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Respiratory function tests 

Respiratory function tests

Chapter:
Respiratory function tests
Author(s):

G.J. Gibson

DOI:
10.1093/med/9780199204854.003.180301_update_001

July 30, 2015: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

Update:

Chapter reviewed by author in December 2012—minor alterations made.

Updated on 30 May 2013. The previous version of this content can be found here.
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date: 24 April 2017

Respiratory function tests are used in diagnosis, assessment and prognosis and in monitoring the effects of treatment of various respiratory conditions. Their use as a diagnostic tool is in recognizing patterns of abnormality which characterize particular types of disease; more often they are used to quantify the severity of functional disturbance or to locate the likely anatomical site(s) of disease (airways, alveoli, or chest wall).

The commonly applied tests are most conveniently classified as (1) tests of respiratory mechanics, (2) carbon monoxide uptake, (3) arterial blood gases and acid–base balance, and (4) exercise.

Tests of respiratory mechanics

Spirometers record the volume of air that is displaced from the lungs in tidal breathing or with forced inspiratory and expiratory manoeuvres. This allows measurement of the tidal volume (VT), inspiratory capacity (IC), forced expiratory volume in 1 s (FEV1) and vital capacity (VC). Residual volume (RV) remains in the lungs after full expiration. Total lung capacity (TLC) represents the volume of air in the lungs after full inspiration—the sum of VC and RV.

RV cannot be measured by spirometric methods: inert gas dilution and whole-body plethysmography are the two main clinical methods used for the measurement of absolute lung volume.

Forced expiratory tests are simple to perform, do not require complex equipment, and are relatively independent of the effort applied by the patient.

The characteristic feature of diffuse airway obstruction is slowing of the rate of expiration, so that the ratio of FEV1 to FVC (or FEV1 to VC) is reduced, which defines an ‘obstructive’ ventilatory defect. In the alternative ‘restrictive’ pattern of ventilatory function TLC is reduced and both FEV1 and VC are reduced in approximate proportion.

Measurement of FEV1 and VC is not sensitive to localized narrowing of the central airway: air flow during forced expiration and inspiration should be examined as maximum flow–volume curves if this is suspected.

Measurements of respiratory muscle function are indicated in evaluation of patients with various neuromuscular diseases.

Carbon monoxide uptake

Carbon monoxide (CO) diffusing capacity (DLco) or transfer factor (TLco) is widely used as a simple test of the integrity of the alveolar capillary membrane and the overall gas exchanging function of the lungs.

Arterial blood gases and acid–base balance

The primary measurements made by modern blood gas analysers are the arterial partial pressures of oxygen (PaO2) and carbon dioxide (PaCo2), and hydrogen ion concentration [H+] or pH.

A reduction in PaO2 can occur by various mechanisms, but in disease the commonest is mismatching of alveolar ventilation (.VA) and perfusion ( .Q).

Respiratory failure is defined in terms of the arterial blood gas tensions as a reduction in PaO2 below 8 kPa (60 mmHg) at sea level, either without (‘type I’) or with (‘type II’, ‘ventilatory failure’) CO2 retention.

The ratio of PaO2/FIo2 is widely used in assessment of patients with severe problems of oxygenation: in acute lung injury a value greater than 300 (PaO2 in mmHg, FIo2 as a fraction) indicates relatively mild hypoxaemia, whilst a value of less than 100 represents very severe disturbance of gas exchange.

Abnormal acid–base disturbances are traditionally classified as one of four types: respiratory acidosis and respiratory alkalosis—where the primary disturbance is reduced or increased CO2 excretion respectively—and metabolic acidosis and metabolic alkalosis—where the primary disturbance is increased or decreased [H+] respectively. A mixed picture is frequently seen.

The likely cause(s) of metabolic acidosis are usefully classified in terms of the ‘anion gap’, which is calculated simply by subtracting the concentrations of the most abundant anions in blood (chloride and bicarbonate) from the most abundant cations (sodium and potassium).

Exercise

Exercise tests can be useful in evaluating the symptom of breathlessness, in the assessment of disability, and in determining the likely factors limiting performance.

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