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Principles of Mechanical Ventilation Mazen Kherallah, MD, FCCP
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Oxygenation Parameters Alveolar P O2 Arterial P O2 Tension-based indices –P (A-a)o2 –P aO2 /P AO2 –P aO2 /F iO2 Respiratory index Pulmonary Shunt
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Distribution of Normal Ventilation-Perfusion Ratios 1 10 0.10
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Oxygenation Status Monitoring Alveolar - arterial Oxygen Tension Difference P (A-a)o2 PAo2 = Fio2 (PB-P H2O ) - Pa co2 /R = (Fio2 713) - (Pa co2 /0.8) at sea level = 150 - (Pa co2 /0.8) at sea level on room air A-a Gradient = PAo2 - PaO2 Normal Value: 5-25 mmHg
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Oxygenation Status Monitoring A-a Gradient Increased: –Decreased Fio2 –V/Q mismatch –Shunting process –Diffusion abnormalities Decreased –Hyperventilation –Increased Fio2
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Pa O2 /PA O2 Remains stable when FiO2 changes Can be used to determined FiO2 needed for desired PO2 –FiO2 needed=[(desired PaO2)/(PaO2/PAO2)+Paco2]/(PB-47) Value of less than 0.75 indicates pulmonary dysfunction due to V/Q abnormality, shunt or diffusion abnormality
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Pa O2 /FI O2 Oxygenation index Value of less than 200 is associated with severe shunt in patients with acute respiratory failure Easy to calculate
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Respiratory Index P(A-a) O2 /PaO2 Normal value 0.1 Values higher than 0.1 indicate respiratory abnormality Better indicator of oxygenation dysfuntion
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Pulmonary Shunt Q S /Q T = (CcO2-CaO2)/(CcO2-CvO2) Q S /Q T = (CcO2-CaO2)/(3.5+ CcO2-CaO2) when pulmonary catheter is not in place
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Oxygenation Status Monitoring Oxygen Delivery Do2 = CI Ca O2 CaO2 = SaO2 1.36 Hgb + (0.0031 PaO2) CI = CO/ BSA Normal Value: 800-1200 mL/min
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Oxygenation Dissociation Curve
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Oxygenation Status Monitoring Oxygen Consumption Vo2=CI (CaO2-CvO2) CaO2 = SaO2 1.36 Hgb + (0.0031 PaO2) CvO2 = SvO2 1.36 Hgb + (0.0031 PvO2) Normal Value: 225-275 mL/min
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Oxygenation Status Monitoring Oxygen Extraction O2 ext = Vo2 / Do2 Normal value: 27%
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Oxygenation Status Monitoring Relationship between Vo2 and Do2
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Oxygenation Status Monitoring Oxygen Transport Variables
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Anatomic and Capillary Shunts
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Dead Space
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Ventilation-Perfusion Inequality Acute Exacerbation of COPD 0.01 0.1 1 10 100
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Ventilation-Perfusion Inequality Asthma 0.01 0.1 1 10 100
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Ventilation-Perfusion Inequality Pulmonary Embolism 0.01 0.1 1 10 100
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Shunting Process ARDS 0.01 1 10 100
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The Effect of Increasing Ventilation- Perfusion Inequality on Arterial Po2 and Pco2
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The effect of changing the inspired oxygen concentration on arterial Po2 for lung’s shunts of 10 to 50%
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Assessment of Hypoxia
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Ventilation Status Monitoring Tidal Volume: Vt Minute ventilation: Vm Respiration Rate: RR CO2 production: Vco2 Dead Space: V DS /V T
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Dead Space Ventilation V D /V T =(Pa CO2 -PE CO2 )/Pa CO2 Normal is 0.2-0.4 PEco2 is measured by collecting condensate from the water trap on the expiratory limb of the ventilator circuit and the measure PCO2 using blood gas analyzer
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Causes of Increased Dead Space Ventilation Pulmonary embolism pulmonary hypoperfusion positive pressure ventilation High rate-low tidal volume ventilation
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Arterial CO2 Pa CO2 = V CO2. 0.863/V E.(1-Vd/Vt)
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High Minute Ventilation Increased CO2 production –Sepsis –Fever –Thyrotoxicosis –High carbohydrate feeding Increased ventilation: –Agitation –Pain –Central hyperventilation –Increased dead space
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Low Minute Ventilation Decreased CO2 production –Hypothermia –Hypothyroidism –Severe sedation –Low carbohydrate feeding –Paralysis Decreased ventilation: –Sedation –Central hypoventilation –Decreased dead space
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Airway Pressure Waveform
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Pulmonary Mechanics Peak pressure Plateau pressure I E Airway Resistance
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Mean Airway Pressure Paw= (PIP-PEEP).(T I /T T )+PEEP.(T E /T T )
