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
Review basic pulmonary mechanics
Describe scalars: pressure, flow & volume
Describe the concept of compliance
Discuss and review pressure–volume, flowvolume loops
Review work of breathing
Application in various clinical scenarios


Spontaneous ventilation
Inspiration
◦ Diaphragm descends and enlarges vertical diameter
of thorax
◦ External intercostal contraction raises ribs

Exhalation
◦ Passive
Transpulmonary
Transairway
Plateau
Alveolar
distention
pressure

Simple model of
respiratory system:
◦ Resistive element
connected to an elastic
element
Interaction between
pressure, volume and
flow follow Newtonian
physics
 Simple but useful
model during
assisted breathing


Newton’s third law of motion
Pappl(t) = Pel (t) + Pres (t)
Ventilation
Pressure
( to deliver
tidal volume)
=
Elastic
Pressure
( to inflate
lungs and
chest wall)
+
P = ΔV X E + Flow x R
Compliance = 1/ E
P = ΔV/ C + Flow x R
Resistive
Pressure
( to make air
flow through
airways)



Pressure, airflow and volume measurements
quantify basic mechanics of the respiratory
system
These are resistance, compliance and work of
breathing
Monitoring and analysis of these parameters
& graphic display of curves and loops during
mechanical ventilation is a useful way to
determine not only how patient are being
ventilated but also a way to assess problems
occurring during ventilation

Dynamic mechanics
◦ Pertain to properties of system during variable flow
◦ Respiratory system resistance, compliance can be
mathematically derived with sample flow, volume
and airway pressures by multiple linear regression
analysis ( or linear least square fitting models)

Static mechanics
◦ Absence of flow
◦ Obtained with airway occlusion on modern
ventilators
 Scalars
 Pressure
 Flow
 Volume
 Loops
 Pressure-Volume
 Flow-Volume

Confirm mode functions

Detect auto-PEEP

Measure the work of breathing

Adjust tidal volume and minimize over distension

Assess the effect of bronchodilator administration

Detect equipment malfunctions

Determine appropriate PEEP level

A pressure - time graphs shows gradual
changes in pressure over time during the
breath cycle
◦ Achieved with a manometer/ pressure gauge at the
airway opening or inside the ventilator
◦ These pressure points are used in the monitoring of
patients, to describe modes of ventilation, and to
calculate a variety of parameters in patients
receiving mechanical ventilation
• During a ventilator driven
breath, the airway
pressure rises to a peak
• This is PIP
PIP is influenced by airway
resistance and compliance
• Plateau Pressure
- Inspiratory pause before
exhalation ( no flow)
- Reflects lung and chest
wall resistance & pressure
in small airways and
alveoli
Resistance = airway resistance
Compliance = compliance of the entire system (lungs, vent circuit, etc)
Δp/Δt = Flow/Compliance
Δp = R * Exp Flow
Δp = R ∗ Flow
Expiration
begins
at
point
E in
and
is passive;
the elastic
recoil
forces
of
the from
thorax
force
air in
AtThere
the
beginning
of
inspiration
the
pressure
points
A and
Bpeak
increases
the
resistances
After
Pressure
may
point
bequickly
Ba
the
slight
pressure
falls
decrease
to plateau
increases
pressure
pressure.
in a between
straight
(points
Dline,
to E)
until
from
the
lung
recruitment
pressure
atand
point
leaks
C the
is
in the
against
atmospheric
out oftothe
the
system.
system.
reached.
This drop
in pressurepressure
is equivalent
thelung
rise in pressure caused by the resistance at the
The
change
in
pressure
is
obtained
by
multiplying
exhalation
resistance
of the
ventilator
The
level
of
the
pressure
at
B
is
equivalent
to
the
product
of inspiratory
resistance
Rflow
and
flow
(V);
(Valid
no
The
The
beginning
level
gradient
of the
ofof
plateau
inspiration.
the pressure
pressure
Thecurve
base
is determined
isline
dependent
between
by
the
on
points
compliance
the
A
and D
and
runs
the
parallel
and
tidal
the
volume
to
overall
the
line if
B by
expiratory
flow
intrinsic
PEEP
exists).
During
compliance.
C. the plateau time no volume is supplied to the lung, and inspiratory flow is zero
The
higher
Once
expiration
the
selected
is Flow
completely
or overall
finished,
pressure
theonce
greater
again
thefurther
reaches
pressure
the
end-expiratory
up to
B.level
The
At
difference
point
C the
between
ventilator
plateau
applied
pressure
theResistance,
set(E)tidal
andvolume
end-expiratory
and
no
pressure
flow
Frise
(PEEP)
is
delivered
is point
obtained
(Flow
by F
Pressure increases rapidly from the lower pressure level (ambient pressure or PEEP) until it reaches
the upper pressure value (PInsp)
Pressure then remains constant for the inspiration time (Tinsp) set on the ventilator.
The drop in pressure during the expiratory phase follows the same curve as in volume-oriented
ventilation, as expiration is a passive process.
Until the next breath, pressure remains at the lower pressure level PEEP.
 As
pressure is preset in pressure
controlled ventilation, Pressuretime diagrams show no changes
or changes which are difficult to
detect as a consequence of
changes in resistance and
compliance of the entire system
Paw (cm H20)
PIP
Transairway
Pressure
Pao
Alveolar
Pressure
Transairway
Pressure
Resistive
Pressure
Elastic
resistance
Time (sec)
Pplateau
When compliance changes, the plateau
and peak pressures change by the same
amount and the pressure difference (ΔP)
remains unchanged
Decreasing compliance → plateau and peak
pressures rise
Increasing compliance → plateau and peak
pressures fall
Increasing Resistance → Peak Pressure Rises
Decreasing Resistance → Peak Pressure Falls
When the inspiratory airway
resistance changes, the peak
pressure changes and the plateau
pressure remains the same
PIP
Bronchospasm
Secretions
Foreign Bodies
Tube Kinks
PIP
Pplat
Pplat
PIP
PIP
Pplat
Pplat
Pulm Edema
Atelectasis
Pneumonia
Pneumothorax
ARDS
Pulmonary
Fibrosis




