Respiratory Medicine. Stephen J. Bourke. Читать онлайн. Newlib. NEWLIB.NET

Автор: Stephen J. Bourke
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Медицина
Год издания: 0
isbn: 9781119774235
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refers to the process of moving air in and out of the lungs.

      Lung volumes

      Spirometry

Schematic illustration of total lung capacity and its subdivisions.

      Vital capacity

      Vital capacity is the volume of air expelled by a full expiration from a position of full inspiration. The patient is usually encouraged to exhale with maximum effort, referred to as forced vital capacity (FVC). VC may also be measured by a slow exhalation, sometimes referred to as ‘slow’ VC. In normal individuals, slow VC and FVC are very similar, but in patients with airway obstruction, air trapping occurs during forced expiration, so that the FVC may be significantly smaller than the slow VC. VC may be reduced by any condition that limits the lung’s ability to achieve a ‘full’ inspiration, such as:

       reduced lung compliance (e.g. lung fibrosis, loss of lung volume)

       chest deformity (e.g. kyphoscoliosis, ankylosing spondylitis)

       muscle weakness (e.g. myopathy, myasthenia gravis).

      It may also be reduced in chronic obstructive pulmonary disease (COPD), when air trapping causes increased residual volume.

      Forced expiratory volume in 1 second and FEV1:FVC ratio

      The forced expiratory volume in 1 second (FEV1) is the volume of air expelled in the first second of a maximal forced expiration from a position of full inspiration. It is reduced in any condition that reduces VC, but is particularly reduced when there is diffuse airway obstruction. In diffuse airway obstruction therefore, the ratio FEV 1 :FVC is lower than would be predicted in health. When interpreting results it is important to understand what normal is. During a forced expiratory manoeuvre, the proportion of air expelled in the first second varies with age. For people in their 20s that volume should be at least 85% of the total, while people in their 80s should be able to expel at least 70% in the first second.

      When the ratio FEV 1 :FVC is reduced, this pattern is referred to as an obstructive defect and is most commonly seen in asthma and COPD. When lung volume is restricted (by, for example, reduced lung compliance chest deformity or muscle weakness), the VC and FEV1 are both reduced, roughly in proportion, so that the FEV1:FVC ratio is essentially normal. This pattern of ventilatory impairment is referred to as a restrictive defect.

      Maximal midexpiratory flow

      In addition to FEV1 and FVC, a number of other indices may be calculated from a forced expiratory spirogram. The forced expiratory flow measured over the middle half of expiration (FEF 25–75% ) reflects changes in the smaller peripheral airways, whereas FEV1 and peak expiratory flow (PEF) particularly are predominantly influenced by diffuse changes in the medium‐sized and larger central airways, at least in health (see Chapter 1).

      Peak expiratory flow

      Schematic illustration of forced expiratory spirogram tracing obtained with a spirometer. Schematic illustration of forced expiratory spirogram tracing obtained with a spirometer.