Mechanics of Ventilation

Objectives 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

Normal Mechanics Spontaneous ventilation Inspiration Exhalation Diaphragm descends and enlarges vertical diameter of thorax External intercostal contraction raises ribs Exhalation Passive

Pressures and Gradients in Lungs Transpulmonary Transairway - During positive-pressure ventilation, the Pbs remains atmospheric, but the pressures at the upper airways (Pawo) and in the conductive airways (airway pressure, or Paw) become positive. Alveolar pressure (PA) then becomes positive, and transpulmonary pressure (PL) increases. - Transrespiratory pressure (Ptr ) is the pressure difference between the airway opening and the body surface: Ptr = Pawo − Pbs . Transrespiratory pressure is used to describe the pressure required to inflate the lungs and airways during positive-pressure ventilation. In this situation, the body surface pressure (Pbs ) is atmospheric and usually is given the value zero; thus Pawo becomes the pressure reading on a ventilator gauge (Paw). Alveolar distention pressure Plateau

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

Elementary Law of Mechanics Newton’s third law of motion Pappl(t) = Pel (t) + Pres (t)

Equation of Motion P = ΔV X E + Flow x R P = ΔV/ C + Flow x R Ventilation Pressure ( to deliver tidal volume) Elastic Pressure ( to inflate lungs and chest wall) Resistive Pressure ( to make air flow through airways) = + P = ΔV X E + Flow x R Compliance = 1/ E P = ΔV/ C + Flow x R

Relevance 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

Mechanics of 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

Basic waveforms Scalars Loops Pressure Flow Volume Pressure-Volume

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

Pressure Scalar 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 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

Pressure Waveform for Volume- Controlled Ventilation Resistance = airway resistance Compliance = compliance of the entire system (lungs, vent circuit, etc) Δp/Δt = Flow/Compliance Δp = R * Exp Flow Δp = R ∗ Flow 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. 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 volume During the plateau time no volume is supplied to the lung, and inspiratory flow is zero The difference between plateau pressure (E) and end-expiratory pressure F (PEEP) is obtained by dividing the delivered volume tidal volume (VT) by compliance (C) 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). Expiration begins at point E and is passive; the elastic recoil forces of the thorax force the air against atmospheric pressure out of the lung The change in pressure is obtained by multiplying exhalation resistance of the ventilator by expiratory flow Once expiration is completely finished, pressure once again reaches the end-expiratory level F (PEEP) 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.

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.

Pressure Waveform for Pressure- Controlled Ventilation 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

PIP Pplateau Paw (cm H20) Pao Resistive Pressure Time (sec) Transairway Pressure Pplateau Paw (cm H20) Pao Alveolar Pressure Transairway Pressure Resistive Pressure Time (sec) 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 Decreasing compliance → plateau and peak pressures rise Increasing compliance → plateau and peak pressures fall

Pressure Waveform: Changes in Airway Resistance 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 vs. Pplat Bronchospasm Secretions Foreign Bodies Tube Kinks Pulm Edema Atelectasis Pneumonia Pneumothorax ARDS Pulmonary Fibrosis Pplat

Mean Airway Pressure (Paw) 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

Increasing Mean Airway Pressure

Pressure – Time Scalar - Information 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

Flow- Time Scalar 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 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

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 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

Expiratory Flow Rate and Changes in Expiratory Resistance 120 . Time in sec V 1 2 3 4 5 6 Bronchospasm COPD Secretions Water in the tubing -120 Low expiratory flow rate Extended exhalation phase Curved contour

A Higher Expiratory Flow Rate and a Decreased Expiratory Time Denote a Lower Expiratory Resistance 120 . Time in sec V 1 2 3 4 5 6 120

Flow- Time Scalar: Low compliance 120 . Time in sec V 1 2 3 4 5 6 -120 Higher peak expiratory flow Shortened Te due to greater elastic recoil

Flow –Time Scalar: Auto PEEP Expiratory Flow: If expiratory time is insufficient to allow flow to reach 0, air trapping occurs (auto-PEEP or intrinsic PEEP) Auto-PEEP results in an increase in lung pressure in volume-controlled ventilation Auto-PEEP can have considerable effects on gas exchange and hemodynamics High respiratory rate Inadequate expiratory time Too long of an inspiratory time Prolonged exhalation due to bronchoconstriction

Flow – Time Scalar - Information 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

Volume –Time Scalar Shows the gradual changes in the volume transferred during inspiration and expiration

Volume - Time Scalar Inspiration SEC 800 ml 2 3 4 5 6 1 VT

Volume -Time Scalar Expiration VT 800 ml 1 2 3 4 5 6 SEC During expiration, seen in red here, the transferred volume decreases, again due to the passive recoil of the lung. Generally, what goes in comes out, unless you have a leak in the patient circuit or the patient, or gas is trapped in the lung. 1 2 3 4 5 6

