Chapter 17: Respiratory Formulas and Parameters

PULMONARY VOLUMES

Minute (or Total) Ventilation

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

Tidal Volume

• V_T = V_A + V_D

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

• V_A = [V_T − Dead Space (V_D)] × RR

  • Dead space volume (V_D) - 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 V_T or 2 mL/kg

Dead Space Calculation

• V_D/V_T = (PaCO₂ - PECO₂)/PaCO₂

  • V_D = Dead space volume

  • V_T = 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 = V_T/(P_plat - PEEP)

  • P_plat – 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 = V_T/(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 V_T/Kg = V_T – [(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 = [I_t/(I_t + E_t)] × (PIP – PEEP)] + PEEP

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

  • I_t = Inspiratory time

  • E_t = Expiratory time

  • PIP = Peak inspiratory pressure

  • PEEP = Positive end-expiratory pressure

OXYGEN PARAMETERS AND LUNG INJURY CLASSIFICATIONS

Alveolar gas equation

• P_AO₂ = (FiO₂ × (P_Atm - P_H20)) - (P_aCO₂/respiratory quotient)

  • FiO₂ = fraction of inspired oxygen

  • P_Atm (sea level) = 760 mmHg

  • P_H20 = 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

• A-a O₂ Gradient= PAO_...