Episode 1 - Acute Respiratory Failure I: Mechanisms & Pathophysiology

Summary

Acute respiratory failure is one of the most common causes of ICU admission. Understanding the pathophysiology and mechanisms of both hypoxemia and hypercapnia is critical for healthcare professionals managing critically ill and injured patients.

Learning Objectives

#1 – Discuss the key equations and concepts used to describe oxygenation and ventilation

#2 – Describe the differences between hypoxia and hypoxemia

#3 – Understand the 6 pathophysiologic causes of hypoxemia

#4 – Apply an “anatomic” approach to understanding causes of hypercapneic respiratory failure 

Take Home Points

• MOST COMMON cause of hypoxemia is V/Q mismatch

• Shunt and headspace ventilation are 2 extremes of V/Q mismatch

• DO2 equation provides an framework for understanding hypoxia and causes of low tissue oxygen tension

Time Stamps

• 00:12 Introduction

• 01:45 Learning Objectives

• 03:17 Definition & Classification

• 07:08 DO2 or oxygen delivery equation

• 10:04 Oxyhemoglobin dissociation curve

• 12:12 paO2 vs. SaO2

• 13:13 A-a O2 difference

• 14:54 P/F ratio

• 15:42 Hypoxia versus hypoxemia

• 18:35 Causes of hypoxemia

• 29:27 Determinants of CO2

Key Equations and Figures

DO2 Equation

Oxygen delivery = Q x CaO2

where Q = HR x SV; CaO2 = (1.34 x Hb x SaO2) + (0.0031 x paO2)

Oxyhemoglobin Dissociation Curve

Alveolar Gas Equation

PAO2 = FiO2 (Patm - PH2O) - (paCO2 / RQ)

= (713 x FiO2) - (1.25 x paCO2)

where Patm = 760 mmHg or 1 atm; pH20 = 47 mmHg; RQ = respiratory quotient = 0.8

P (A - a O2) = PAO2 - PaO2 (normal = 5-10 mmHg)

Determinants of paCO2

paCO2 = K x CO2 production / alveolar ventilation

= K VCO2 / VA x (1-Vd/Vt)

where K = constant; VCO2 = CO2 production; VA = alveolar ventilation; Vd = deadspace ventilation; Vt = tidal volume

Recommended Reading

David T. Neilipovitz. Acute Resuscitation and Crisis Management: Acute Critical Events Simulation (ACES). University of Ottawa Press, 2005.