Age-Related Changes in the Respiratory System

Respiratory Function Tests

The commonly used respiratory function tests are presented in this chapter. In addition, patterns of lung function abnormality seen in some of the common types of condition are also presented. Breathing parameters include the following:

• Forced expiratory volume (L) in 1 second, FEV ₁ . This is the volume of air expired during the first second of a forced expiratory maneuver from vital capacity (maximal inspiration); it is measured by spirometry.

• Forced vital capacity (L), FVC. This is the total volume of air expired during forced expiration from the end of maximum inspiration. A slow vital capacity (SVC) is the volume of air expired, but this time through an unforced maneuver. In the young, these are similar, but in emphysema, where there is loss of elastic recoil, FVC may fall disproportionately more than SVC. These are also measured by spirometry.

• Peak expiratory flow rate (L/min), PEFR. This is the maximal expiratory flow rate measured using a peak flow meter, a more portable method; therefore, serial home measurements may be performed by patients.

The following parameters require more detailed lung physiology testing:

• Total lung capacity (L), TLC. This is the volume of air contained in the lung at the end of maximal inspiration; it is measured by helium dilution or body plethysmography together with the next two tests.

• Functional residual capacity (L), FRC. This is the amount of air left in the lungs after a tidal breath out and indicates the amount of air that stays in the lungs during normal breathing.

• Residual volume (L), RV. This is the amount of air left in the lungs after a maximal exhalation. Not all the air in the lungs can ever be expired.

• Transfer factor (mmol/min), TL _CO . This is a measure of the ability of the lung to oxygenate hemoglobin. It is usually measured with a single breath hold technique using low-concentration carbon monoxide.

• Transfer coefficient (mmol/min/k/Pa/L _BTPS ), K _CO . This is the TL _CO corrected for the lung volume.

In addition, blood gas measurements are often performed to assess acid-base balance and oxygenation. The most important measures for respiratory disease are the partial pressure of oxygen (Pa o ₂ ), partial pressure of carbon dioxide (Pa co ₂ ), and the pH. A low Pa o ₂ (hypoxemia) with a normal Pa co ₂ indicates type I respiratory failure. An increased Pa co ₂ with hypoxemia indicates type II respiratory failure. A rapidly rising Pa co ₂ will result in a fall in the pH—for example, that seen in an acute exacerbation of chronic obstructive pulmonary disease (COPD). Renal compensation occurs in response to a chronically high Pa co ₂ , with correction of the pH to normal or near-normal levels, but this renal compensation takes several days to occur. Hyperventilation, associated with excess expiration of CO ₂ , as seen in anxiety attacks but also in altered respiratory control such as Cheyne-Stokes respiration, will result in an increase in pH as a result of a drop in Pa co ₂ . Pure anxiety-related hyperventilation will not cause hypoxemia but other causes for this altered respiratory control may cause hypoxemia.

There are two main characteristic patterns of respiratory disease based on spirometric evaluation, the obstructive and restrictive patterns.

An obstructive pattern is seen in several situations including in patients with asthma and COPD. It is characterized by the following:

• Reduced FEV ₁ and PEFR

• Normal or reduced FVC (if FVC is reduced, it is disproportionately less reduced than FEV ₁ )

• Reduced FEV ₁ /FVC ratio

A restrictive pattern is characterized by the following:

• Reduced FEV ₁

• Reduced FVC

• Normal or high FEV ₁ /FVC ratio

Conditions relating to both these spirometric patterns, with more detail about lung function patterns and the use of other lung physiology parameters to characterize and diagnose conditions, will be discussed elsewhere in this text.