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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