Reflects level of airway pressure and
duration of elevation
Area under Pressure-Time curve
Pressure Targeted Ventilation
◦ Mean Paw = (PIP – PEEP) X (Ti/Tt) + PEEP
Volume Control Ventilation
◦ Mean Paw = 0.5 X (PIP – PEEP) X (Ti/Tt) + PEEP
Ti = Inspiratory time and Tt = Total cycle time
Mode
Volume or Pressure targeted
Triggering
Negative deflection preceding inspiration
I:E Ratio
Calculated from lengths of insp to exp
Peak Airway Pressure
Highest point in pressure tracing
Plateau Pressure
Inspiratory pause
Mean Airway Pressure
Area under inspiratory curve
Set PEEP
Start of inspiratory tracing above baseline
Auto PEEP
Expiratory tracing ending above set PEEP
Airway obstruction
Disproportionate rise in PIP
Response to therapy
Decrease in PIP




Reveals gradual change in Inspiratory and
expiratory flow
The transferred volume (Tidal Volume) is the
integration of flow over time and is equivalent
to area under the curve
Inspiratory flow is influenced by set ventilator
mode
Respiratory compliance and resistance can be
assessed only in expiratory phase



Only volume targeted ventilation offers a choice in flow wave
pattern
In pressure targeted ventilation, to maintain constancy of
pressure, decelerating waveform is necessary
With each flow pattern the maximum flow rate is the same
while inspiratory time
 Slow rise to peak flows - thought to
improve oxygenation by allowing time for
gas distribution but may result in ‘flow
starvation’
 Constant flow and decelerating flow are
the standard forms for ventilator control.
No evidence exists to suggest that using
other flow forms improves clinical
outcomes
Tplateau
 If
at the end offlow
Decelerating
is
inspiration
and at the
typical of pressureend
of expiration
flow
controlled
ventilation
=0
 The flow falls constantly
 C
= VThaving
/ ΔP reached an
after
 ΔP
= PIPhigh
- PEEP
initially
value
 Flow in the expiratory phase
 Under normal conditions
permits conclusions to be drawn
the the
flow
returns
toand
zero
about
overall
resistance
compliance of the lung and the
during the course of
system
inspiration
Expiratory Flow Rate and Changes
in Expiratory Resistance
120
.
Time in sec
V
1
-120
2
3
Low expiratory flow rate
Extended exhalation phase
Curved contour
4
5
6
Bronchospasm
COPD
Secretions
Water in the tubing
A Higher Expiratory Flow Rate and a
Decreased Expiratory Time Denote a Lower
Expiratory Resistance
120
.
Time in sec
V
1
120
2
3
4
5
6
Flow- Time Scalar: Low compliance
120
.
Time in sec
V
1
-120
2
3
Higher peak expiratory flow
Shortened Te due to greater
elastic recoil
4
5
6
Flow –Time Scalar: Auto PEEP
Expiratory
Flow:
respiratory
rate
Auto-PEEP resultsHigh
in an
increase
in
lung
pressure in
volume-controlled
If expiratory
time
is insufficient
allow flow to reach
Inadequate
expiratorytotime
ventilation
0, air trapping occurs
or intrinsic
PEEP)
Tooconsiderable
long(auto-PEEP
of an inspiratory
time
Auto-PEEP can have
effects on gas exchange
and
Prolonged
exhalation due to
hemodynamics bronchoconstriction
Volume target
Square wave-form
Pressure target mode
Decelerating flow patter on inspiration
Auto-Peep
Failure of exp flow to return to baseline
Airway obstruction
PEF is low, Prolonged expiratory flow
Bronchodilator
response
Reversal or improvement of airflow pattern
Air Leak
Decreased PEF

