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.
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, http://otwo.com/emergency-cpap/o_two-single-use-cpap/.