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Today’s simulation led to a discussion of the optimal therapy for treatment of the acutely dyspneic, diaphoretic, tachycardic, hypertensive distressed patient with acute pulmonary edema. After discussing the routine medical therapies — nitro SL, nitro gtt, ACE inhibitors, and furosemide — the discussion turned to the role of non-invasive ventilation (NIV).

It is no longer thought to be controversial that a trial of NIV is a crucial intervention for patients presenting with acute pulmonary edema prior to committing to endotracheal intubation (assuming that the patient has the mental status to tolerate NIV). The controversy lies in which modality of NIV to use — continuous positive airway pressure (CPAP) vs. bilevel non-invasive positive pressure ventilation (NIPPV), commonly referred to by the proprietary name BiPAP®.

Theoretically, it seems that BiPAP would be superior to CPAP since in addition to the basal positive pressure of CPAP which helps to stent open alveoli prone to collapse due to the weighty edema filling the lungs, there is an augmentation with positive pressure during inhalation to reduce the work of breathing. It would seem to follow that BiPAP would likely be superior to CPAP in reducing rates of intubation and possibly even mortality, while both modalities would be superior to just oxygen alone as they prevent derecruitment of alveoli.

Numerous trials have studied whether NIV is superior to oxygen alone when used to augment medical therapy for APE. Recent meta-analyses in JAMA (PMID: 16380593) and Critical Care (PMID: 16646987 and 16569254) have concluded that both CPAP and BiPAP are effective in the treatment of acute pulmonary edema with respect to the endpoints of mortality and subsequent need for intubation. However, all three meta-analyses find that BiPAP is not superior to CPAP with respect to either of these two endpoints.

All three articles make mention of a 1997 comparison study of BiPAP and CPAP by Mehta, et al. published in Critical Care Medicine (PMID: 9142026) which was terminated after interim analysis indicated that the patients randomized to the BiPAP arm of the study suffered greater myocardial infarction rates than those receiving CPAP (71% vs 31%). The articles mention that subsequent studies have failed to show this disparity and that the numbers in this trial we very small. Despite these assurances, none of the authors’ conclusions recommend a strategy of BiPAP over CPAP in lieu of the fact that the “physiological benefits [of BiPAP] did not translate into primary outcomes.” (JAMA). The JAMA article goes on to conclude that

the question of whether one technique offers advantage over the other and what subset of patients would benefit more with either one of these techniques remains unresolved.

The first of the Critical Care articles concludes that CPAP should be the NIV of choice because “from a practical point of view CPAP has been shown to be cheaper and easier to use” while the second recommends whichever modality is available.

ACEP also weighed in on this controversy in early 2007 when it published its Clinical Policy on patients presenting to the ED with acute heart failure syndromes. ACEP recommends the use of CPAP as a level B recommendation and downgrades the use of BiPAP to a level C recommendation citing the possible increase in myocardial infarction in conjunction with the lack of observed benefit over CPAP in the two main endpoints, mortality and reduction in the need for intubation.

Bottom line: It’s the smart and safe move to favor CPAP over BiPAP unless and until BiPAP is shown unequivocally to be more effective than CPAP with similar safety profile.

Given our recent guest speaker and spirited discussion, I thought it would be worthwhile to discuss a review from the September ‘06 Annals: the Use of Noninvasive Ventilation in ED Patients with Acute Cardiogenic Pulmonary Edema. We know ACEP will soon be publishing a new clinical policy on heart failure, it’ll be interesting to see how their interpretation of the literature squares with that of Collins et al.

And, even though we’ve all seen these masks in action, it’s probably worth repeating that CPAP is continuous positive airway pressure, regardless of inspiration or expiration. CPAP has been shown to reduce the work of breathing and decrease LV afterload, while maintaining cardiac index.

Noninvasive positive pressure support — sold under the trade name BiPAP – works similarly, but with less positive pressure during exhalation; it’s inspiratory pressure support plus PEEP (this variant called C-Flex kind of demonstrates it, but with an exhalation pression of zero). In theory, BiPAP should reduce the work of breathing even more than CPAP, and physiologically would seem to be of more benefit in obstruction airway disease (asthma, COPD).

For a more in-depth review of the mechanisms, indications and contraindications, check out this eMedicine article. For a practical guide with some key citations, see EMCrit.org. Basically, both CPAP and BiPAP work in part by raising intrathoracic pressure, which decreases preload AND afterload (which probably benefits patients with cardiac dysfunction). Neither therapy is definitive for cardiogenic pulmonary edema; they’re temporizing measures while your nitrates and diuretics kick in.

Previous studies have shown that CPAP decreases intubation rates in patients with acute cardiogenic pulmonary edema (ACPE). A systemic review in 1998 bolstered the claim, but that review included trials with non-ED patients.

The first big BiPAP study (Mehta, 1997) showed an increase in respiratory function and hemodynamic improvement compared to CPAP, but similar rates of mortality in hospital, and similar intubation rates. Plus, notably, the BiPAP patients experienced more MI! (it’s worth noting that that arm of the study received, by chance more patients complaining of substernal chest pain, and a 2004 study comparing CPAP and BiPAP showed no difference in MI).

Since then, many small trials between CPAP and BiPAP suggested no difference between these noninvasive ventilatory modes compared to standard-of-care, which other studies suggested a decrease in intubation and mortality. But this paper is the first systematic review of CPAP and BiPAP in ED ACPE patients.

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