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Tidal breathing is normal, resting breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such breath. The average human respiratory rate is 30–60 breaths per minute at birth, [2] decreasing to 12–20 breaths per minute in adults. [3]
Pulmonary compliance is calculated using the following equation, where ΔV is the change in volume, and ΔP is the change in pleural pressure: = For example, if a patient inhales 500 mL of air from a spirometer with an intrapleural pressure before inspiration of −5 cm H 2 O and −10 cm H 2 O at the end of inspiration.
The amount of He in the spirometer is known at the beginning of the test (concentration × volume = amount). The patient is then asked to breathe (normal breaths) in the mixture starting from FRC (functional residual capacity), which is the gas volume in the lung after a normal breath out. The spirometer measures helium concentration.
The spirometry test is performed using a device called a spirometer, [2] which comes in several different varieties. Most spirometers display the following graphs, called spirograms: a volume-time curve, showing volume (litres) along the Y-axis and time (seconds) along the X-axis
Output of a spirometer. Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. It is approximately equal to Forced Vital Capacity (FVC). [1] [2]
Pulmonary function testing (PFT) is a complete evaluation of the respiratory system including patient history, physical examinations, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity of pulmonary impairment. [1]
80 to 100 percent of the usual or normal peak flow readings are clear. A peak flow reading in the green zone indicates that the asthma is under good control. Yellow Zone 50 to 79 percent of the usual or normal peak flow readings Indicates caution. It may mean respiratory airways are narrowing and additional medication may be required. Red Zone
The closing capacity (CC) is the volume in the lungs at which its smallest airways, the respiratory bronchioles, collapse. It is defined mathematically as the sum of the closing volume and the residual volume. The alveoli lack supporting cartilage and so depend on other factors to keep them open.