13th January 2024

Muscle Tone

As one progresses through the sleep cycle, one’s muscle activity and tone decreases. One loses the wakefulness drive to breathe, and our breathing lightens: the respiratory rate drops, the minute ventilation drops, and muscles become flaccid. This loss of ventilatory motor output is noted in the upper airway muscles, chest wall, and accessory muscles.

Upper Airway

The loss of tonicity in the upper airway manifests itself as a narrowing of the upper airway lumen and increased flaccidity of the upper airway walls. This, in turn, causes increased upper airway resistances and more turbulent airflow. In healthy individuals, this tends not to cause an issue. However, some people are at increased risk of having more substantial upper airway narrowing or even collapse leading. If the upper airway resistance increases enough (i.e. there is sufficient narrowing of the lumen), one can have noisier breathing or snoring (especially if the soft palate vibrates or flaps with inhalation). This can lead to reduced (but not absent) airflow. For some individuals, the increased resistance and the floppiness of their upper airway structures lead to collapse and obstruction, completely stopping airflow and leading to an obstructive apnoeic episode. This, in turn, leads to a rise in pCO2 and increased airway pressure until it triggers an arousal and a surge in catecholamines, leading to increased wakefulness and, therefore, increased drive to the respiratory muscles.

Chest Wall and Diaphragm

The reduction of tonicity in the chest wall muscles increases as our sleep deepens. The diaphragm remains the main respiratory pump during sleep but loses the assistance of the intercostal and accessory respiratory muscles. This leads to a reduction in the minute ventilation. In healthy individuals, the body can manage. However, for certain patient groups, the loss of the supporting respiratory muscles can lead to worsening of their breathing and gas exchange. They develop hypercarbia/hypoxia, which in turn triggers arousals.