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.”
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 60% at flows of 10 to 15 L/min. A nonrebreather mask with a reservoir can deliver 95% FIo2 with a supply flow rate of 10 to 12 L/min.
The nature and severity of respiratory disorders can be estimated by comparing the measured arterial concentration of oxygen to the expected concentration of oxygen. The approximate arterial oxygen concentration (Pao2) values that are expected in normal persons who are inhaling various concentrations of oxygen are listed in Table 16-1.
TABLE 16-1Expected Pao2 in Patients Inhaling Various Concentrations of Oxygen (mm Hg) |Favorite Table|Download (.pdf) TABLE 16-1 Expected Pao2 in Patients Inhaling Various Concentrations of Oxygen (mm Hg)
|Fraction of inspired oxygen ||0.21 ...|