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Patient Care

The Truth About CPAP

In 2013 the journal Prehospital and Disaster Medicine published a literature review of all English-language studies of CPAP in the prehospital setting published through May 2012.1 The results were as follows:

The evidence suggests the use of CPAP therapy in the prehospital environment may be beneficial to patients with acute pulmonary edema, as it can potentially decrease the need for endotracheal intubation, improve vital signs during transport to hospital, and improve short-term mortality.

That same year two separate studies found no benefit to the provision of CPAP in the prehospital setting.2,3 Two years later the Canadian Journal of Emergency Medicine published a study that reiterated those findings:4 

Despite the robust in-hospital data supporting its use, we could not find benefit from CPAP in our prehospital setting with respect to morbidity, mortality, and length of stay. 

The purpose of this article is to discuss the mechanics of gas flow and pressure to the end of formulating a hypothesis that explains the conflicting conclusions on prehospital CPAP performance since its inception in the late 1990s.5 

A Noninvasive Advance

CPAP is an acronym for continuous positive airway pressure. It was first used on humans in 1980 by Australian physician Colin Sullivan as a treatment for sleep apnea. As the medical community became familiar with CPAP, the intervention proved to be advantageous not just to sleep apnea patients but also to people suffering from bronchospasm and pulmonary edema.

CPAP was beneficial to patients approaching respiratory failure because it provided ventilatory assistance similar to that accomplished by bag-valve mask or automated ventilator. Since CPAP did not require a patient to be either chemically or physiologically obtunded, it was categorized as noninvasive ventilation.

During inhalation the positive airway pressure delivered by CPAP reduces the amount of work a patient must do to increase the volume of their thoracic cavity. By providing a gas pressure that exceeds atmospheric pressure, CPAP pushes air past areas of bronchospasm or other obstruction so gas exchange at the alveoli requires less work on behalf of the patient. Although CPAP can provide 100% O2, research has revealed that patients may receive equivalent benefit from CPAP delivering less than 100% FiO2.6 

During exhalation CPAP maintains airway pressure in excess of atmospheric pressure via positive end expiratory pressure (PEEP). The ongoing positive pressure continues to work against the force of bronchospasm, aiding in the removal of CO2 from the lungs while helping keep alveoli inflated.

Why Low-Flow?

In the prehospital setting CPAP is limited primarily by the quantity of compressed oxygen required to operate the equipment. Early prehospital CPAP routinely expended compressed oxygen at a rate of 60 lpm. The reason for such high expenditure was to ensure the patient received a volume of gas that surpassed that demanded by their inspiratory rate. Meeting a patient’s rate of inspiration is not as simple as matching their minute volume; rather it’s the rate at which a patient inhales their tidal volume that must be surpassed. 

Consider, for example, that a patient with a 500-cc tidal volume who takes one full second to inhale is taking in gas at a rate of 30 lpm. Thus in order for this patient to receive continuous positive airway pressure, gas must be flowing through the CPAP device at a rate of no less than 30 lpm. Given the propensity for a patient in respiratory distress to take less than one second to inhale, it is reasonable to provide O2 for CPAP at a rate of 60 lpm in order to ensure their volumetric inspiratory demand is met.

Contemporary prehospital CPAP devices are marketed by touting relatively low rates of consumption of compressed medical O2 and focusing on FiO2 and PEEP, with little or no mention of inspiratory pressure.7,8 Such rhetoric is in conflict with the name continuous positive airway pressure, for all low-flow CPAP devices increase gas volume by decreasing the pressure of the gas delivered to the patient.

Boyle’s law describes the relationship between gas volume and gas pressure: When the volume of a gas is increased, its pressure is decreased, and vice versa. This can be observed every time we inhale. During inhalation we increase the volume of our thoracic cavity, thereby lowering the pressure of the gas within that cavity and subsequently sucking in air from the atmosphere.

Low-flow CPAP devices increase the volume of O2 going to the patient by forcing compressed O2 through a narrowed passageway. By forcing gas through a narrowing passage, the velocity of the gas is increased, and pressure is decreased. This is Bernoulli’s principle. When Bernoulli’s principle occurs within a narrowing section of tubing, it is termed the Venturi effect. As one may suspect, a passage engineered to force a fluid through a narrowed section of tubing is called a Venturi. A Venturi is extremely reliable for delivering controlled amounts of fluid to a designated area, which is why they are used to provide specific FiO2 to patients via the Venturi mask and precise volumes of gasoline to an engine through the Venturi within a carburetor.

