High-Flow Nasal Cannula Outside the ICU: A Systematic Review and Meta-Analysis

Background: Use of high-flow nasal cannula (HFNC) expanded from ICUs to internal medicine/respiratory wards during and after the COVID-19 pandemic, but safety and effectiveness in non-ICU settings remain uncertain. Methods: We performed a systematic review and meta-analysis of adults (≥18 years) initiated on HFNC in non-ICU wards. Primary outcomes were in-hospital (or 28-day) mortality and ICU transfer; where available, we compared mortality for HFNC vs. conventional oxygen therapy (COT) in do-not-intubate (DNI) cohorts. Observational studies and trials were eligible. Random-effects models synthesized proportions and risk ratios; risk of bias (ROBINS-I/RoB 2) and certainty (GRADE) were assessed. Results: Ten studies met the inclusion criteria for any-ward HFNC; subsets contributed data to pooled analyses. Across all non-ICU wards (general wards plus step-up IMCU/HDU), pooled mortality was 14.0% (95% CI 4.6-35.5; I² ≈ 92%). Pooled ICU transfer after ward/step-up HFNC start was 20.0% (95% CI 6.3-48.1; I² ≈ 97%). Restricted to internal medicine/respiratory wards, pooled mortality was 19.8% (95% CI 7.1-44.2; I² ≈ 95%) and ICU transfer 31.2% (95% CI 9.9-65.0; I² ≈ 97%). In step-up units (IMCU/HDU), ICU transfer appeared lower and less variable (22.0% [95% CI 16.5-28.8]; I² ≈ 10%), suggesting environment-dependent outcomes. In a multicenter DNI COVID-19 cohort, HFNC vs. COT showed no clear mortality difference (RR ≈ 0.90, 95% CI 0.75-1.08; adjusted OR ≈ 0.72, 95% CI 0.34-1.54). Certainty of evidence for all critical outcomes was very low due to observational design, high inconsistency, and imprecision. Conclusions: HFNC outside the ICU is feasible, but it is related to nontrivial mortality and frequent escalation-particularly on general wards-while step-up units demonstrate more reproducible trajectories. Outcomes appear strongly conditioned by care environment, staffing, monitoring, and escalation pathways. Given very low certainty and substantial heterogeneity, institutions should pair ward HFNC with protocolized reassessment and rapid response/ICU outreach, and future research should prospectively compare ward HFNC pathways against optimized COT/NIV using standardized outcomes.