Time ( I:E ratio)
◦ During spontaneous breathing Normal I : E ratio is 1:2
◦ If exhalation time is too short “breath stalking” occurs resulting in an
increase in End Expiratory Pressure also called AUTO – PEEP.
◦ Depending on the disease process such as ARDS, I:E ratio is changed to
improve ventilation
◦ Increase inspiratory time to improve oxygenation
◦ Increase expiratory time to promote co2 wash out

23. FIO2
◦ Start with maximum Fio2 of 100%
◦ Keep watch on spo2 and serial ABG
◦ Taper down soon to minimum requirement
◦ Avoid oxygen toxicity

24. TRIGGER, LIMIT, CYCLE

25. PHASE VARIABLES
◦ Variable used to initiate different phases of a breath
◦ Inspiration phase :
A. Trigger – initiates inspiration
B. Limit – ends inspiratory flow
C. Cycle – ends Inspiration
o Expiration phase :
o As expiration is a passive process, no
variable is needed to start or end
Expiratory flow or Expiration
Baseline variable : PEEP

26. A) TRIGGER : Variable which initiates inspiration
two types:
1. Machine or time triggered
2. Patient triggered
MACHINE/TIME TRIGGERED :
◦ Breath is initiated at fixed time interval
◦ Determined by set respiratory rate, irrespective of patient effort
PATIENT TRIGGERED :
◦ Breath initiated on detecting a measured patient signal during trigger window,
irrespective of machine trigger signal
◦ Patient triggers could be measured
a) Pressure
b) Volume / Flow
c) Diaphragmatic electrical activity

27. MACHINE/TIME TRIGGER
◦ Breath is initiated at fixed time interval
◦ Determined by set respiratory rate, irrespective of patient effort

28. PATIENT TRIGGER

29. 1.PRESSURE TRIGGER :-
a) It relies on the monitoring of circuit or airway pressure
b)inspiratory efforts → airway pressure drops below current baseline pressure →
triggers breath
c)Pressure trigger sensitivity – set at a negative value in cmH2O, e.g –0.5, –1.0, or –2.0
d)set PEEP is 5 cmH2O & triggering sensitivity is –2.0 cmH2O, the ventilator starts
delivering inspiratory gas if the airway pressure drops to or below 3 cmH2O.
e)So, a pressure trigger of –0.5 cmH2O is more sensitive than –2.0 cmH2O.
f)More sensitive trigger – Disadv is Auto triggering

30. 2.FLOW/VOLUME TRIGGER :- Relies on circuit or airway flow monitoring
◦ In late Expiration - the patient inhales, a part of base flow, resulting in an inspiratory
airway flow and a decreased flow in circuit
◦ If the detected inspiratory airway flow or the difference between flow A and B reaches a
defined threshold, the ventilator is triggered
sensitivity is expressed in litres
per minute, such as 0.5, 1.0,
2.0, or 5.0 litres per minute
More sensitive trigger – Disadv
is Auto triggering

31. B)LIMIT :-
◦ LIMIT IS SAFETY MEASURE TO PREVENT VENTILATOR INDUCED LUNG INJURY.
◦ LIMIT DOES NOT ENDS (CYCLES) INSPIRATION
◦ BREATH CAN BE :
◦ PRESSURE LIMITED: When set pressure limit is reached, inspiratory flow stops, to prevent
further rise in airway pressure.
◦ VOLUME LIMITED: When set volume limit is reached, inspiratory flow stops, to prevent
further rise in tidal volume.

32. 3.CYCLING – Variable which ends inspiration
◦ Two types
1. Machine cycled breath
2. Patient cycled breath
Machine cycled Breath :
1. Inspiration is terminated by machine
2. Example : volume cycle, time cycle
3. Patient cannot change Inspiratory time (Ti) with inspiratory or expiratory
efforts
Patient cycled Breath :
1. Inspiration can be terminated by patient
2. Example : pressure cycle, flow cycle
3. Patient can change Inspiratory time (Ti) with inspiratory or expiratory
efforts

33. TIME CYCLING
◦ A ventilator switches from inspiration to
expiration when the set inspiratory time is
completed.
◦ Time cycling applies to all control breaths
and all assisted breaths. (Backup mode)
◦ Time cycling can be realized with
following methods:
(a) the Ti method,
(b) the I:E (inspiratory/expiratory) ratio
method,
VOLUME CONTROLLED MODE PRESSURE CONTROLLED MODE

35. VOLUME CYCLED BREATH
1. Inspiration is terminated when pre-set tidal volume is
delivered, airway pressures increase in response to delivered
breath.
2. Airway pressure (PIP) is determined by set TV and compliance
3. If inspiratory pressure exceeds pressure limit, the machine
cycles into expiration even if the selected volume has not
been delivered, it protects against pulmonary barotrauma.
VOLUME CONTROLLED MODE

