Pulmonary function tests

Lung volumes are measured by spirometry and once quantified can be compared against predicted values for the size,

Figure 8.10. Spirometric tracing of lung volumes. ERV: end residual volume; IC: isovolumetric contraction.

age, and gender of the patient. An example of a spirometry tracing is provided in Fig. 8.10 and an explanation of these volumes is provided below:

• Tidal volume (VT) is the volume of air inspired or expired per breath.

Normal tidal volume = 500 ml

• Minute ventilation is the volume of air inspired or expired over 1 min.

Minute ventilation = VT X respiratory rate Normal minute ventilation = 5-7l/min

• Total lung capacity (TLC) is the volume of air in the lungs after maximal inspiration (6l). TLC equals the sum of the vital capacity (VC) and the residual volume (Rv).

• VC is the maximal volume of air that can be exhaled after a maximal inspiration. It depends on the strength of the respiratory muscles and the resistance of the lungs and chest wall. Normal VC is approximately 4.81.

• Forced expiratory volume in 1s (FEV1) is the volume of air that can be forcibly exhaled in 1 s.

• Functional residual capacity (FRC) is the volume of air left in the lungs after exhalation (2.21). It is a critical volume because it allows for gas exchange throughout the entire respiratory cycle. FRC decreases with anesthesia, obesity, surgery (particularly thoracic and abdominal), and increasing age. When the FRC is too low, atelectasis occurs resulting in hypoxemia. Recruitment maneuvers to open alveoli (e.g. positive end-expiratory pressure - PEEP) are aimed at increasing FRC.

• Rv is the amount of air remaining in the lungs after a maximal expiration (1.21).

Diffusion capacity of carbon monoxide (DLCO) is the ability of CO to diffuse across intact alveoli. This study is often performed in conjunction with spirometry. The patient inhales a small known quantity of CO, holds his breath for 10 s, and then exhales. The expired gas is analyzed for CO. A low DLCO implies that there is a ventilation/perfusion (V/Q) mismatch and is often seen in disorders that disrupt normal alveolar architecture (emphysema or pulmonary fibrosis). It can also be present in severe anemia. Alternatively, DLCO may be increased in patients with polycythemia.

Measurements of VC and FEV1 are useful in determining whether obstructive, restrictive, or mixed airways disease is present. FEV1 is also one of the best predictors of outcome for patients about to undergo pulmonary resection (e.g. in lung cancer). In general, a patient with a FEV1 greater than 21 or 60% predicted and DLCO > 60% is at low risk of complications even for pneumonectomy.

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