Respiratory function tests

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