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Methods to Increase Mean Airway Pressure Increase in tidal volume Increase in respiratory frequency Reduction in T E Decrease in respiratory flow rate: increase in T I Addition of end-inspiratory pause Addition of PEEP
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Equation of Motion
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Work of Breathing Mechanical work is performed when a force moves its point of application through a distance In the case of three dimensional fluid system, work is done when a pressure (P) changes the volume (V) of the system W = P.V: {PIP-(0.5). (Pplat)/100}.v T 0.5 J/L
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Static Pressure-volume curve in ARDS
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Ventilatory System
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Control Variables during Inspiration
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Phase Variables
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Modes of Ventilation
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Breath Type during Mechanical Ventilation
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Pressure Waveforms
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Flow, Pressure, and Volume Waveforms with Constant Flow, Volume Ventilation
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Flow, Pressure, and Volume Waveforms with Decelerating Ramp Flow, Volume Ventilation
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Waveforms for Decelerating and Accelerating Ramp Flows
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Full and Partial Decelerating Ramp Flow with Volume Ventilation
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Flow, Pressure, and Volume Waveforms with Pressure Ventilation
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Full and Modified Sine-flow Waveforms during Volume Ventilation
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Flow, Pressure, and Volume Waveforms with Pressure Support Ventilation
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Active Inspiration during Positive Pressure Ventilation
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Airway Flow Waveform during Mechanical Ventilation
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Airway Volume Waveform during Mechanical Ventilation
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Flow-Volume and Pressure-Volume loops with COPD
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Changes in Flow-Volume and Pressure- Volume loops with Bronchodilators
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Pressure-Volume Loop Work Performed to Trigger the Ventilator
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Pressure-Volume Loop Lung/Chest Wall Compliance
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Dynamic Pressure-Volume LOOP Restrictive Work
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Inspiratory Work of Breathing
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Pressure-Volume Loop Deflection Points
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Modes of Mechanical Ventilation Volume-Cycled Control Mode Ventilation
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Modes of Mechanical Ventilation Assist-Control Ventilation
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Indications: – for patients who are awake, moderately sedated or paralyzed and able to initiates ventilation –increase metabolic demands: infection, burns, multisystem organ failure –Respiratory muscle strengthening and weaning Limitations: –patient-ventilator dysynchrony –ventilator assisted hyperventilation in agitated patients with increased inspiratory drive –auto-PEEP in COPD patients
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Modes of Mechanical Ventilation Intermittent Mandatory Ventilation
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Modes of Mechanical Ventilation Synchronized Intermittent Mandatory Ventilation
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Pressure Waveform for SIMV
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Synchronized Intermittent Mandatory Ventilation Indications: –patients with minimal spontaneous respiratory efforts –respiratory muscle conditioning –ventilator weaning Limitations: –patient-ventilator dysynchrony especially in agitated patients –nonphysiologic way of respiratory muscle conditioning
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Modes of Mechanical Ventilation Pressure Support Ventilation
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Indications: –weaning –more physiologic conditioning of respiratory muscles: low pressure-high volume load –improved patient- ventilator dysynchrony Limitations:
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Modes of Mechanical Ventilation Inverse Ratio Ventilation
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Auto-PEEP
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Normal Lung Mechanics and Gas Exchange
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Severe Airflow Obstruction
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Acute on Chronic Respiratory Failure
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Acute Hypoxemic Respiratory Failure
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