Aims: Acetazolamide inhibits proximal tubular sodium and bicarbonate re-absorption and improved decongestive response in acute heart failure in the ADVOR trial. It is unknown whether bicarbonate levels alter the decongestive response to acetazolamide., Methods and Results: This is a sub-analysis of the randomized, double-blind, placebo-controlled ADVOR trial that randomized 519 patients with acute heart failure and volume overload in a 1:1 ratio to intravenous acetazolamide (500 mg/day) or matching placebo on top of standardized intravenous loop diuretics (dose equivalent of twice oral maintenance dose). The primary endpoint was complete decongestion after 3 days of treatment (morning of day 4). Impact of baseline HCO3 levels on the treatment effect of acetazolamide was assessed. : Of the 519 enrolled patients, 516 (99.4%) had a baseline HCO3 measurement. Continuous HCO3 modelling illustrated a higher proportional treatment effect for acetazolamide if baseline HCO3 ≥ 27 mmol/l. A total of 234 (45%) had a baseline HCO3 ≥ 27 mmol/l. Randomization towards acetazolamide improved decongestive response over the entire range of baseline HCO3- levels (P = 0.004); however, patients with elevated baseline HCO3 exhibited a significant higher response to acetazolamide [primary endpoint: no vs. elevated HCO3; OR 1.37 (0.79-2.37) vs. OR 2.39 (1.35-4.22), P-interaction = 0.065), with higher proportional diuretic and natriuretic response (both P-interaction < 0.001), greater reduction in congestion score on consecutive days (treatment × time by HCO3-interaction <0.001) and length of stay (P-interaction = 0.019). The larger proportional treatment effect was mainly explained by the development of diminished decongestive response in the placebo arm (loop diuretics only), both with regard to reaching the primary endpoint of decongestion as well as reduction in congestion score. Development of elevated HCO3 further worsened decongestive response in the placebo arm (P-interaction = 0.041). A loop diuretic only strategy was associated with an increase in the HCO3 during the treatment phase which was prevented by acetazolamide (day 3: placebo 74.8% vs. acetazolamide 41.3%, P < 0.001)., Conclusion: Acetazolamide improves decongestive response over the entire range of HCO3- levels; however, the treatment response is magnified in patients with baseline or loop diuretic-induced elevated HCO3 (marker of proximal nephron NaHCO3 retention) by specifically counteracting this component of diuretic resistance., Competing Interests: Conflict of interest P.M.: received consultancy fees from CLS Vifor Pharma. F.H.V.: none. J.D.: none. P.N.: none. E.M.: FWO research grant—1SF6822N. S.N.A.: none. J.M.T.M.: received consultancy fees from Boehringer Ingelheim and a Personal grant from the Dutch Heart Foundation. L.H.: none. K.D.: speakers fee from Abbott, Boehringer ingelheim and Astrazeneca and fees for participation in DSMB of FIRE1, REPREIVE, SEQUANA Medical. A.M.: none. G.F.: grants from the European Commission, and speaker fees or DSMB fees from Bayer and Boehringer Ingelheim and fees from the Heart Failure Association. Serves as committee member for Medtronics, Bayer, Boehringer Ingelhein, Vifor, Amgen, Servier, Novartis. F.R.: not received personal payments by pharmaceutical companies or device manufacturers in the last 3 years. The Department of Cardiology (University Hospital of Zurich/University of Zurich) reports research, educational and/or travel grants from Abbott, Amgen, Astra Zeneca, Bayer, Berlin Heart, B. Braun, Biosense Webster, Biosensors Europe AG, Biotronik, BMS, Boehringer Ingelheim, Boston Scientific, Bracco, Cardinal Health Switzerland, Corteria, Daiichi, Diatools AG, Edwards Lifesciences, Fresenius, Guidant Europe NV (BS), Hamilton Health Sciences, Kaneka Corporation, Kantar, Labormedizinisches Zentrum, Medtronic, MSD, Mundipharma Medical Company, Novartis, Novo Nordisk, Orion, Pfizer, Quintiles Switzerland Sarl, Sahajanand IN, Sanofi, Sarstedt AG, Servier, SIS Medical, SSS International Clinical Research, Terumo Deutschland, Swiss National Foundation, Trama Solutions, V-Wave, Vascular Medical, Vifor, Wissens Plus, ZOLL. The research and educational grants do not impact on Prof. Ruschitzka’s personal remuneration. The Department of Cardiology received consultancy fees form AstraZeneca (IMC), Bayer, Boehringer Ingelheim, Citi Research, Klub Class, Novo Nordisk, Radcliffe Group, Stiftung Pfizer Forschungspreis, Vifor. The Department of Cardiology (University Hospital of Zurich/University of Zurich) reports speaker fees from Abbott, Amgen, AstraZeneca (A + Science AB), Bayer (At the Limits), Boehringer Ingelheim, Boston Scientific (CCE Services), Brigham and Women’s Hospital Boston, C.T.I. GmbH, Hôpitaux Universitaires des Genève (GECORE), Luzerner Kantonsspital, Sanofi-Aventis, Servier, Medscape (WebMD), Medtronic, Medworld, Novartis, Roche, Ruwag, Swiss Heart Failure Academy, The Hong Kong Heart Failure Society, Trama Solutions SL, Inselspital Bern, Charité—Universitätsmedizin Berlin (Medical Education Global Solutions), Romanian Society of Cardiology, ÖKG—Österreichische Gesellschaft für Kardiologie. W.H.W.T.: received grants from NIH and consultancy fees from Owkin, Relypsa, ProCardia, Sequana Medical, Cardiol Therapeutics, Genomics plc, Zehna Therapeutics, Renovacor, Whiteswell, Boston Scientific, Kiniksa Pharmaceuticals, CardiaTec Biosciences, Applier Therapeutics. M.D.: received consultancy fees from Astra Zeneca and Boehringer Ingelheim. W.M.: speakers fees from Medtronic, Abbott, Novartis, Vifor Pharma, AstraZenica, Boehringer Ingelheim, Pfizer, Novo Nordisk., (© The Author(s) 2023. Published by Oxford University Press on behalf of the European Society of Cardiology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)