Which diagnostic test is used to measure the efficiency of gas transfer in the lung?

The main clinical roles of respiratory function tests include diagnosis, assessment of severity, monitoring treatment and evaluation of prognosis.

Spirometry

Spirometry (figure 1) is the most important function test – it measures vital capacity (VC) and forced expiratory volume in 1 second (FEV1). This permits differentiation between restrictive and obstructive respiratory diseases. If expired volume is measured by electrical integration of airflow (using a pneumotachograph), maximum flow–volume curves can also be registered. These tests are used to measure the effect of bronchodilating drugs on reversibility of obstruction as well as to determine responsiveness to bronchial provocation tests. Simple instruments for patient home use include peak flow meters, which measure the degree of obstruction.

Lung capacity and airway resistance

The total lung capacity can be determined using either gas dilution techniques or body plethysmography. The latter method also allows the measurement of airway resistance. The forced oscillation technique, which measures the resistance of the total respiratory system, has the advantage that the patient does not need to perform specific breathing manoeuvres.

Diffusing capacity

The diffusing capacity of the lung for carbon monoxide (also known as transfer factor), which is usually performed as a single-breath test, measures the overall gas-exchange function of the lung.

Blood gas analysis

Arterial blood gas (ABG) measurement to determine the arterial oxygen tension (PaO2 ) and arterial carbon dioxide tension (PaCO2) is one of the most useful diagnostic tests: blood can be sampled directly from an artery, or an estimate can be obtained from capillary blood from, for instance, a warmed earlobe. ABG measurement allows the diagnosis of hypoxaemia (decreased PaO2) with or without hypercapnia (increased PaCO2), a sensitive index of inefficient pulmonary gas exchange, which is also used for defining respiratory failure. PaO2 measurement after breathing 100% oxygen is sometimes used to estimate the anatomical right-to-left shunt. Arterial oxygen saturation (SaO2) represents the percentage of binding sites on the haemoglobin molecule occupied by oxygen and offers a noninvasive method of estimating arterial blood oxygenation; it is measured directly by an oximeter with a probe attached to either the finger or the earlobe. PaCO2 can also be estimated noninvasively, using a transcutaneous electrode but such devices are not yet as widely used as oximeters. ABG measurement also allows evaluation of acid–base disorders.

Cardiopulmonary exercise testing

Cardiopulmonary exercise testing (CPET), with determination of minute ventilation, cardiac and respiratory frequency, oxygen uptake and carbon dioxide output, is an objective measure of exercise capacity (spiroergometry). Simpler tests use capillary oxygen partial pressure measurements during exercise on an ergometer or symptom-limited walking tests, such as the 6-min shuttle walk test, with measurement of SaO2 using an oximeter.

Respiratory muscle function measurement

Respiratory muscle function is commonly assessed by measuring maximal pressures generated at the mouth during maximal inspiratory and expiratory efforts against an occluded airway.

Control of ventilation

Tests of ventilatory control include the hyperoxic rebreathing method and the hypoxia-withdrawal method. Simpler, but less specific, is the measurement of the mouth occlusion pressure.

Diagnosis of sleep breathing disorders

The diagnosis of sleep-related respiratory disorders requires special tests. The gold standard is polysomnography, but simpler tests are available for screening purposes (‘respiratory polysomnography’).

Right heart catheterisation

Right heart catheterisation is used in the differential diagnosis of pulmonary hypertension.

Intensive care monitoring

The management of respiratory failure in the intensive care unit requires, in addition to frequent checking of ABGs, the measurement of several special parameters (e.g. tidal volume, inspiratory and expiratory pressures); in mechanically ventilated patients, these are often measured automatically by the ventilator.

See the entire Principles of respiratory investigation Chapter

Pulmonary function tests are a group of tests that measure breathing and how well the lungs are functioning.

Spirometry measures airflow. By measuring how much air you exhale, and how quickly you exhale, spirometry can evaluate a broad range of lung diseases. In a spirometry test, while you are sitting, you breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air that you breathe in and out over a period of time. When standing, some numbers might be slightly different.

For some of the test measurements, you can breathe normally and quietly. Other tests require forced inhalation or exhalation after a deep breath. Sometimes, you will be asked to inhale a different gas or a medicine to see how it changes your test results.

