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July 1, 2011
The basics of CPAP
The Way We Breathe
An alveolus is a very small structure with a large amount of surface area, allowing gases to diffuse through it efficiently. The millions of alveolus we have in our lungs are called alveoli. Because of inherent moisture in the wall, surface tension builds that would collapse the alveoli if it weren't for the atmospheric pressure inside the lung and a liquid that reduces surface tension, called pulmonary surfactant.
To make air move, the diaphragm and the accessory muscles in the chest contract, causing the chest cavity to enlarge. This causes the pressure inside the lungs to drop and become negative in relation to the atmosphere, allowing air to passively flow in as we inhale. Upon expiration, the same muscles relax, causing the chest to shrink in volume. This causes the pressure to rise inside the chest, forcing air out of the lungs.
Air moves in and out of the alveoli through a series of tubes called bronchioles. These tubes are like the roots of a tree, combining together and becoming larger in diameter. The two largest tubes are called bronchi and they meet together at the carina. From there, the air moves out of the lower airway through the trachea and is eventually released into the atmosphere through the upper airway.
What Can Go Wrong
Another example is where the bronchioles become irritated due to exposure to things like pollen, causing them to inflame and swell, reducing the diameter of the tubes. This makes it harder for airflow to occur; the wheezing you hear upon auscultation during an asthma attack is the sound made as air moves through the constricted bronchioles. In addition, excessive mucous is produced, which clog the bronchioles even further. Long-term lung diseases such as emphysema and chronic bronchitis can flare up, or exacerbate, causing acute shortness of breath.
CPAP works the same way — a circuit creates extra pressure at the end of the patient's exhalation phase, keeping the bronchioles and alveoli open. The pressure isn't much — about 5 — 10 cm of water pressure is all that's necessary. In pulmonary edema, this pressure forces the excess fluid to leave the interstitial tissue surrounding the alveoli, allowing better exchange of gases. The bronchioles also increase in diameter, promoting better airflow down to the alveoli.
In order for CPAP to work, there must be a pressure-tight seal throughout the system. CPAP accomplishes this with a well-fitted mask that the patient wears over the mouth and nose. The circuit is attached to the mask and the patient is encouraged to breathe as normally as possible. Many patients will begin to feel relief within seconds to minutes of application, as their lungs begin to exchange gases more easily. Many providers report that the change in their patient's condition is so dramatic that they have to convince hospital staff of the original severity level!
There are a few precautions. The main concern is that CPAP can, and will, lower blood pressure. This makes sense: as pressure increases inside the lungs, it also increases pressure on the heart chambers and great vessels, causing cardiac output to drop. If the patient is hypotensive to begin with — i.e. the patient is experiencing cardiogenic shock with pulmonary edema — CPAP is not indicated. The patient must also be breathing and able to follow commands in order for CPAP to work effectively.
There are many devices available for prehospital CPAP use such as the O2-RESQ or the PortO2Vent. While it remains primarily an ALS procedure, states like Wisconsin, Tennessee, Oklahoma and others that authorize CPAP at the BLS level. It is an easy technology to understand and implement, and it can be a real benefit to patients with difficulty breathing.