John J. Marini

1
Advanced Mechanical Ventilation
John J. Marini Alain F. Broccard University of
Minnesota Regions Hospital Minneapolis / St.
Paul USA
2
Advanced Mechanical Ventilation Outline

• Consequences of Elevated Alveolar Pressure
• Implications of Heterogeneous Lung Unit Inflation
• Adjuncts to Ventilation
• Prone Positioning
• Recruitment Maneuvers
• Difficult Management Problems
• Acute Lung Injury
• Severe Airflow Obstruction
• An Approach to Withdrawing Ventilator Support
• Advanced Modes for Implementing Ventilation
• Case Scenarios

3
Consequences of Elevated Alveolar Pressure--1

• Mechanical ventilation expands the lungs and
  chest wall by pressurizing the airway during
  inflation. The stretched lungs and chest wall
  develop recoil tension that drives expiration.
• Positive pressure developed in the pleural space
  may have adverse effects on venous return,
  cardiac output and dead space creation.
• Stretching the lung refreshes the alveolar gas,
  but excessive stretch subjects the tissue to
  tensile stresses which may exceed the structural
  tolerance limits of this delicate membrane.
• Disrupted alveolar membranes allow gas to seep
  into the interstitial compartment, where it
  collects, and migrates toward regions with lower
  tissue pressures.
• Interstitial, mediastinal, and subcutaneous
  emphysema are frequently the consequences. Less
  commonly, pneumoperitoneum, pneumothorax, and
  tension cysts may form.
• Rarely, a communication between the high pressure
  gas pocket and the pulmonary veins generates
  systemic gas emboli.

4
Partitioning of Alveolar Pressure is a Function
of Lung and Chest Wall Compliances
Lungs are smaller and pleural pressures are
higher when the chest wall is stiff.
5
Hemodynamic Effects of Lung Inflation

• Lung inflation by positive pressure causes
• Increased pleural pressure and impeded venous
  return
• Increased pulmonary vascular resistance
• Compression of the inferior vena cava
• Retardation of heart rate increases
• These effects are much less obvious in the
  presence of
• Adequate circulating volume
• Adequate vascular tone
• Spontaneous breathing efforts
• Preserved adrenergic responsiveness

6
Hemodynamic Effects of Lung Inflation
With Low Lung Compliance, High Levels of
PEEP are
Generally Well Tolerated.
7
Effect of lung expansion on pulmonary
vasculature. Capillaries that are embedded in
the alveolar walls undergo compression even as
interstitial vessels dilate. The net result is
usually an increase in pulmonary vascular
resistance, unless recruitment of collapsed units
occurs.
8
Conflicting Actions of Higher Airway Pressure

• Lung Unit Recruitment and Maintenance of Aerated
  Volume
• Gas exchange
• Improved Oxygenation
• Distribution of Ventilation
• Lung protection
• Parenchymal Damage
• Airway trauma
• Increased Lung Distention
• Impaired Hemodynamics
• Increased Dead Space
• Potential to Increase Tissue Stress
• Only If Plateau Pressure Rises

9
Gas Extravasation Barotrauma
10
Diseased Lungs Do Not Fully Collapse, Despite
Tension Pneumothorax
and They cannot always be fully opened
Dimensions of a fully Collapsed Normal Lung
11
Tension Cysts
12
Tidally Phasic Systemic Gas Embolism
End-Inspiration
End-Expiration
13
Consequences of Elevated Alveolar Pressure--2

• In recent years there has been intense interest
  in another and perhaps common consequence of
  excessive inflation pressure--Ventilator-Induced
  Lung Injury (VILI).
• VILI appears to develop either as a result of
  structural breakdown of the tissue by mechanical
  forces or by mechano-signaling of inflammation
  due to repeated application of excessive tensile
  forces.
• Although still controversial, it is generally
  agreed that damage can result from
  overstretching of lung units that are already
  open or from shearing forces generated at the
  junction of open and collapsed tissue.
• Tidally recurrent opening and closure of small
  airways under high pressure is thought to be
  important in the generation of VILI.
• Prevention of VILI is among the highest
  priorities of the ICU clinician caring for the
  ventilated patient and is the purpose motivating
  adoption of Lung Protective ventilation
  strategies.
• Translocation of inflammatory products, bacteria,
  and even gas may contribute to remote damage in
  systemic organs and help explain why lung
  protective strategies are associated with lower
  risk for morbidity and death.

