Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 3: Mechanics of Ventilation

3-1	The alveolar pressure is 0 cm H2O in both instances (end-expiration and end-inspiration) because atmospheric and alveolar pressures have equalized, as is indicated by the lack of gas flow in both situations.

3-2	The lung contains the same volume in both cases because the pressures distending the lung are identical in both cases (i.e., the transpulmonary pressures or the differences between alveolar and intrapleural pressures are identical).

3-3	An increased residual volume (RV) means there is more air left in the lung at the end of a maximal-effort exhalation; this results in a decreased vital capacity if total lung capacity remains constant. An increased RV also increases the functional residual capacity, which decreases the inspiratory capacity.

3-4	The person with high lung compliance (low elastance) requires the longest passive exhalation time because of reduced elastic lung recoil forces. A lung with high elastance (low compliance) has a high recoil force and thus empties more quickly.

3-5	A low functional residual capacity means the lung volume at the end of a normal exhalation is abnormally low, implying that some alveoli are airless and collapsed (atelectasis). This condition reduces lung compliance and makes the lung more difficult to inflate. Collapsed alveoli strongly oppose inflation because of high surface tension; high critical opening pressures increase the work of breathing.

3-6	Loss of blood flow implies loss of nutrition to alveolar structures. This interferes with surfactant synthesis, leading to high alveolar surface tension, atelectasis, and difficult lung inflation.

3-7	A lung with low compliance is stiff and difficult to inflate. More pressure is required to inflate the lung to a given volume, regardless of whether pressure is measured at the peak of inspiration, before airflow has ceased (peak pressure), or at the end of inspiration during a pause when airflow is absent (plateau pressure). Peak pressure is generated by the sum of elastic plus frictional resistances to lung inflation, whereas plateau pressure is generated by elastic lung recoil forces only. The difference between peak and plateau pressures is a reflection of only frictional airflow resistance. Thus low lung compliance by itself does not affect the difference between peak and plateau pressures during constant volume mechanical ventilation.

3-8	Exhaled vital capacity and residual volume are increased because airway compression traps gas inside of the lungs, decreasing the volume of gas that can be forcefully exhaled.

3-9	In Figure 3-26, there is a point at which the Ppl surrounding the airway is greater than the pressure inside of it; this greater outside pressure compresses the airway, creating a narrowed choke point that controls how much flow can pass through it. Therefore, the driving pressure responsible for the expiratory flow rate must be equal to the difference between PA and the choke point pressure (Ppl). It should be apparent that the lower pressure downstream from the choke point (e.g., Pao) has nothing to do with the driving pressure. In contrast, in Figure 3-25 (note the portion of the airway still inside the thorax) pressure inside the airway is greater than outside pressure (Ppl) at all points, and a narrowed choke point never develops; thus the driving pressure responsible for the expiratory flow rate in this figure is equal to the difference between PA and Pao.

3-10	Narrowed airways in asthma slow airflow, prolonging the emptying or filling time    for a given inflation pressure; this produces an abnormally long time constant.

3-11	Auto-PEEP occurs when the breathing pattern does not allow the lungs enough time for complete exhalation to the normal end-tidal exhalation volume (i.e., the normal functional residual capacity). An inspiratory time to expiratory time ratio of 2:1 is most likely to produce auto-PEEP because the time allowed for exhalation is shortened.