Shows the gradual changes in the volume
transferred during inspiration and expiration
800 ml
Inspiration
VT
SEC
1
2
3
4
5
6
800 ml
Expiration
VT
SEC
1
2
3
4
5
6
I-Time
E-Time
1.2
A
B
VT
Liters
SEC
1
2
-0.4
A = inspiratory volume
B = expiratory volume
3
4
5
6
800 ml
Expiration
VT
SEC
1
2
3
4
5
Angle of volume rise
drops as the flow
decelerate
6

Can be described as the relative ease with
which the structure distends
◦ Two types of forces oppose inflation of the lungs:
elastic forces and frictional forces
 Elastic forces arise from the elastic properties of the
lungs and chest wall
 Frictional forces are the result of two factors:
 The resistance of the tissues and organs
 The resistance to gas flow through the airways

In the clinical setting, compliance
measurements are used to describe the
elastic forces that oppose lung inflation

C=ΔV/ΔP

Compliance has two components
◦ Static compliance
◦ Dynamic compliance
Cstat = VT/ Pplat-PEEP
Normal Cstat in a ventilated
patient: 70 -100 mL/cm H2O
 Static compliance
measurements are
made during static or
no-flow conditions
 Static compliance
monitors elastic
resistance only
 Includes recoil of
lung and thorax
 Therefore, the
plateau pressure is
used for the
calculation
• Consolidation
• Collapse
Decreased
• Pulmonary edema
• ARDS
• Pneumothorax
• Abdominal distention
• Obesity/ Scoliosis
Increased
• Emphysema




Dynamic compliance is the total impedance to
inflation and represents the sum of all forces
opposing movement of gas into the lung
Indicative of the “lungs and airway resistance”
The PIP indicates the energy needed to
overcome the elastic and airway resistance
Cdyn = VT/ PIP-PEEP
Cdyn
Cstat
Decrease
Unchanged
Increase
Decrease
Increase
Unchanged
Decrease
Increase
Increase PIP, unchanged Pplat: Increase Raw
Improved Raw e.g., cleared secretions,
bronchodilators
Increased PIP and Pplat :Dec lung
compliance and Raw
Improved PIP and Pplat: Improved lung
compliance and airway resistance



Dynamic compliance of
obtained through least
square fit method
The inspiratory curve
of the dynamic P-V
loop closely
approximates the static
curve
Slope = C = Δ V / Δ P
Shift in lung compliance
curve yield different VT
 Pressure-
Volume
 Flow- Volume
 P-V and F-V loops provide
provide dynamic trends in
respiratory system compliance
and resistance

