Advanced Pulmonary Mechanics during Mechanical Ventilation

1
Advanced Pulmonary Mechanics during Mechanical
Ventilation
2
Points of Discussion

• Basics

• Abnormalities

• Equation of motion
• Dynamic Compliance
• Pressure-volume loop
• Flow-volume loop
• Work of breathing
• Lower and upper inflection points
• Hysterexis
• Intrathoracic Pressures

• Air-leak
• Air trapping
• Increased airway resistance
• Inadequate flow support
• Inadequate sensitivity
• Atelectasis
• Inadequate PEEP
• Airway obstruction
• Over-distension

3
Tube Spring Model
Resistive Forces
Elastic Forces
4
Static and Dynamic Pressures
Pressure
PIP
Flow-Resistive Pressure difference (Pres)
Pplat
Alveolar Distending (recoil) Pressure difference
(Pdis)
PEEP
time
5
Airway Resistance
Pressure difference Flow Rate x Resistance
dP Q x R
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Resistive and Elastive Forces
DYNAMIC CHARACTERISTICS dP dV / Cdyn
RESISTANCE dPresistive R x Flow
STATIC COMPLIANCE dPdistensive dV / Cst
dP dPresist. dP dist. dP R x Flow
dV / C st
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Basic Calculations
dP R x Flow dV / C st
Pressure

• Cst dV / (Pplat-PEEP)
• R (PIP-Pplat) / Flow

Pplat
PEEP
time
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Assessment of static P-V curveSuper-syringe
method

• Stepwise inflation from a big syringe with
  multiply occlusions at each volumes to record
  recoil pressure
• Time consuming
• Cumbersome to perform
• Difficult to standardize
• Patient must be paralysed, connected to a special
  equipment
• Great risk of oxygen desaturation

Volume
Pressure
9
Assessment of static P-V curveSlow Flow Single
Inflation Method

• Slow (5-10 lpm) inspiratory flow with large Vt
  and ZEEP
• The inspiratory curve of the dynamic P-V loop
  closely approximates the static curve
• The flow-resistive pressure component could be
  subtracted
• Easy to perform, fast and relatively comfortable
• Servillo AJRCCM 1997
• Lu AJRCCM 1999

Volume
Static curve
UPIflex
inspiration
LPIflex
Pressure
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FRC and PV Loop
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Components of Pressure-Volume Loop
VT
Expiration
Volume (mL)
Inspiration
PIP
Paw (cm H2O)
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Pressure-Volume Loop(Type of Breath)
E
E
Vol (ml)
E
I
I
I
Paw (cm H2O)
Spontaneous
Controlled
Assisted
I Inspiration E Expiration
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PEEP and P-V Loop
Volume (mL)
Paw (cm H2O)
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Inflection Points

• Upper Inflection Point Represents pressure
  resulting in regional overdistension
• Lower Inflection Point Represents minimal
  pressure for adequate alveolar recruitment

Volume (mL)
Pressure (cm H2O)
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Decreased Compliance
Volume(ml)
Pressure (cm H2O)
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Lung Compliance Changes and the P-V Loop
Volume Targeted Ventilation
Preset VT
Increased
Normal
Decreased
Volume (mL)
Paw (cm H2O)
PIP levels
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Lung Compliance Changes and the P-V Loop
VT levels
Increased
Normal
Pressure Targeted Ventilation
Decreased
Volume (mL)
Preset PIP
Paw (cm H2O)
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Hysteresis
Volume (ml)
Normal Hysteresis
Abnormal Hysteresis
Pressure (cm H2O)
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Flow-Volume Loop
Inspiration
PIFR
Volume (ml)
FRC
VT
Flow (L/min)
PEFR
Expiration
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Positive Pressure Ventilation The Equation of
Motion

• In a passive subject, airway pressure represents
  the entire pressure (P) applied across the
  respiratory system.
• The work required to deliver a tidal breath (Wb)
  tidal volume (VT) x airway pressure
• The pressure (P) associated with the delivery of
  a tidal breath is defined by the simplified
  equation of motion of the respiratory system
  (lungs chest wall)

