Obstructive sleep apnea - Continuous Positive Airway Pressure Therapy - technical
Obstructive sleep apnea is a fairly common condition with significant adverse health outcome. The treatment of choice is nasal continuous positive airway pressure (CPAP), which restores upper airway patency by increasing intraluminal pressure and hence acting as a ‘pneumatic splint’ maintaining patency throughout the entire upper airway. Determination of the optimal CPAP pressure requires ‘titration’ during polysomnography. The criteria for initiating nasal CPAP therapy are continuously evolving.
Nasal CPAP can be delivered as a CPAP, as bi-level positive pressure therapy (BPAP) or as an auto-titrating device. While these modalities improve comfort; there is no evidence that adherence with therapy is enhanced. An area of controversy is whether patients with mild disease would benefit from CPAP therapy. Effectiveness of therapy requires adherence to therapy; unfortunately, adherence with nasal CPAP remains suboptimal. Intensive support may improve adherence with potential improvement in patient outcome.
Obstructive sleep apnea syndrome (OSAS) is a common disorder with significant adverse health consequences. Specifically, OSAS is associated with reduced quality of life, increased risk of motor vehicle accidents, systemic hypertension, and cardiac consequences. Epidemiologic studies suggest a prevalence of 2% of women and 4%of men. Continuous positive airway pressure (CPAP) is an effective therapy and is the most widely used modality in patients with clinically significant sleep-disordered breathing.
Mechanism of Action
The upper airway is an extrathoracic conduit, void of structural support. Therefore, upper airway patency depends on upper airway caliber, upper airway muscle tone, and the transmural pressure across pharyngeal wall. The occurrence of upper airway obstruction indicates the development of a collapsing transmural pressure across a highly compliant pharyngeal wall. Nasal CPAP restores upper airway patency by increasing intraluminal pressure above the positive critical transmural pressure across the pharyngeal wall. This pressure is referred to as the ‘critical opening pressure’. Thus, nasal CPAP serves as a ‘pneumatic splint’ maintaining patency throughout the entire upper airway.
To initiate nasal CPAP therapy, the appropriate ‘critical opening pressure’ should be determined. Treatment with an optimal pressure is of utmost importance. If subtherapeutic pressure is used, snoring, sleep fragmentation, and daytime sleepiness may persist. Conversely, excessive pressure could result in patient discomfort, sleep fragmentation, or development of central apnea. Therefore, this is accomplished in the sleep laboratory during a sleep study (polysomnography) by titrating the pressure level upward until resolution of respiratory events, oxyhemoglobin desaturation, and sleep fragmentation. Various body positions and both non-rapid-eyemovement (non-REM) and REM sleep should be studied to determine optimal CPAP pressure under these conditions.
Who Should Be Treated with Nasal CPAP?
The severity of sleep apnea influences the decision to initiate therapy with nasal CPAP. Most sleep laboratories use the combined number of apneas and hypopneas per hour of sleep (known as apnea/hypopnea index (AHI)) as the ‘metric’ of the severity. Current reimbursement (treatment) guidelines in the US require five apneas per hour of sleep. However, AHI is not an optimal ‘metric’ as mild disease (by AHI) may have significant adverse consequences. In fact, several studies have shown that the treatment of mild disease may be beneficial in terms of daytime function or alertness.
Another limitation of AHI as a metric is the lack of standardization across different sleep laboratories. There are multiple operational definitions for hypopnea incorporating variable degrees of oxyhemoglobin desaturation, reduced effort, or subsequent arousal. Such variations and the use of different flow sensors contribute to the variation in AHI. Interestingly, AHI for sleep apnea is used in a manner analogous to blood pressure in systemic hypertension. However, this analogy is premature given the variations in the measurement methods, instrumentations, or determinants of adverse outcome.
Does CPAP Titration Require a Full-Night Polysomnography?
Studies have shown that a split-night study can identify an effective pressure in 78% of patients. However, a significant discrepancy in final CPAP pressure occurs when titration time is less than 3 h. In general, a measurable proportion will require changes in the prescription or the interface. Nevertheless, split-night studies are often necessary to reduce wait time and expedite treatment. The concern that split-night studies may decrease compliance was not substantiated by empirical evidence. However, there is some evidence that compliance may be less than optimal following split-night studies in patients with mild disease.
