Chapter 16: Blood Gases

INTRODUCTION

Blood gases provide important clinical information for patients with respiratory disorders, compromised circulation, or abnormal metabolism. In recent years, a number of surrogates for blood gas analysis have entered the daily practice of emergency medicine. Leveraging the benefits of blood gases requires an understanding of the underlying physiology, appropriate use of arterial and venous blood gases, and knowledge of the advantages and limitations of noninvasive monitoring methods. This chapter will be limited to evaluation of oxygen and carbon dioxide levels; for information on carbon monoxide, please refer to chapter 222, "Carbon Monoxide."

RESPIRATORY PHYSIOLOGY

Several factors contribute to overall gas exchange in the lungs. Each breath (tidal volume) is composed of functional movement of air in and out of the alveolus and nonfunctional movement of air through bronchioles, bronchi, trachea, and nonperfused areas of lung (physiologic dead space). The physiologic dead space is approximately 30% of the tidal volume. The air remaining in the chest at the end of exhalation is referred to as the functional residual capacity. Dead space and the functional residual capacity do not contribute to gas exchange. The amount of air inhaled and exhaled in a minute is referred to as minute ventilation and is the product of the respiratory rate and tidal volume. Relatively small changes in the functional alveolar space demand large increases in the minute ventilation to maintain the same rate of gas exchange. Either raising the fraction of inspired oxygen (Fio₂) or increasing the surface area or functional residual capacity of the lung can increase total alveolar oxygen (O₂) content. Positive-pressure ventilation increases the functional residual capacity through recruitment of collapsed nonventilated alveolar space.

ESTIMATING OXYGEN DELIVERY TO THE ALVEOLAR SPACE

The Fio₂ is the fraction or percentage of oxygen in the space being measured. At sea level, room air is 21% oxygen (20% is often used for ease of calculation). As Fio₂ increases, the alveolar concentration of oxygen (Pao₂) proportionately increases. Unless the patient is part of a closed system, like a ventilator circuit, the Fio₂ is only estimated. Each liter per minute of O₂ flow delivered via nasal cannula increases the Fio₂ by about 4%. Flow rates >4 L/min through a nasal cannula are poorly tolerated because of upper airway irritation. A simple O₂ mask provides an Fio₂ of 35% to 60% at flows of 10 to 15 L/min. A nonrebreather mask with a reservoir can deliver 95% O₂ at a flow rate of 10 to 12 L/min.

Comparing the measured arterial concentration of oxygen to the expected concentration of oxygen can be useful for determining the nature and severity of respiratory disorders. The approximate arterial oxygen concentration (Pao₂) values that are ...