Noninvasive Ventilation in Pulmonary Edema: CPAP or BiPAP?

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.