14
Recognized Mechanisms of Airspace Injury
Airway Trauma
Stretch
Shear
15
Mechanisms of Ventilator-Induced Lung Injury
(VILI)

• High airway pressures may injure the lungs by
  repeated overstretching of open alveoli, by
  exposing delicate terminal airways to high
  pressure, or by generating shearing forces that
  tear fragile tissues.
• These latter shearing forces tend to occur as
  small lung units open and close with each tidal
  cycle and are amplified when the unit opens only
  after high pressures are reached.
• To avoid VILI, end-inspiratory lung pressure
  (plateau) should be kept from rising too high,
  and when high plateau pressures are required,
  sufficient PEEP should be applied to keep
  unstable lung units from opening and closing with
  each tidal cycle.
• Independent of opening and closure, tissue strain
  is dramatically amplified at the junctions of
  open and closed lung units when high alveolar
  pressures are reached.
• Extremely high tissue strains may rip the
  alveolar gas-blood interface. Repeated
  application of more moderate strains incite
  inflammation.
• Such factors as breath frequency, micro-vascular
  pressure, temperature, and body position modify
  VILI expression.

16
Pathways to VILI
End-Expiration
Moderate Stress/Strain
Tidal Forces (Transpulmonary and Microvascular
Pressures)
Extreme Stress/Strain
Rupture
Signaling
Mechano signaling via integrins, cytoskeleton,
ion channels
inflammatory cascade
Cellular Infiltration and Inflammation
Marini / Gattinoni CCM 2004
17
Microvascular Fracture in ARDS

A Portal for Gas Bacteria?
1 ?
Hotchkiss et al Crit Care Med 2002
18
The Problem of Heterogeneity

• The heterogeneous nature of regional mechanical
  properties presents major difficulty for the
  clinician, who must apply only a single pressure
  or flow profile to the airway opening.
• Heterogeneity means that some lung units may be
  overstretched while others remain airless at the
  same measured airway pressure.
• Finding just the right balance of tidal volume
  and PEEP to keep the lung as open as possible
  without generating excessive regional tissue
  stresses is a major goal of modern practice.
• Prone positioning tends to reduce the regional
  gradients of pleural and trans-pulmonary pressure.

19
Spectrum of Regional Opening Pressures (Supine
Position)
Superimposed Pressure

Lung Units at Risk for Tidal Opening Closure
(from Gattinoni)
20
Different lung regions may be overstretched or
underinflated, even as measures of total lung
mechanics appear within normal limits.
UPPER LUNG
TOTAL LUNG
Lung Volume
Alveolar Pressure
LOWER LUNG
21
Recruitment Parallels Volume As A Function of
Airway Pressure
Recruitment and Inflation ()
Frequency Distribution of Opening Pressures ()
Airway Pressure (cmH2O)
22
Opening and Closing Pressures in ARDS
High pressures may be needed to open some lung
units, but once open, many units stay open at
lower pressure.
50

23
Zone of ? Risk
24
Dependent to Non-dependent Progression of Injury

25
Histopathology of VILI
Belperio et al, J Clin Invest Dec 2002
110(11)1703-1716
26
Links Between VILI and MSOF
Biotrauma and Mediator De-compartmentalization
Slutsky, Chest 116(1)9S-16S
27
Airway Orientation in Supine Position
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Prone Positioning Evens The Distribution of
Pleural Transpulmonary Pressures
30
Prone Positioning Relieves Lung Compression by
the Heart
Supine
Prone
31
Proning May Benefit the Most Seriously Ill ARDS
Subset
0.5
0.4
0.3
Mortality Rate
0.2
0.1
0.0
gt 49
40- 49
31- 40
0 - 31
SAPS II
Quartiles of SAPS II
32
Proning Helped Most in High VT Subgroup At Risk
For VILI
0.5
0.4
0.3
Mortality Rate
0.2
0.1
0.0
lt 8.2
8.2- 9.7
9.7- 12
gt 12
VT/Kg
Quartiles of VT /Predicted body weight
33
How Much Collapse Is DangerousDepends on the
Plateau
34
Recruiting Maneuvers in ARDS

• The purpose of a recruiting maneuver is to open
  collapsed lung tissue so it can remain open
  during tidal ventilation with lower pressures and
  PEEP, thereby improving gas exchange and helping
  to eliminate high stress interfaces.
• Although applying high pressure is fundamental to
  recruitment, sustaining high pressure is also
  important.
• Methods of performing a recruiting maneuver
  include single sustained inflations and
  ventilation with high PEEP .