36. FLOW CYCLED BREATH
◦ Flow cycling is designed for active
patients and is a key feature of support
breaths.
◦ Flow cycling works with the descending
part of inspiratory flow
◦ The peak inspiratory flow, regardless of
its absolute height, is taken as 100%. A
ventilator cycles from inspiration to
expiration when the inspiratory flow
falls to a preset percentage
o If the set percentage is very low (e.g. 10%) and there is a massive leak at the circuit or
airway, flow cycling can fail because the inspiratory flow does not fall to the set level.
The consequence is endless inspiration, which is clinically unacceptable
o Because flow cycling can fail, backup time cycling should be part of the ventilator
design

37. PRESSURE CYCLED BREATH
◦ Inspiration is terminated when specific airway pressure is attained

38. SCALARS

39. SCALARS :-
◦ The three scalars are – flow curve, volume curve, pressure curve
◦ In a scalar graphics, time is conventionally shown on the horizontal axis (X) where
as the flow, volume, and pressure are plotted on the vertical axis (Y)

40. PRESSURE Vs TIME SCALAR

41. PRESSURE Vs TIME SCALAR
SPONTANEOUS VS MACHINE BREATH
1. Spontaneous breath (unassisted) - inspiration below the baseline and expiration
is traced above.
2. Machine Breath - Inspiration starts above baseline reaches peak and after
exhalation returns to baseline ( zero pressure or basal pressure level/PEEP)

42. MECHANICAL BREATH
Controlled Vs Assisted Breath
a. TIME/MACHINE Trigger – Controlled breath
◦ Breaths are initiated at the baseline (without negative pressure) at fixed intervals
b. PATIENT Trigger – Assisted Breath
◦ the patient initiates the breath by generating a negative pressure and associated flow.
◦ This event can be observed on the P-T scalar where a small negative deflection below the
baseline precedes a mechanical breath. This negative pressure deflection may not be visible
if flow compensation or flow trigger is active.

45. 1.Peak inspiratory pressure (PIP)
◦ It is the highest pressure created during a positive pressure breath
◦ In volume-controlled ventilation, the PIP is reached at the end of inspiration
◦ In PCV –pressure is preset
◦ PIP will vary with changes in lung compliance and airway resistance

46. PIP must overcome
1. the resistance of the airways to
flow and
2. resistive and elastic properties
(compliance) of the lung

47. Dynamic Compliance
◦ compliance of the lung when air is moving through the airways.
◦ Dynamic Compliance = Tidal Volume / (PIP – PEEP)
❖ Components of Dynamic Compliance :-
◦ It reflects how much pressure is required to over come the resistance of the airways as
well as how much pressure is required to overcome the elastic properties of the alveoli
and fill them with air.
❖ Normal Values for Dynamic Compliance :-
◦ In a non intubated patient, the dynamic compliance is normally 30 to 40 mL/cm H2O
❖ Effects of Lung Disease on Dynamic Compliance :-
◦ Lung diseases that decrease lung compliance or increase airway resistance will result in a
decrease in the dynamic compliance.

48. ❑ An abnormally high PIP takes two forms: pressure overshooting and a pressure
spike
A. Pressure overshooting appears at the beginning of inspiration due to circuit
pressurization that is too fast.
B. A pressure spike occurs at the end of inspiration when patient actively exhales to
terminate inspiratory phase.

49. RISE TIME
◦ Rise time is the time it takes for airway pressure to reach a preset maximum value.
◦ A rapid rise time value will allow instantaneous delivery of flow at the start of the
breath, resulting in an immediate rise in pressure to the preset level.
◦ When inspiratory flow is delivered too fast, turbulence can result in a higher pressure
than preset airway pressure.
◦ Conversely, a slow rise time inhibits initial flow delivery (inadequate inspiratory
flow rate), thus delaying the pressure rise to the preset level

50. INADEQUATE INSPIRATORY FLOW
Inadequate flow that is indicated
• when the pressure rises very slowly or
• sometimes is indicated when there is a depression in the inspiratory limb of
the pressure contour

51. 2. Plateau Pressure :-
◦ pressure required to overcome the elastic properties of the lungs.
◦ How to measure :
◦ an inspiratory pause is set or the inspiratory hold key is pressed until the
value is obtained.
◦ After TV is delivered, exhalation valve is kept in closed position.

52. i. Plateau pressure can be monitored in passive patients only.
ii. Conventionally, plateau pressure is measured in volume modes during an
inspiratory pause.
iii. Less conventionally, plateau pressure can also be measured in pressure
breaths if the inspiratory time is sufficiently long so that the airway flow
reaches zero.

53. ◦ Significance :-
◦ it represents the alveolar pressure when the lungs are inflated.
◦ Pplat should ideally be kept at less than 30 cm H2O
◦ Interventions should aim at lowering the plateau pressure in order to
avoid barotrauma of the lungs

54. STATIC COMPLIANCE
a) pressure needed to overcome the elastic properties of the lung.
b) It is calculated by dividing the exhaled tidal volume by the plateau pressure minus the
baseline pressure (PEEP)
c) calculated when there is no air movement in the lung (end inspiratory pause)
Normal Value for Static Compliance :-
a) The normal value is 60 to 100 mL/cm H2O.
Effects of Lung Disease on Static Compliance
a) Lung diseases that decrease lung compliance will result in a decrease in the static
compliance.
b) As a patient’s lung recovers, the static compliance will improve (serial monitoring)
c) The lower the plateau pressure, the higher the static lung compliance.

