Introduction
Once a clinician develops a suspicion of pulmonary hypertension based on the patient’s history, physical examination, chest X-ray, and ECG, it is necessary to establish the presence or absence of a restrictive pulmonary pathology or an obstruction of airflow. Unless hypoxia is a pre-existing condition, pulmonary hypertension cannot be attributed to airway restriction or pulmonary pathologies such as connective tissue disease until actual pulmonary function is significantly reduced. Although high-resolution computer assisted tomography (CT) of the chest can be used to rule out occult interstitial lung disease, this procedure is more costly, but can be used in the event PFTs are unable to contribute to the diagnosis.
Types of Pulmonary Function Tests
Spirometry – the most commonly used lung function screening tool and usually the health care provider’s first option. Spirometry testing is typically administered in an ambulatory setting, and measures air flow into and out of the lungs.
Lung Diffusing Capacity (DLco) – this test measures the molecular movement of gases across the alveolar-vascular border. The test commonly uses carbon monoxide (CO) as the tracer since it has a high affinity for hemoglobin, hence the test is referred to as the diffusing capacity of the lung for carbon monoxide test or DLco. Changes in DLco are very dependent on blood hemoglobin concentration, thus corrections must be made if blood testing indicates the presence of anemia in the patient.
How are the Tests Performed and Interpreted?
Spirometry-Spirometry is an expiratory maneuver which measures exhaled volume or vital capacity; it does not measure residual lung volume. The test utilizes a spirometer, which directly measures the volume of air displaced or measures airflow. During the procedure the patient is seated and inhales maximally from tidal respiration to total lung capacity. Based on expiratory flow data a spirogram is then generated (Figure 1).
Courtesy of Dr. Sean Studer
Spirometry is a simpler form of full pulmonary function testing (PFTs). Results must be combined with full pulmonary function testing that includes volume measurements and lung diffusing capacity.
- Forced Expiratory Volume over 1 sec (FEV1) – this parameter is reduced when obstructive lung disease is present. If airway flow is obstructed, this may lead to hypoxia. Hypoxia over time can lead to PH (WHO Group 3 pulmonary hypertension).
- Forced Vital Capacity (FVC) – this is the volume of air which can be forcibly expelled after a full inhalation. A measure of lung volume, it is usually reduced in diseases such as pulmonary fibrosis (e.g. WHO Group 3 pulmonary hypertension).
- Volume-Time Tracing and Flow – Volume Loop-a flow-volume loop displays airflow (in L/sec) as it relates to lung volume (in L) during maximal inspiration from complete exhalation (residual volume [RV]), and during maximum expiration from complete inhalation (TLC). The principal advantage of the flow-volume loop is that it can show whether airflow is appropriate for a particular lung volume.
Airflow is normally slower at low lung volumes. Patients with pulmonary fibrosis or interstitial pulmonary fibrosis (as for some patients in Group 3 pulmonary hypertension) have low lung volumes so airflow appears decreased if measured alone. When airflow is shown as a function of lung volume, it becomes apparent that airflow is actually higher than normal (as a result of the increased elastic recoil characteristic of fibrotic lungs). A decrease in lung capacities, changes in flow-volume loop, or volume time traces may suggest to the clinician that changes in lung or pulmonary vascular structures or increases in pulmonary arterial pressures have limited the normal ability of the lungs to expand on inspiration.
Lung Diffusing Capacity-during this test, the patient inhales a test gas of 10% helium, 0.3% carbon monoxide, 21% oxygen, and nitrogen. After inhalation, the patient holds his/her breath for 10 seconds to allow equilibration of alveolar gas with blood. The patient then exhales to wash out a conservative overestimate of the mechanical and anatomical lung dead space. Following this, an alveolar sample is collected.
- DLco -calculated from the total volume of the lung, breath-hold time, and the initial and final alveolar concentrations of CO. Exhaled helium concentration is used to determine a single breath estimate of total lung capacity and the initial alveolar concentration of CO.
Calculated DLco=(TLC single breath) x (rate of CO uptake during the 10 sec breath hold)
Reduced DLco (<80% of normal) may indicate presence of interstitial lung disease, early stages of pulmonary hypertension, or acute/recurrent pulmonary thromboembolism (to name a few). Reduced DLco may also indicate pulmonary edema, mixed connective tissue disease, or progressive systemic sclerosis. These conditions all are known to contribute to development of PH.
Summary
Spirometry, a simple set of office based procedures, can give the clinician an indication of airway obstruction, reduced lung volume, and reduced airway flow. These conditions can contribute to the development of pulmonary hypertension by creating a hypoxic situation that can create lung injury leading to increased pulmonary vascular resistance or PAH. Some classes of patients with pulmonary fibrosis, interstitial lung disease, systemic sclerosis, scleroderma, and other connective tissue disease may also be partly identified through reductions in lung volumes and airway flow rates as visualized through spirometry.
Lung diffusion capacity can highlight a potential contributory process to the development of pulmonary hypertension. Taken together, results of pulmonary function testing help the clinician to determine if any other lung diseases may be contributing to the suspicion of pulmonary hypertension.
References
1. Weinberger SE and Rosen IM. Disturbances of respiratory function. In Harrison’s: Principles Of Internal Medicine 17th Ed., 2008:1586-1592.
2. Perez, LL. “Office Spirometry”. Osteopathic Family Physician, 2013;5(2):65–69.
3. Gildea TR and McCarthy K. Pulmonary Function Testing. In: Disease Management Project. Cleveland Clinic Center for Continuing Education. http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/pulmonary/pulmonary-function-testing/ 2000-2011.
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