Chapter 8: Hypoxemic Respiratory Failure

INTRODUCTION

Hypoxia occurs when tissues do not receive adequate oxygen to meet their metabolic demands. It is nonspecific and can mean reduced arterial partial pressure of oxygen (PaO₂; hypoxemic hypoxia), reduced hemoglobin for oxygen transportation (anemic hypoxia), decreased cardiac output or regional decrease in perfusion (stagnant hypoxia), disarray in cellular metabolism of oxygen such as in cyanide (histotoxic hypoxia), or mitochondrial dysfunction in sepsis (cytopathic dysoxia).¹

Type 1 respiratory failure is defined as PaO₂ less than 60 mmHg with an equivalent arterial oxygen saturation (SaO₂) of 90% and normal or near or low arterial partial pressure of carbon dioxide (PaCO₂). Type 2 respiratory failure involves hypercapnia and is defined as a PaCO₂ of 46 mmHg or higher, regardless of oxygen saturation. Type 1 respiratory failure will be discussed in this chapter.

OXYGEN

Two percent of oxygen is dissolved in plasma (PaO₂) and 98% is bounded to hemoglobin (SaO₂). One gram of hemoglobin (Hgb) carries 1.34 mL O₂ per gram. Oxygen delivery is based on the formula DO₂ = CaO₂ × CO, where DO₂ is the diffusing capacity of oxygen, CaO₂ is the arterial concentration of oxygen, and CO is cardiac output. Oxygen content is derived from the formula CO₂ = (1.39 × Hgb × SaO₂/100) + (0.003 × PO₂), where CO₂ is the concentration of oxygen, Hgb is the hemoglobin, SaO₂ is fraction of oxygen saturated hemoglobin over total hemoglobin, and PO₂ is the partial pressure of oxygen. Oxygen consumption is derived from the formula V.O₂ = CO × (CaO₂ – CvO₂), where CvO₂ is the venous concentration of oxygen. The alveolar–arterial (A-a) oxygen gradient is PAO₂ – PaO₂, where PAO₂ = (FiO₂ × [P_ATM – PH₂O]) – (PaCO₂/R). In these formulas, PAO₂ is the alveolar oxygen tension; PaO₂ is the arterial pressure of oxygen; FiO₂ is the fraction of inspired oxygen, which is 0.21 on room air; P_ATM is atmospheric pressure, which is 760 mmHg at sea level, PH₂O is the partial pressure of water, which is 47 mmHg at 37°C; PaCO₂ is arterial carbon dioxide tension; and R is 0.8 at steady state. The normal A-a gradient varies with age and is based on the formula 2.5 + 0.21 × age in years.

A common, noninvasive way to measure oxygen bounded to hemoglobin is oxygen saturation via pulse ...