56. Components of Airway Resistance
a) gas flowing through an airway will encounter friction as it rubs the sides of the
airways and encounters irregularities along the walls of the airway.
b) irregularities - secretions on the walls of the airway , inflammation or narrowing of
the walls of the airway.
c) flow is altered at every bifurcation of the airway.
Normal Value for Airway Resistance
1. natural airways → 0.5 to 2.5 cm H2O/L/sec in adult patients.
2. In intubated patients → higher (at least double - Approx) because of the resistance
of the endotracheal (ET) tube
3.Transairway Pressure :-

57. TRANSAIRWAY PRESSURE (Pta =PIP – Pplat)
◦It reflects the pressure required to overcome airway resistance.
◦Causes → Bronchospasm, main stem intubation, airway secretions, and other types
of airway obstructions (ET tube Kinking..etc).
◦Management → airway suctioning, administration of bronchodilators, or removal of
obstruction such as a kink in the inspiratory line of the circuit.

59. 4.PEEP :-
▪ measured by applying end expiratory pause for 0.5 – 2.0 sec.
▪ For a valid measurement, the patient must be relaxed and breathing in synchrony with
the ventilator, as active breathing invalidates the measurement.
▪ Total PEEP = Set PEEP + Auto PEEP

60. Auto-PEEP
◦ If the expiratory phase is terminated prematurely, it causes Incomplete emptying
of the lungs leading to accumulation of gas.
◦ Pressure produced by this trapped gas is called auto-PEEP, intrinsic PEEP, or
occult PEEP.
◦ Auto-PEEP leads to progressive increase in end expiratory lung volume and thus
causes dynamic hyperinflation

61. ◦ In spontaneously breathing patients, measurement of esophageal pressure (Pes) can
be used to determine auto-PEEP
◦ The end-expiratory pause method can underestimate auto-PEEP when some airways
close during exhalation, as may occur during ventilation of the lungs of patients with
severe asthma.

62. 5.Mean airway pressure (MAP) :-
◦ Mean airway pressure is the average pressure applied over one mechanical breath.
◦ PEEP is part of the mean airway pressure.

63. SIGNIFICANCE OF MAP :
◦ The heart, large blood vessels, and the lungs are soft structures sitting inside the
thoracic cavity.
◦ A positive pressure applied to the airway and lungs compresses the circulatory organs.
◦ A high MAP can cause elevated pulmonary vascular resistance, decreased cardiac
output, and even decreased system blood pressure.
◦ Mean airway pressure is a good indicator of how much these intrathoracic structures
are compressed.
◦ To minimize this unwanted effect, mean airway pressure should be kept as low as
clinically acceptable

64. ESOPHAGEAL PRESSURE
◦ Pleural pressure (Ppl) cannot be easily measured directly.
◦ Measured by using esophageal balloon catheter - thin catheter with multiple
small holes in the distal 5–7 cm of its length. the balloon is inflated with a small
amount of air (0.5 mL).
◦ The proximal end of the catheter is attached to a pressure transducer.
◦ The catheter is inserted orally or nasally to 35–40 cm from the airway opening.
◦ After the balloon is inflated and the pressure is measured, If the catheter is in the
esophagus, cardiac oscillations should be visible on the Pes waveform.

66. ◦ There are potential sources of error in the use of
Pes to estimate Ppl.
◦ It is important to appreciate that the Pes
estimates Ppl mid-thorax.
◦ The Ppl is more negative in the non-dependent
thorax and more positive in the dependent thorax.
◦ The weight of the heart can bias the Pes by as
much as 5 cm H2O.

67. TRANSPULMONARY PRESSURE(PL)
◦ It is the difference between pressure measured at the mouth (Paw) and
esophageal (pleural) pressure.
◦ During no flow (inspiratory or expiratory pause maneuvers), PL becomes the
alveolar distending pressure.

68. DRIVING PRESSURE
◦ It is calculated by Plateau pressure – PEEP
◦ It is the distending pressure of Respiratory system
◦ It is considered as surrogate of transpulmonary pressure(PL) as measurement of PL
requires estimation of Ppl which is invasive and complicating
◦ Lower the driving pressure, less is chance of VILI especially in ARDS.

69. VOLUME VS TIME SCALAR
Tidal volume refers to the volume of gas that one inhales (inspiratory tidal
volume) or exhales (expiratory tidal volume) during one breath

70. ◦ Normally, both tidal volumes are almost identical.
a. Inspiratory tidal volume (VTI) is the monitored volume of gas going into the
lungs. It represents the maximum tidal volume that the ventilated patient can
possibly receive.
b. Expiratory tidal volume (VTE) is the monitored volume of gas leaving the lungs.
It represents the minimum tidal volume that the ventilated patient can possibly
receive.
c. VTE is commonly displayed, because it is considered to be more clinically
relevant.

