Respiratory equations

Question 1

Fick’s principle

Answer 1

Oxygen Consumption (VO2) = Cardiac Output (Q) × Arterial-Venous Oxygen Content Difference (CaO2 - CvO2)

Question 2

Oxygen Delivery (DO2)

Answer 2

DO2 = CO x Arterial Oxygen Content (CaO2) = CO x (1.34 x Hb x SaO2 + 0.003 x PaO2)

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Question 3

A-a Gradient

Answer 3

(150) - (1.25 * PaCO2) - PaO2

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Question 4

Minute Ventilation (VE)

Answer 4

VE = Tidal Volume (Vt) * Respiratory Rate (RR) (L/min)

Question 5

Respiratory Dead Space (VD)

Answer 5

VD = Vt * (PaCO2 - PeCO2) / PaCO2
Where PeCO2 is the partial pressure of CO2 at the end of expiration.

Question 6

Compliance (C)

Answer 6

C = ΔV / ΔP = (Tidal Volume) / Driving Pressure (ml/cmH2O)

Question 7

Driving Pressure

Answer 7

Driving Pressure = Plateau Pressure - PEEP

Question 8

Transalveolar pressure

Answer 8

Transalveolar Pressure = Plateau Pressure - Esophageal Pressure

Note: esophageal pressure is as a surrogate for pleural pressure

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Question 9

Rapid shallow breathing index (RSBI)

Answer 9

RSBI = f/Vt

Where Vt is tidal volume and f is the respiratory rate.

Question 10

Predicted body weight (for tidal volumes)

Answer 10

• Males: PBW (kg) = 50 + 0.91 (height (cm) – 152.4)
• Females: PBW (kg) = 45.5 + 0.91 (height (cm) – 152.4)

MCQs