35
Theoretical Effect of Sustained Inflation on
Tidal Cycling
Benefit from a recruiting maneuver is usually
transient if PEEP remains unchanged afterward.
VOLUME ( TLC)
Rimensberger ICM 2000
36
Three Types of Recruitment Maneuvers
S-C Lim, et al Crit Care Med 2004
37
How is the Injured Lung Best Recruited?

• Prone positioning
• Adequate PEEP
• Adequate tidal volume (and/or intermittent
  sighs?)
• Recruiting maneuvers
• Minimize edema (?)
• Lowest acceptable FiO2 (?)
• Spontaneous breathing efforts (?)

38
Severe Airflow Obstruction

• A major objective of ventilating patients with
  severe airflow obstruction is to relieve the work
  of breathing and to minimize auto-PEEP.
• Reducing minute ventilation requirements will
  help impressively in reducing gas trapping.
• When auto-PEEP is present, it has important
  consequences for hemodynamics, triggering effort
  and work of breathing.
• In patients whose expiratory flows are flow
  limited during tidal breathing, offsetting
  auto-PEEP with external PEEP may even the gas
  distribution and reduce breathing effort.

39
Auto-PEEP Adds To the Breathing Workload
The pressure-volume areas correspond to the
inspiratory mechanical workloads of auto-PEEP
(AP) flow resistance and tidal elastance.
40
Gas Trapping in Severe Airflow Obstruction

• Disadvantages the respiratory muscles and
  increases the work of tidal breathing
• Often causes hemodynamic compromise, especially
  during passive inflation
• Raises plateau and mean airway pressures,
  predisposing to barotrauma
• Varies with body position and from site to site
  within the lung

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Volume Losses in Recumbent Positions
Note that COPD patients lose much less lung
volume than normals do, due to gas trapping and
need to keep the lungs more inflated to minimize
the severity of obstruction. Orthopnea may result.
43
PEEP in Airflow Obstruction

• Effects Depend on Type and Severity of Airflow
  Obstruction
• Generally Helpful if PEEP ? Original Auto-PEEP
• Potential Benefits
• Decreased Work of Breathing
• Increased VT During PSV or PCV
• (?) Improved Distribution of Ventilation
• (?) Decreased Dyspnea

44
Inhalation Lung Scans in the Lateral Decubitus
Position for a Normal Subject and COPD Patient
No PEEP PEEP10
Normal
Addition of 10 cm H2O PEEP re-opens dependent
airways in COPD
COPD
45
Flow Limitation Waterfall
PEEP
46
Adding PEEP that approximates auto-PEEP may
reduce the difference in pressure between
alveolus (Palv) and airway opening, thereby
lowering the negative pleural (Pes) pressure
needed to begin inspiration and trigger
ventilation.
47
Adding PEEP Lessens the Heterogeneity of
End-expiratory Alveolar Pressures and Even the
Distribution of Subsequent Inspiratory Flow.
48
PEEP may offset (COPD) or add to auto-PEEP
(Asthma), depending on flow limitation. Note
that adding 8 cmH2O PEEP to 10 cmH2O of intrinsic
PEEP may either reduce effort (Pes, solid arrow)
or cause further hyper-inflation (dashed arrow).
Ranieri et al, Clinics in Chest Medicine 1996
17(3)379-94
49
Conventional Modes of Ventilatory Support
The traditional modes of mechanical
ventilationFlow-regulated volume Assist Control
(Volume Control, AMV, AC)) or Pressure-Targeted
Assist Control (Pressure Control), Synchronized
Intermittent Mandatory Ventilation (SIMV)with
flow or pressure targeted mandatory cycles),
Continuously Positive Airway Pressure (CPAP) and
Pressure Support can be used to manage virtually
any patient when accompanied by adequate sedation
and settings well adjusted for the patients
needs. Their properties are discussed in the
Basic Mechanical Ventilation unit of this
series.
50
Positive Airway Pressure Can Be Either Pressure
or Flow ControlledBut Not Both Simultaneously
Dependent Variable
Set Variable
Set Variable
Dependent Variable
51
Decelerating flow profile is an option in flow
controlled ventilation but a dependent variable
in pressure control.
Peak pressure is a function of flow plateau
pressure is not
Decelerating Flow
Pressure Control
52
Patient-Ventilator Interactions

