Spirometric Measurement of Respiratory Mechanics

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Respiratory mechanics is the physical action of the ribs, chest wall and diaphragm during respiration. It also includes breathing patterns, mechanoreceptors, neural and reflex controls, airflow and lung volume. A study of these helps clinical officers in diagnosing respiratory diseases. Being able to identify these diseases at an early stage is particularly important as it helps ensure the health of individuals. In countries like Australia where respiratory diseases affect a third of the children’s population and 10% of the adult population, there is a need to develop reliable ways of determining the health of respiratory mechanics.

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This would allow doctors or clinical officers to identify respiratory diseases and reduce their effects on individuals. Among the most common respiratory diseases are asthma, chronic obstructive pulmonary disease (COPD) which is mainly caused by smoking, and obstructive pulmonary sleep apnoea (OSA) (Comino et al 1996, p. 403-406). These 3 conditions are chronic and can persist throughout a patient’s life. Beyond these, there are other common infections such as flu, pneumonia and colds which also affect respiratory mechanics.


All these measures are useful in determining evidence of respiratory disease. The most common is FEV1, FVC and FEV1/FVC ratio. A healthy person is able to exhale at least 80% of their FVC in the first second of forced expiration (FEV1) (Hyatt et al 1958, p. 331-336). However, in obstructive respiratory disorders FEV1 falls in proportion to the severity of the obstruction, FVC remains unaffected unless the infection is very severe.

In determining if there is evidence of respiratory disease there is need to consider other effects such as gender differences(since men have lungs around 20% larger than women), age and body size. To ensure that values realized are reliable one needs to express FEV1 and FVC values as a percentage of the average values predicted for a person of the corresponding gender, age and body size (Macklem 1967, p. 395-401).

Sucking air through a drinking straw causes it to collapse as the amount of air breathed out is more than the air inside the straw. It collapses near the mouth as the amount of the air at the top is not replaced as fast as that at the bottom of the straw because the top is blocked and still the intensity of the breath is more close to the mouth than far away from the mouth. Airway obstruction acute upper is demonstrated by this as it is characterized by a collapse or blockage of the upper airway.

Functional residual capacity (FRC) is the volume of gas in the lungs at the end of a normal tidal volume exhalation. It can be gotten by summing up the expiratory reserve volume and the residual volume. Both expiratory reserve volume and residual volume can be measured using a spirometer, thus it becomes possible to measure FRC using a spirometer though not directly.

FVC, FEF50, FEV1, PEFR. FVC gives the maximum volume of air either exhaled or inhaled thus is a sensitive measure of a small airway. FEF50 is maximum expiratory flow at a point where 50% of FVC has been expired, this is more sensitive than FEV1 which is less reproducible in the measurement of small airways narrowing (Hyatt, 1958 p. 331). PEFR is not a sensitive measure as it occurs very early in the forced expiratory maneuver.

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Initially expiratory airflow is effort dependent due to the huge volume of air in the lungs, as this volume decreases due to continued exhalation expiratory airflow becomes effort-independent.

This is because FEV1 measures the volume expired within the first second of maximal expiration after a maximum inspiration whereas FVC measures the maximum volume of air inspired or exhaled.


Brown, R, Woolcock, N, J. Macklem, P.T. (1969). Physiological effects of experimental airway obstruction with beads. J. Appl. Physiol. 27:328-355.

Comino, E.J., Mtchell, A. Bauman, R.L, Henry, C.F.Robertson, M.J. Abramson, R. Landau, I. (1996). Asthma management in eastern Australia, 1990 and 1993. Med. J. Aust.164: 403-406.

Fry, D.L., and Hyatt, R.E. (1960). Pulmonary mechanics. A unified analysis of the relationship between pressure, volume and gas flow in the lungs of normal and diseased human subjects. Am.J. Med. 29: 672-689.

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Hurry, V.M., Peat, J.K, and Woolcock, A.J.(1988). Prevalence of respiratory symptoms, bronchial hyperresponsiveness and atopy in schoolchildren living in the Villawood area of Sydney. Aust. N.Z. J. Med. 18: 745-752.

Hyatt, R.E, Schilder, D.P., and Fry, D.L. (1958). Relationship between maximum expiratory flow and degree of lung inflation. J. Appl. Physiol. 13:331-336.

Macklem, P.T., and Mead, J. (1967). Resistance of central and peripheral airways measured by a retrograde catheter. J. Appl. Physiol. 22: 395-401.

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NursingBird. (2022, March 8). Spirometric Measurement of Respiratory Mechanics. Retrieved from https://nursingbird.com/spirometric-measurement-of-respiratory-mechanics/


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NursingBird. "Spirometric Measurement of Respiratory Mechanics." March 8, 2022. https://nursingbird.com/spirometric-measurement-of-respiratory-mechanics/.