Course n : Course 3 Title: RESPIRATORY PHYSIOLOGY, PHYSICS AND PATHOLOGY IN RELATION TO ANAESTHESIA AND INTENSIVE CARE Sub-category: Intensive Care for Respiratory Distress Topic: Pulmonary Function and Respiratory Mechanics in ICU Date: January 15-17, 2016 Language: English City: Karachi Country: Pakistan Speaker: Dr. Hameed Ullah
Conflict of interest None to declare.
Definition Expression of lung function through measures of: Pressure. Flow. Derived variables: Volume. Compliance. Resistance. Work of breathing.
Respiratory mechanics Waveform time-based graphics. Pressure-time. Flow-time. Volume time. Loops and curves. One parameter is plotted against another parameter, where one variable is derived (volume). Flow-volume. Pressure-volume.
Waveforms and loops Bedside monitoring available on most ventilators. Assumes: Lung as a single compartment. Linear response over a range of linear volume. Useful to: Evaluate lung function. Optimize mechanical ventilatory support. Assess response to therapy.
Respiratory mechanics Relation between pressure, flow and tidal volume. Static mechanics: Done with flow interruption zero flow. Dynamic mechanics: Displays C RS, R aw and auto-peep without inspiratory and expiratory pause maneuvers. Ventilator software use linear regression analysis from constantly changing variables from 100 or more equations per breath.
Equation of motion P RS = P VENT + P MUS P VENT + P MUS = V T / C RS + R aw x V I + PEEP + Inertance PEEP = PEEP VENT + PEEP I Inertance inertia to flow low in adults ignored. During mechanical ventilation P MUS = zero. PEEP recruitment? / work of breathing?
Airway pressure Ideally measured at distal airways. Distal to expiratory valve during inspiratory phase. Distal to inspiratory valve during expiratory phase. Flow is ZERO in: Expiratory limb during inspiration. Inspiratory limb during expiration. Should approximate distal airway pressure.
Airway pressure Displayed as a function of time. Shape of airway pressure waveform depends upon: Flow. Tidal volume. Lung mechanics. Spontaneous breathing efforts.
Alveolar pressure Volume controlled ventilation: Tidal volume. Compliance. PEEP. Pressure controlled ventilation: Airway pressure above PEEP. Time constants. PEEP.
Plateau pressure Measure of end-inspiratory distending pressure. End-inspiratory transpulmonary pressure (stress) might be a better indicator of the potential for lung injury than plateau pressure alone. Use of esophageal manometry resurgence. End-expiratory transpulmonary pressure might be useful to guide the setting of PEEP to counterbalance the collapsing effect of chest wall.
Plateau pressure Due to R aw, proximal airway pressure (PIP) will always be greater than P alv. Plateau pressure (P plat ) is measured by applying endexpiratory breath hold for 0.5 2 seconds.
Plateau pressure
Plateau pressure Due to R aw, proximal airway pressure (PIP) will always be greater than P alv. Plateau pressure (P plat ) is measured by applying endexpiratory breath hold for 0.5 2 seconds. It depends upon same factors as P alv.
Auto-PEEP Pressure produced by incomplete emptying of lungs. Increases end-expiratory lung volume. Causing dynamic hyperinflation. Measured by application of end-expiratory pause for 0.5 2 seconds. Pressure in excess of PEEP set on ventilator. Active breathing invalidates the measurement. Underestimated if airways close at different times. Esophageal pressure (P es ) spontaneous breath.
Auto-PEEP
Auto-PEEP Auto-PEEP is a function of: Airway resistance. Compliance. Breathing rate. Inspiratory time. Tidal volume.
Auto-PEEP
Auto-PEEP
Mean airway pressure Mean airway pressure is determined by: Peak inspiratory pressure. Fraction of inspiratory time. PEEP. Triangular vs. square pressure waveform. P alv may be different for Mean airway pressure if inspiratory and expiratory resistance are different often happens in lung disease.
Asynchrony Airway pressure waveform varies. During pressure support patient exhales to terminate inspiratory phase. May bias measurement of respiratory mechanics particularly P plat.
Asynchrony
Stress index Assesses shape of the pressure-time curve during constant flow ventilation. Normal recruitment without over distension. Over distension alter PEEP, V T or both. Recruitment increase PEEP.
Stress index
Stress index
Stress index
Time constant Determines the rate of change in the volume of a lung unit that is passively inflated or deflated. Lung units with high resistance will have long time constants and require more time to fill or empty. Simple method divide volume by flow.
Inspiratory flow Volume control ventilation set on ventilator. Pressure control ventilation ventilator calculates the flow based upon: Airway resistance. Time constant. Tidal volume. PEEP.
Expiratory flow Normally passive. Depends upon: Airway resistance. Alveolar pressure. Time of exhalation. Volume. Determines the cause of auto-peep. Auto-PEEP decreases trigger sensitivity fatigue.
Expiratory flow
Tidal volume Not measured directly by most ventilators. Measured by flow sensors placed in the ventilators not the patient end. Volumes delivered are closely approximated. Leak difference between inspiratory / expiratory.
Respiratory system compliance... Tidal volume divided by pressure required. C RS = ΔV / ΔP = V T / (P plat PEEP) Lung + chest wall. Best PEEP: Highest compliance. Lowest driving pressure (P plat PEEP).
Respiratory system compliance... Regional variation. Compliance measured is a function of many different regional compliances. In health, the difference is not significant In disease, its may contribute significantly to distribution of ventilation.
Airway resistance Changes with inspiration, expiration and diseases. Estimated from: Peak pressure. Plateau pressure. End-inspiratory flow. Raised in: Secretions. Bronchospasm. Small diameter ETT.
Pressure-volume curves Displayed with volume as a function of pressure. Slope of P-V curve is compliance. Inflection points. Used to assess PEEP induced lung recruitment.
Pressure-volume curves
Pressure-volume curves
Pressure-volume curves
Flow-volume loops Displayed flow as a function of volume. Inspiratory positive. Expiratory negative. Vice versa. May be useful in identifying: Secretions. Response to bronchodilation.
Flow-volume loops
Summary A variety of respiratory mechanics parameters are available. Help in: Identifying the pathophysiological process. Make ventilation advantageous. Minimize ventilator induced lung injury.