• Coordination of the patients needs for flow,
  power, and cycle timing with outputs of the
  machine determine how well the ventilator
  simulates an auxiliary muscle under the patients
  control.
• Flow controlled, volume cycled ventilator modes
  offer almost unlimited power but specify the
  cycle timing and flow profile.
• Traditional pressure targeted modes (Pressure
  Control (PCV) and Pressure support (PSV)) provide
  no greater power amplitude than that set by the
  clinician but do not limit flow. PCV has a cycle
  time fixed by the clinician, and in that sense is
  as inflexible as VCV.
• Pressure Support (PSV) allows the patient to
  determine both flow and, within certain limits,
  cycle length as well. Because flow is determined
  by respiratory mechanics as well as by effort,
  adjustments may be needed to the inspiratory flow
  offswitch criterion of PSV so as to coordinate
  with the patients needs.
• Modification of the rate at which the pressure
  target is reached at the beginning of inspiration
  (the ramp or attack rate) may be needed to help
  ensure comfort

53
Pressure Support off-switch is a set flow value
or a set of peak inspiratory flow. The patient
with airflow obstruction may need to put on the
brake with muscular effort to slow flow quickly
enough to satisfy his intrinsic neural timing.
54
Tapered inspiratory attack rate and a higher
percentage of peak flow off switch criterion are
often more appropriate in airflow obstruction
than are the default values in PSV.
Airflow Obstruction
55
Although early flows are adequate, mid-cycle
efforts may not be matched by Decelerating Flow
Control (VCV).Pressure Controlled breaths (PCV)
do not restrict flow.Since the flow demands of
severely obstructed patients may be nearly
unchanging in severe airflow obstruction ,
decelerating VCV may not be the best choice.
56
Interactions Between Pressure Controlled
Ventilation and Lung Mechanics

• The gradient of pressure that determines tidal
  volume is the difference between end-inspiratory
  (Plateau) and end-expiratory alveolar pressure
  (total PEEP).
• In PCV, tidal volume can be reduced if the
  inspiratory time or the expiratory time is too
  brief to allow the airway and alveolar pressures
  to equilibrate.
• The development of auto-PEEP reduces the gradient
  for inspiratory flow and curtails the potential
  tidal volume available with any given set
  inspiratory airway pressure.
• These conditions are most likely to arise in the
  setting of severe airflow obstruction. Therefore,
  modifications of the breathing frequency and
  inspiratory cycle period (duty cycle) may
  powerfully impact ventilation efficiency.

57
Airway Pressure
Alveolar Pressure
Residual Flows
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Paw Reflects Effort and Dys-Synchrony During
Constant Flow Ventilation
Deformed Airway Pressure Waveforms
60
High Pressure Alarm
Variable Tidal Volume or Pressure Limit Alarm
During Pressure Control
61
What to Do When the Patent and Ventilator are
Out of Synch?

• Frequent pressure alarms during Volume Assist
  Control or Tidal Volume alarms during Pressure
  Control both mean that the patient is not
  receiving the desired tidal volume.
• If not explained by an important underlying
  change in the circuit properties (e.g.,
  disconnection or plug) or in the impedance of the
  respiratory system, this often arises from a
  timing collision between the patient and
  ventilators cycling rhythms.
• To quickly regain smooth control without deep
  sedation, the patient must be given back some
  degree of control over cycle timing.
• This timing control is conferred by introducing
  flow off-switched cycles in the form of high
  level pressure support. Pressure Support alone,
  or SIMV at a relatively low mandated rate
  together with PSV, are the logical choices to
  provide adequate power, achieve flow synchrony
  and yield cycle timing to the patient. Pressure
  Controlled SIMV, where the inspiratory time of
  the PCV cycle is set to be a bit shorter than the
  observed PSV cycle, is a good strategy for this
  purpose.

