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Mechanics of Ventilation
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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
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Normal Mechanics Spontaneous ventilation Inspiration Exhalation
Diaphragm descends and enlarges vertical diameter of thorax External intercostal contraction raises ribs Exhalation Passive
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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
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Mechanics of Spontaneous Ventilation
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Mechanics of Positive Pressure Ventilation
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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
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Elementary Law of Mechanics
Newton’s third law of motion Pappl(t) = Pel (t) + Pres (t)
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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
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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
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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
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Basic waveforms Scalars Loops Pressure Flow Volume Pressure-Volume
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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
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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
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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
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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.
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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.
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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
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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
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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
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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
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PIP vs. Pplat Bronchospasm Secretions Foreign Bodies Tube Kinks
Pulm Edema Atelectasis Pneumonia Pneumothorax ARDS Pulmonary Fibrosis Pplat
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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
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Increasing Mean Airway Pressure
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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
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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
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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
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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
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Volume Targeted Ventilation
Tplateau
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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
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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
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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
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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
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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
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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
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Volume –Time Scalar Shows the gradual changes in the volume transferred during inspiration and expiration
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Volume - Time Scalar Inspiration SEC 800 ml 2 3 4 5 6 1 VT
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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
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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
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Volume -Time Scalar : PCV
800 ml Expiration VT SEC 1 2 3 4 5 6 Angle of volume rise drops as the flow decelerate
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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
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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
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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: mL/cm H2O
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Static Compliance Decreased Increased Emphysema Consolidation Collapse
Pulmonary edema ARDS Pneumothorax Abdominal distention Obesity/ Scoliosis Decreased Emphysema Increased
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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
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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
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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
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Loops Pressure- Volume Flow- Volume
P-V and F-V loops provide provide dynamic trends in respiratory system compliance and resistance
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Lung Compliance Changes and the P-V Loop
Increased Normal Decreased VT levels Pressure Targeted Ventilation Volume (mL) Paw (cm H2O) Preset PIP
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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)
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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)
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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
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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
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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
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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
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Work of Breathing B A: Resistive Work B: Elastic Work A Volume (ml)
Pressure (cm H2O)
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Work of Breathing WOB equals area under the changing pressure curve as volume moves from zero to its peak at end inspiration
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P-V Loops and WOB The greater the area comprised by A & B, the greater the work
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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
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Flow -Volume Loop Volume Control
Tidal Volume Peak Inspiratory Flow Peak Expiratory Flow Inspiration Flow Volume FRC Expiration PEFR
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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.
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F-V Loop: Air Leak Normal Abnormal Inspiration Expiration Volume (ml)
Flow (L/min) Volume (ml) Air Leak in mL Normal Abnormal Expiration
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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
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F-V Loop: Airway Secretions
Inspiration Flow (L/min) Volume (ml) Normal Abnormal Expiration
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F-V Loop – Auto PEEP Does not return to baseline
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Summary Ventilator waveforms provides much information on airway and lung mechanics Assist in monitoring clinical course and response to therapy
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References West B J, Respiratory Physiology. Ninth edition. Philadelphia: Lippincott Williams & Wilkins; 2012 Cairo J.M., Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications. 5th ed. St Louis: Elsevier Mosby; 2012. Wyka K, Mathews P, Rutkowski J. Foundations of Respiratory Care. 2nd ed. NY. Delmad; 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 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; 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 acute respiratory failure, Med Intensiva, 2012;36(4): Lucangelo U, Bernabe F, Blanch L; Respiratory Mechanics Derived from Signals in the Ventilatory Circuit, Resp Care; 2005;50(1):55-67 Chatburn R, Fundementals of Mechanical Ventilation, Mandu Press, Cleveland, OH. 2003
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