Chapter 16: Blood Gases, Pulse Oximetry, and Capnography

INTRODUCTION

Blood gases provide important clinical information for patients with respiratory disorders, compromised circulation, or abnormal metabolism. This chapter briefly reviews respiratory physiology to aid selection of assessment options and discusses appropriate use of arterial and venous blood gases and the advantages and limitations of noninvasive monitoring methods. The material focuses on the 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) moves air in and out of the alveolus for gas exchange but also moves air through large airways and nonperfused areas of lung, the 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 the functional residual capacity. Dead space and the functional residual capacity do not contribute to gas exchange. Minute ventilation is a product of the respiratory rate and tidal volume. Relatively small changes in the usable alveolar space require large increases in the minute ventilation to maintain the same rate of gas exchange. Raising the fraction of inspired oxygen (FIo₂) or increasing the surface area or functional residual capacity of the lung can increase total alveolar oxygen content. Positive-pressure ventilation increases the functional residual capacity through recruitment of collapsed nonventilated alveolar space.

ESTIMATING OXYGEN DELIVERY

The FIo₂ is the percentage of oxygen in each breath. At sea level, room air is 21% oxygen. As FIo₂ increases, so does the alveolar concentration of oxygen (PAo₂). Each liter per minute of oxygen 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, although some noninvasive devices can supply a higher FIo₂ via nasal cannula. A simple mask provides an FIo₂ of 35% to 50% at flow rates of 5 to 10 L/min. A nonrebreather mask with a reservoir can deliver near 100% FIo₂ with a supply flow rate of 10 to 15L/min (Table 16-1).