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What Determines the Major Lung Volumes?

Total lung capacity is determined by the ability of the inspiratory pump (brain, nerves, muscle) to expand the chest wall and lungs which have a strong tendency to recoil inwards at high lung volumes.  Any breakdown in the ability of pump to function will result in a smaller total lung capacity  (restrictive lung disease).  Neuromuscular disease is an example of this.  Diseases which increase inward recoil of the lung (pulmonary fibrosis) will lead to a smaller TLC.  Diseases which lead to a reduction in inward recoil of the lung (emphysema) result in an increase in TLC known as hyperinflation.

Residual volume (RV) is determined in healthy younger individuals by the competition between the strength of the expiratory muscles and compressibility of the chest wall.  However, by the onset of middle age or in obstructive lung disease RV appears to be determined by a "flow limitation";  expiratory flow rates at low lung volumes are so low that expiration is prolonged and is not completed down to the original RV by the time the subject gives up the effort and takes another breath.

Vital capacity (VC) is determined by the difference between TLC and RV and changes with variations in RV or TLC.  It is easily measured and reliable and can check the measured validity of a measured change in RV or TLC.  FRC is the relaxation volume at the end of expiration.  It is not a reliable measurement and requires excellent cooperation on the part of the subject.  In patients with obstructive lung disease FRC may be elevated.  This imposes a significant extra load on the inspiratory muscles which can results in muscle fatigue.

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What Determines Airflow Through the Bronchial System?

Air flows through a tube if there is a pressure difference between the ends.  In the respiratory system the pressure difference is between the alveolar pressure and the pressure at the airway opening or mouth.  Flow may be laminar (smooth) or turbulent dependent on characteristics of the gas and the tube through which it is traveling.  Most of the resistance to airflow occurs in the first few divisions of the airways.  The more distal airway divisions, because of their large cross-sectional area, constitute a silent zone of airway resistance.

Resistance to flow is not constant at all lung volumes.  As the lung expands, airways enlarge reducing the airways resistance at high lung volumes.  Airways resistance increases at lower lung volumes.  A plot of airways resistance vs. lung volume is shown in Fig 4.

Fig 4

plot of airways resistance vs. lung volume

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Other factors besides lung volume can affect airway resistance.  Smooth muscle within the wall of the same bronchi can contract and increase airways resistance.  Secretions in airways or edema in the airway wall can also increase airways resistance.  In patients with emphysema, loss of tethering of small airways open during exhalation leads to collapse and an increase in resistance to airflow.  All obstructive lung diseases are characterized by an increase in resistance to expiratory flow.

Measurement of Expiratory Flows

Measurement of expiratory flow is extremely useful to us particularly in identifying obstructive lung disease but in a number of other ways also.  Expiratory flows are measured during the forced expiratory spirogram (Figure 2).  Again, the patient breaths to TLC and forcefully exhales to residual volume generating the expiratory spirogram with volume plotted against time.

  The spirogram can be broken up into subdivisions.  The ones which we are most concerned about are

  1. the FVC which has been mentioned previously and represents the entire volume exhaled from the lungs in a forced breath,
  2. the FEV1 or the volume of gas exhaled in the first one second of exhalation,
  3. the FEF25-75 which is the flow of gas exhaled during the middle half of the vital capacity previously known as the maximal mid expiratory flow or (MMFR). 

The forced expiratory maneuver has been called "an unnatural act" because it is rarely if ever performed during daily activities.  Nevertheless, it probes a very important pathophysiologic limit.  Beyond a modest expiratory effort, the limit to flow is effort-independent; pushing harder does absolutely no good.  The limit, however, is markedly volume dependent ranging in healthy persons from 10 liters per second at high lung volumes to near zero flow at RV.  The limit is lowered at all lung volumes by primary narrowing of airways or narrowing due to decrease in lung recoil (emphysema) and is responsible for the ventilatory impairment seen in these obstructive lung diseases. 

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Flow Volume Loop Analysis

In addition to portraying the spirogram as volume plotted against time, it can also be plotted as flow against volume as shown below in figure 5.  This can be particularly helpful in identifying obstruction lesions of the upper airway. 

Fig 5

flow against volume

Fig 6: Intra and extrathoracic large airway obstructing lesions

Intra and extrathoracic large airway obstructing lesions

Fig 7: Flow-volume loops in intra and extrathoracic lesions

Flow-volume loops in intra and extrathoracic lesions

Diffusing Capacity

The diffusing capacity is a measure of the transport of gas across the alveolo-capillary membrane. The techniques of this measurement is discussed will be discussed with you. The diffusing capacity reflects the surface area of the alveolo-capillary membrane as well as its thickness and the driving pressure for gas across the membrane. It can be reduced in diseases such as emphysema, pulmonary fibrosis, or pulmonary vascular disease. It can also be reduced in patients with anemia.

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