## PULMONARY VOLUMES

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

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

• Approximately 30% of VT or 2 mL/kg

• 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

• 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

## OXYGEN PARAMETERS AND LUNG INJURY CLASSIFICATIONS

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

Alveolar-arterial gradient: measure of lung function