Define functional residual capacity (10% marks). Outline the functions (70% marks) of the functional residual capacity and the factors affecting it (20% marks).
This question was in two parts with the percentage of marks allocated an indication of the relevant time or detail expected per part. The second part of the question also contained two distinct headings which should have been used in the answer. As an outline question, dot points with a brief explanation of each point were expected. Most candidates drew diagrams, few of which added value. For a diagram to add value it should be accurate, have labelled axes, a scale with numerical values and units. As a general rule, diagrams should also be explained and help to illustrate or relate to a written point.
For factors affecting FRC, to score full marks, it should be clearly stated if the factor causes an increase or decrease in FRC. This topic is well covered in the recommended respiratory texts.
The FRC is:
Functions of the FRC are:
Factors affecting the FRC are:
It would be unreasonable to expect a massive amount of detail for these factors in a two-minute answer, but just as a reference, a longer list is offered here:
|Factors which increase FRC||Factors which decrease FRC|
|Factors which influence lung size|
|Increased height||Short stature|
|Male gender||Female gender|
|Age: ratio of FRC to total lung capacity increases, but absolute FRC remains stable
(Wahba et al, 1983)
|Factors which influence lung compliance|
|Increased compliance due to disease, eg. emphysema||Decreased lung compliance due to disease, eg. ARDS|
|Increased end-expiratory pressure, eg. PEEP or auto-PEEP||Negative end-expiratory pressure|
|Factors which influence chest expansion and chest wall compliance|
|Open chest or mediastinum||Increased intraabdominal pressure:
pregnancy, ascites, abdominal surgery
|Decreased respiratory muscle tone, eg. anaesthesia/sedation|
|Upright position and prone position||Supine and head down position|
|Circumferential burns, chest binder devices (eg. post mastectomy)|
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