306e Disturbances of Respiratory Function

Question 1

alveolar surface area

Answer 1

typically 70 m2

Question 2

volume of a thoracic cavity

Answer 2

typically 7 L

Question 3

zero inflation pressure, even normal lungs retain some

air in the alveoli. why?

Answer 3

because the small peripheral airways are tethered open by radially outward pull from inflated lung
parenchyma attached to adventitia; as the lung deflates during exhalation, those small airways are pulled open
progressively less, and eventually they close, trapping some gas in the alveoli.

Question 4

what is the passive resting point of the respiratory system?

Answer 4

It is attained when alveolar gas pressure equals body surface pressure (i.e., when the transrespiratory system pressure is zero). At this volume (called the functional residual capacity [FRC]), the outward recoil of the chest wall is balanced exactly by the inward recoil of the lung

Question 5

how much is the frictional resistance of normal pulmonary airways?

Answer 5

Raw , normally less than 2 cmH O/L per second

Question 6

Explain the dynamic airflow limitation.

Answer 6

An important anatomic feature of the pulmonary airways is its treelike branching structure. Therefore the area of the airways becomes very large toward the lung periphery.
Because flow (volume/time) is constant along the airway tree, the velocity of airflow (flow/summed crosssectional
area) is much greater in the central airways than in the peripheral airways. During exhalation, gas leaving the
alveoli must therefore gain velocity as it proceeds toward the mouth. The energy required for this “convective”
acceleration is drawn from the component of gas energy

Question 7

How do you normally achieve modest increase of ventilation? And pronounced increase of ventilation?

Answer 7

A modest increase of ventilation is most efficiently achieved by increasing tidal volume but not respiratory rate, which is the normal ventilatory response to lowerlevel exercise. At high levels of exercise, deep breathing persists, but respiratory rate also increases.

Question 8

Explain the sensation of difficulty in inhaling in obstructive lung diseases.

Answer 8

With repetition of incomplete exhalations of each tidal breath, the operating FRC becomes
dynamically elevated, sometimes to a level that approaches TLC. At these high lung volumes, the respiratory system is much less compliant than at normal breathing volumes, and thus the elastic work of each tidal breath is also increased. The dynamic pulmonary hyperinflation that accompanies severe airflow obstruction causes patients to sense difficulty in inhaling—even though the root cause of this pathophysiologic abnormality is expiratory airflow obstruction.

Question 9

anatomic dead space (VD)

Answer 9

gas in the conducting airways that has the first contact with the alveoli

Question 10

CO2 fraction in alveolar gas

Answer 10

typically ~5.6%

Question 11

respiratory quotient

Answer 11

The rate of oxygen uptake is related to
the average rate of metabolic CO production, and their ratio—the “respiratory quotient” (R = ˙VCO2/˙VO2)—
depends largely on the fuel being metabolized. For a typical American diet, R is usually around 0.85.

Question 12

Explain the concepts of dead space and shunt in ventilation perfusion mismatch

Answer 12

ventilation of unperfused lung distal to a pulmonary embolus, in which ventilation of the physiologic dead space is “wasted” in the sense that it does not contribute to gas exchange; and (2) perfusion of nonventilated lung (a “shunt”), which allows venous blood to pass through the lung unaltered.

Question 13

Which functional measurements can be done to determine TLC and RV?

Answer 13

two approaches are commonly used:

inert gas dilution and body plethysmography

Question 14

How does race influence lung volumes?

Answer 14

TLC values are ~12% lower in African Americans and 6% lower in Asian Americans than in Caucasian Americans

Question 15

which variability from the predicted value is allowed in lung function studies in order to be considered normal?

Answer 15

85–115% of the predicted value

Question 16

In the lung, most resistance originates from where?

Answer 16

the central airways

Question 17

how do you measure respiratory muscle strength?

Answer 17

the patient is instructed to exhale or inhale with maximal effort against a closed shutter while pressure is monitored at the mouth. Pressures greater than ±60 cmH O at FRC are considered adequate

Question 18

DLco2 decreases and increases in which diseases?

Answer 18

DL decreases in diseases that thicken or destroy alveolar membranes (e.g., pulmonary fibrosis, emphysema), curtail the pulmonary vasculature (e.g., pulmonary hypertension), or reduce alveolar capillary hemoglobin (e.g., anemia). Singlebreath diffusing capacity may be elevated in acute congestive heart failure, asthma, polycythemia, and pulmonary hemorrhage