62
Advanced Interactive Modes of Mechanical
Ventilation

• Airway pressure release/BiPAP/Bi-Level
• Combination or Dual control modes
• Proportional assist ventilation
• Adaptive support ventilation
• Automatic ET tube compensation

63
Combination Dual Control Modes

• Combination or dual control modes combine
  features of pressure and volume targeting to
  accomplish ventilatory objectives which might
  remain unmet by either used independently.
• Combination modes are pressure targeted
• Partial support is generally provided by
  pressure support
• Full support is provided by Pressure Control

64
Combination Dual Control Modes

• Volume Assured Pressure Support(Pressure
  Augmentation)
• Volume Support(Variable Pressure Support)
• Pressure Regulated Volume Control(Variable
  Pressure Control, or Autoflow)
• Airway Pressure Release
• (Bi-Level, Bi-PAP)

65
Pressure Regulated Volume ControlCharacteristics

• Guaranteed tidal volume using pressure control
  waveform
• Pressure target is adjusted to least value that
  satisfies the targeted tidal volume minimum
• Settings Minimum VE Minimum
  Frequency Inspiratory Time per Cycle

66
Compliance Changes During Pressure Controlled
Ventilation
67
PRVC Automatically Adjusts To Compliance Changes
68
Several modes allow the physician to allow for
variability in patient efforts while achieving a
targeted goal. Volume support monitors minute
ventilation and tidal volume , changing the level
of pressure support to achieve a volume target.
Volume assured pressure support allows the
patient to breathe with pressure support,
supplementing the breath with constant flow when
needed to achieve the targeted tidal volume
within an allocated time. Proportional assist
(see later) varies pressure output in direct
relation to patient effort.
69
Airway Pressure Release and Bi-Level Airway
Pressure

• Over the past 20 years, modes of ventilation that
  move the airway pressure baseline around which
  spontaneous breaths occur between two levels have
  been employed in a variety of clinical settings.
• Although the machines contribution to
  ventilation may resemble inverse ratio
  ventilation, patient breathing cycles occur
  through an open (as opposed to closed) circuit,
  so that airway pressure never exceeds that value
  desired by the clinician.
• Transpulmonary pressurethe pressure across the
  lungcan approach dangerous levels, however, and
  mean airway pressure is relatively high.

70
Inverse Ratio Airway Pressure Release (APRV), and
Bi-Level (Bi-PAP)
71
Bi-Pap Airway Pressure ReleaseCharacteristics

• Allow spontaneous breaths superimposed on a set
  number of pressure controlled ventilator cycles
• Reduce peak airway pressures
• Open circuit / enhanced synchrony between
  patient effort and machine response
• Settings Pinsp and Pexp (Phigh and Plow)
  Thigh and Tlow

72
Unlike PCV, BiPAP Allows Spontaneous Breathing
During Both Phases of Machines Cycle
73
Bi-Level Ventilation
74
Bi-Level VentilationWith Pressure Support
75
Modes That Vary Their Output to Maintain
Appropriate Physiology

• Proportional Assist Ventilation(Proportional
  Pressure Support)
• - Support pressure parallels patient effort
• Adaptive Support Ventilation
• - Adjusts Pinsp and PC-SIMV rate to meet
  optimum breathing pattern target
• Neurally Adjusted Ventilatory Assist
• - Ventilator output is keyed to neural signal

76
Proportional Assist Ventilation (PAV)

• Proportional assist ventilation attempts to
  regulate the pressure output of the ventilator
  moment by moment in accord with the patients
  demands for flow and volume.
• Thus, when the patient wants more, (s)he gets
  more help when less, (s)he gets less. The timing
  and power synchrony are therefore nearly
  optimalat least in concept.
• To regulate pressure, PAV uses the monitored flow
  and a calculated assessment of the mechanical
  properties of the respiratory system as inputs
  into the equation of motion of the respiratory
  system.
• More than with any other currently available
  mode, PAV acts as an auxiliary muscle whose
  strength is regulated by the proportionality
  constant determined by the clinician.
• At present, PAV cannot account for auto-PEEP, and
  there is a small chance for inappropriate support
  to be applied.

