The Controller: Directing the Orchestra

Definitions

As we begin our exploration of the control of ventilation, it is important to be sure we are all using terms in the same way. Some of these words were familiar to you before you embarked on your medical studies, but the meanings you associated with them at that time may not be the same as the ones we use in the care of patients.

Hyperventilation is a term that describes breathing beyond that which is required to meet the metabolic needs of the body as reflected in the production of carbon dioxide. Recall the equation (discussed in Chapter 5) that relates P_aCO₂ to carbon dioxide production and alveolar ventilation:

When alveolar ventilation () is well matched to carbon dioxide production (), P_aCO₂ is normal. Thus, if a person is hyperventilating, it means that the P_aCO₂ is lower than the normal range, that is, is below approximately 38 mm Hg. On the other hand, hypoventilation refers to breathing that is less than required to meet the metabolic needs of the body, and it is characterized by a P_aCO₂ that is above the normal range, that is, greater than approximately 42 mm Hg.

Hyperpnea is increased breathing that matches the metabolic needs of the body, again as reflected in the production of carbon dioxide. If ventilation is increasing in concert with carbon dioxide production, P_aCO₂ remains within the normal range. During most of exercise, for example, ventilation increases in proportion to metabolic needs, and you would refer to the increased breathing as hyperpnea.

When a person increases ventilation, one of two strategies may be used: the respiratory rate may be increased, or the tidal volume (V_T) may be increased (of course, you can do both at the same time). Tachypnea refers to an increase in respiratory rate above the normal range (this term is usually reserved for rates greater than or equal to 20 breaths per minute). You must remember, however, that a person can be tachypneic without hyperventilating. As discussed in Chapter 5, if rapid breathing is accompanied by small tidal volumes, the alveolar ventilation may be reduced, even if total ventilation is increased. This could result in an increase in P_aCO₂, and you would describe the patient as being tachypneic and yet showing evidence of alveolar hypoventilation. Finally, bradypnea is a word used to describe respiratory rates below the normal range (this term is usually reserved for rates less than 10 breaths per minute).