ASSESSMENT OF OSA (OBSTRUCTIVE SLEEP APNOEA)
Although a physical examination and detailed medical history can tell a great deal about the likelihood of OSA, confirmation of the syndrome can only be done in properly equipped sleep laboratories which are now established in many large public hospitals and some private institutions. Formal sleep studies (technically referred to as polysomnography) make it possible to observe the patient in a clinical setting under the supervision of trained personnel.
Many of the functional and physiological changes mentioned previously, such as airway obstruction, hypoxaemia and irregularities of heart rate, are monitored during the study, giving investigators enough information to decide on the best course of action for the hapless snorer. Most people are apprehensive about the need for hospital admission and comprehensive testing procedures, and it is important that they be informed and reassured of the non invasive nature of these procedures. A nervous patient is likely to have a poor night’s sleep from which little useful information can be extracted.
Oxygen saturation
Probably the single most important measure of the severity of OSA is the level to which blood oxygen falls. Blood oxygen concentration, or saturation, can be estimated quite accurately by attaching a probe to either a finger or ear lobe. A special light emitted from the probe is able to detect oxygen saturation in the blood supply of the site to which it is attached, which in turn reflects the circulating oxygen level.
Sleep states
Airway obstruction is more pronounced in deep sleep, particularly during REM, and it is therefore important to document sleep stages to confirm that the patient slept soundly, preferably with several episodes of REM. Sleep states are monitored via several electrodes which are glued to various sites on the head which detect electrical activity °f the brain, eye movements and muscle tone, all of which help to define sleep states.
Chest wall movement: Obstructive and Central
The two broad categories of sleep apnoea, central and obstructive, are defined by the type of respiratory efforts made. One way of measuring this is to look at the movement of the chest wall as it expands and contracts with each breath, in conjunction with a detector of airflow at the opening of the nasal passage. When the upper airway collapses in obstructive sleep apnoea, the chest wall continues to move but there will be no detectable airflow through the nose. Falling oxygen levels stimulate the patient to take bigger breaths resulting in ever increasing signals from the monitor of chest wall movement. Eventually, the obstruction is overcome, airflow is again detected and breathing returns to normal until the next obstructive event. With central apnoea, the chest wall shows little or no sign of movement, indicating an absence of respiratory effort. This is also accompanied by a cessation of airflow.
Heart monitoring
The final obligatory assault comes in the form of a heart monitor (or ECG), the electrodes of which are attached to the chest. The ECG records the heart rate and provides information about the electrical impulses which stimulate the heart to beat regularly.
The process of “wiring up” for a sleep study can be an intimidating experience for those with an innate fear of hospitals and high technology. A small percentage of patients never relax enough to get a good night’s sleep, but most adapt quickly to the novel circumstances and sleep soundly enough for the purpose of the investigation. In this respect, patients with advanced OSA have no problems, often falling asleep while being attached to their monitoring equipment.
The capacity to measure oxygen saturation, sleep states, chest wall movement, nasal airflow and heart rate is a minimum requirement for a unit specializing in sleep-related breathing disorders. Some units, however, are capable of measuring other parameters of sleep and breathing and have the resources to screen several patients on the same night.
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