Typical Volume Curve I-Time E-Time 1.2 A B VT Liters 1 2 3 4 5 6 -0.4 SEC The volume-time curve shows the gradual changes in the volume that is delivered during inspiration and expiration. During the inspiratory flow phase the volume increases continuously until the set tidal volume is achieved or the high pressure alarm limit has been reached, or I-Time has expired. 1 2 3 4 5 6 -0.4 A = inspiratory volume B = expiratory volume

Volume -Time Scalar : PCV 800 ml Expiration VT SEC 1 2 3 4 5 6 Angle of volume rise drops as the flow decelerate

Lung Compliance 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

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

Static Compliance ( Cstat) 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 Cstat = VT/ Pplat-PEEP Normal Cstat in a ventilated patient: 70 -100 mL/cm H2O

Static Compliance Decreased Increased Emphysema Consolidation Collapse Pulmonary edema ARDS Pneumothorax Abdominal distention Obesity/ Scoliosis Decreased Emphysema Increased

Dynamic 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

Static and dynamic compliance Cdyn Cstat Decrease Unchanged Increase PIP, unchanged Pplat: Increase Raw Increase 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

Compliance 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

Loops Pressure- Volume Flow- Volume P-V and F-V loops provide provide dynamic trends in respiratory system compliance and resistance

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 Paw VT 20 40 60 -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 Paw Inspiration VT 20 40 60 -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

Pressure-Volume Loop Counterclockwise Paw Expiration Inspiration VT When preset VT is reached expiration begins and returns to FRC LITERS 0.6 Expiration 0.4 Hysteresis Inspiration 0.2 Paw cmH2O -60 40 20 20 40 60 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 Paw Inspiration VT Clockwise 0.6 0.4 0.2 cmH2O -60 LITERS 0.6 0.4 Inspiration 0.2 Paw cmH2O -60 40 20 20 40 60

Spontaneous Breath Paw Inspiration Expiration VT Clockwise 0.6 0.4 0.2 LITERS 0.6 0.4 Inspiration Expiration 0.2 Paw cmH2O -60 40 20 20 40 60

Assisted Breath Paw Assisted Breath VT 0.6 0.4 0.2 cmH2O -60 40 20 20 LITERS 0.6 0.4 Assisted Breath 0.2 Paw cmH2O -60 40 20 20 40 60

Assisted Breath Paw Assisted Breath Inspiration VT 0.6 0.4 0.2 cmH2O LITERS 0.6 0.4 Assisted Breath 0.2 Inspiration Paw cmH2O -60 40 20 20 40 60

Assisted Breath Paw Expiration Assisted Breath Inspiration VT Clockwise to Counterclockwise LITERS 0.6 Expiration 0.4 Assisted Breath 0.2 Inspiration Paw cmH2O -60 40 20 20 40 60

Components of Pressure-Volume Loop Tidal Dynamic compliance Δ V / Δ P VT PIP Normal compliance is 50 – 80mL/cm H20 Slope set at 45 degree Volume (mL) PEEP FRC: Balance between lung recoil and chest wall expansion Paw (cm H2O) FRC

Lung Compliance Changes and the P-V Loop Volume Targeted Ventilation Preset VT Change in slope COMPLIANCE Increased Normal Decreased Volume (mL) Paw (cm H2O) Decreased compliance: more pressure to deliver volume PIP levels

Lung Compliance Changes and the P-V Loop Increased Normal Decreased VT levels Pressure Targeted Ventilation Volume (mL) Paw (cm H2O) Preset PIP

Change in resistance Normal Hysteresis Abnormal Hysteresis Volume (ml) If resistance changes during constant flow ventilation the steepness of the right branch of the loop remains unchanged, but changes position Normal Hysteresis Abnormal Hysteresis Pressure (cm H2O)

Inflection Points Point of change in line of a slope 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) Upper Inflection Point Volume (mL) Lower Inflection Point Pressure (cm H2O)

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

Inflection Point 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

With little or no change in VT Over distension Beaking With little or no change in VT Normal Abnormal Volume (ml) Pressure (cm H2O) Paw rises

Work of Breathing B A: Resistive Work B: Elastic Work A Volume (ml) Pressure (cm H2O)

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 WOB The 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 Tidal Volume Peak Inspiratory Flow Peak Expiratory Flow Inspiration Flow Volume FRC Expiration 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 Leak Normal Abnormal Inspiration Expiration Volume (ml) Flow (L/min) Volume (ml) Air Leak in mL Normal Abnormal Expiration

F-V Loop: Increased Airway Resistance BEFORE AFTER Worse Better 3 3 2 1 3 V . INSP 2 2 1 1 . . V V VT 1 1 2 2 “Scooped out” pattern & decreased PEFR 3 3 EXP

F-V Loop: Airway Secretions Inspiration Flow (L/min) Volume (ml) Normal Abnormal Expiration

F-V Loop – Auto PEEP Does not return to baseline

Summary Ventilator waveforms provides much information on airway and lung mechanics Assist in monitoring clinical course and response to therapy

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