Key Clinical Questions
What information do pulmonary function tests provide in addition to the history and physical examination?
What specific tests might you order to evaluate the acutely ill hospitalized patient and how does each test influence diagnostic evaluation or management?
What operations require preoperative pulmonary function tests as part of the preoperative evaluation?
What are the predictors of increased postoperative risk?
Pulmonary function tests (PFTs) objectively assess lung function. Along with measurement of arterial blood gases (ABGs), PFTs are used to evaluate how much a patient's symptoms or known lung disease impairs daily activities and the tests are helpful in management, such as when to treat a patient and in what setting. The purpose of PFTs is to evaluate dyspnea by assessing the mechanical function of the respiratory system, to quantitate the loss of lung function, and to monitor disease progression and response to treatment. PFTs also predict postoperative risk of pulmonary complications and which patients will likely have adequate pulmonary function after lung resection. Serial evaluations monitor respiratory muscular strength in progressive neuromuscular diseases such as Guillain-Barre, myasthesia gravis, and muscular dystrophy.
PFTs estimate the following:
Volumes or the ability of the lungs to fully expand (TLC, FRC, RV).
Flow rates or the rate of inflow and outflow of air (FEV1, forced expiratory flow [25%-75%]).
Maximum voluntary ventilation or airflow through major airways by rapid inspiration and expiration maneuvers (MVV).
Maximum inspiratory and expiratory pressure, a measure of respiratory muscle strength (Pi[max], Pe[max]).
Diffusing capacity (DLCO) or measurement of the ability of oxygen to get into the blood.
Interpretation will be (1) normal, (2) obstructive, (3) restrictive, or (4) combined obstructive and restrictive. For the majority of PFTs to be meaningful, patients must be able to physically perform the tests and to follow instructions. With the exception of oximetry, ABGs, and simple spirometry, PFTs are usually performed in the outpatient setting. Hospitalists should be able to (1) recognize patterns of pulmonary involvement when they review outside medical records, (2) know when to order specific tests to evaluate acutely ill patients, and (3) avoid unnecessary ordering of PFTs when they are of limited utility in hospitalized patients.
PFTs will detect significant increased resistance to airflow (airway obstruction) and increased resistance to expansion (parenchymal disease, weakness of respiratory muscles or abnormalities of the chest wall or diaphragm). ABGs supplement PFTs by measuring the effect of pulmonary and other illnesses on oxygenation and ventilation (Figure 110-1).
Lung Volumes. (Reproduced, with permission, from Weinberger SE. Principles of Pulmonary Medicine, 4th ed. Philadelphia: Saunders; 2004.)