Sleep and breathing

Significant physiologic changes in breathing take place during normal sleep related to alterations in respiratory drive and musculature.

Sleep onset
Set point of ventilation is different in wakefulness and sleep. pCO2 is higher and ventilation is lower in sleep. Sleep onset in normal subjects is not immediate, but oscillates between arousal, stage I and II sleep before steady NREM sleep is obtained. So falling asleep results in decreased ventilation and a higher pCO2, above the wakefulness set point. On wakefulness, this constitutes an error signal which provokes hyperventilation until the wakefulness set point is reached. When the subject falls asleep, ventilation decreases and pCO2 rises, resulting in hypoventilation or even apnea. These oscillations continue until steady state sleep is obtained.

Ventilation
Breathing is remarkably regular, both in amplitude and frequency in steady NREM sleep. Steady NREM sleep has the lowest indices of variability of all sleep stages.Minute ventilation decreases by 13% in steady stage II sleep and by 15% in steady slow wave sleep (Stage III and Stage IV sleep). Mean inspiratory flow is decreased but inspiratory duration and respiratory cycle duration are unchanged, resulting in an overall decreased tidal volume.

Rib cage and abdominal muscle contributions
Rib cage contribution to ventilation increases during NREM sleep, mostly by lateral movement, and is detected by an increase in EMG amplitude during breathing.Diaphragm activity is little increased or unchanged and abdominal muscle activity is slightly increased during these sleep stages.

Upper airway resistance
Airway resistance increases by about 230% during NREM sleep. Elastic and flow resistive properties of the lung do not change during NREM sleep. The increase in resistance comes primarily from the upper airway in the retroepiglottic region. Tonic activity of the pharyngeal dilator muscles of the upper airway decreases during the NREM sleep, contributing to the increased resistance, which is reflected in increased esophageal pressure swings during sleep. The other ventilatory muscles compensate for the increased resistance, and so the airflow decreases much less than the increase in resistance

Arterial blood gases
pCO2 increases by 3-7mmHg, pO2 drops by 3-9mmHg and SaO2 drops by 2% or less. These changes occur despite a reduced metabolic rate, reflected by a 10-20% decrease in O2 consumption, suggesting overall hypoventilation instead of decreased production/metabolism.

Pulmonary arterial pressure
Periodic oscillations of the pulmonary arterial pressure occur with respiration. Pulmonary arterial systolic and diastolic pressure and PAD increase by 4-5mm in NREM sleep

Effects of arousals
Induced transient arousal from NREM sleep cause the following: Increase EMG activity of the diaphragm 150%, increased activity of upper airway dilating muscles 250%, increased airflow and tidal volume 160% and decreased upper airway resistance.

Ventilation
Irregular breathing with sudden changes in both amplitude and frequency at times interrupted by central apneas lasting 10-30seconds are noted in REM (Rapid Eye Movement) sleep. (These are physiologic changes and are different from abnormal breathing patterns noted in sleep disordered breathing). These breathing irregularities are not random, but correspond to bursts of eye movements. This breathing pattern is not controlled by the chemoreceptors, but is due to the activation of behavioral respiratory control system by REM sleep processes. Quantitative measure of airflow is quite variable in this sleep stage and has been shown to be increased, decreased or unchanged. Tidal volume has also been shown to be increased, decreased or unchanged by quantitative measures in REM sleep. So breathing during REM sleep is somewhat discordant

Rib cage and abdominal muscle contributions
Intercostal muscle activity decreases in REM sleep and contribution of rib cage to respiration decreases during REM sleep. This is due to REM related supraspinal inhibition of alpha motoneuron drive and specific sdepression of fusimotor function. Diaphraghmatic activity correspondingly increases during REM sleep. Although paradoxical thoracoabdominal movements are not observed, the thoracic and abdominal displacements are not exactly in phase. This decrease in intercostal muscle activity is primarily responsible for hypoventilation that occurs in patients with borderline pulmonary function.

Upper airway function
Upper airway resistance is expected to be highest during REM sleep because of atonia of the pharyngeal dilator muscles and partial airway collapse. Many studies have shown this, but not all. Some have shown unchanged airway resistance during REM sleep, others have shown it to increase to NREM levels.

Arterial blood gases
Hypoxemia due to hypoventilation is noted in REM sleep but this is less well studied than NREM sleep. These changes are equal to or greater than NREM sleep

Pulmonary arterial pressure
Pulmonary arterial pressure fluctuates with respiration and rises during REM sleep.

Effect of arousals
Arousals cause return of airway resistance and airflow to near awake values. Refer arousals in NREM sleep.

Upper airway resistance syndrome
Upper Airway Resistance Syndrome (UARS)

Obstructive sleep apnea (including hypopnea) syndrome
Sleep apnea

Central sleep apnea syndrome
Sleep apnea