Lung volume measurement can be done in two ways:

• The most accurate way is called body plethysmography. You sit in a clear airtight box that looks like a phone booth. The technologist asks you to breathe in and out of a mouthpiece. Changes in pressure inside the box help determine the lung volume.
• Lung volume can also be measured when you breathe nitrogen or helium gas through a tube for a certain period of time. The concentration of the gas in a chamber attached to the tube is measured to estimate the lung volume.

To measure diffusion capacity, you breathe a harmless gas, called a tracer gas, for a very short time, often for only one breath. The concentration of the gas in the air you breathe out is measured. The difference in the amount of gas inhaled and exhaled measures how effectively gas travels from the lungs into the blood. This test allows the health care provider to estimate how well the lungs move oxygen from the air into the bloodstream.

Do not eat a heavy meal before the test. Do not smoke for 4 to 6 hours before the test. You will get specific instructions if you need to stop using bronchodilators or other inhaled medicines. You may have to breathe in medicine before or during the test.

Since the test involves some forced breathing and rapid breathing, you may have some temporary shortness of breath or lightheadedness. You also might have some coughing. You breathe through a tight-fitting mouthpiece and you will have nose clips. If you are claustrophobic, the part of the test in the closed booth may feel uncomfortable.

Follow instructions for using the mouthpiece of the spirometer. A poor seal around the mouthpiece may cause results that aren't accurate.

Pulmonary function tests are done to:

• Help in the diagnosis of certain types of lung disease, such as asthma, bronchitis, and emphysema
• Find the cause of shortness of breath
• Measure whether exposure to chemicals at work affects lung function
• Check lung function before someone has surgery
• Assess the effect of medicines
• Measure progress in disease treatment
• Measure the response to treatment in cardiopulmonary vascular disease

Normal values are based on your age, height, ethnicity, and sex. Normal results are expressed as a percentage. A value is usually considered abnormal if it is approximately less than 80% of your predicted value.

Normal value ranges may vary slightly among different laboratories, based on slightly different ways to determine normal values. Talk to your provider about the meaning of your specific test results.

Different measurements that may be found on your report after pulmonary function tests commonly include:

• Diffusion capacity to carbon monoxide (DLCO)
• Expiratory reserve volume (ERV)
• Forced vital capacity (FVC)
• Forced expiratory volume in 1 second (FEV1)
• Forced expiratory flow 25% to 75% (FEF25-75)
• Functional residual capacity (FRC)
• Maximum voluntary ventilation (MVV)
• Residual volume (RV)
• Peak expiratory flow (PEF)
• Slow vital capacity (SVC)
• Total lung capacity (TLC)

Abnormal results usually mean that you may have chest or lung disease.

Some lung diseases (such as emphysema, asthma, chronic bronchitis, and infections) can make the lungs contain too much air and take longer to empty. These lung diseases are called obstructive lung disorders.

Other lung diseases make the lungs scarred and smaller so that they contain too little air and are poor at transferring oxygen into the blood. Examples of these types of illnesses include:

• Extreme overweight
• Pulmonary fibrosis (scarring or thickening of the lung tissue)
• Sarcoidosis and scleroderma

Muscular weakness can also cause abnormal test results, even if the lungs are normal, that is, similar to the diseases that cause smaller lungs.

There is a small risk for collapsed lung (pneumothorax) in people with a certain type of lung disease. The test should not be given to a person who has experienced a recent heart attack, has certain other types of heart disease, or has had a recent collapsed lung.

PFTs; Spirometry; Spirogram; Lung function tests; Lung volume; Plethysmography

Bhakta NR, Kaminsky DA. Pulmonary function testing: physiologic and testing principles. In: Broaddus VC, Ernst JD, King TE, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 31.

Scanlon PD. Respiratory function: mechanisms and testing. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 79.

Wald O, Izhar U, Sugarbaker DJ. Lung, chest wall, pleura, and mediastinum. In: Townsend CM Jr, Beauchamp RD, Evers BM, Mattox KL, eds. Sabiston Textbook of Surgery. 21st ed. Philadelphia, PA: Elsevier; 2022:chap 58.

Updated by: Denis Hadjiliadis, MD, MHS, Paul F. Harron, Jr. Associate Professor of Medicine, Pulmonary, Allergy, and Critical Care, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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