71. Recognition of common abnormalities on V-T Scalar:
◦ AIR LEAKS
◦ ACTIVE EXHALATION
◦ CHANGE IN PULMONARY MECHANICS

72. AIR LEAKS
air leaks can be due to volume loss
a) through the circuit,
b) chest tube,
c) bronchopulmonary fistula,
d) around the cuff of the endotracheal (ET) tube

73. Identification of Air leak :-
a. expiratory tracing smoothly descends and then plateaus, but does not reach
baseline, it indicates the presence of a leak in the system.
b. The volume of the leak can be easily estimated by measuring the distance
from the plateau to the end of the expiratory tracing.

75. ACTIVE EXHALATION
◦ Forced exhalation is seen on V -T tracing as a tracing that extends below the zero line.
◦ It can also occur if the flow transducer is out of calibration

76. Changes in Pulmonary Mechanics
◦If the ventilator is set on any pressure-controlled mode of ventilation, the
V-T waveform could reveal changes in pulmonary mechanics.
◦ In PCV, airway pressure is preset, hence decrease in Tidal volume can be due to
◦ Increase in airway resistance or
◦ Decrease in lung compliance,

77. FLOW VS TIME

78. Spontaneous breath
a. The inspiratory flow is traced above the baseline,
b. whereas expiratory flow is indicated below the baseline.
c. The F-T curve for the inspiratory portion of a spontaneous breath resembles
a sine wave flow pattern

79. MECHANICAL BREATH
a. The inspiratory flow pattern is traced above the baseline and in constant flow
delivery (VCV) and it is square in shape.
b. There is a significant tracing below the baseline representing the expiratory flow
which is dependent on the patients lung characteristics and effort
c. Only flow vs time graph demonstrates significant tracing below the baseline

80. INSPIRATORY FLOW
FOUR PATTERNS - SQUARE, DESCENDING, ASCENDING and SINE WAVE
a)The square(constant) flow pattern delivers the breath at a constant flow rate.
b) The decelerating flow (or ramp) pattern delivers the fastest flow at the start of the
breath and the flow decreases throughout inspiration.
◦ the descending ramp flow pattern is associated with a lower PIP than the square flow pattern
and may meet initial flow demand better.
◦ theorized to improve gas distribution and therefore oxygenation

81. During volume control ventilation, the inspiratory flow is constant, which is set on the
ventilator.

82. ◦ During pressure control ventilation, the
inspiratory flow is variable, depends on
◦ Pressure applied to the airway above PEEP
◦ Compliance of RS
◦ Airway resistance
◦ Inspiratory time
◦ Time constant

83. EXPIRATORY FLOW
◦ Expiration is a passive manoeuvre in both spontaneous and mechanical breath
◦ Expiratory flow decays to zero before next mechanical breath is initiated thus
duration of expiratory flow is shorter than allocated expiratory time

84. ◦ An estimation of the time needed to complete the process of lung inflation or
deflation
◦It determines the rate of change in the volume of a lung unit that is passively
inflated or deflated
◦The time constant is the product of the monitored compliance (C) and resistance
(R) of the respiratory system →Time constant (RC) = (R) × (C)
◦Lung units with a higher resistance and/or compliance will have a longer time
constant and require more time to fill and to empty.
◦ In contrast, lung units with a lower resistance and/or compliance will have a
lower time constant and thus require less time to fill and to empty.

85. ❑There is a 63% volume change in 1 RC
❑87% volume change in 2 time constant,
❑ 95% volume change in 3 time constant ,
❑ 98% volume change in 4 time constant ,
❑ 99% volume change in 5 time constant .
◦ It is generally agreed that, to be adequate, inspiratory time should be at least
three inspiratory time constants long, while expiratory time should be at least
three expiratory time constants long
• The time constant is clinically significant, as it is how we can estimate the
minimal Ti and Te required for a given patient.

86. RECOGNITION OF COMMON ABNORMALITIES
1.ACTIVE INSPIRATION OR ASYNCHRONY

87. 2.AUTOPEEP OR AIRTRAPPING
◦ It is the presence of unintended baseline pressure.
◦ It can exist with or without set (external) PEEP applied to the alveoli at end expiration
◦ How to identify AUTOPEEP in F – T Scalar:-
◦ Normally expiratory flow pattern returns to the baseline prior to the next breath
◦ In autopeep, expiratory flow doesnot return to the zero line and subsequent inspiration
begins below the baseline.

88. 3.AIRWAY OBSTRUCTION AND ACTIVE EXHALATION
◦ Exhalation is normally passive
◦ But Patients active efforts to exhale seen in
1. Airway Obstruction :-
◦ In increased airway resistance due to either bronchospasm or accumulation of
secretions in the airway may result in decreased PEFR and a prolonged expiratory
flow.
2. Active Exhalation :-
◦ If the patient begins to actively exhale using expiratory muscles this may result in
an increase in PEFR and a shorter duration of expiratory flow.

