Heart Failure Beyond the Heart: Digestive-System Involvement and Its Diagnostic and Prognostic Implications—A Systematic Review

Background/Objectives: Heart failure (HF) is a systemic syndrome in which congestion, hypoperfusion, inflammation and neurohormonal activation may affect the liver, splanchnic circulation, intestinal barrier, pancreas, biliary system and nutritional state. This systematic review synthesised clinical evidence on digestive-system involvement in adults with established HF and examined its diagnostic and prognostic implications within a cautious cardio-gastrointestinal syndrome (CGIS) framework. Methods: The structured synthesis was based on an audited historical review register. The original project record reported 815 records before deduplication, although source-specific yields and the citation-level deduplication trail could not be reconstructed from the preserved files. A reproducibility–validation rerun was executed on 8 August 2026 in PubMed/MEDLINE, Scopus and Web of Science Core Collection, targeting literature through 1 July 2026; exact day-level cutoff reproducibility differed by database. The rerun recovered all ten benchmark reference-set reports but was not used to redefine the historical corpus and does not establish search exhaustiveness. The final 29-report register was re-audited for eligibility, design, analysed sample, principal findings and cohort overlap. Owing to substantial heterogeneity, findings were synthesised narratively. Results: 29 eligible reports represented approximately 26 cohort families. Hepatic and splanchnic involvement was the most mature evidence cluster, followed by intestinal congestion/barrier dysfunction. Pancreatic, biliary, microbiota and nutritional findings were less independently replicated. Liver biochemistry, composite hepatic scores and venous Doppler phenotypes showed marker-specific prognostic associations. Intestinal oedema was associated with impaired oral diuretic response. Direct pancreatic exocrine-insufficiency evidence was limited to faecal-elastase assessment. Conclusions: Digestive involvement in HF is clinically coherent but unevenly evidenced. CGIS is best regarded as a hypothesis-generating framework rather than a validated diagnostic entity, and prospective standardised studies are required before organ-specific biomarkers or imaging thresholds enter routine algorithms.