Pressure-Volume Loop
◦ Pressure X - axis & Volume Y- axis
◦ Important for understanding optimal alveolar
recruitment (volume at which compliance is
maximized) and to measure patient compliance
◦ Static P-V: super syringe method
 Time consuming, error prone
◦ Dynamic P-V loops generated during mechanical
ventilation with slow steady flow and corrected
for airway resistance
VT
• Pressure and Volume
changes plotted
against each other
LITERS
0.6
• Elliptical or Football
shaped
0.4
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Pressure-Volume Loop
VT
On a ventilatorinitiated mandatory
breath, the loop
starts in left hand
corner
LITERS
0.6
0.4
Progresses counter
clockwise
Inspiration
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Pressure-Volume Loop
VT
When preset VT is
reached expiration
begins and returns to
FRC
Counterclockwise
LITERS
0.6
Expiration
Hysteresis
0.4
Inspiration
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Hysteresis refers to When
unrecoverable
energy, or delayed recovery
the forward path is different from the reverse
of energy due to alveolar
recruitment/
path, then
this is referredde
to recruitment;
as hysteresis
surfactant; stress relaxation; and gas absorption during the
measurement of P-V curves
Spontaneous Breath
VT
Clockwise
LITERS
0.6
0.4
Inspiration
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Spontaneous Breath
VT
Clockwise
LITERS
0.6
0.4
Inspiration
Expiration
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Assisted Breath
VT
LITERS
0.6
0.4
Assisted Breath
0.2
Paw
cmH2O -60
40
20
0
20
40
60
Assisted Breath
VT
LITERS
0.6
0.4
Assisted Breath
0.2
Inspiration
Paw
cmH2O -60
40
20
0
20
40
60
Assisted Breath
VT
Clockwise to Counterclockwise
LITERS
0.6
Expiration
0.4
Assisted Breath
0.2
Inspiration
Paw
cmH2O -60
40
20
0
20
40
60
Tidal Dynamic
compliance
ΔV/ΔP
VT
•
Volume
(mL)
FRC: Balance
between lung
recoil and chest
wall expansion
FRC
•
PEEP
Paw (cm H2O)
PIP
Normal
compliance is
50 – 80mL/cm
H20
Slope set at 45
degree
Volume Targeted Ventilation
Preset VT
Change in slope
COMPLIANCE
Increased
Normal
Decreased
Volume (mL)
Paw (cm H2O)
PIP levels
Decreased
compliance:
more pressure to
deliver volume
VT levels
Increased
Normal
Decreased
Pressure Targeted Ventilation
COMPLIANCE
Volume (mL)
Paw (cm H2O)
Preset PIP
Volume (ml)
Normal Hysteresis
If resistance changes during
constant flow ventilation the
steepness of the right branch
of the loop remains
unchanged, but changes
position
Abnormal Hysteresis
Pressure (cm H2O)
Upper Inflection Point
Point of change in
line
Lower
ofInflection
a slopePoint:
Volume (mL)
Represents minimal pressure
for adequate alveolar
recruitment (alveoli begin to fill
rapidly and alveolar
recruitment begins)
 Upper Inflection Point:
Represents pressure resulting
in regional over distension
(the lung’s maximum volume is
reached in the face of
continued inspiratory flow)
Lower Inflection Point
Pressure (cm H2O)


Initially, the volume per unit
pressure rise is slow
At the lower inflection
point, the lung-opening
pressure is reached and the
rise shows a more rapid
increase in volume per unit
pressure
◦ Point at which alveolar
recruitment begins


Lung recruitment may
continue until the upper
inflection point
At the upper inflection
point, the compliance limit
is reached the slope
decreases again



Ventilation should take place within the
linear compliance area as dangerous shear
forces may occur outside of this area
Some advocate setting PEEP at the LIP of
expiratory curve. This prevents cyclical
derecruitment injury
The ventilation volume (in CMV, SIMV) or
inspiratory pressures (in BIPAP, PCV) must
then be selected such that the upper
inflection point not be exceeded

Alveolar over distention
◦ Occur when the volume capacity of lung
has been exceeded and addition pressure
causes very little change in volume
◦ May result in barotrauma, decreased
venous return, etc
◦ Correction involves decreasing the tidal
volume or pressure target
Beaking
With little or no change in VT
Volume (ml)
Normal
Abnormal
Pressure (cm H2O)
Paw rises
• WOB equals area under the changing pressure curve as
volume moves from zero to its peak at end inspiration
The greater the area comprised by A & B, the greater the work

Flow-Volume Loop
◦ Flow X axis and Volume Y- axis
◦ Used to gain information about airway resistance
and response to bronchodilators
◦ In PFT’s inspiratory curve is below horizontal axis
and expiratory curve above X- axis
◦ Depending on brand of ventilator, orientation
may vary
Tidal Volume
Peak Inspiratory Flow
Peak Expiratory Flow
Inspiration
Volume
FRC
Expiration
PEFR
- Pressure Control
Volume target has
constant flow pattern,
in pressure control,
due to decelerating
flow pattern the F-V
appears as two
opposing expiratory
curves.
Inspiration
Flow
(L/min)
Volume (ml)
Air Leak in mL
Normal
Abnormal
Expiration
BEFORE
AFTER
Better
Worse
3
3
3
INSP
2
2
2
1
1
1
.
.
V
.
V
V
VT
1
1
1
2
2
2
3
3
“Scooped out”
pattern & decreased PEFR
3
EXP
Inspiration
Flow
(L/min)
Volume (ml)
Normal
Abnormal
Expiration
Does not return
to baseline
 Ventilator
waveforms provides
much information on airway and
lung mechanics
 Assist in monitoring clinical
course and response to therapy

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