Comprehensive pulmonary function testing is usually done in a pulmonary function laboratory. Simpler tests can also be done in a physician’s office or in the home. Testing most often is used to identify airflow abnormality, reduced lung volume (restrictive abnormality), diffusing capacity changes (as in interstitial disease), and blood gas abnormality (too much carbon dioxide, too little oxygenation). The following tests are commonly performed.
Spirometry includes a number of simple measurements. Vital capacity (VC) measures the total volume of air one can breathe out completely after inhaling a full breath. VC is usually done forced, as fast as possible, and is known as FVC. When this fast forced breathing effort is performed, the volume breathed out in the first second is known as FEV1. The ratio of FEV1/FVC is a useful parameter for assessing airflow limitation and airflow obstruction. (Airflow obstruction is often defined as FEV1/FVC of less than 70%.) VC is sometimes done slowly and is called SVC. SVC avoids dynamic airway compression and may yield a larger VC than FVC, but cannot be used to assess airflow. The maximum airflow, or peak expiratory flow (PEF), and other airflow measurements can be made from the same forced expiration. These tests can be done while standing, sitting, or lying supine. A significant change in VC when measured supine can uncover respiratory muscle (especially diaphragm) weakness.
Tests used to evaluate neuromuscular diaphragmatic function include VC (sitting and supine), maximum inspiratory force (MIF), maximum expiratory force (MEF), maximum voluntary ventilation (MVV), and nocturnal oximetry. MIF and MEF are measured by maximally breathing in and out with full effort, through a closed mouth tube attached to a pressure measuring device. This measurement reflects inspiratory and expiratory muscle power. MVV is measured by breathing in and out deeply and rapidly for 12 seconds, the only “endurance” respiratory test. MVV is a more global measurement of maximal breathing capacity and respiratory muscle endurance and may be lower than predicted from multiplying the FEV1 by 40.
Lung volume testing measures VC and the residual volume (RV) of air still in the lung after breathing all the way out. This allows calculation of the total lung capacity (TLC), which may be reduced due to causes such as neuromuscular disease, kyphoscoliosis, and pulmonary fibrosis.
Diffusing capacity (DLCO) is a measurement of gas transport (transfer) across the lung’s alveolar air sacs and capillaries into the blood. Causes of a reduced DLCO include intrinsic parenchymal diseases such as COPD (emphysema), chronic pulmonary embolism, interstitial pulmonary disease and fibrosis. For post-polio/neuromuscular patients, it is often hard to hold the breathing long enough to get a result. If it is done, the decrease often parallels the degree of VC reduction.
Blood oxygen, carbon dioxide, bicarbonate, and pH are measured from an arterial blood specimen. Arterial blood gas (ABG) measurements can be done at rest or with exercise, breathing room air or oxygen. The oxygen saturation can be measured noninvasively using an oximeter. One type of oximeter has a memory module so that it can record 8-12 hours of oxygen and pulse rate data; this is useful to evaluate data noninvasively overnight (nocturnal oximetry).
Sleep studies are preferably performed in a sleep laboratory with an overnight study recording multiple variables simultaneously to assess sleep disorders for post-polio/neuromuscular patients (such as sleep apnea or underventilation). Since most sleep labs do not measure CO2 levels (nor do home studies measure brain waves to determine if sleep occurs or the stages of sleep), wrong or underdiagnosis may result. Sleep laboratories vary in their ability to accommodate persons with disabilities. If one needs a sleep study, it is useful to confirm in advance whether the lab can meet your needs during the test. These studies include EEG (brain wave), ECG (electrocardiogram), airflow measured at nose and mouth, oximetry, muscle measurements of chest and abdomen, and often a video record of sleep movements. As respiratory muscle weakness develops in neuromuscular disease, night-time underventilation and a drop in oxygen saturation may be an early finding. Occasionally, nocturnal oximetry alone at home may provide sufficient information. This is better reserved for checking whether the settings on the ventilator are sufficient.
A polio survivor with an abnormal sleep study should consult a pulmonologist/sleep specialist experienced in neuromuscular diseases to determine the best treatment (CPAP alone is inadequate for post-polio/neuromuscular hypoventilation; BiPAP or other assisted ventilation for respiratory muscle weakness is more physiologic).
A polio survivor scheduled for pulmonary function tests should at least have spirometry with SVC (if possible sitting and supine), FVC, FEV1; MIF and MEF; and oximetry on room air. Nocturnal oximetry should also be considered since it is a simple useful test for abnormalities that often occur first during sleep.
Home or office monitoring of VC, PEF, and oximetry can be done with simple devices. For home monitoring, people often use a peak flow meter or an incentive spirometer (both are relatively inexpensive), to assess whether they are stable or worsening, especially during a respiratory infection. A nurse, a respiratory therapist, and most health care facilities can easily measure the oxygen saturation on home visits with a small portable oximeter, most commonly with a small clip on a fingertip or earlobe. Personal pulse oximeters are also available for purchase by the general public, but the readings should be checked against a professional pulse oximeter because accuracies vary among devices. Accuracy may also be diminished in persons of color.