The low pressure of the gas flowing through a low-flow CPAP device may in turn be utilized to suck in higher-pressure air from the atmosphere. This “sucking in” is entrainment. There is only one problem with entrainment when it comes to low-flow CPAP: When CPAP is in use, the CPAP mask is sealed against the patient’s face and occludes the flow of oxygen, rather than permitting it to expand out into the atmosphere as it does with a Venturi mask.

Since the downstream portion of the gas flow through a low-flow CPAP mask is occluded by the patient’s face, the pressure of the gas inside the mask rises to match atmospheric pressure as flow is stifled and rerouted out through the entrainment port when the patient is not inhaling. 

This is to say that when in use, the only time entrainment is present in low-flow CPAP is during inhalation. Those who have used high-flow and low-flow CPAP devices have likely noticed how much easier it is to attain a seal on the patient with a low-flow CPAP mask. This is because the only time the pressure inside the low-flow CPAP mask is higher than the atmospheric pressure is when the patient exhales. The patient using low-flow CPAP receives gas volume not by positive pressure but by the negative pressure created by their own labor, just as if they were using a nonrebreather mask.

The Lowdown on Low-Flow

From this discussion one may hypothesize that the reason reviews of prehospital CPAP have gone from being generally positive to finding it generally lacking in benefit is due to the prehospital movement away from high-flow (60+ lpm) to low-flow CPAP. Low-flow CPAP does not provide positive pressure to the patient during inhalation and is essentially nothing more than PEEP with supplemental O2

Furthermore, recent peer-reviewed studies of CPAP have found significant variance in the volume of gas delivered by CPAP devices manufactured by different companies.9,10 While the benefits of PEEP are numerous, even early literature (1988) found “to minimize work of breathing, airway pressure should not fluctuate during spontaneous breathing with continuous positive airway pressure.”11 

Furthermore, the fact remains that PEEP alone does not constitute continuous positive airway pressure. It is a disservice to our patients to tell them they’re receiving CPAP when they are not receiving positive airway pressure during their most labor-intensive moments of breathing.  


1. Williams B, Boyle M, Robertson N, Giddings C. When pressure is positive: A literature review of the prehospital use of continuous positive airway pressure. Preh Dis Med, 2013; 28(1): 52–60.

2. Cheskes S, Turner L, Thomson S, Aljerian N. The impact of prehospital continuous positive airway pressure on the rate of intubation and mortality from acute out-of-hospital respiratory emergencies. Preh Emerg Care, 2013; 17(4): 435–41.

3. Aguilar SA, Lee J, Dunford JV, Castillo E, et al. Assessment of the addition of prehospital continuous positive airway pressure (CPAP) to an urban emergency medical services (EMS) system in persons with severe respiratory distress. J Emerg Med, 2013; 45(2): 210–9.

4. Willmore A, Dionne R, Maloney J, et al. Effectiveness and safety of a prehospital program of continuous positive airway pressure (CPAP) in an urban setting. Clinical J Emerg Med, 2015; 17(6): 609–16.

5. Hastings D, Monahan J, Gray C, et al. CPAP. A supportive adjunct for congestive heart failure in the prehospital setting. J Emerg Med Serv, 1998; 23(9): 58–65.

6. Bledsoe BE, Anderson E, Hodnick R, et al. Low-fractional oxygen concentration continuous positive airway pressure is effective in the prehospital setting. Preh Emerg Care, 2011; 16(2): 217–21.

7. O-Two Medical Technologies. Single Use CPAP,

8. Pulmodyne. GO-PAP,

9. Brusasco C, Corradi F, De Ferrari A, et al. CPAP devices for emergency prehospital use: A bench study. Resp Care, 2015; 60(12): 1,777–85.

10. Vargas M, Marra A, Vivona L, et al. Performances of CPAP devices with an oronasal mask. Resp Care, 2018; 63(8): 1,033–9.

11. Banner MJ, Downs JB, Kirby RR, et al. Effects of expiratory flow resistance on inspiratory work of breathing. Chest, 1988; 93(4): 795–9.

Jim Miller  is a North Carolina paramedic and has been in EMS for 20 years.



Submitted on 05/02/2019

1) With such significant differences, how did the FDA allow these devices to market?
2) Does the author have a recommendation for the best CPAPs for prehospital use in light of this?


1.) This is a good question, however I am not familiar with either the process of registering a medical product with the FDA or with the rigors and methods by which the FDA carries out field sampling and testing. I did phone the FDA field office in Atlanta three times in 2015 and attempted to file a complaint regarding “a medical device which calls itself CPAP but isn’t”, but was informed that complaints about medical devices can only be filed by patients who have used the devices-- not by the practitioners who implement them professionally. The person at the FDA branch office did take down my number (all three times) and stated that they would “forward my concerns to a manager who would get back to me”, but no return call was ever made. That same year I was successful in filing a false advertising claim with the Federal Trade Commission (complaint reference #65857883), but I have heard nothing about the complaint since that time.