89. 4.RESPONSE TO BRONCHODILATOR
◦ In bronchoconstriction - PEFR is reduced and expiratory flow returns to the
baseline very slowly.
◦ Administration of a bronchodilator improves PEFR and allows for an expiratory
flow to return to baseline within a normal time

91. LOOPS

92. LOOPS :-
◦ They are two dimensional graphic displays of two scalar curves
◦ TWO TYPES
◦ Pressure volume loops
◦ Flow volume loops

93. PRESSURE VOLUME LOOP
a. airway pressure is plotted on the x-axis, and volume is plotted on the y-axis.
b. The inspiratory curve goes upward, and the expiratory curve goes downward.
c. Spontaneous breaths without pressure support goes clockwise, and Controlled
breath goes counter-clockwise.
d. In an assisted mechanical breath, the tracing can begin clockwise indicating patient’s
effort but resumes in counter clockwise fashion for the mechanical delivery.

94. ◦ Inspiration begins from the functional residual capacity (FRC) level and terminates
when the preset parameter (volume or pressure) is achieved.
◦ The tracing continues during expiration and returns to FRC at end of exhalation.
◦ When PEEP is applied, the PVL shifts to the level of PEEP on the horizontal scale.

95. ◦ Inflection Points and Alveolar Overdistention
a. Inflection points in a PVL can represent sudden changes in alveolar opening and closing.
b. The lower inflection point represents opening pressure at which alveoli open/fill at a
faster rate.
c. whereas the upper inflection points represent either the presence of alveolar
overdistention (during inspiration) or lung recoil and airway resistance characteristics
(during exhalation).

96. ❑ The beak-shaped part of the PVL at end of inspiration, which lends it its
penguin-like shape, is the region of pressure where rising pressure does not
lead to increasing volume. The lung is simply overstretched

98. 1.Change in pulmonary compliance :-
a. In a volume-controlled mode :-
◦ A shift to the right of the inspiratory limb of the PVL ,indicates that a higher
pressure is required to overcome either airway resistance (decreased dynamic
compliance) or lung recoil (static compliance).

99. b. In a pressure-controlled mode :-
◦ for a preset pressure, downward shift of the inspiratory limb of the PVL will be
associated with a lower Vt as a result of decreased lung compliance

100. 2.INCREASED AIRWAY RESISTANCE:-
◦ An increased airway resistance is associated with the abnormal widening of the
inspiratory tracing.
◦ Patients with obstructive disorders exhibit a wide pressure – volume loop
◦ This abnormal shape of P-V loop is referred to as an increased “hysteresis”

101. 3.AIR LEAK :-
◦ When the expiratory limb of the PVL does not return to zero volume, an air leak
is present

102. 4.INADEQUATE INSPIRATORY FLOW RATE :-
◦ Inappropriate inspiratory flow rates are recognized from a scooped out pattern

103. FLOW – VOLUME LOOPS
a. airway flow is plotted on the y-axis, and volume is
plotted on the x-axis.
b. Inspiration is above the horizontal line, and
expiration is below.
c. The shape of the inspiratory curve matches the
ventilator settings.
d. The shape of the expiratory flow curve
represents passive exhalation.

105. Recognition of Common Abnormalities – F -V Loop
1.AIRLEAK :-
◦ Ideally, expired volume should be equal to the inspired volume. With an air leak,
however, expired volume is less than inspired volume.
◦ A leak can be identified from a flow – volume loop when the volume doesnot
return to the frc (zero volume level).
◦ The deficit of volume indicates the magnitude of air leak.

108. 2.INCREASED AIRWAY RESISTANCE :-
◦ Increased airway resistance, such as in asthma and chronic obstructive
pulmonary disease (COPD), is commonly associated with a “scooping” of
the expiratory tracing of the FVL and a decreased PEFR.
◦ A positive response to bronchodilator will show an improvement on both the
configuration of the expiratory tracing and the PEFR
◦ A continued scooped out appearance and a low PEFR indicates ineffectiveness of
the bronchodilator therapy.

110. 3.AIRWAY SECRETIONS/ACCUMULATION OF CONDENSATE :-
▪ The presence of secretions in the large airways, as well as excessive
fluid condensation in the ventilator circuit, appear as a distinctive pattern
in the FVL known as a “sawtooth” pattern.
▪ This occurs mostly in the expiratory component of the curve.
▪ However if the situation is not corrected, the pattern will also appear in the
inspiratory curve.

112. 4.AUTO- PEEP / AIR TRAPPING :-
◦ In an air trapping or auto -PEEP situation the flow doesnot return to the ZERO level.
◦ Since the next inspiration must begin from the ZERO flow level, the tracing jumps
abruptly, from the trapped level to the zero level and proceeds with next breath.

