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Understanding Why We Use Spirometry: Part One

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ABSTRACT Booker, R. (2007) Understanding why we use spirometry: part one. Nursing Times; 103: 45, 46–48.

Rachel Booker discusses why spirometry is used and the interpretation of results. In part two she describes the procedure for using spirometry.

Carrying out accurate spirometry testing: part 2

AUTHOR Rachel Booker, DN(Cert) HV, RGN, is independent specialist respiratory nurse and freelance medical writer.

ABSTRACT Booker, R. (2007) Understanding why we use spirometry: part one. Nursing Times; 103: 45, 46–48.

Rachel Booker discusses why spirometry is used and the interpretation of results. In part two she describes the procedure for using spirometry.

Spirometers measure lung volumes and airflow rates. The essential lung volumes to record with simple spirometry are:

- The vital capacity (VC). The volume measured at the mouth, between the positions of full inspiration and full expiration;

- The forced vital capacity (FVC). The maximum volume of air that can be expired from the lungs during a forced and complete expiration from a position of full inspiration;

- The forced expired volume in one second (FEV1). The maximum volume of air that can be expelled from the lungs in the first second of a forced expiration from a position of full inspiration (Miller et al, 2005).

In a healthy person the FVC and VC should be the same. In those with obstructive airways disease, the VC may be greater than the FVC due to compression and collapse of narrowed airways during a forced manoeuvre.

FEV1 is a simple and reproducible measurement with clearly defined reference values. It is affected in all patterns of lung disease and is a good predictor of mortality.

Lung volumes are presented as absolute values, in litres, and as a percentage of the reference (predicted) value for someone of that age, gender, height and ethnic group. Reference values are determined from large population studies and
are the mean value for that population group. The reference values recommended for use in white Europeans are those of the European Respiratory Society (Quanjer et al, 1993). Correction factors for different ethnic groups can be applied to these values, but difficulties do arise with individuals of mixed ethnic origin. When a correction factor is applied, it must be applied consistently.

A lung volume of 80–120% of the reference value is generally considered to be within normal limits, although this rule does not necessarily apply in older people or adolescents. Population studies in these groups are lacking and applying reference values extrapolated from studies in other populations can be misleading. A helpful rule is to interpret the spirometry alongside the clinical
history and other investigations, not in isolation.


Measurement of airflow rates

Reduced airflow is the hallmark of obstructive lung disease. The essential flow-rate measurements are:

- Peak expiratory flow;

- The FEV1 as a ratio or percentage of the FVC or VC (if that is greater). This is the FEV1/FVC (or FEV1/VC) and is also sometimes expressed as the FEV1% or forced expiratory ratio (FER) (Box 1).

Peak expiratory flow is achieved in the first 10 milliseconds of a forced expiration and represents airflow from the central conducting airways. In healthy individuals around three-quarters of the FVC can be exhaled in the first second. In other words, the FEV1 should be approximately three-quarters (75%) of the FVC, giving a ratio of FEV1 to FVC (FEV1/FVC) of around 0.75. This can also be expressed as an FEV1% of 75%.

In obstructive airways disease these values are reduced. An FEV1/FVC of less than 0.70 is generally considered diagnostic of airflow obstruction, although it may be not be abnormal in an older subject who is asymptomatic.

Graphical presentation of results
Forced expiratory manoeuvres are presented graphically in two ways:

- The volume/time graph;
- The expiratory flow/volume graph.

Graphs are essential tools that are needed to check the technical acceptability of a patient’s effort and validate an interpretation of lung volume and airflow rate data.

The volume/time graph plots the volume of air exhaled forcibly against time, with volume in litres on the vertical axis of the graph and time in seconds on the horizontal axis. The rapid exhalation of three-quarters of the FVC in the first second in a healthy person gives the volume/time graph its typical shape – a steep start and a plateau at 4–6 seconds (Fig 1).

The expiratory flow/volume graph plots flow rate against volume exhaled. The flow rate (in litres/second or litres/minute) is on the vertical axis and volume (litres) is on the horizontal. Peak expiratory flow rate is reached very rapidly, giving a steep, almost vertical, rise on the graph. In health, the flow rate then decreases steadily until all air has been exhaled (on the graph the flow rate merges smoothly with the horizontal axis at FVC, Fig 2).

Interpretation
Spirometry results can be broadly categorised as:
- Normal;
- Restricted;
- Obstructed;
- Severely obstructed.
- The effect of different defects on lung volumes and flow rates is shown in Table

Restrictive respiratory conditions
Conditions producing restrictive spirometry defects are those that reduce the ability of the lungs to inflate fully, either by restricting movement of the rib cage (extrapulmonary) or by stiffening or fibrosing lung tissue (intrapulmonary) (Table 2). Lung volumes are reduced, but airways are unaffected. Flow rates are normal. The volume/time graph is a normal shape, but is small. The flow/volume graph is narrow and domed (Fig 3).

Obstructive respiratory conditions
Conditions causing obstructed spirometry are more common, for example, asthma and COPD. The characteristic feature is reduced airflow. Reduction in airflow reduces the volume of air that can be exhaled in the first second of a forced exhalation. The volume of the FEV1 is reduced and the ratio of FEV1 to FVC falls. The volume/time graph will have a flattened look and the flow/volume graph will be ‘scooped’ out and concave (Fig 3). The FVC may be well maintained, but the time taken to exhale to FVC will be prolonged.

Severely obstructed airways
In severely obstructed airways, compression and collapse during a forced manoeuvre prevents full exhalation and reduces the FVC. More can usually be exhaled in a relaxed manoeuvre and the VC will be higher than the FVC. The reduction in FVC may, paradoxically, raise the FEV1/FVC.

The full extent of airflow obstruction will be revealed by looking at the FEV1/VC and checking the graphs. Severe airflow obstruction will cause a marked flattening of the expiratory representation on the volume/time graph and a very prolonged expiratory time.

Dynamic airway collapse rapidly reduces expiratory flow from smaller airways producing a dramatic scooping of the flow/volume graph and a failure to reach expected FVC (Fig 3).

Conclusion
In summary, spirometry is an essential test for any patient presenting with cardiorespiratory symptoms. It is becoming more widely available and is frequently performed by nurses.


REFERENCES

Miller, M.R. et al (2005a) ATS/ERS task force: standardisation of lung function testing: standardisation of spirometry.
European Respiratory Journal; 26: 2, 319–338.

Miller, M.R. et al (2005b) ATS/ERS task force: standardisation of lung function testing: general considerations for lung function testing.
European Respiratory Journal; 26: 1, 153–161.

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