2.) What is “best” in EMS has always seemed to me to be an emotional conversation rather than a rational one, and I would not presume to know what is best for any EMS system other than my own. I can say with 100% confidence however, that if a patient is told that they are receiving CPAP and they are continuously NOT receiving positive airway pressure during inhalation, that they are being lied to.


Submitted byjmiller on 05/02/2019

thank you

Submitted bypaamedictoto on 05/16/2019

I am writing in reference to your Cpap article “The Truth About Cpap” in the May 2019, Vol. 48, NO. 5 EMS World Magazine issue. As a Registered Respiratory Therapist and a Paramedic with 24 of experience, I felt the needed to address a few concerns about your article.

In your article one statement you made about the Cpap in the “Pre-Hospital” setting is a “Low Flow” device? You mention it provides 60 liters per minute of flow. I was confused on what you were trying to convey in your article. I was confused and felt there is a misunderstanding/communication of information that was given to you after reading the article.

Cpap, Bipap, AVAPS are modes of non-invasive therapies that are used in the hospitals, LTACHs, nursing homes and home care settings for Acute and Chronic respiratory problems. They provide Positive Airway Pressure therapy (PAP). These devices are measured in Centimeters of Waters (cmH20), not flow. Its acronym can be inter changeable as (PAP/CPAP/PEEP/EPAP) depending on how you are using it. These devices also provide other measurements, parameters, and comfort measures such as: Inspiratory Time, Rise Time, Trigger, RAMP, EPR (Exhalation Pressure Relief) aka C-flex. These devices provide two types of sources. 1) Positive Airway Pressures which range anywhere from 5-50 cmH20 and 2) It also provides oxygen (Fi02) from 21% to 100%. These devices are used on patients for such medical issues as: COPD, Asthma, OSA, OHS (Obesity Hypoventilation Syndrome), NMD (Neuromuscular diseases), CHF, secretion management, Hypercarbia, and Hypoxia again in both the Acute and Chronic.

Many if not all Pre-Hospital Cpaps gets its pressures from a single source which is oxygen. The patient is receiving 100% Fi02 by a 50 psi regulator. The patient is receiving a max of 10 cmH20 (PEEP), not 60 liters of flow.

Work of Breathing (WOB) correlates with Minute Ventilation or Minute Volume, Respiratory Rate, and Tidal Volume so understanding the definition can help.

Minute Ventilation or Minute Volume (MV/VE) is the amount air moved in and out in one minute, which is measured in liters. It is a very important parameter in respiratory care due to it relationship between carbon dioxide levels in the blood, proper removal of CO2, and proper oxygenation. Minute Ventilation/Volume is treated in the clinical practice of respiratory therapy as a flow rate. During an average adults normal breathing 12 breaths a minute (about 1 breath every 5 seconds), their tidal volumes are about 0.5 liters of air per breath. The volume of air breaths each minute (MV) equals 6 liters. (0.5 Liters x 12 breaths/min = 6 liter/minute).
Tidal Volume (VT or TV) Is the amount or volume of air moved in and out of the lungs during breathing. It is measured in Liters.
Respiratory Rate or Frequency (RR/F) is the number of active breaths taken in a minute.

Besides Cpap therapy, there is a device called the High Flow Nasal Cannula (HFNC) ie: Opti Flow, Airvo, Vapotherm, these devices provide “Flows” at 10-60 Liters per minute, onto top of providing a separate oxygen (Fi02). There are also other Nasal Cannula High Flow devices that can provide flows up to 15 LPM from just an oxygen source/regulator, there is no separate “Air” flow mixtures. It’s Fi02 is based off of the LPM ie: 10 lpm, would be about 60% Fi02 and about 1 cmH20 (PEEP). In general, for every 10 liters of flow, equals 1 cmH20 (PAP/CPAP/PEEP/EPAP). So, if you were providing a patient with 60 liters of flow on a HFNC, this patient would be getting roughly 6 cmH20 or PEEP.

Enclosing, I have used both CPAP, Bipap, AVAPS and HFNC therapy in both the prehospital and hospital environment and YES, it really works very well in the recovery of severe respiratory distress in patients of all ages. These therapies work well enough that patients avoid being intubated and in return are being discharged within few hours in the emergency room or quicker discharge from the hospital avoiding prolonged stays due to the ICU admission.

