How does CPAP improve oxygenation and ventilation with certain respiratory problems?

Continuous Positive Airway Pressure Therapy

CPAP remains the therapeutic mainstay for primary treatment of OSA. It serves as a pneumatic stent for the upper airway and is effective in reducing the physiologic abnormalities measured on polysomnography. Additionally, CPAP is thought to augment lung volumes and elicit a reflex that increases tone in the upper airway musculature. Overall, it has been shown to reduce AHI, improve quality of life, and reduce cardiovascular risk.

There are many manufacturers of CPAP devices and many interfaces that help maximize comfort with treatment. Expiratory pressure release (EPR) is available through several CPAP manufacturers. EPR does not seem to compromise the effectiveness of CPAP therapy and improves the patient’s sense of comfort with therapy, but it does not seem to systematically improve the level of adherence. Automatically adjusting positive airway pressure (APAP) is similar to CPAP, but instead of delivering a constant pressure, APAP adjusts delivered pressure breath to breath.

Bilevel respiratory-assist devices deliver alternating levels of positive airway pressure and may be considered an alternative therapeutic option when standard CPAP is not tolerated or when oxygen saturation is not raised sufficiently with standard CPAP. In some cases of severe OSA (in particular among patients with underlying pulmonary conditions), supplemental oxygen can be used in conjunction with CPAP therapy.

The main disadvantage with positive airway pressure treatment is poor compliance. CPAP adherence rates—defined as at least 4 hours per night—70% of nights are between only 39% and 50% overall CPAP compliance. Treatment is effective as long as the patient uses the device for the entire night, every night. APAP, CPAP desensitization, and the use of integrated heated humidifiers helped improve adherence to therapy.

Positive Pressure/Oxygen Therapy

CPAP delivered noninvasively through nasal or oronasal interface is a highly efficacious therapy for pediatric OSA, though long-term compliance is often poor. CPAP is typically reserved for moderate–severe OSA, not amenable to surgical or pharmacologic treatment. A properly fitted mask and adequate age-appropriate behavioral training is crucial to the success of CPAP therapy. The minimum daily duration of CPAP therapy required to reverse the consequences of OSA is unknown. In a prospective study of children with severe OSA, CPAP compliance monitored electronically after approximately 1 month of therapy revealed that CPAP usage >5 h per night was <50%. The reported side effects of CPAP in children include skin erythema, eye irritation, congestion, and rhinorrhea. CPAP appears to be equally efficacious compared to bilevel ventilation for the treatment of OSA in children.

Positive End-Expiratory Pressure

Positive end-expiratory pressure (PEEP) decreases the proportion of shunt units by recruiting the nonfunctional gas exchanging units, thereby improving functional residual capacity and arterial oxygenation. Additionally, by decreasing cardiac output (Q), PEEP produces a parallel fall in intrapulmonary shunt. However, even when Q is preserved, application of PEEP results in decreased shunt due to the redistribution of blood flow from shunt units to normal units because of alveolar recruitment.32 With constant Q, PEEP decreases venous admixture and increases mixed venous Po2. Yet PEEP tends to increase zone 1 and 2 regions within the lung, possibly increasing the vertical gradient of perfusion.

PEEP also affects dead space. Low levels of PEEP decrease dead space by reductions in shunt and mid-range VA/Q heterogeneity, but high levels of PEEP increase dead space. The increase in dead space with high PEEP results from overinflation of some lung units, leading to compression of capillaries and increases in anatomic dead space by distention.

Titration

EEP is used to relieve obstruction, if any is present. In most instances, pressure support is set as default – that is, PSmin is set at 3 cmH2O and PSmax is 15 cmH2O. In patients without any airway obstruction, the manufacturer recommends keeping the EEP at its minimum level of 4 cmH2O. The maximal pressure that can be generated by VPAP-AdaptSV is 25 cmH2O.

During the titration of the VPAP-AdaptSV device, any obstruction is relieved by increasing the EEP, while keeping the pressure support constant at default values; commonly, the initial EEP setting is 2 cmH2O below the CPAP level used to control obstruction during the CPAP titration study. With an ASV technology, the central apneas are initially converted to hypopneas. The manufacturer recommends waiting 40 min for the breathing to stabilize before increasing the EEP setting. After that time, central apneas are typically eliminated and any residual hypopneas are treated as obstructive events, with an increase in EEP.

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