113. MODE OF VENTILATION
◦ SPONTANEOUS
◦ CONVENTIONAL

114. SPONTANEOUS MODE OF VENTILATION
A. Spontaneous breaths in ventilated patients are a great challenge, and should
be used only in patients who are stable and in good clinical condition.
B. TARGET PATIENTS :
1. Their typical application is for weaning trials, also known as spontaneous
breathing trials.
2. Patient breathes spontaneously at a moderate positive baseline pressure
(PEEP).

115. ii. If pressure support is set to zero or close to zero, the patient has to do all the required
work of breathing to satisfy the ventilatory demand, hence w

116. Conventional ventilation modes :-
◦

117. 1.VOLUME ASSIST / CONTROL MODE
◦

121. ◦ Intended for ventilated patients who are passive
◦ NO WOB for the patients
◦ It is critical to set tidal volume and rate so that the resultant alveolar ventilation
matches the patient’s current demand
Indications :-
1. Apnoeic patients
2. Sedated or paralyzed patients
3. Cerebral malfunction
4. Spinal cord or phrenic nerve injury
5. Seizures
6. Closed head injury or after neurosurgery

122. ◦ Every breath is initiated by the patient and is seen as a small negative
Deflection on P-T scalar and is mandatory breath, delivering clinician set
tidal volume.
◦ Clinician sets only the backup frequency to ascertain ventilation in
unforeseen situation such Apnoea.

123. ◦ In a passive patient, all breaths are volume control breaths, and the monitored rate and
the set rate are equal.
◦ In an active patient, some or all breaths are volume assist breaths, and the monitored
rate is higher than the set rate
◦ Disadvantages :-
◦ There is a high incidence of patient- ventilator asynchrony in active patients with a strong drive.
◦ The resultant peak airway pressure can be uncomfortably high resulting in Barotrauma.
❖ Contraindications :
1. Irregular respiratory rate
2. Cheyne stokes respiration
3. Hiccups

128. Stress Index
◦ The stress index is used to assess the shape of the pressure- time curve during
constant flow-volume control ventilation, in passive patient
1. A linear increase in pressure (constant compliance, stress index = 1) suggests
adequate alveolar recruitment without over-distention.
2. If compliance worsens as the lungs are inflated (progressive decrease in
compliance, upward concavity, stress index >1), this suggests over-distention,
and the recommendation is to decrease the PEEP, VT, or both.
3. If compliance improves as the lungs are inflated (progressive increase in
compliance, downward concavity, stress index <1), this suggests tidal
recruitment and potential for additional recruitment, and the
recommendation is to increase PEEP.

129. Driving pressure ΔP = Pplat –PEEP
If the increase in PEEP level leads to increased aeration of lung tissue through
recruitment, a decrease in ΔP is expected. On the other hand, if PEEP increases and does
not recruit lung tissue, the lungs may become overstretched, and ΔP may remain
unchanged or even increase over time

130. ◦ PEEP-induced increase in end-expiratory lung volume
1. might be the result of recruitment, or
2. it might be the result of over-distention of already open alveoli.
Thus, end-expiratory lung volume by itself might not be useful to assess PEEP
response

135. 2.PRESSURE ASSIST/CONTROL MODE
◦

136. ◦

137. ◦ flow time scalar shows decelerating flow through out the inspiratory phase
1. flow is generated due to pressure gradient i.e., clinician sets a desired positive
pressure at the airway opening, at that time alveolar pressure is same as
atmospheric pressure
2. At the beginning of inspiration there
is big pressure gradient, as the
alveoli starts filling, pressure gradient
progressively decreases hence
flow also starts decreasing.
◦ Clinician sets target pressure and RR.
◦ TV Delivered varies Depending on
compliance and resistance.

139. TARGETED PATIENTS :-
I. The pressure A/ C mode is suitable for passive or partially active patients.
II. It can also be used in active patients with weak respiratory drive, because this
mode allows the patient to influence rate, inspiratory flow, and tidal volume.
ADVANTAGES :
I. lower incidence of patient- ventilator asynchrony.
II. this mode enables the ventilator to compensate for moderate levels of gas leakage.
DISADVANTAGES :
I. operator cannot directly control tidal volume. The resultant tidal volume may be
unstable when the patient’s breathing effort and/ or respiratory mechanics change.
Key alarms :- Low volume alarm – set at 100 – 200 ml below the initial ventilating
volume

142. Transition from VCV TO PCV Mode

143. 3.Pressure support ventilation (PSV) mode
TARGET PATIENTS :
◦ Indicated for active patients only, but their own efforts are inadequate to
meet their required ventilatory demand.
❖ Indications for PSV : a).NM DISORDERS b).AE of COPD
c). To augment spontaneous component of SIMV breath
d). during weaning from mechanical ventilation
❖ Contraindicated for the passive patients.
❑ Caution : Apnoea (backup) ventilation should be activated in this mode.
➢ This mode enables the ventilator to adequately compensate for moderate
levels of gas leakage.
◦ In pressure support ventilation, the baseline pressure (PEEP) is constant