P VT/CR VT/Ti x RR PEEP total
P elastic
P resistive
P elastic
Where CR compliance of the respiratory system,
Ti inspiratory time and VT/Ti Flow, RR
resistance of the respiratory system and PEEP
total the alveolar pressure at the end of
expiration external PEEP auto (or intrinsic)
PEEP, if any. Auto PEEP PEEP total P
extrinsic (PEEP dialed in the ventilator) adds to
the inspiratory pressure one needs to generate a
tidal breath.
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Work of Breathing
Volume (ml)
B
A Resistive Work B Elastic Work
A
Pressure (cm H2O)
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Work of Breathing

• WOB is a major source of caloric expenditure and
  oxygen consumption
• Appr. 70 to overcome elastic forces, 30
  flow-resistive work
• Patient work is a one of the most sensitive
  indicator of ventilator dependency
• Comparison of Ventilator and Patient work is a
  useful indicator during weaning process
• WOB may be altered by changes in compliance,
  resistance, patient effort, level of support,
  PEEP, improper Ti, demand system sensitivity,
  mode setting
• Elevated WOB may contraindicate the weaning
  process

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WOB Measurements
V

• WOB ?0 ti P x Vdt
• Elasic work ABCA
• Resistive work
• Inspiratory ADCA
• Expiratory ACEA

B
C
E
D
P
A
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Work of Breathing Measurements

• WOB ?0 ti P x Vdt
• Paw Ventilator Work The physical force required
  to move gas into the lung, represents the total
  work of the resp. system (patient ventilator)
• Peso Patient Work done by respiratory muscles,
  represents the pulmonary work of breathing
• Paw-Ptr Imposed Work by the Endotracheal tube

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P-V Loop and WOB
V
Normal Compliance Increased Resistance
Decreased Compliance Normal Resistance
P
V
Normal Compliance Normal Resistance
V
P
P
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Work of Breathing

• Work per breath is depicted as a pressure-volume
  area
• Work per breath (Wbreath) P x tidal volume (VT)
• Wmin wbreath x respiratory rate

WEL elastic work
WR resistive work
Volume
Volume
Volume
VT
Pressure
Pressure
Pressure
The total work of breathing can be partitioned
between an elastic and resistive work. By
analogy, the pressure needed to inflate a balloon
through a straw varies one needs to overcome the
resistance of the straw and the elasticity of the
balloon.
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Intrinsic PEEP and Work of Breathing
When present, intrinsic PEEP contributes to the
work of breaking and can be offset by applying
external PEEP.
Volume
VT
VT
Dynamic Hyperinflation
FRC
Pressure
PEEPi
PEEPi intrinsic or auto PEEP green triangle
tidal elastic work red loop flow resistive
work blue rectangle work expended in
offsetting intrinsic PEEP (an expiratory driver)
during inflation
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The Pressure and Work of Breathing can be
Entirely Provided by the Ventilator (Passive
Patient)
Ventilator


?


?
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The Work of Breathing can be Shared Between the
Ventilator and the Patient
The ventilator generates positive pressure within
the airway and the patients inspiratory muscles
generate negative pressure in the pleural space.
AC mode
PAW
patient
machine
PES
time
Paw Airway pressure, Pes esophageal pressure
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Work of breath
Resistive Work
Elastic Work of Lung
Pressure
Elastic Work of Chest
Paw
Pes
Work to inflate the chest wall
Inflation
Deflation
Volume
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Relationship Between the Set Pressure Support
Level and the Patients Breathing Effort
The changes in Pes (esophageal pressure) and in
the diaphragmatic activity (EMG) associated with
the increase in the level of mask pressure (Pmask
pressure support) indicate transfer of the work
of breathing from the patient to the ventilator.
Carrey et al. Chest. 199097150.
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Partitioning of the Workload Between the
Ventilator and the Patient

