review basic pulmonary mechanics describe scalars: pressure, flow & volume describe the...
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Mechanics of Ventilation
Review basic pulmonary mechanics Describe scalars: pressure, flow & volume Describe the concept of compliance Discuss and review pressure–volume, flow-
volume loops Review work of breathing Application in various clinical scenarios
Objectives
Spontaneous ventilation Inspiration
◦ Diaphragm descends and enlarges vertical diameter of thorax
◦ External intercostal contraction raises ribs
Exhalation◦ Passive
Normal Mechanics
Pressures and Gradients in Lungs
Transairway
Transpulmonary
Plateau Alveolar distention pressure
Mechanics of Spontaneous Ventilation
Mechanics of Positive Pressure Ventilation
Elementary Law of Mechanics 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
Elementary Law of Mechanics
Pappl(t) = Pel (t) + Pres (t)
Equation of Motion
Ventilation Pressure( to delivertidal volume)
=
Resistive Pressure( to make air flow through airways)
ElasticPressure( to inflate lungs and chest wall)
+
P = ΔV X E + Flow x R
Compliance = 1/ E
P = ΔV/ C + Flow x R
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
Relevance
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
Mechanics of Ventilation
Scalars Pressure Flow Volume
Loops Pressure-Volume Flow-Volume
Basic waveforms
Uses of Flow, Volume, and Pressure Graphic Display 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
Pressure Scalar
Pressure Scalar• During a ventilator driven
breath, the airway pressure rises to a peak
• This is PIPPIP 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
Pressure Waveform for Volume- Controlled Ventilation
Resistance = airway resistanceCompliance = compliance of the entire system (lungs, vent circuit, etc)
At the beginning of inspiration the pressure between points A and B increases from the resistances in the system. The level of the pressure at B is equivalent to the product of resistance R and flow (V); (Valid if no intrinsic PEEP exists).The higher the selected Flow or overall Resistance, the greater the pressure rise up to point B.Reduced inspiratory Flow and low Resistance lead to a low pressure at point B.
Δp = R ∗ Flow
Δp/Δt = Flow/Compliance
After point B the pressure increases in a straight line, until the peak pressure at point C is reached. The gradient of the pressure curve is dependent on the inspiratory flow and the overall compliance.At point C the ventilator applied the set tidal volume and no further flow is delivered (Flow = 0).
Pressure quickly falls to plateau pressure. This drop in pressure is equivalent to the rise in pressure caused by the resistance at the beginning of inspiration. The base line between points A and D runs parallel to the line B - C.
There may be a slight decrease in pressure (points D to E) from lung recruitment and leaks in the system. The level of the plateau pressure is determined by the compliance and the tidal volumeDuring the plateau time no volume is supplied to the lung, and inspiratory flow is zeroThe difference between plateau pressure (E) and end-expiratory pressure F (PEEP) is obtained by dividing the delivered volume tidal volume (VT) by compliance (C)
Δp = R * Exp Flow
Expiration begins at point E and is passive; the elastic recoil forces of the thorax force the air against atmospheric pressure out of the lungThe change in pressure is obtained by multiplying exhalation resistance of the ventilator by expiratory flowOnce expiration is completely finished, pressure once again reaches the end-expiratory level F (PEEP)
Pressure Waveform for Pressure- Controlled Ventilation
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, Pressure-time diagrams show no changes or changes which are difficult to detect as a consequence of changes in resistance and compliance of the entire system
Pressure Waveform for Pressure- Controlled Ventilation
Alveolar Pressure
PIPTransairway Pressure
Pplateau
Time (sec)
Paw (
cm H
20)
Transairway Pressure
Pao
Resistive Pressure
Elastic resistance
