Understanding the pathophysiology of difficulty breathing and different methods of delivering airway pressure
By Steven C. LeCroy, MA, CRT, EMTP
In the late 1970s, legendary broadcaster Paul Harvey started each show with, “You know what the news is, in a minute you're going to hear the rest of the story.” That’s what this article is about, the rest of the story when it comes to single-pressure continuous positive airway pressure (CPAP) and double or bilevel pressure CPAP.
The value of CPAP therapy is well documented. If you listen to some clinicians, CPAP therapy is the cure for what ails you (though CPAP doesn’t cure anything). For the purposes of this article, single-pressure CPAP is defined as continuous positive airway pressure therapy with one level of pressure. Bilevel CPAP is continuous positive airway pressure therapy with two levels of pressure, called inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP).
Both therapies have their supporters. But when we go from talking the talk to walking the walk, what do we want and need to know?
- Are two levels of pressure better than one?
- What starting pressures should be used?
- What types of patients would respond better to each type of therapy?
- Does either therapy improve outcomes or reduce length of stay for patients in the hospital?
- For those that are non-clinical, can either therapy save money?
Bilevel vs. single-pressure therapy
Bilevel CPAP is commonly referred to as bilevel positive airway pressure (BiPAP), trademarked by Respironics or variable positive airway pressure (VPAP), trademarked by Resmed.
With all the confusion, it’s important to state up front that bilevel is not better than CPAP, it’s just different therapy. Bilevel may be more effective for some patients or even the preferred treatment for some patients with difficulty breathing, but not better.
Many, EMS agencies that lack bilevel capabilities use CPAP for all patients with difficulty breathing, regardless of the underlying cause.
Anecdotally, during the 2018 National Association of EMS Physicians conference (NAEMSP), I asked several EMS physicians if they routinely change therapy from CPAP to bilevel when patients arrive at the hospital. The answer was a resounding “yes.” When I pointed out that current research does not strongly support one over the other, each one reported that, in their experience, patients do better on bilevel therapy. However, when I asked respiratory therapists the same question, most felt the doctors were overutilizing bilevel therapy. How’s that for an enigma? If bilevel is better, why doesn’t EMS use bilevel? If it’s not better, why do physicians routinely change therapy? One aspect prohibiting EMS use of bilevel therapy is cost. Disposable CPAP devices, including bilevel devices, can be left with the patient in the ED, preventing cleaning delays, and eliminating the prohibitive cost. As far as physicians routinely using bilevel over CPAP, I believe some anecdotally think it’s better, others think that by the time the patient gets to the ED they are tired and need the extra help bilevel provides, and some may not know the difference.
Clinicians who have worked in EMS over time know that patients with difficulty breathing or shortness of breath don’t often call at the first sign of a problem. So, if we follow the thinking that the patient is tiring or having trouble doing the work, then the obvious question would be, which therapy is non-invasive ventilation (NIV)? Whether single-pressure CPAP is non-invasive is debated. I, for one, believe it’s not. Single-pressure CPAP is for a spontaneously breathing patients who can still do the work but would benefit if the work was easier. Many of these patients have an oxygenation issue and are best described as hypoxic, difficulty breathing patients. Bilevel is generally for spontaneously breathing patients who need oxygenation and some help doing the work, best described as a hypercapnic difficulty breathing patients.
CPAP makes the work easier, bilevel helps the patient do the work.
When a bilevel device cycles pressure from low (expiratory pressure or EPAP) to high (inspiratory pressure or IPAP), that kick-in pressure helps the patient inhale, making it a form of NIV. Some bilevel devices also offer a backup rate often described as a spontaneous or time bilevel mode, providing a breath if the patient becomes apneic. This capability is not seen with disposable bilevel CPAP devices. So, instead of pigeonholing a patient under difficulty breathing, why not assess which patients are hypoxic, hypercapnic or both. The best solution, especially in the prehospital setting or when there is a shortage of bilevel ventilators, might be a disposable device that offers both bilevel pressures and single-pressure CPAP.
What’s a beneficial starting pressure? I don’t believe there is a universal answer when it comes to using single-pressure CPAP or bilevel devices for difficulty breathing. When initiating CPAP of any type, I believe the safest approach is to start low and work your way up. It’s important to consider that there can be negative effects of increased airway pressure, such as a drop-in blood pressure or an increase in the work of breathing.
Using the lowest pressure that improves the patient’s respiratory status should be the goal. In the prehospital setting, research indicates the maximum pressure should be 10 cmH2O. In the hospital environment, higher pressures can be used due to a more controlled setting. With bilevel, the same thinking applies. Start with the lowest pressure that improves the patient’s respiratory status. Good initial bilevel starting pressures would be 10 cmH2O for the inspiratory pressure (IPAP) and 5 cmH2O for the expiratory pressure (EPAP). These numbers can always be adjusted based on patient assessment. Nevertheless, the difference between the IPAP and the EPAP pressures, often referred to as pressure support, should always be at least 5 cmH2O. Some of the advanced bilevel devices offer automatic settings that adjust pressure levels according to patient need.
Choosing the appropriate airway management therapy
CPAP in general has been shown to reduce intubations and reduce admissions to critical care units. These benefits consequently reduce patients’ length of stay in the hospital. Most studies that report outcomes from CPAP therapy do not differentiate between bilevel and single-level CPAP.
So, back to the basic question. Does bilevel produce a better outcome? The popular opinion appears to be yes, but what we don’t see is the supporting documentation. Most everyone agrees that positive pressure therapy works. What everyone doesn’t agree on is which form of therapy works best. I know one pulmonologist who believes all patients with difficulty breathing should be CPAP candidates, regardless of the underlying cause. What he doesn’t say is which type.
I’m not a supporter of “always” and “never” when it comes to medicine. A better sequence of questions would be:
- Should choosing the appropriate therapy be based on patient assessment?
- Is the patient hypoxic or hypercapnic or both?
- Do we understand the pathophysiology of different medical conditions that may benefit from not only positive pressure, but pressure delivered in different ways?
The jury is still out on bilevel making a difference, but with advances in technology and further studies, we may soon have our answer.
And now you know the rest of the story, at least for now.
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