Respiratory Formulas and Parameters

PULMONARY VOLUMES

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Minute (or Total) Ventilation

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

Tidal Volume

  • VT = VA + VD

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

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)

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.

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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Jan 14, 2019 | Posted by in PEDIATRICS | Comments Off on Respiratory Formulas and Parameters

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