Selection of the Optimal Interface
CPAP interface is critical to the delivery of comfortable and effective CPAP pressure. Nasal masks are the most common type of CPAP interface. Oro-nasal, oral masks, or hybrid masks are available. Optimal fit is essential for patient comfort; poor fit may adversely affect compliance. Identifying the optimal interface requires patience, perseverance, and readiness for trial and error.
Which Device Is Best for My Patient?
This is the default mode in most patients. The prescribed CPAP pressure refers to the end-expiratory pressure. Most patients can be managed effectively with nasal CPAP with an effective pressure ranging from 5 to 15cmH2O. Most patients do not tolerate higher pressure, especially during expiration.
Bi-Level Positive Pressure
Bi-level positive pressure (BPAP) devices allow for independent titration of the inspiratory and expiratory pressures delivered to the patient. The positive pressure required to maintain upper airway patency is higher during inspiration relative to expiration owing to the presence of negative intraluminal pressure during inspiration. BPAP devices are more comfortable, especially with high-pressure requirements. BPAP devices are indicated in patients who have difficulty exhaling against the positive pressure, especially those who require high-pressure levels. In addition, patients who have nocturnal ventilatory failure or hypoventilation are also candidates for BPAP therapy. In this group, the term noninvasive positive pressure ventilation (NIPPV) is preferable since the device serves as a ventilatory support and not for upper airway patency per se.
The mask pressure changes throughout the night based on indirect indications of changing upper airway mechanics. The primary ‘signal’ driving the change varies among devices. Typically, third-party payers require intolerance to conventional CPAP or failure to determine optimal pressure for approval.
Positive pressure therapy requires high flow of ambient air to pass through the circuit. This may cause significant dryness of the nasal and pharyngeal mucosa. The result is mucosal edema, increased nasal resistance, diminished effectiveness, and lack of adherence with therapy.
Humidity in the breathing circuit can be increased by cold (passover) or heated humidification. There is evidence that heated humidification decreases nasal resistance and increases patient comfort. Humidification can alleviate dryness, heated humidification, but not passover. Humidification may enhance compliance with nasal CPAP.
Other data demonstrate improved comfort but no effect on long-term use.
Adherence with CPAP Therapy
Two terms are often used to describe patient’s fidelity to a treatment plan. The first term is compliance, which indicates following directives. The second term is adherence, which indicates an active role for the patient. Although the terms are different, different studies use them interchangeably.
Adherence with Chronic Therapy
Adherence to chronic therapy is suboptimal. For example, studies have shown that adherence with inhaled antiasthma therapy and oral antihypertensive is suboptimal in one-half of the patients. Interestingly, this is noted even in patients with chronic renal failure requiring hemodialysis; one-third of the patients missed treatments over a 6-month period.
Adherence to Nasal CPAP Therapy
Several prospective, objective studies have investigated patient compliance with nasal CPAP therapy for sleep apnea. There is a discrepancy between subjective and objective compliance. One-half of the patients are regular users and about one-half of the patients are intermittent users. The pattern of use is established by day 4 of treatment.
How Can We Optimize Compliance/Adherence?
The ability to identify determinants of adherence is essential to improving patient outcome. Studies have focused on several pertinent variables. For example, there is no evidence that positive pressure modalities influence adherence to CPAP as there is no evidence of improved compliance with bi-level devices. In contrast, patients receiving ‘intensive support’ have a higher likelihood of CPAP usage than standard support patients do. Likewise, patients who are self-referred have greater CPAP usage than those whose referral is initiated by bed partner. Similarly, patients with subjective complaints of excessive daytime sleepiness, however, show greater hours of CPAP use versus those with minimal daytime complaints. Finally, conclusive evidence of improved compliance with auto-CPAP, bi-level, self-titration, and humidification is also lacking. There is some evidence that psychological/educational interventions improve CPAP usage (The Cochrane Database of Systematic Reviews 2005). A recent study has identified three variables that predict compliance: female gender, improvement in ESS scores with CPAP, and advanced age. In summary, adherence to nasal CPAP therapy is a critical component of the management of sleep apnea. The available literature does not provide firm guidelines on optimizing adherence. Intensive support is the key intervention to enhance patient adherence to CPAP therapy.
Nasal CPAP is the treatment of choice for obstructive sleep apnea. Attention to detail is required to ensure that optimal mode, interface, and settings are provided for each patient. Despite its effectiveness, compliance with nasal CPAP therapy remains suboptimal. Intensive follow-up and support can improve compliance.