Respectfully Submitted,
Joey Toto, RRT, MPS, BS, EMT-P

I apologize for the poor communication on my part. You may notice that both of the last two sentences of the article start with “furthermore”, and that was a bit bush league of me as well.

Three separate studies have concluded that CPAP on ambulances is not as effective as CPAP in the hospital. I would like to believe that these findings are a reflection of the inefficacy of the equipment rather than of the practitioners who use it.

By “low-flow” CPAP I mean non-powered, single source oxygen delivery devices which are advertised as CPAP and operate within a 5 - 25 liter-per-minute range of compressed oxygen flow. These are devices in which you literally disconnect the nonrebreather mask from the Christmas tree and plug the CPAP device in its place, possibly followed by reducing the flow of oxygen at the regulator. There are three devices like this that I know of: the Boussignac CPAP System1, the Rescuer® Disposable Emergency CPAP System2, and the Pulmodyne GoPAP3.

A patient with a tidal volume of 500mL who takes one second to inhale is using the muscles of their thoracic cavity to displace ambient atmospheric pressure at a rate of 500mL per second.

An O2 bottle attached to the Rescuer® CPAP System is displacing ambient atmospheric pressure at a rate between 5 - 15Lpm, depending on how the user sets the O2 regulator. This is a flow rate of 83.3 - 250mL per second-- far less than what our 500mL-per-second patient demands. The Boussignac is better with a range of 15-25Lpm (250 - 417mL per second), but still falls short of the demands of our hypothetical patient’s needs.

You can increase the volume of a gas, but only at the expense of either lowering the gas pressure or adding energy to the system. None of these devices adds energy to the compressed gas which flows through it. Also, discussion of increasing gas volume inside a CPAP mask is somewhat moot because the volume of the mask itself is constant.

All of these devices do provide PEEP, and that is good, but they aren’t CPAP.


Submitted byEric Gjerde on 05/21/2019

This is an excellent review of the current disposable CPAP devices on the market. Good work Mr. Miller!
In full disclosure, I am an ex Respiratory Therapist and now President CEO of Airon Corporation. Airon designs and manufacture CPAP systems and ventilators with CPAP for hospitals and EMS. Our key technology is ICU quality CPAP put into transport devices. These are demand flow systems that work just like ICU ventilators - you set the CPAP level and the system provides whatever flow is needed to meet patient demand, while maintaining the desired CPAP during both inspiration and expiration. Our systems can provide up to 140 L/min flow during inspiration, if that is what the patient wants. Then the flow drops to 10 L'min during expiration so there is no expiratory flow resistance.
We have been selling our MACS CPAP system to EMS for 12 years with over 2,000 units in use around the world. My biggest disappointment has been the huge adoption of disposable CPAP by EMS. As Mr. Miller describes, these are NOT CPAP systems, but rather very high work of breathing PEEP systems. How they ever got past the FDA is beyond me. It was a serious effort to get the FDA to allow us to call our systems as true CPAP devices.
If you really want to support your patients respiratory efforts with CPAP, please choose a professional grade true CPAP system, not those terrible disposable CPAP masks.
Eric Gjerde RRT, MBA

Submitted byjseiverth on 09/03/2019

This article has compelled me to look a bit more into some of the products offered in our area. Interestingly, of the two brands evaluated thus far (3 + Models), all claim to sustain peak inspiratory flows (inspiratory flow rates) ranging from 65 lpm to 120 lpm. This claim is contrary to what this article suggests. However, when measuring via a manometer, all had significant pressure drops during inhalation. The problem with the measurement, I suppose, would be that the manometer itself is not measuring "in line" with the flow of the inspiratory pressure. Perhaps the true inspiratory measurement would be best measured at the end of inhalation, prior to exhalation.

Unfortunately, it does not appear that there has been a study performed for any of the products we are evaluating. It would be nice to substantiate the IFR claims of these products compared to the products that have already been tested in References 9 and 10.

And I must also say, while the RRT comments above are accurate statements, he seems to be missing the point of this article. These devices can only supply so much flow and any less than what the patient demands is a dangerous situation. To the patient, it probably feels like trying to breath in through a straw.

As I continue to ponder the importance of this article, perhaps these devices should be called EPAP devices, not CPAP. There seems to be no conceivable way that these devices can produce enough inspiratory flow to maintain a constant pressure upon inhalation as is in exhalation.

One way to perhaps overcome this drop in inspiratory pressure is to flow a cannula under the mask to help stabilize pressure during inhalation. This seems to work well with the device that we utilize.

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