The dorsal and ventral respiratory groups have many afferent connections.Themostimportant afferent fibers are from a sensor in the lower medulla that monitors cerebrospinal fluid pH. The cerebrospinal fluid pH depends on blood pCO2 and reflects the acid-base balance status. The dorsal and ventral respiratory groups react to the signals from this sensor by modifying their discharge frequency and intensity. The discharge frequency and intensity of the dorsal and ventral respiratory groups dictate the respiratory rate and tidal volume. The dorsal and ventral respiratory groups also receive and integrate information regarding lung volume, airflow through the upper airway, and arterial oxygenation through the fifth and the tenth cranial nerve connections. In addition, fibers from structures in the pons and from higher central nervous system locations connect with the dorsal and ventral respiratory groups and help modulate breathing during all behavioral stages and sustain breathing during active sleep. During active sleep, respiration becomes less dependent on cerebrospinal fluid pH than while awake or during quiet sleep. Two important pontine centers are: (1) the apneustic center, which provides inspiratory drive to the medullary respiratory centers, and (2) the pneumotaxic center, which suppresses the apneustic center. The apneustic center helps in the transition from inspiration to expiration.
In addition to understanding the normal neuroanatomy of breathing, the evaluation of a neonate with apnea requires some understanding of polysomnography.

POLYSOMNOGRAPHY

Polysomnography with pH probe is a laboratory investigation often used in the evaluation of apnea. The polysomnogram monitors brain, cardiac, and respiratory functions; oxygen and CO2 concentrations; body movements; and esophageal pH. The polysomnogram is useful because it detects apnea; allows classification of the apnea as central, obstructive, or mixed; determines the behavioral stage during which apnea occurred; establishes the cardiac and electroencephalographic consequences of the apnea; and occasionally reveals the cause of apnea. The polysomnogram also allows detection and quantification of periodic breathing.
Apnea is an abnormal respiratory pause. A respiratory pause refers to a cessation in nasal and oral airflow that lasts longer than 3 seconds. A respiratory pause is abnormal if: (1) it is associated with pathological respiratory, cardiovascular, and neurological consequences; (2) it is of a type not normally seen in the neonatal period; (3) it occurs with a frequency above that of normal neonates; (4) it is triggered by electroencephalographic seizures or gastroesophageal reflux; or (5) it lasts more than 20 seconds in a premature infant or 15 seconds in a fullterm neonate.