Continuous positive airway pressure (CPAP) has become the standard of care for treating obstructive sleep apnea (OSA). It is estimated that 18 million Americans have symptoms of OSA and could be candidates for testing and treatment.

For patients, CPAP has several issues that must be addressed to maximize compliance. The most important is comfort of the interface, which should maintain a good seal over the patient?s airway to hold therapeutic pressure.

The second issue is comfort of the flow into the airway. The perception is that nasal CPAP flow generators create a virtual wind tunnel in the patient?s upper airway, especially if the patient?s mouth remains open. Humidification is needed to counteract the drying effect of this tornado in the nose if the mouth is open.

Is That How CPAP works?
CPAP works by creating a positive pressure in the patient’s airway high enough to keep the airway open at all levels of sleep. The pressure acts as a “pneumatic splint” to prevent collapse. An example might be to put enough air in your bicycle tire to prevent it from collapsing when you get on your bike. The difference in this situation is your bike tire is sealed closed and not continuously breathing or leaking air.

Instead, CPAP systems provide a continuous high flow to 1) allow an exhaust flow to exit the system close to or in the mask, to vent carbon dioxide that the patient is exhaling and 2) compensate for leaks around the mask or patient’s airway.

If there are no leaks, other than through the vent port, there is no additional flow going in and out of the patient’s airway—other than the flow of air the patient normally breathes. This is because the human respiratory system is a dead-end street. Gas going into the airway has no place to go except back out again. A good seal on a CPAP device creates a “deadhead” pressure in the lung and effectively holds the airway open, much like the inflated bicycle tire lifts the bicycle and keeps it from touching the pavement.

But CPAP systems are designed to continuously leak and still maintain pressure while they are leaking. CPAP systems are sophisticated blowers, which generate high flows that create therapeutic pressure while compensating for these leaks. But these leaks can have a major drying effect on the patient.

The body requires humidity of 44 mg of water in every liter of gas that enters the lungs.

The air that the blower uses is room air with the same humidity content as that of the room. When their mouths are closed, patients on nasal CPAP receive the same humidity while on the system as when they are off the system. Any additional humidity that is needed is normally provided by the patient’s nose, whether they are on or off CPAP.

The nose is the ideal humidification system. Its design gives a large surface area for inhaled gas to pass over, blood flow provides heat and moisture, and it even acts as a filter. The patient needs to be well hydrated for the nose to do its job, yet when the body is in balance, the nose is the humidifier of choice. If the patient is dehydrated, has excess secretions, is mouth breathing, or is in a very cold, dry environment (Minnesota in winter), a supplemental humidifier can assist the nose. Also, if there is a high flow of gas passing through the nose and mouth, supplemental humidification may be necessary. These “mouth leaks” are common with CPAP therapy and are the major reason humidifiers are routinely used. But if there is no mouth leak, is a humidifier necessary? Probably not. The patient’s nose will work just fine as the humidifier.

The body requires a humidity level of 44 mg of water in every liter of gas that enters the lungs at body temperature (37 degrees C or 98.6 degrees F). If the inhaled gas does not contain that humidity content, the nose or upper airway will compensate and add water to the gas.

If a humidifier can effectively add humidity to the inspired gas, the body will not need to compensate—which can prevent drying of secretions, drying of tissue, and patient discomfort.

How Does a Humidifier Work?
Humidifiers are simply kettles of water that evaporate. The evaporated water turns to vapor, which mixes with the gas. Various design features are used to increase the evaporation rate. All humidifiers used with CPAP therapy are passover type. Flow from the CPAP is directed to a humidifier chamber and then to the smooth bore tubing, the mask, and ultimately the patient. These humidifier chambers come in all sizes and shapes, can have baffling to increase surface area for gas/water interface, and may or may not be heated. So with all this variety, what are the important issues for humidification?

• Water temperature
• Humidity of the air
• Area of the water surface
• Temperature of the air
• Water currents convecting heat
• Airflow past the water surface

This graph shows the comparison of water surface area to absolute humidity.

The newest CPAP generators and humidifiers are very small. Patients may prefer these since they take up less space on the nightstand and travel conveniently. But size does make a difference because a small surface area does not allow for enough transfer of water to the gas at high flow rates. If there is baffling, there is a longer path for the gas to go through, which can add to the humidification.

Bench testing of several CPAP humidifiers shows the performance difference of each unit. Surface area is an important factor even when small humidifiers are heated.

RainOut
A challenging issue for humidification systems is to maintain heat and humidity through the breathing circuit to the patient’s airway. The gas in a heated breathing system is typically warmer than the room air; therefore, the tubing will be cooled by room temperature. As warm moist air comes in contact with a cooler tubing surface, water in the gas will rain out. This will limit the absolute humidity in the gas, plus create pooling of water in the breathing circuit. This water in the circuit has several negative effects on therapy.

Water in CPAP tubing will add resistance to flow, an annoying bubbling sound, and a source for bacterial contamination. If an auto-adjusting CPAP blower is used, the oscillation flow pattern created by water in the circuit can confuse the flow sensor to think there is a changing breathing pattern from the patient, and the unit may respond inappropriately. A solution to this situation would be to keep the CPAP tubing at a constant temperature.

One manufacturer has introduced a heated wire circuit for CPAP therapy. This will prevent rainout and maintain constant temperature in the circuit. The wires in the tubing act like the heated wires in a rear-window defogger by heating the tubing surface to prevent condensation. This effective humidification therapy comes at a price, yet if performance is the objective, the product should provide results.

Graph shows the differences between room humidity/cool and heated humidification.

Humidification has become the standard of care for CPAP therapy, and most providers start with humidifiers. If necessary, a humidifier can improve patient compliance, prevent complications, and possibly reduce pressure necessary to maintain an open airway. The clinician must do a complete patient assessment and understand the capabilities and limitations of the available humidifiers to provide the most effective therapy. In short, “dew” diligence is recommended.

The Basics
Humidification may not be needed for routine CPAP therapy if there is no inadvertent leak in the breathing system, the room humidity is adequate, and the patient’s respiratory tract is not compromised. If humidification of the inspired gas is attempted, a nonheated humidifier should have adequate surface area to add therapeutic humidity to the gas. Size and baffling of cool humidifier systems help allow effective use of surface area.

Heated humidifiers have the capability to add volume of water vapor to the inspired gas and are more effective in increasing water content of inspired air compared to cool humidifiers. But this improvement in humidity comes at a price of increased complexity and maintenance, and the annoyance of condensation of water vapor in the tubing. DP

RespiratoryStimulant Helps Sleep Apnea? Researchers at the Department of Veterans Affairs Medical Center and the Department of Medicine at the University of Cincinnati College of Medicine, Cincinnati, have shed new light on sleep apnea. Shahrokh Javaheri, MD, found that since sleep apnea is associated with heart failure, patients who took a single dose of acetazolamide?a mild diuretic and respiratory stimulant?exhibited less sleep apnea, improved blood oxygen levels, and fewer daytime symptoms of sleepiness. The results of the double-blind, placebo-controlled study appear in the January 2006 issue of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society. ?An important finding of the double-blind study was the significant improvement in patient perception of improved sleep quality, waking up more refreshed, with less daytime fatigue and sleepiness while taking acetazolamide, compared with placebo,? said Javaheri.

Robert McCoy, RRT, is managing director of Valley Inspired Products Inc, Apple Valley, Minn. McCoy can be reached via e-mail: bmccoy@inspiredrc.com.  

Bob Virag is a principle in Alveoli Medical LLC, a Chesterfield, MO-based company that does testing, research, and consulting.