CPAP w/o Pulmonary Edema

[johnmedic]

Looking for some advice on CPAP outside of Pulm Edema, are there any indications that you folks use it for?

CPAP of course works wonders for pulmonary edema.. but what about patients with low tidal volumes? I'm a Medic student & have been told by most medics that by increasing PEEP you won't increase tidal volume enough to make it worth a hoot.. & I concur, intubation & bronchodilators are definitive treatment.

But today I just got questioned by a couple precepting medics on why I wouldn't consider CPAP, they advised me that the patient (expiratory wheezes, sats ~80, low tidal volume, Chronic Bronchitis w/o any pulm edema) would have benefitted by CPAP's ability to increase tidal volume if Duoneb treatment had been ineffective. (it was)

Thoughts?

BTW if this is a scenario question let me know & I'll move the thread.

 

[usafmedic45]

would have benefitted by CPAP's ability to increase tidal volume if Duoneb treatment had been ineffective

It's not that cut and dry. Remember the problem with COPD is not an inability to inhale, it's an inability to exhale in most cases (since it's an obstructive and not restrictive respiratory disease, hence the "O" in it). In emphysema, it's due primarily to the loss of elastic recoil through tissue destruction. In asthma and chronic bronchitis, it's primary due to the loss of airway diameter associated with bronchospasm, bronchiolar wall inflammation and the accumulation of mucus. That's why when you see the lungs of one of these folks at autopsy, the lungs are classically hyperinflated. Clinically, this manifests itself as tachypnea with small tidal volumes, flattening of the diaphragmatic arches on a chest x-ray, and the increased anteroposterior diameter more commonly called a "barrel chest" in very chronic patients.

The indication for CPAP in ventilatory failure (which is NOT the same thing as a COPD exacerbation) is to reduce the work of breathing and not to increase the tidal volumes.

but what about patients with low tidal volumes? I'm a Medic student & have been told by most medics that by increasing PEEP you won't increase tidal volume enough to make it worth a hoot.. & I concur, intubation & bronchodilators are definitive treatment.

Five things....
1. Most patients with pulmonary edema do have low tidal volumes.
2. Increasing the tidal volume (without changing something else, like slowing the breathing down) in someone who is already breath stacking (which is exactly what happens in a lot of COPD related ventilatory failures is not necessarily a good thing.
3. This patient's "low tidal volume" may be their normal. In moderate to severe COPD patients, it's not unusual to see them have a baseline tidal volume of 300-400 mL or less. Also, even experienced RTs have a hard time judging a tidal volume accurately without the use of equipment that most ambulances do not have.
4. Most medics don't know their *** from a hole in the ground when it comes to mechanical ventilation which is what CPAP/PEEP.
5. CPAP and PEEP are physiologically the same thing. The only difference is whether the patient is breathing or if their ventilations are being delivered mechanically (by BVM or a ventilator). In the former, it's CPAP. In the latter it's PEEP. And no, you can't use a PEEP valve on a BVM to deliver CPAP.


In the case you're describing, chances are some inhaled and systemic steroids and adequate oxygen therapy (up to and including a NRB if necessary) would have solved the problem without the need to break out the CPAP.

 

[18G]

The main reason that CPAP can be effective in severe COPD and asthma is that it reduces the overall workload of breathing and helps overcome the increase in positive pressure that is present within the chest.

A patient with air trapping has an increased intrathoracic pressure. In order for air to move at all a pressure gradient needs to be created meaning the positive pressure in the chest must be overcome with a higher pressure outside the chest. Think of it as resistance. How do you get something to move against a resistance? You overpower it. In the case of COPD/asthma, you "over power" the positive pressure present in the chest to make air flow in. A patient has to work really, really hard and use the accessory muscles and tons of extra energy to make air flow in and out when experiencing an exacerbation.

By using CPAP we can create a positive pressure for the patient which makes it easier to breathe. The patient can save their energy and let the CPAP do the work so they don't have to be the ones creating the higher pressure needed to breathe. CPAP in my opinion is much more preferred over intubation which essentially just increases the airway resistance and does nothing to fix the underlying problem.

While these patients do have problems with air trapping (auto-peep), CPAP works on expiration by exerting PEEP to hold the airways open so they do not collapse. All the effort to exhale can make the airways narrow and prone to collapse which then will need extra pressure to re-expand them on inspiration thus taking more energy and more oxygen expense that the patient doesn't have.

CPAP makes nebulized meds more effective in these patient groups and while CPAP doesn't primarily increase tidal volume, it does buy time for medications to work which improve tidal volume and at the same time reducing energy expenditure and making it easier to breathe.

Dr. Keith Wesley explains this as part of the JEMS presentations online regarding CPAP and he highly encourages CPAP in COPD and asthma.

 

[johnmedic]

Excellent, thank you both! Usafmedic great points & info, I had to reread your post but it covered alot! & 18g you cleared up my confusion on how increasing PEEP in a patient with expiratory wheezing will decrease work-of-breathing.

 

[usafmedic45]

Dr. Keith Wesley explains this as part of the JEMS presentations online regarding CPAP and he highly encourages CPAP in COPD and asthma.

Although this case doesn't sound like the type of case where the use of CPAP is necessary. Maybe if the patient didn't respond to multiple doses of bronchodilators, etc. But tachypnea and a moderately low SpO2 isn't going to make me immediately assume the patient is in impending ventilatory failure, especially given how insufficient the oxygen therapy for COPD patients is in EMS due to the lingering myth of hypoxic drive.

18g you cleared up my confusion on how increasing PEEP in a patient with expiratory wheezing will decrease work-of-breathing.

Remember that you can increase the PEEP to the point where the patient can't breathe. This is especially a problem when you have a patient who is air trapping, breathing rapidly and you have a high PEEP setting. It's just like most other treatments, you can "overdose" the patient on PEEP or CPAP.

 

[18G]

Remember that you can increase the PEEP to the point where the patient can't breathe. This is especially a problem when you have a patient who is air trapping, breathing rapidly and you have a high PEEP setting. It's just like most other treatments, you can "overdose" the patient on PEEP or CPAP.

True.. true... CPAP is just an adjunct therapy to buy time to allow medications to open the patients airways which will help resolve the air trapping. CPAP alone with no concomitant medications would prob end up being really detrimental and end up like you said.

 

[usafmedic45]

CPAP is just an adjunct therapy to buy time to allow medications to open the patients airways which will help resolve the air trapping.

Well, it's useful in severe refractory cases. It's not obviously something you want to break out on every patient who doesn't respond to that first dose of medication. This is especially the case with bronchodilators where the medication should be kicking in about the time most of us are pulling up to the hospital. Remember, albuterol takes about 15-20 minutes to start to have any broncodilatory effect. Any relief reported before that is placebo effect or due to the saline.

 

[Akulahawk]

CPAP with a tri-mix or Heliox as a breathing gas gets kind of expensive... and the patients can sound kind of funny.

 

[Journey]

Any relief reported before that is placebo effect or due to the saline.

It would not necessarily be a placebo effect since you are probably administering the medication by oxygen which is also a drug. An increase in the PaO2 can make a patient feel somewhat better.

CPAP with a tri-mix or Heliox as a breathing gas gets kind of expensive... and the patients can sound kind of funny.

You might be confusing the use of heliox for COPD with the treatment of asthma. It is now recognized that although asthma is considered COPD it is to be treated differently. At this time there is not sufficient evidence that COPD is benefited by the use of heliox. Asthma however is benefited by heliox. If the patients are sick enough to require heliox, I doubt if they will be doing much talking for anybody's amusement.