Noninvasive ventilation or continuous positive airway pressure in pulmonary edema patients with respiratory acidosis? Look at the bicarbonates

Abstract

Dear Editor, We read with interest the article of Nouira et al. [1] comparing noninvasive pressure support ventilation (NIPSV) and continuous positive airway pressure (CPAP) in the treatment of acute cardiogenic pulmonary edema (CPE). In the post hoc analysis carried out in hypercapnic patients, the authors found that these subjects seem not to respond successfully to NIPSV more than CPAP. These data support those by Bellone et al. [2], who failed to demonstrate a difference in efficacy between NIPSV and CPAP in hypercapnic CPE patients. We agree with the explanation of these findings given by Nouira and coworkers, who considered the application of positive end-expiratory pressure (PEEP) more relevant in CPE patients than relief of inspiratory workload by pressure support. However, some concerns may arise about those patients with chronic pump failure who develop acute on chronic respiratory acidosis because of an episode of CPE. Early recognition of these patients could be based on both past medical history and increased values of bicarbonates on basal blood gas analysis. From a pathophysiological point of view, CPE patients with acute on chronic respiratory acidosis could benefit more from inspiratory support delivered by NIPSV rather than a single level of PEEP delivered by CPAP. To explore this hypothesis, we retrospectively analyzed 378 consecutive CPE patients treated with CPAP in the Emergency Department [3]. There were 194 (51%) hypercapnic patients (PaCO2 [45 mmHg), among whom three groups of subjects were identified: those with acute respiratory acidosis on admission before CPAP treatment (pH\7.35 with PaCO2 [45 mmHg and 22 \ HCO3 \ 26 mEq/L), group A; those with ‘‘definite’’ acute on chronic respiratory acidosis (pH\7.35 with PaCO2 [45 mmHg and HCO3 [ 30 mEq/ L), group B; and those with ‘‘intermediate’’ acute on chronic respiratory acidosis (pH\7.35 with PaCO2 [45 mmHg and 26 B HCO3 B 30 mEq/L), group C. The number of patients with chronic obstructive pulmonary diseases was significantly higher in group B in comparison with group A (11 patients, 65% versus 19 patients, 25%, respectively, p = 0.002). We found patients in group B to have worse clinical outcomes in comparison with group A, with a significantly higher rate of switch from CPAP to NIPSV (Table 1). Group C accounted for 30 patients, of whom none had treatment failure. The evaluation of which of NIPSV and CPAP is preferable for CPE patients with acute on chronic respiratory acidosis should be explored in future clinical trails. However, we should acknowledge that the percentage of these subjects among all CPE patients who need noninvasive ventilation could be low, around 10% or less [3]. Thus, our hypothesis that NIPSV would be superior to CPAP in CPE patients with acute on chronic respiratory acidosis needs to be explored in large randomized controlled trials (RCTs). In the meantime, interesting data could also come from secondary analysis of studies performed by Nouira et al. as well as the 3CPO trialists [1, 4].