Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

TABLE 21-2 Pulmonary FunctionTests Test Symbol

Measurement*

Maximal voluntary ventilation MVV

Maximum amount of air that can be breathed in a given time

Forced vital capacity

FVC

Maximum amount of air that can be rapidly and forcefully exhaled from the lungs after full inspiration.The expired volume is plotted against time.

Forced expiratory volume achieved in 1 s Percentage of forced vital capacity Forced midexpiratory flow rate

FEV 1.0

Volume of air expired in the first second of FVC

(FEV 1.0

/FVC%)

Volume of air expired in the first second, expressed as a percentage of FVC

× 100

FEF 25%–75%

The forced midexpiratory flow rate determined by locating the points on the volume-time curve recording obtained during FVC corresponding to 25% and 75% of FVC and drawing a straight line through these points. The slope of this line represents the average midexpiratory flow rate. FIF is the volume inspired from RV at the point of measurement. FIF 25%–75% is the slope of a line between the points on the volume pressure tracing corresponding to 25% and 75% of the inspired volume.

Forced inspiratory flow rate

FIF 25%–75%

*By convention, all the lung volumes and rates of flow are expressed in terms of body temperature and pressure and saturated with water vapor (BTPS), which allows for a comparison of the pulmonary function data from laboratories with different ambient temperatures and altitudes.

Efficiency and Work of Breathing The efficiency of breathing is determined by matching the TV and respiratory rate in a manner that provides an optimal minute volume while minimizing the work of breathing. The minute volume, or total ventilation, is the amount of air that is exchanged in 1 minute (TV multiplied by the respiratory rate). It is determined by the metabolic needs of the body, which during normal activity are about 6000 mL (500 mL TV × respiratory rate of 12 breaths/min). The work of breathing is determined by the amount of effort required to move air through the conducting air- ways and by the ease of lung expansion. Because expan- sion of the lungs is difficult for persons with stiff and noncompliant lungs, they usually find it easier to breathe if they keep their TV low and breathe at a more rapid rate (e.g., 300 mL × 20 breaths/min = 6000 mL/min) to achieve their minute volume and meet their oxygen needs. In contrast, persons with obstructive airway dis- ease usually find it less difficult to inflate their lungs but expend more energy in moving air through the airways. As a result, these persons tend to take deeper breaths and breathe at a slower rate (e.g., 600 mL × 10 breaths/ min = 6000 mL) to achieve their oxygen needs.

■■ Lung compliance or ease with which the lungs can be inflated reflects the elastic forces of the lung tissue and the surface tension in the alveoli. Surfactant molecules, produced by type II alveolar cells, reduce the surface tension in the lungs, thereby increasing lung compliance and ease of inflation. ■■ Airway resistance refers to the impediment to flow that the air encounters as it moves through the airways. It varies with airway radius and lung volume, being greatest in the bronchi with medium-sized radii and lowest in the bronchioles with their smaller radii. Airway resistance decreases as the lungs expand and pull the airways open during inspiration and it increases as the lungs deflate during expiration. ■■ Lung volumes reflect the amount of air that is exchanged during normal and forced breathing. The minute volume (tidal volume [TV] multiplied by the respiratory rate) is determined by the metabolic needs of the body. ■■ The efficiency of breathing is determined by matching theTV and respiratory rate in a manner that provides an optimal minute volume while minimizing the work of breathing. Persons with stiff and noncompliant lungs usually find it easier to keep theirTV low and breathe at a more rapid rate, whereas those with increased airway resistance usually find it less difficult to inflate their lungs and increase theirTV, while breathing at a slower rate.

SUMMARY CONCEPTS

■■ Breathing involves the movement of atmospheric air into and out of the alveolar structures in the lungs. It requires a system of open airways and alternating pressure changes resulting from the action of the respiratory muscles in changing the volume of the chest cage.