Respiratory Function Test

What Are Respiratory Tests?

Respiratory function tests are part of the clinical assessment of many respiratory diseases. The tests can measure individual parts of the respiratory process and, therefore, need to be selected appropriately. Spirometry is the basic screening test for assessing mechanical load problems. Arterial blood gas analysis yields considerable information about gas exchange efficiency while tests of gas transfer assess alveolar-capillary surface function. When specifically indicated, assessing bronchial reactivity and the response to exercise can help in the evaluation of breathlessness

Types of Respiratory Tests

Simple measurements of respiratory load

Increases in the respiratory load to breathing are very common. Resistive load increases, such as asthma, obstructive bronchitis, cystic fibrosis, and emphysema, impair airflow. Elastic load increases such as interstitial fibrosis, muscle paralysis, and obesity impair lung inflation. The quantitation of respiratory load involves determining the vital capacity and the speed of maximal expiratory flow.

The peak flow meter is widely promoted as a simple lung function monitor. Serial measurements in conditions such as asthma provide valuable information about disease progress. However, peak expiratory flow (the earliest portion of forced expiration) is very effort-dependent. Also, peak flow measurements give no information about elastic load abnormalities.

Lung Volume Measurement

Body plethysmography (pleth-iz-MOG-re-fe) is a test that measures how much air is present in your lungs when you take a deep breath. It also measures how much air remains in your lungs after you breathe out fully.

During the test, you sit inside a glass booth and breathe into a tube that’s attached to a computer.

For other lung function tests, you might breathe in nitrogen or helium gas and then blow it out. The gas you breathe out is measured to show how much air your lungs can hold.

Lung volume measurement can help diagnose pulmonary fibrosis or a stiff or weak chest wall.

Asthma

Asthma is a common cause of airflow limitation. The reversibility of the obstruction of the airways is usually assessed by spirometry before and after a bronchodilator aerosol. An increase of 10% or more in either vital capacity or FEV1 is taken to indicate significant reversibility, although, of course, not necessarily the maximum reversibility achievable.

When the suspicion of asthma is not confirmed by spirometry, a challenging procedure can be used to assess abnormal bronchial reactivity. This may involve the patient exercising or inhaling histamine, methacholine, hypertonic saline or cold air. Each challenge has its own protocol and risks, and these challenges are best performed in a well-supervised laboratory. Bronchial hyperreactivity is not synonymous with asthma. Although the vast majority of patients with ongoing asthma will have brisk reactivity, most people with a past history of asthma will have intermediate reactivity and some asymptomatic people with no past history will have a degree of bronchial reactivity. Bronchial reactivity is often expressed as the percentage concentration or dose of an agent that produces an acute fall of 20% in FEV1 (PC20 or PD20). Laboratories performing these challenges will usually have established their `normal reactivity’ values.

Tests To Measure Oxygen Level

Pulse oximetry and arterial blood gas tests show how much oxygen is in your blood. During pulse oximetry, a small sensor is attached to your finger or ear. The sensor uses light to estimate how much oxygen is in your blood. This test is painless and no needles are used.

For an arterial blood gas test, a blood sample is taken from an artery, usually in your wrist. The sample is sent to a laboratory, where its oxygen level is measured. You may feel some discomfort during an arterial blood gas test because a needle is used to take the blood sample.

Simple measurements of gas exchange

Normal gas exchange requires adequate alveolar ventilation, normal ventilation/blood flow relationships and adequate alveolar-capillary membrane surface area. There are tests of varying sophistication which specifically examine each of these functions.

Tests To Measure Oxygen Level

Pulse oximetry and arterial blood gas tests show how much oxygen is in your blood. During pulse oximetry, a small sensor is attached to your finger or ear. The sensor uses light to estimate how much oxygen is in your blood. This test is painless and no needles are used.

For an arterial blood gas test, a blood sample is taken from an artery, usually in your wrist. The sample is sent to a laboratory, where its oxygen level is measured. You may feel some discomfort during an arterial blood gas test because a needle is used to take the blood sample.

Alveolar ventilation

This is not easy to measure directly, as it is not a simple function of the volume of expired air passing the mouth each minute (i.e. the minute ventilation). The size of the dead space (alveolar dead space, connecting tubing volume, and tracheobronchial tree) is often uncertain. This uncertainty, combined with the influence of the breathing pattern, means that minute ventilation may be a very misleading estimate of alveolar ventilation. To overcome this difficulty, the arterial carbon dioxide tension is used as an inversely proportional index of `effective’ alveolar ventilation. Hence, a normal arterial carbon dioxide tension is taken to indicate satisfactory alveolar ventilation. Elevated or reduced carbon dioxide tensions reflect alveolar hypoventilation or hyperventilation respectively.

Ventilation/blood flow relationships

These are most simply assessed by considering the lungs as a gas exchanger. Its efficiency is rated by the size of the difference between the amounts of oxygen and carbon dioxide in the blood and in the air. If the lungs are working efficiently the differences in the composition will be small. Non-uniformity of ventilation/blood flow ratios will result in abnormally wide differences – the alveolar-arterial PO2 and arterial-alveolar PCO2 gradients will be abnormal. The oxygen tension gradient is normally less than 10% of the inspired oxygen tension. This simple index can be calculated using the alveolar gas equation.

Alveolar-capillary surface area

This is assessed by one of several techniques measuring the uptake of carbon monoxide, a gas with an affinity for blood and which is easily analyzed. Although sometimes designated as tests of diffusion, these techniques are much more influenced by effective alveolar-capillary area and therefore are now more commonly termed gas transfer tests. Although many factors influence the result, these tests are usually abnormal in diffuse interstitial inflammatory and fibrotic processes and in emphysema. They are useful in the subclassification of restrictive conditions (those with and without gas transfer impairment) and in determining the probable extent of emphysema in patients with chronic airflow obstruction. They are commonly used in following patients’ response to therapy in such conditions as sarcoidosis and fibrosing alveolitis.

Simple exercise testing

Tests performed during exercise provide information about overall fitness and the appropriateness of cardiorespiratory responses. They can be elaborate procedures following cardiac output, pulmonary hemodynamics, gas exchange and anaerobic metabolism measurements at varying grades of exercise, but this type of study has little place in everyday practice. Observations made during a six-minute walk test can provide useful objective information provided the subject is induced to co-operate fully. The actual distance walked, the degree of breathlessness experienced and the change in blood oxygen level (assessed by portable oximetry) is data which can be obtained simply. These data are required before some authorities will agree to provide portable domiciliary oxygen. The extent of exercise limitation due to mechanical load excess agrees reasonably well with the degree of impairment on spirometry

When to use respiratory function tests?

The most common reason for studying pulmonary function is in the analysis of breathlessness. The application of simple tests of load (spirometry3), gas exchange (arterial blood gas analysis5) and the gas transfer will usually allow conclusions as to whether or not the complaint is reasonably based.

In hospital practice, the gas exchanging aspects of pulmonary function become important in the assessment and management of acute respiratory failure.

Respiratory function tests are also widely used to assess fitness for surgery, fitness to undertake certain occupations or to assess the degree of impairment in work-related lung conditions.