Pressure Waveform: Changes in Compliance
When compliance changes, the plateau and peak pressures change by the same amount and the pressure difference (ΔP) remains unchanged
Increasing compliance → plateau and peak pressures fall
Decreasing compliance → plateau and peak pressures rise
Pressure Waveform: Changes in Airway Resistance
When the inspiratory airway resistance changes, the peak pressure changes and the plateau pressure remains the same
Increasing Resistance → Peak Pressure Rises
Decreasing Resistance → Peak Pressure Falls
PIP vs. Pplat
PIP
Pplat
PIP
PIP
PIP
Pplat
Pplat
Pplat
BronchospasmSecretionsForeign BodiesTube Kinks
Pulm EdemaAtelectasisPneumoniaPneumothoraxARDSPulmonary 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
Mean Airway Pressure (Paw)
Ti = Inspiratory time and Tt = Total cycle time
Increasing Mean Airway Pressure
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
Pressure – Time Scalar - Information
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
Flow- Time Scalar
Flow – Time Scalar
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
Flow – Time Scalar
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
Volume Targeted Ventilation
Tplateau
Decelerating flow is typical of pressure-controlled ventilation
The flow falls constantly after having reached an initially high value
Under normal conditions the flow returns to zero during the course of inspiration
Pressure Targeted Ventilation If at the end of
inspiration and at the end of expiration flow = 0
C = VT/ ΔP ΔP = PIP - PEEP Flow in the expiratory phase
permits conclusions to be drawn about the overall resistance and compliance of the lung and the system
1 2 3 4 5 6
Time in sec
120
-120
V
.
Expiratory Flow Rate and Changes in Expiratory Resistance
Low expiratory flow rateExtended exhalation phaseCurved contour
Bronchospasm COPDSecretions Water in the tubing
A Higher Expiratory Flow Rate and a Decreased Expiratory Time Denote a Lower
Expiratory Resistance
1 2 3 4 5 6
Time in sec
120
120
V
.
1 2 3 4 5 6
Time in sec
120
-120
V
.
Flow- Time Scalar: Low compliance
Higher peak expiratory flow Shortened Te due to greater elastic recoil
Flow –Time Scalar: Auto PEEP
High respiratory rate
Inadequate expiratory time
Too long of an inspiratory time
Prolonged exhalation due to bronchoconstriction
Auto-PEEP results in an increase in lung pressure in volume-controlled ventilationAuto-PEEP can have considerable effects on gas exchange and hemodynamics
Expiratory Flow:If expiratory time is insufficient to allow flow to reach 0, air trapping occurs (auto-PEEP or intrinsic PEEP)
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
Flow – Time Scalar - Information
Shows the gradual changes in the volume transferred during inspiration and expiration
Volume –Time Scalar
Volume - Time Scalar
Inspiration
SEC
800 ml
2 3 4 5 61
VT
Volume -Time Scalar
Expiration
SEC
800 ml
2 3 4 5 61
VT
Typical Volume Curve
1 2 3 4 5 6
SEC
1.2
-0.4
VT
Liters
I-TimeE-Time
A B
A = inspiratory volume
B = expiratory volume
Volume -Time Scalar : PCV
Expiration
SEC
800 ml
2 3 4 5 61
VT
Angle of volume rise drops as the flow decelerate
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
Lung Compliance
In the clinical setting, compliance measurements are used to describe the elastic forces that oppose lung inflation
C = Δ V / Δ P
Compliance has two components◦ Static compliance◦ Dynamic compliance
Compliance
Static Compliance ( Cstat)
Cstat = VT/ Pplat-PEEP
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
Normal Cstat in a ventilated patient: 70 -100 mL/cm H2O
• Consolidation• Collapse• Pulmonary edema• ARDS• Pneumothorax• Abdominal distention• Obesity/ Scoliosis
Decreased
• EmphysemaIncreased
Static Compliance
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
Dynamic compliance
Static and dynamic complianceCdyn Cstat
Decrease UnchangedIncrease PIP, unchanged Pplat: Increase Raw
Increase Unchanged
Improved Raw e.g., cleared secretions, bronchodilators
Decrease Decrease
Increased PIP and Pplat :Dec lung compliance and Raw
Increase Increase