77
Proportional Assist Amplifies Muscular Effort
Muscular effort (Pmus) and airway pressure
assistance (Paw) are better matched for
Proportional Assist (PAV) than for Pressure
Support (PSV).
78
Goals of Adaptive Support Ventilation

• Reduce Dead-space Ventilation
• Avoid Auto-PEEP
• Discourage Rapid Shallow Breathing
• Mirror Changing Patient Activity

79
Adaptive Lung Ventilation

• Synchronized Intermittent Pressure
  Control Rate Inspiratory Pressure
• PSV cycling characteristics
• Physician Settings Ideal body weight
• Minute Volume Maximal Allowed Pressure

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Neural Control of Ventilatory Assist (NAVA)

• Although still in its development phase, the
  possibility of controlling ventilator output by
  sensing the neural traffic flowing to the
  diaphragm promises to further enhance synchrony.
• NAVA senses the desired assist using an array of
  esophageal EMG electrodes positioned to detect
  the diaphragms contraction signal.
• The reliability of neurally-controlled ventilator
  assistance needs to be determined before its
  deployment in the clinical setting.

82
Neural Control of Ventilatory Assist (NAVA)
Ideal Technology
Central Nervous System ? Phrenic
Nerve ? Diaphragm Excitation ? Diaphragm
Contraction ? Chest Wall and Lung
Expansion ? Airway Pressure, Flow and Volume
New Technology
Ventilator Unit
Neuro-Ventilatory Coupling
Current Technology
83
Electrode Array in Neurally Adjusted Ventilatory
Assist (NAVA)
Sinderby et al, Nature Medicine 5(12)1433-1436
84
NAVA Provides Flexible Response to Effort
Volume PAW DGM EMG
Sinderby et al, Nature Medicine 5(12)1433-1436
85
Automatic Tube Compensation

• The endotracheal tube offers resistance to
  ventilation both on inspiration and on
  expiration.
• A low level of pressure support can help overcome
  this pressure cost, but its effect varies with
  flow rate.
• Automatic tube compensation (ATC) adjusts its
  pressure output in accordance with flow,
  theoretically giving an appropriate amount of
  pressure support as needed as the cycle proceeds
  and flow demands vary within and between
  subsequent breaths.
• Some variants of ATC drop airway pressure in the
  early portion of expiration to help speed
  expiration.
• Supplemental pressure support can be provided to
  assist in tidal breath delivery.

86
External and Tracheal Pressures Differ Because of
Tube Resistance
ATC offsets a fraction of tube resistance
87
Valve Control Maintains Tracheal Pressure During
ATC
Pressure Support
Pressure Support
ATC
ATC
Fabry et al, ICM 199723545-552
88
ATC Adjusts Inspiratory Pressure to Need
Postop Critically Ill
89
Discontinuation of Mechanical Ventilation

• To discontinue mechanical ventilation requires
• Patient preparation
• Assessment of readiness
• For independent breathing
• For extubation
• A brief trial of minimally assisted breathing
• An assessment of probable upper airway patency
  after extubation
• Either abrupt or gradual withdrawal of positive
  pressure, depending on the patients readiness

90
Preparation Factors Affecting Ventilatory Demand
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92
The frequency to tidal volume ratio (or rapid
shallow breathing index, RSBI) is a simple and
useful integrative indicator of the balance
between power supply and power demand. A rapid
shallow breathing index lt 100 generally indicates
adequate power reserve. In this instance, the
RSBI indicated that spontaneous breathing without
pressure support was not tolerable, likely due in
part to the development of gas trapping.
Even when the mechanical requirements of the
respiratory system can be met by adequate
ventilation reserve, congestive heart failure,
arrhythmia or ischemia may cause failure of
spontaneous breathing.
93
Integrative Indices Predicting Success
94
Measured Indices Must Be Combined With Clinical
Observations
95
Three Methods for Gradually Withdrawing
Ventilator Support
Although the majority of patients do not require
gradual withdrawal of ventilation, those that do
tend to do better with graded pressure supported
weaning than with abrupt transitions from
Assist/Control to CPAP or with SIMV used with
only minimal pressure support.
96
Extubation Criteria

• Ability to protect upper airway
• Effective cough
• Alertness
• Improving clinical condition
• Adequate lumen of trachea and larynx
• Leak test during airway pressurization with the
  cuff deflated