Why Water Reservoirs Are Used with PAP Therapy
Heated humidification with water reservoirs is used with PAP therapy primarily to reduce uncomfortable side effects—including dry mouth, nasal dryness, nasal congestion, throat irritation, and nosebleeds—that can lead patients to discontinue treatment. 1
Primary Mechanism and Clinical Benefits
The fundamental problem is that PAP devices deliver pressurized air at significantly reduced relative humidity compared to ambient air, and this drying effect worsens as therapeutic pressures increase. 1, 2
The American Academy of Sleep Medicine recommends clinicians generally use heated humidification with PAP devices to reduce side effects, based on meta-analyses of 9 randomized controlled trials demonstrating clinically significant reductions in:
- Dry mouth and throat 1
- Nasal discharge and congestion 1
- Dry nose and nosebleeds 1
- Sinus pain, headache, and sore throat 1
- Hoarse voice and reduced smell 1
Research confirms that CPAP alone drops relative humidity from 80% during normal breathing to 63% with mouth closed and 39% with mouth open, while heated humidification restores humidity to 81-82% even during mouth leaks. 3
Impact on Treatment Adherence
While heated humidification significantly improves comfort, the evidence shows no clinically significant improvement in PAP adherence, sleepiness, or quality of life compared to PAP without humidification. 1 This means the water reservoir functions primarily as a side-effect management tool rather than directly improving therapeutic outcomes.
When Humidification Is Most Critical
Heated humidification should be available for all patients undergoing PAP titration and is specifically indicated when patients complain of significant oral or nasal dryness. 1
High-risk scenarios requiring humidification include:
- High therapeutic pressures (especially NPPV/BiPAP settings), since increasing pressure further lowers delivered relative humidity 1, 2
- High intentional or unintentional mask leak, which exacerbates mucosal drying 1
- Dry climates (American southwest, winter in cold climates) 1
- Patients with baseline nasal congestion or history of severe mucosal dryness 1
Technical Considerations
The water reservoir works through a heated humidifier that warms water to produce water vapor (not aerosolized droplets), which is then delivered through the PAP circuit. 4 Research demonstrates that convection-type humidifiers produce only water vapor without aerosolizing water, meaning tap water is safe to use—sterile or distilled water is unnecessary for infection prevention. 4
Common Pitfall: "Rain Out"
Condensation of water vapor into the PAP circuit, face, and airway ("rain out") can occur with humidification, potentially deterring use. 1 This is mitigated by using heated tubing, which maintains temperature throughout the circuit and prevents condensation. 2
Cost and Maintenance Burden
Humidification increases ongoing costs (purchasing water, heated hoses) and maintenance requirements (regular cleaning of the chamber to prevent mineral buildup). 1, 2 However, the American Academy of Sleep Medicine judges these burdens acceptable given the significant reduction in side effects that may otherwise lead to treatment discontinuation. 1
Clinical Algorithm for Humidification Use
- Make heated humidification available for all PAP patients 1, 2
- Initiate humidification at PAP start for patients with:
- Add humidification during therapy if patients develop dry mouth, nasal dryness, congestion, or throat irritation 1
- Adjust temperature settings before discontinuing if thermal discomfort occurs 2
- Use heated tubing to prevent rain out 2
Do not assume dry mouth or nasal discomfort is normal with PAP therapy—these symptoms indicate correctable issues including inadequate humidification, mask leak, or inappropriate settings. 2