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PULMONARY VOLUMES

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Minute (or Total) Ventilation

  • MV = Tidal Volume (VT) × Respiratory Rate (RR)

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Tidal Volume

  • VT = VA + VD

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Alveolar Ventilation (VA): inversely proportional to PaCO2, doubling of VA will halve PaCO2

  • VA = [VT − Dead Space (VD)] × RR

    • Dead space volume (VD) - anatomic and physiologic components

      • Anatomic dead space is the volume of conducting airways

      • Physiologic dead space = Alveolar dead space + Anatomic dead space

      • Alveolar dead space is difficult to determine, so use physiologic components as a surrogate

      • Approximately 30% of VT or 2 mL/kg

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Dead Space Calculation

  • VD/VT = (PaCO2 - PECO2)/PaCO2

    • VD = Dead space volume

    • VT = Tidal volume

    • PaCO2 = Arterial CO2

    • 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

  • Static Compliance = VT/(Pplat - PEEP)

    • Pplat – Plateau pressure

    • PEEP – Positive end-expiratory pressure

    • Nl: 60 to 200 mL/cmH2O in adults

    • >1 mL/cm H2O/kg in pediatrics

    • Reflects pressure to overcome elastic forces of respiratory system

  • Dynamic Compliance = VT/(PIP - PEEP)

    • PIP – Peak inspiratory pressure

    • Nl: 50 to 175 mL/cm H20 (∼10%–20% less than static)

    • Reflects lung compliance + pressure to overcome airway resistance

    • *The difference is an indirect index of flow-resistive properties of the respiratory system

  • Effective VT/Kg = VT – [(PIP – PEEP) × tubing compliance factor]/weight in kg

    • Typically automatically compensated by ventilator setup parameters

  • Resistance – (Poiseuille's law) R = 8 nl/πr4

    • Resistance to flow inversely related to the fourth power of the radius.

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Mean Airway Pressure:

  • MAP = [It/(It + Et)] × (PIP – PEEP)] + PEEP

  • Correlated to oxygenation, an increase in MAP will improve alveolar volume and improve oxygenation

    • It = Inspiratory time

    • Et = Expiratory time

    • PIP = Peak inspiratory pressure

    • PEEP = Positive end-expiratory pressure

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OXYGEN PARAMETERS AND LUNG INJURY CLASSIFICATIONS

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Alveolar gas equation

  • PAO2 = (FiO2 × (PAtm - PH20)) - (PaCO2/respiratory quotient)

    • FiO2 = fraction of inspired oxygen

    • PAtm (sea level) = 760 mmHg

    • PH20 = 47 mmHg

    • Normal RQ = 0.8

  • Pearls:

    • Alveolar PO2 is determined mostly by level of alveolar ventilation

    • If FiO2 is held constant and PaCO2 increases, PAO2 and PaO2 will always decrease

    • Hypoxia due to hypoventilation is easily overcome by increasing FiO2

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Alveolar-arterial gradient: measure of lung function

  • A-a O2 Gradient= PAO...

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