Respiratory rhythm is usually regular and continues throughout life without conscious effort. Indeed, consciousness of one's breathing often signifies the presence of extreme environmental conditions or pathological states. When respirations become arrhythmic, the term periodic breathing is applied, while apnea refers to an absence of breathing that is usually temporary. Apneas are obstructive when caused by upper-airway blockage despite efferent impulses to breathe, or central when without respiratory movements, or may be of mixed type. However other apneas exist, including obstructive expiratory apnea, obstructive hypoventilation, and central hypoventilation syndrome ("Ondine's curse"); these will be discussed in Chap. 25.
The term dyspnea was introduced earlier to describe a patient's sensation of inadequate or stressful respiration, or an exaggerated consciousness of a need for increased respiratory effort. Clinically, dyspnea usually implies labored respiration and the use of some or all accessory respiratory muscles, as in many patients with acute lung injury and the acute respiratory distress syndrome (Chap. 28). Many factors induce dyspnea, but it is important to remember that it is a sensation that may not have an underlying pathophysiological explanation in every case.
Periodic breathing is characterized as clusters of breaths separated by intervals of apnea or near-apnea. A clinically useful definition of such periodic breathing might be when at least three respiratory pauses, each lasting 3 seconds or longer, are separated by periods of normal breathing, each lasting less than 20 seconds. Cheyne-Stokes breathing as commonly seen in congestive heart failure is an example of such periodic breathing (Fig. 11.11). Periodic breathing usually occurs during sleep and can occur in healthy individuals, and the apnea there is usually of central rather than obstructive origin.
Cheyne-Stokes breathing in a patient with congestive heart failure. Shown from top to bottom are the patient's arterial saturation by pulse oximetry (Sao2), airflow rate measured at the mouth, chest movements by thoracic tension belt, and abdominal wall movements by a similar device. Note the reproducible periodicity, with the complete cycle repeated every 70-90 seconds.
Periodic breathing is a normal respiratory pattern in premature infants during active REM-sleep and non-REM sleep. It persists in infants, but decreases as a percentage of the population as children mature. Among term infants, periodic breathing usually is confined to REM sleep. Persistence of periodic breathing during longer portions of sleep may be abnormal, and reflect immaturity or an abnormality of brainstem respiratory control (Chap. 39). Sleep apnea occurs in 2%-3% of children, 3%-7% of middle-aged adults, and 10%-15% of otherwise healthy adults who are > 65 years old. Periodic breathing is generally considered to be a consequence of chemoreceptor function. When Paco2 or PAco2 is far below normal eupneic values, or when Pao2 or PAo2 is far above, then at least transient apnea will almost certainly ensue. Thus, there are apneic thresholds for both hypocapnia and hyperoxia that may differ strikingly among healthy individuals. Whether their periodic breathing is more, the result of activation of central versus peripheral chemoreceptors remains unresolved to date.