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Humidity Matters: The Science of Why

Humidity Matters: The Science of Why

Do I really need to use a humidifier with my CPAP machine?

Yes, you really should use a humidifier.
Using a humidifier adds a bit of friction to the CPAP routine. Setting up takes extra steps and there is additional daily maintenance of the equipment. Humidification adds cost and some 3rd party payers do not recognize its importance, but humidification has been proven in numerous studies to increase adherence with sleep therapy. Heated humidification, in particular, has been shown to augment the sense of waking up well-rested compared to cool air humidification and no humidification.

Common complications of CPAP include a dry mouth, nose, and throat. Some people complain that CPAP causes them to sneeze or gives them a stuffy nose. Others observe that CPAP makes their nose run or feel like the CPAP causes their nose to burn. Some people even get nose bleeds. Humidifiers prevent and minimize these unpleasant side effects.

 

The science of respiratory humidification

Humidification is the process of adding water vapor to the air. A major function of the nose is to warm, moisturize, and purify air before it reaches the lungs. As air enters the nose, it swirls through a roller coaster like structure called the turbinates to get warmed, humdified, and purified. The nasal turbinates contain three shell like bones called conchae that are covered with soft tissue and mucosa. Tiny nose hairs called cilia blow back and forth to filter out allergens and dust. The mouth and throat also add moisture to the air we breathe.

Although the nose and upper airway are not bypassed during CPAP therapy, the rate of air flow is much higher than during normal breathing. The nose and upper airway simply cannot keep up with the pace and are unable to adequately warm, moisturize and purify all the air being breathed.

When a full face mask is used, the CPAP machine puts out even higher flow rates than when a nasal mask is used. Leaks around the mask cause the machine to increase the flow rate. Higher CPAP settings also necessitate more flow.

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What happens when the airways don't get enough humidity?

Alveoli are the microscopic units in the lungs where gas exchange occurs. At the alveoli, oxygen crosses over to the blood stream and carbon dioxide crosses over from the blood stream to be exhaled. The alveoli function optimally at a humidity level of 44 mg of water vapor per liter of air. The entire upper airway (nose, mouth, throat) normally provides 75% of the needed humidity to air flow before it arrives in the lungs.   

Under normal conditions the nose and upper airway humidify air flow to about 33 mg/L. As air passes through the trachea, the humidification levels reach 36-40 mg/L. The passage of air through the numerous tiny airway branches called the bronchioles folds in the last bit of humidity needed to hit the optimal level of 44 mg/L.

When the humidification level falls below 25 mg/L, the upper and lower airways start to suffer from mucosal dysfunction. Symptoms of mucosal dysfunction include sore throat, dry nose and dry mouth.  Physiologically, the nasal cilia slow down and blood flow to the airways slows. There may also be a decrease in normal nasal secretions, that causes further drying of the nose.

In the absence of enough humidity, the nose and upper airway get wounded and are going to be less effective at warming, humidifying, and purifying air during normal breathing.

Once you develop nasal dryness, your body responds by breathing more through the mouth. This causes a leak in the nasal mask and the CPAP machine increases flow to compensate. Then your mouth gets dry and the problem perpetuates.

The nose is an important filter for dust, pollen, germs, and allergens. Due to the higher flow rates generated by the CPAP machine, the nose will have a more difficult time filtering everything out. More allergens, dust, and germs will get by the nose. Using CPAP every night without a humidifier can impair the nose’s filtering function further. It’s a bit of a vicious cycle.

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Humidifying CPAP

Hopefully, I’ve convinced you that there is real science behind the recommendation to add humidity to your CPAP machine. Now, let’s look at the available mechanisms for humidifying your airway during CPAP or BIPAP therapy.

When you start shopping, three options emerge:

  • Cool humidification (passive)
  • Heated Humidification (active and passive)
  • Waterless humidification with Heat Moisture Exchangers

Heated humidification is the best choice. It’s been demonstrated to reduce symptoms of dryness and its use has been correlated in several studies with users waking up more refreshed than with non-heated humidifiers.

Heat Moisture Exchangers (HME)s with CPAP and BIPAP are not ideal. HMEs have a potential role to play to meet needs for a night or two but should not be your go to plan.

When cool humidification is used, air flow travels across a reservoir of sterile water and picks up water vapor before it reaches your mask. Cool humidification “pushes” less water vapor into the gas flow than heated humidification does.

Heated humidifiers warm the sterile water. When gas passes through the water reservoir, water vapor from the warm water crosses into the cool air more abundantly. Air flow exits the humidifier reservoir, warm and moist, but it will start to cool as it travels through the circuit to the mask and you. The cool down triggers condensation (think morning dew) inside the CPAP tubing. That cool down is often described as “rain out” and it can be problematic: water may drip into the mask, or pool in the circuit and add resistance to the CPAP flow.

Pairing a heated humidifier with a heated circuit can reduce rain out. Heated circuits have a heated coil inside that keeps the circuit warm so that the temperature gradient between the humidifier and you is reduced to preventing the gas from cooling down (and releasing moisture).

Many current generation machines also have a climate control option for their heated humidifiers.  When used with a heated circuit, the machine monitors room temperature. The temperature of the heated coil and the heater are automatically adjusted to optimize for the room’s conditions to minimize rain out.

Heat Moisture Exchangers (HME) are a waterless humidity device that are sometimes called artificial noses. An HME captures exhaled moisture from exhaled air (which is presumably moister than the air being pushed in by the CPAP machine). The exhaled humidity is trapped inside a porous or spongy material. During inspiration, the airflow from the CPAP machine passes over the trapped humidity and picks some of it up. HMEs are brilliant little devices and do not require an external water source, but an HME is not nearly as efficient as humidifiers at raising the humidity level.

An HME plugs in between the mask and the CPAP circuit. An HME adds weight to the circuit and may tug downward on the mask and make it more difficult to get a good seal. Some people complain that an HME may smell bad after some use. An HME typically needs to be replaced at least every thirty days. 

HMEs are still a better choice than no humidification. If you are going camping for a few days, for example, an HME may be a great choice. Since an HME adds a bit of dead space and resistance to the circuit, they can cause carbon dioxide levels to go up a bit. HMEs should be avoided or used with caution if you have any other lung disease issues, such as asthma or COPD.

If you are planning to use an HME on vacation, try it ahead of time for a night or two at home to you determine if you have any adverse effects or if it makes it more difficult to breathe.

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Trying out heated humidification

If you have a heated humidifier, consider picking up heated tubing.  You may be surprised what a difference it makes.  If you are content with your cool air, passive humidifier, kudos to you for using it! Think about trying some heated humidity, though, next time you have a respiratory illness (cold, flu, etc.).  You may be surprised at how much better it makes you feel.

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