145. ◦ The tidal volume depends in PSV on
1. The set pressure,
2.Airway resistance (Raw),
3.lung compliance (Cl) and
4.patient effort
It is the most comfortable mode for this patient population, because they can
influence the actual rate, inspiratory time, inspiratory flow, and tidal volume

146. 4.Volume SIMV mode
◦

149. ❖
◦
❖

150. 5.Pressure SIMV mode
◦

154. 6.Continuous Positive Airway Pressure + Pressure Support
1. CPAP displays a spontaneous breath on a P-T waveform that starts at the preset
pressure level (baseline).
2. Pressure support (PS) augments spontaneous breaths with positive pressure to
provide a higher volume for all spontaneous breaths.
3. A pressure-supported breath is by definition patient triggered, pressure limited,
and flow cycled.
4. Inspiration ends when a preset expiratory flow reaches a predetermined value
(typically 25% of the peak flow). The delivered volume is dependent on the
patient’s lung compliance, airway resistance, and inspiratory demand.

156. PATIENT – VENTILATOR ASYNCHRONY

157. PATIENT – VENTILATOR ASYNCHRONY
◦ Phenomenon that occurs when patients are unable to breathe
comfortably with mechanical ventilator.
◦ “fighting the ventilator” is something used to describe an individual
who is apparently doing well while receiving mechanical ventilation
but suddenly develops acute respiratory distress.
◦ Particularly challenging as patient is unable to verbalize his or her
discomfort

158. 1. Inappropriate Trigger sensitivity
2. Inappropriate inspiratory flow setting
3. Inappropriate cycle variable
4. Inappropriate PEEP setting
5. Inappropriate ventilatory support mode

159. A.TRIGGER ASYNCHRONY :-
◦ three types
a. Ineffective or missed triggering
b. Double triggering
c. Auto triggering

160. Ineffective or missed trigger and delayed triggering :-
patients inspiratory effort doesnot trigger the ventilator into inspiration
Best seen with the flow time and pressure – time waveforms

161. ◦ Wasted effort – trigger is not sensitive enough (high level of trigger)

164. ◦ Ventilatory parameters associated with high incidence of ineffective triggering are with
1. poorly sensitive trigger
2.high tidal volume
3.high peak inspiratory pressure
4.high level of pressure support
❖Ineffective triggering causes increased WOB, wasted efforts and inspiratory muscle
dysfunction and ultrastructural changes in muscle.
◦ Solution :-
1. adjust the trigger to a lower setting (decrease flow trigger from 2 l/m to 0.8 l/m
2. sedate the patient and move to a mandatory mode

165. AUTO PEEP
◦ Measurement :
◦ perform expiratory hold manoeuvre
◦ Expiratory circuit occlusion for 3 – 5 seconds, allows alveolar pressure to equilibrate
with airway pressure
◦ if measured PEEP is more than Set PEEP then autoPEEP is present.
◦ Magnitude of AUTOPEEP is directly measured from the P-T scalar.
◦ Total PEEP = external PEEP + Intrinsic PEEP

166. Mechanism of AUTOPEEP :-
◦ Occurs when expiratory time is shorter than the time needed to fully deflate the lungs
◦ It causes progressive air trapping, identified from the F-T waveform.
◦ This accumulation of air increases alveolar pressure at the end of expiration

167. ◦ The presence of autopeep may result from :
1. Inadequate expiratory time
2. Too high respiratory rate
3. Long inspiratory time
4. Prolonged exhalation due to bronchoconstriction or loss of recoiling force as
observed in advanced emphysematous patients

168. Effects of autoPEEP :-
◦ Predisposes to increased WOB, Barotrauma, hemodynamic instability.
◦ If autopeep is not recognised, Hemodynamic instability may lead to inappropriate fluid
restriction or unnecessary vasopressor
◦ Autopeep also interferes with Weaning therapy

169. How to Eliminate Auto-PEEP :-
1.Changes that reduce airway resistance can help reduce auto-PEEP.
a) suctioning the patient,
b) administering a bronchodilator or
c) anti-inflammatory agent, or
d) replacing an artificial airway that is kinked or has dried accumulation
of secretions.
2.Shortening the inspiratory time and lengthening the expiratory time.
3. Reducing an above-normal minute ventilation
a) by a reduction in the respiratory rate or tidal volume, or
b) by sedating the patient

170. 4. Auto-PEEP associated with premature airway collapse is more likely to be
resolved with applied PEEP (external, therapeutic PEEP - 80 to 90% of
AutoPEEP).
❖ Weakness: nutrition/ reduce sedation/ physiotherapy

171. Shortening the inspiratory time and lengthening the expiratory time

172. 2.DOUBLE TRIGGERING :-
◦ Double triggering is evidence that the ventilator has not met the patients
demand for tidal volume.
◦ Patient neural inspiratory time is more than the ventilator inspiratory time
◦ Causes :
1. High respiratory drive,
2. Low tidal volume
3. Short inspiratory time

