Sleep and breathing. Pathophysiology of collapse
Theories of balance of forces and the balance of pressure in the collapse of the UA.
The air permeability depends on a complex multifactorial system, that combines mechanical factors in the muscle action on one hand and on the other hand the balance of pressures, that interact to keep the light of pharyngeal size in the different luminal, nasopharynx, oropharynx and hypopharynx sections. The mechanical action of the upper airway depends on the work of two muscle groups, that interact in complex anatomical relationship to ensure opening during breathing. The muscles, that generate pressure, mainly the diaphragm and the muscles of the UA: suprahyoid, infrahyoid, elevators , pharyngeal constrictor, soft palate and tongue musculature. In respiratory mechanics, they acquire a primary importance, involved muscles as the genioglossus, tensor palatini, the diaphragm and intercostal muscles. The process of ventilation is required to reproduce the normal pattern a balance of forces between the negative pressure exerted by the contraction of the diaphragm in the intrathoracic area and tends to collapse the pharynx, and the contraction of the muscles of the pharynx dilator action . It is a delicate balance between the respiratory muscles and pharyngeal muscles, that is easy to crack for any anomaly. Maintain patency of the airways depends on the action opposed extralumial pressure exerted between the diaphragm and the intercostal muscles and the action of dilator muscles covered by the neural system.
During inspiration, negative pressure of the airway causes a tendency in the pharyngeal walls to contract. The soft condition of this section makes it vulnerable to collapse. To counter this effect, the action of dilator muscles genioglossus, geniohyoid, sternohyoid and tensor veli is essential. Under normal conditions, contraction of these muscles must be simultaneous with inspiratory muscle, counteracting the drag force, that they cause. The most important inspiratory muscle is the diaphragm. It is a big muscle contraction due to scroll down the abdominal contents increasing the vertical diameter of the rib cage to attract by itself the lungs, forcing them to expand and to penetrate air inside. When relaxation occurs it goes back to its original position, causing expiration. The external intercostal muscles situated in each of the intercostal spaces to contract, produce a rise in the whole rib cage.
The human body changes during sleep to suit the situation of rest and presents substantial changes regarding to the operation of the vigil. These amendments are not pathologic by themselves, they are intelligent mechanisms, that puts the body in place to address the physiological circumstances of the dream state. Many of these circumstances can cause ailments in line with other situations, that induce fragmentation of normal sleep pattern . Breathing movements are regulated automatically according to the needs of our body in order to avoid disruption and maintain constant levels of oxygen and carbon dioxide in blood. The respiratory system is controlled and regulated by the brain stem, that controls the inspiratory and expiratory muscles. The respiratory center has two groups of neurons, some stimulate the inspiratory muscles and inhibit the expiratory, and others perform the opposite function. These centers enable automatic operation of the respiratory system through receptors. The function of these brain centers also coincides with the voluntary control exercised by the individual in his own breathing. During sleep, breathing becomes a completely involuntary act, and in the absence of pathology, physiological peculiarities of the UA are not a problem for its proper functioning. The enlargement of the oropharyngeal muscles during inspiration guarantees the opening of the UA and is sufficient to counteract the negative pressure exerted by the diaphragm and intercostal muscles. But this pulse pressures and forces in the respiratory act, necessary to maintain the patency of the UA, can be unbalanced during sleep. This mismatch is the cause of the malfunction of the respiratory structures, that cause the reduction in pharyngeal caliber.
Closing of the UA can be caused by various abnormalities, mainly mechanical, anatomical and functional (breath control) abnormalities. In the collapse of the UA existen two circumstances, first, the subject stays by itself under controle of the involuntary respiration, to which can be added, that the sleep onset is marked by a decrease in muscle tone, a physiologic reduction of muscle activity also extended to the muscles of the pharynx. This inevitably leads to decreased muscle tone and pharyngeal lumen diameter. In stages III and IV NREM sleep and REM sleep, muscles are more relaxed and muscle weakness also affects the upper airway. Individuals in these stages reach the deepest sleep and it is the time, when the pharynx is more prone to collapse. In these states, caused by the vulnerability of the pharynx, ocurrs pharyngeal obstruction in patients with sleep apnea with subsequent occurrence of apneas and hypopneas. Obstruction or collapse can occur at any point of the pharynx. This process is more pronounced during REM phase characterized by an absolute relaxation of muscle tone. This decrease in muscle tone does not affect equally all the involved muscles with respiratory function. In NREM sleep stages the activity of the intercostal muscles causes 60% of tidal volume. In REM, the responsible muscle for keeping the tidal volume is the diaphragm exempt of the muscular generalized atony.
Starling resistor model
It is difficult to quantify the mechanical and neuromuscular favoring the decrease in the size of the upper airway during sleep. One of the most widely used approach has been developed by Starling to recreate the functionality of the UA as a flow control, treated like a collapsible tube. The pharynx should be a distensible conduit for speech and swallowing. This pharyngeal distensibility may be an inconvenience for the tract functions. To explain the dynamics of fluids, that occur during breathing he uses a collapsible tube model, which is known as the Starling Resistor. These two rigid tubes connected by a simple collapsible tube, which involves inside a cube, that can be added with water to increase the pressure from outside, causing more or less a collapse to itself. By this model three possible states of the tube and flow conditions can be simulated, depending on the magnitude of the different pressures acting on the tube. Each of the extreme situations is determined by the balance resulting from the pressure values inside and outside the tube.
Thus we can distinguish the following situations:
- "Rigid"tube and free flow: When the pressure inside the tube is much higher, than the outside pressure and the pressure upstream is greater, than the critical pressure.
- Partially collapsible tube and vibration: When the pressure inside the tube is close to outside pressure and upstream pressure is greater than the critical pressure.
- Very collapsible tube and choked flow: When the pressure inside the tube is much lower than outside pressure and upstream pressure is less than the critical pressure. In the Starling resistor model, the collapse occurs, when negative pressure is lower than in the segment of exit or critical pressure (Pcrit) is associated with predisposition to collapse and the development of simple snoring in the partially collapsed and vibration phase to the profile of patients, who develop apnea, which reproduced a collapsible tube model and choked flow.