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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2012 Waugh JB, Deshpande VM, Harwood RJ, Brown M. Rapid Interpretation of Ventilator

Waveforms. 2nd edition, Upper saddle River, New Jersey: Prentice-Hall. Inc. 2007 Rittner F, Döring M. Curves and Loops in Mechanical Ventilation. Telford, Pa: Draeger

Medical; 1996. Rittner F, Doring M. Curves and Loops in Mechanical Ventilation. Drager Medical 2006.

6-44 Tobin J, Principles and Practice of Mechanical Ventilation, 3rd edition. The Mcgaw-Hill

companies, 2013 Grinnan DC, Truwit J, Clinical review: Respiratory mechanics in spontaneous and assisted

ventialtion, Critical Care 2005;9; 473- 483 Jubran A, Monitoring Mechanics during ventialtion, Sem Resp Crit Care M; 1999;20;65-79 Hess D, Kackmarek R, Essentials of Mechanical Ventilation, 3rd edition, The McGraw Hill

Education, 2014 Correger E, Muria G, Chacon E et al, Interpretation of ventilator curves in patients with

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Ventilatory Circuit, Resp Care; 2005;50(1):55-67 Chatburn R, Fundementals of Mechanical Ventilation, Mandu Press, Cleveland, OH. 2003