173. Double triggering is present when there are 2 consecutive inspiratory breaths and
between each breath the mean inspiratory time is less than half of what is set on the
ventilator for eg – if mean inspiratory time is 1.0 second and the time between each
breath drops to 0.5 seconds or less it is considered double triggering. It may result in
higher pressure in the second breath

175. ◦ Solution :
1. Changing to a lower expiratory flow trigger tends to prolong insufflation
time, and increase the tidal volume.
2. Switch from Assist VC mode to assist PC mode or PSV

176. 3.AUTO TRIGGERING :-
◦ the trigger is too sensitive so that Non-respiratory factors trigger the ventilator.
◦ the ventilator improperly recognizes a flow or pressure variation in the circuit as
being patient spontaneous respiratory muscle effort and triggers breath
Causes :
• Cardiac oscillations
• Leak from the circuit
• Leak from the chest drain (eg. a bronchopleural fistula)
• Water condensation sloshing and bubbling in the circuit
• Large volume of respiratory secretions, eg. bronchiectasis
• Swallowing or vomiting
• Inappropriate sensitivity settings

179. Solution :-
◦ Increase intrinsic respiratory drive
➢ Lower level of sedation
◦ Optimize sensitivity setting
◦ Correct factors leading to flow/pressure wave form distortion.
◦ Minimize air leak
◦ Remove condensate

180. B. Flow dysynchrony – Inadequate inspiratory flow
◦ If patients spontaneous inspiratory flow rate is greater than inspiratory flow rate
set on the ventilator, this will result in flow asynchrony.
◦ This causes an increased WOB as the patient has to work harder to get the flow
into the lungs
◦ Solution :
◦ Increase flow rate with rise time ( shorter I time) or
◦ Change to pressure targeted ventilation

183. C.PREMATURE CYCLING AND DELAYED CYCLING
Premature cycling :-
◦ It occurs when the ventilator ends the inspiratory flow sooner than desired
by the patient, that is, the ventilator inspiratory time is shorter than patient
neural inspiratory time .
Causes :-
A. Ventilator related cause :
Inspiratory time is too short relative to patient inspiratory time
◦ Management is by –
1. In VCV mode decrease inspiratory flow or increase tidal volume,
2. In PCV mode increase inspiratory time

184. B. Patient related cause :
◦ Restrictive respiratory mechanics in PSV
(as in pulmonary fibrosis – recovering from ARDS, high respiratory drive & short time
constant)
◦ Management is by :
◦ In PSV mode , decrease the cycling threshold percentage criterion or increase
Pressure support
the ventilator's inspiratory time was
shorter than the patient’s respiratory
muscle inspiratory time . This is shown in
the expiratory segment of the flow curve,
which tends to return to baseline or to
become positive as a result of the
patient's inspiratory effort

185. ◦ On the right the cutoff cycling point was reduced to 5%, prolonging the
inspiratory time and improving synchrony

186. • waveforms represent a patient with restrictive lung disease experiencing
premature cycling.
• Asynchrony is attenuated by decreasing the cycling threshold percentage of
peak flow.

187. Delayed cycling :-
◦ It is due to the ventilator delivers a breath with a longer inspiratory time than is
desired by the patient
❑ Causes :
◦ Ventilator related cause :
inspiratory time is longer than patient neural inspiratory time.
◦ Management is by :
In VCV, increase inspiratory flow In PCV, decrease inspiratory time
❖ Patient related cause:
Obstructive respiratory mechanics in PSV, as in COPD
❖ Management is by :
In PSV, increase the cycling threshold percentage criterion, or decrease PS, or
decrease rise time

188. • Flow and pressure waveforms, represent a patient with COPD.
• Asynchrony is corrected by increasing the threshold percentage of peak inspiratory
flow for termination of inspiration

189. TROUBLE SHOOTING

192. ◦ THANK YOU

193. ADVANCED MODES
◦ PRVC
◦ APRV
◦ VAPS
◦ ASV

194. PRVC

201. ◦ SET :-
◦ Minimum respiratory rate
◦ Target tidal volume
◦ Upper pressure limit
◦ Maximum delivered pressure = 5 cm H2O below pressure alarm limit
◦ FIO2
◦ Inspiratory time or I:E ratio

203. ◦ Longer inspiratory time
◦ Improved oxygenation
◦ Higher mean airway pressure
◦ Re-distribution
◦ Lower peak airway pressure
◦ More time available to deliver set tidal volume
◦ Shorter inspiratory time
◦ Less risk of gas trapping and PEEPi
◦ Less effect on cardiovascular system

204. ADVANTAGES :-
◦ Decelerating inspiratory flow pattern
◦ Pressure automatically adjusted for changes in compliance and
resistance within a set range
◦ Tidal volume guaranteed
◦ Limits volume trauma
◦ Prevents hypoventilation
◦ DISADVANTAGES:-
◦ Pressure delivered is dependent on tidal volume achieved on last breath
◦ Intermittent patient effort ⇒ variable tidal volumes
◦ Less suitable for patients with asthma or COPD