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
Compliance
Shift in lung compliance curve yield different VT
Pressure- VolumeFlow- VolumeP-V and F-V loops provide provide dynamic trends in respiratory system compliance and resistance
Loops
Loops 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
Pressure-Volume Loop
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
VT• Pressure and Volume changes plotted against each other
• Elliptical or Football shaped
Pressure-Volume Loop
Inspiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
VT
On a ventilator-initiated mandatory breath, the loop starts in left hand corner
Progresses counter clockwise
Expiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
Inspiration
VT Counterclockwise
Pressure-Volume Loop
Hysteresis
When preset VT is reached expiration begins and returns to FRC
Hysteresis refers to unrecoverable energy, or delayed recovery of energy due to alveolar recruitment/ de recruitment; surfactant; stress relaxation; and gas absorption during the measurement of P-V curves
When the forward path is different from the reverse path, then this is referred to as hysteresis
Spontaneous Breath
Inspiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
VT
Clockwise
InspirationExpiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
VT
Clockwise
Spontaneous Breath
Assisted Breath
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
Assisted Breath
VT
Inspiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
Assisted Breath
VT
Assisted Breath
Inspiration
Expiration
0 20 40 602040-60
0.2
LITERS
0.4
0.6
Paw
cmH2O
Assisted Breath
VT Clockwise to Counterclockwise
Assisted Breath
Volume (mL)
VT
PIPPaw (cm H2O)
PEEP
Components of Pressure-Volume Loop
FRC
FRC: Balance between lung recoil and chest wall expansion
Tidal Dynamic complianceΔ V / Δ P
• Normal compliance is 50 – 80mL/cm H20
• Slope set at 45 degree
Lung Compliance Changes and the P-V Loop
Volume (mL)
PIP levels
Preset VT
Paw (cm H2O)
COMPLIANCEIncreasedNormalDecreased
Volume Targeted Ventilation
Decreased compliance: more pressure to deliver volume
Change in slope
Lung Compliance Changes and the P-V Loop
Volume (mL)
Preset PIP
VT
levels
Paw (cm H2O)
COMPLIANCEIncreasedNormalDecreased
Pre
ssu
re Ta
rgete
d
Ven
tilatio
n
Change in resistance
Volume (ml)
Pressure (cm H2O)
Abnormal Hysteresis
Normal Hysteresis
If resistance changes during constant flow ventilation the steepness of the right branch of the loop remains unchanged, but changes position
Inflection Points
Pressure (cm H2O)
Volu
me (
mL)
Lower Inflection Point
Upper Inflection Point Lower Inflection Point: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)
Point of change in line of a slope
Inflection Points 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
Inflection Points 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
Inflection Point
Over distension
Volu
me (
ml)
Pressure (cm H2O)
With little or no change in VT
Paw rises
NormalAbnormal
Beaking
Work of Breathing
• WOB equals area under the changing pressure curve as volume moves from zero to its peak at end inspiration
P-V Loops and WOBThe greater the area comprised by A & B, the greater the work
Flow- Volume Loops 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
Flow -Volume Loop Volume Control
Flow
Volume
Peak Expiratory Flow
Peak Inspiratory Flow
Tidal Volume
Inspiration
Expiration
FRC
PEFR
F-V Loop - 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.
F-V Loop: Air LeakInspiration
Expiration
Volume (ml)
Flow (L/min)
Air Leak in mL
NormalAbnormal
2
1
1
2
3
3
V.
VT
INSP
EXP
BEFORE AFTER
Worse Better
2
1
1
2
3
3
V.
2
1
1
2
3
3
V.
F-V Loop: Increased Airway Resistance
“Scooped out” pattern & decreased PEFR
Inspiration
Expiration
Volume (ml)
Flow (L/min)
NormalAbnormal
F-V Loop: Airway Secretions
F-V Loop – Auto PEEP
Does not returnto baseline
Ventilator waveforms provides much information on airway and lung mechanics
Assist in monitoring clinical course and response to therapy
Summary
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