Respiratory Mechanics

Question 1

Define compliance. Define resistance. Contrast airway compliance and airway resistance.

Answer 1

Compliance is the change in volume that occurs in response to a change in pressure. Resistance is the change in pressure along a tube divided by flow. Compliance is a measure of the ease with which a structure such as an alveolus is distended. Resistance is a measure of the ease with which a fluid (gas, liquid) flows through a tube (such as a bronchus).

Question 2

Describe the relationship between volume and pressure for an alveolus with a large compliance. Are alveoli with large compliances easier or harder to distend?

Answer 2

An alveolus with a large compliance will have a large increase in volume for a small increase in pressure. Alveoli with large compliances are easy to distend.

Question 3

What cells secrete surfactant? Describe the composition of surfactant.

Answer 3

Surfactant is secreted by type II alveolar epithelial cells. Surfactant is a lipoprotein mixture. Dipalmitoyl lecithin is the major phospholipid of surfactant.

Question 4

Discuss the three primary functions of surfactant.

Answer 4

Surfactant: (1) acts like a detergent to decrease surface tension, so pulmonary compliance is increased and the work of breathing is reduced, (2) permits alveolar stability by keeping small alveoli from collapsing into larger alveoli, and (3) helps keep alveoli dry.

Question 5

As alveolar size decreases, what happens to surface tension in healthy individuals? What is the significance of this? What law applies.

Answer 5

Normally, surface tension decreases as alveoli become smaller. (The surface tension of surfactant decreases as surface area decreases.) It is this property of surfactant that keeps pressure equalized among alveoli and prevents small alveoli from collapsing and emptying into larger ones. The law of LaPlace applies.

Question 6

Define functional residual capacity (FRC).

Answer 6

Functional residual capacity is the volume of gas left in the lungs after a normal exhalation.

Question 7

In what direction does the chest wall naturally recoil? In what direction do the lungs naturally recoil? When is the chest wall recoil exactly balanced by the lung recoil?

Answer 7

The chest wall (thorax) naturally recoils outward, and the lungs naturally recoil inward. At functional residual capacity (FRC), the outward chest recoil equals the inward lung recoil.

Question 8

More than two-thirds of the work of breathing is used to overcome what?

Answer 8

More than two-thirds of the work of breathing is used to overcome elastic recoil of the lungs and the thorax.

Question 9

What is the cause of exhalation during the normal respiratory cycle?

Answer 9

Passive elastic recoil of the lungs is responsible for exhalation during normal tidal breathing.

Question 10

How does the intrapleural pressure fluctuate during normal tidal breathing?

Answer 10

Intrapleural pressure is negative at the onset of inspiration and becomes more negative during inspiration. During expiration, intrapleural pressure becomes less negative.

Question 11

During a normal respiratory cycle, when is the intrapleural pressure positive?

Answer 11

Intrapleural pressure is never positive, it is always negative (subatmospheric) during a normal inspiratory-expiratory cycle.

Question 12

What happens to intrapulmonary pressure during normal inspiration? Expiration? When is intrapulmonary pressure zero?

Answer 12

Intrapulmonary pressure becomes negative (subatmospheric) during inspiration and positive (above atmospheric pressure) during expiration. Intrapulmonary pressure is zero at end-expiration and at end-inspiration.

Question 13

Compare intrapleural pressure in the dependent versus non-dependent lung?

Answer 13

Intrapleural pressure is greater (less negative) in dependent lung and lower (more negative) in non-dependent lung.

Question 14

How does intrapleural pressure vary from apex to base at end-expiration in the upright position?

Answer 14

Intrapleural pressure is lowest (most negative) in the apex and greatest (least negative) at the base. Intrapleural pressure is greatest (least negative) in the dependent lung which is the base in the standing or sitting (upright) person.

Question 15

What is the intrapleural pressure in the base to apex direction in the supine position? Prone position? Lateral decubitus position?

Answer 15

The intrapleural pressure is the same at the base as at the apex in the supine, prone, and lateral decubitus positions. Intrapleural pressure changes in the vertical direction, not in the horizontal direction.

Question 16

Does the gradient in intrapleural pressure found in the awake, spontaneously breathing patient change when patient is anesthetized and paralyzed?

Answer 16

No, the gradient in intrapleural pressure does not change when the awake, spontaneously breathing patient is anesthetized and paralyzed. Intrapleural pressure is greater (less negative) in dependent lung compared with nondependent lung regardless of whether the patient is awake or anesthetized/paralyzed.

Question 17

What happens to intrapleural pressure during inspiration if the patient is on a positive pressure mechanical ventilator?

Answer 17

During inspiration, intrapleural pressure increases (becomes less negative and may become positive) if the patient is on a positive pressure ventilator.

Question 18

Describe the Valsalva maneuver. What happens to intrapleural and intrapulmonary pressures, heart rate, cardiac output and blood pressure during this maneuver?

Answer 18

The Valsalva maneuver is accomplished by performing a forced expiration with the glottis closed. All intrathoracic pressures, including intrapleural and intrapulmonary pressures, increase; intrapleural pressure changes from negative to positive, so venous return to the right ventricle decreases; consequently, cardiac output and blood pressure decrease; the decrease in blood pressure results in a reflex increase in heart rate (baroreceptor reflex).

Question 19

Total lung capacity (TLC) is the sum of what two lung

capacities? Total lung capacity (TLC) is the sum of what four lung volumes?

Answer 19

Total lung capacity is inspiratory capacity plus functional residual capacity. Total lung capacity is inspiratory reserve volume plus tidal volume plus expiratory reserve volume plus residual volume.

Question 20

The residual volume (RV) is normally what percent of the total lung capacity?

Answer 20

Residual volume is normally 20-25% of total lung capacity.

Question 21

Define closing volume (CV).

Answer 21

Closing volume is the volume of gas that can be exhaled during a force expiration after airways begin to close.

Question 22

What two lung volumes make up the functional residual capacity (FRC)?

Answer 22

Functional residual capacity (FRC) is the sum of the expiratory reserve volume (ERV) and residual volume (RV).