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)
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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