PULMONARY VOLUMES
Minute (or Total) Ventilation
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MV = Tidal Volume (VT) × Respiratory Rate (RR)
Tidal Volume
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VT = VA + VD
Alveolar Ventilation (VA): inversely proportional to PaCO2, doubling of VA will halve PaCO2
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VA = [VT − Dead Space (VD)] × RR
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Dead space volume (VD) – anatomic and physiologic components
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Anatomic dead space is the volume of conducting airways
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Physiologic dead space = Alveolar dead space + Anatomic dead space
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Alveolar dead space is difficult to determine, so use physiologic components as a surrogate
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Approximately 30% of VT or 2 mL/kg
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Dead Space Calculation
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VD/VT = (PaCO2 – PECO2)/PaCO2
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VD = Dead space volume
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VT = Tidal volume
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PaCO2 = Arterial CO2
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PECO2 = Mean expired CO2 in a breath (obtain from exhaled CO2 monitor)
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GAS MOVEMENT
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Compliance = Change in Volume/Change in Pressure
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Static Compliance = VT/(Pplat – PEEP)
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Pplat – Plateau pressure
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PEEP – Positive end-expiratory pressure
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Nl: 60 to 200 mL/cmH2O in adults
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>1 mL/cm H2O/kg in pediatrics
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Reflects pressure to overcome elastic forces of respiratory system
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Dynamic Compliance = VT/(PIP – PEEP)
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PIP – Peak inspiratory pressure
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Nl: 50 to 175 mL/cm H20 (∼10%–20% less than static)
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Reflects lung compliance + pressure to overcome airway resistance
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*The difference is an indirect index of flow-resistive properties of the respiratory system
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Effective VT/Kg = VT – [(PIP – PEEP) × tubing compliance factor]/weight in kg
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Typically automatically compensated by ventilator setup parameters
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Resistance – (Poiseuille’s law) R = 8 nl/πr4
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Resistance to flow inversely related to the fourth power of the radius.
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Mean Airway Pressure:
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MAP = [It/(It + Et)] × (PIP – PEEP)] + PEEP
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Correlated to oxygenation, an increase in MAP will improve alveolar volume and improve oxygenation
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It = Inspiratory time
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Et = Expiratory time
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PIP = Peak inspiratory pressure
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PEEP = Positive end-expiratory pressure
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