• How the work of breathing partitions between the
  patient and the ventilator
• depends on
• Mode of ventilation (e.g., in assist control
  most of the work is usually done by the
  ventilator)
• Patient effort and synchrony with the mode of
  ventilation
• Specific settings of a given mode (e.g., level
  of pressure in PS and set rate in SIMV)

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Intrathoracic pressures
PROX. AIRWAY PRESSURE
TRACHEAL PRESSURE
PLEURAL PRESSURE
ALVEOLAR PRESSURE
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3 Levels of Lung Mechanics
DYNAMIC CHARACTERISTICS dP dV / Cdyn
RESISTANCE dP R x Flow
STATIC COMPLIANCE dP dV / Cst
AIRWAY RESISTANCE dP Raw x Flow
LUNG COMPLIANCE dP dV / CL
IMPOSED RESISTANCE dP Rimp x Flow
CHEST WALL COMPLIANCE dP dV / Ccw
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A closer look at lung mechanics

• Crs Vt / dPdist (aw)
• Ccw Vt / dPdist (pl)
• CL Vt / Pdist (aw - pl)
• or (tr - pl)
• Rrs Presist (aw) /Flow
• RL Presist (tr) / Flow
• Rimp Presist (aw-tr)/ Flow

Paw
Ptr
Ppl
P
dP resist
dPdist
t
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Respiratory Mechanics in ARF
Volume

• Reduced range of volume excursion Low compliance
• Flattering at low and high volumes Lower and
  upper inflection points
• Bigatello Br J Anaest 1996

NORMAL
ARDS
Pressure
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Lung Protective Strategy

1. Set PEEP above the lower Pflex to keep the lung
   open and avoid alveolar collapse
2. Apply small Vt to minimize stretching forces
3. Set Pplat below the upper Pflex to avoid regional
   overdistension

Volume
Pressure
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Abnormalities

• Air-leak
• Air trapping
• Increased airway resistance
• Inadequate flow support
• Inadequate sensitivity
• Atelectasis
• Inadequate PEEP
• Airway obstruction
• Over-distension

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Air Leak
Volume (ml)
Air Leak
Pressure (cm H2O)
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Air Leak
Inspiration
Flow (L/min)
Volume (ml)
Air Leak in mL
Normal Abnormal
Expiration
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Air Leak
Volume (mL)
Time (sec)
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Air Trapping
Inspiration
Time (sec)
Flow (L/min)

Expiration
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Air Trapping
Inspiration
Flow (L/min)
Does not return to baseline
Volume (ml)
Normal Abnormal
Expiration
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Response to Bronchodilator
After
Before
Time (sec)
Flow (L/min)
Long TE
PEFR
Shorter TE
Higher PEFR
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Increased Airway Resistance
Inspiration
Flow (L/min)
Volume (ml)
Normal Abnormal
Scooped out pattern
Decreased PEFR
Expiration
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Increased Raw
Higher PTA
Vol (mL)
Normal Slope
Lower Slope
Pressure (cm H2O)
47
Inadequate Inspiratory Flow
Volume (ml)
Active Inspiration
Inappropriate Flow
Paw (cm H2O)
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Airway Secretions/Water in the Circuit
Inspiration
Flow (L/min)
Volume (ml)
Normal Abnormal
Expiration
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Airway Obstruction
F
F
V
V
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Optimising PEEP
V
V
P
P
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Inadequate Sensitivity
Volume (mL)
Paw (cm H2O)
Increased WOB
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Atelectasis
V
V
P
P
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Overdistension
With little or no change in VT
Normal Abnormal
Volume (ml)
Pressure (cm H2O)
Paw rises
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Overdistension

• Overdistension occurs when the volume limit of
  some components of the lung has been exceeded
• Abrupt decrease in compliance at the termination
  of inspiration
• Results in a terminal Beaking of the P/V Loop

Volume
Pressure
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Overdistension Index
Volume
C20
Cdyn
Pressure
0.8 Pmax Pmax
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