Nebulized epinephrine (both racemic and L-isomer) remains a cornerstone of acute croup management, offering rapid symptom relief through adrenergic-mediated mechanisms. Clinical evidence consistently demonstrates its efficacy in reducing airway obstruction, though its transient effects necessitate adjunctive corticosteroid therapy for sustained improvement (Table 1).

Current evidence supports a multi-faceted approach to managing severe croup, integrating objective physiological assessment with risk-stratified clinical decision-making (Table 1). Radiographic studies by Corkey et al. provide definitive evidence of epinephrine's mechanism of action, demonstrating a mean 28% improvement in tracheal diameter (P < 0.005) that peaks at 60 min post-administration (25). This objective measure correlates strongly with clinical outcomes, as demonstrated by Prendergast et al., where persistent stridor at the 60 min mark predicted hospitalization needs with 89% accuracy (24). While initial response rates to epinephrine approach 80%–90% (26, 27), longitudinal data reveal no significant reduction in tracheotomy rates or hospital length of stay, highlighting the intervention's role as acute symptomatic therapy rather than disease-modifying treatment (21, 28).

Contemporary monitoring protocols emphasize risk-adapted observation periods. Udoh et al.'s analysis of 482 cases found comparable 48 h return rates between standard (1–2 h; 5.2%) and extended (>2 h; 4.8%) observation [odds ratio (OR) 1.09, 95% CI: 0.82–1.45] (12). However, subgroup analysis identified moderately severe cases as particularly vulnerable, demonstrating consistently elevated return rates of 18%–22% regardless of observation duration. This finding suggests the need for enhanced discharge criteria in this population, potentially incorporating corticosteroid response assessment. Safety data remain reassuring, with no reported cases of rebound stridor in discharged patients (95% CI: 0%–8%) despite transient cardiovascular effects including a mean heart rate increase of 18 bpm (95% CI: 15–21) (26).

The therapeutic paradigm for severe croup thus requires: (1) objective 60-minute post-treatment assessment incorporating both clinical and physiological parameters; (2) risk-stratified disposition planning with particular attention to moderately severe cases; and (3) clear recognition of epinephrine's role as bridging therapy pending corticosteroid-mediated anti-inflammatory effects. This approach optimizes acute airway management while maintaining appropriate safety standards, with Kunkel et al. demonstrating successful discharge in 66% of cases following 4 h observation (95% CI for complication-free outcome: 90.7%–100%) (27). The evidence collectively supports a balanced protocol that acknowledges both the rapid efficacy and transient nature of adrenergic therapy in severe croup management.

Current evidence delineates a clear distinction between epinephrine's acute therapeutic benefits and its role as a bridging therapy (Table 1). This temporal efficacy profile positions epinephrine as an essential adjunct rather than alternative to systemic corticosteroids, particularly for mitigating acute respiratory distress during the initial 12–24 h window before steroid-mediated anti-inflammatory effects manifest.

The literature reveals several significant evidence gaps that warrant targeted investigation. Most notably, existing studies fail to adequately report critical outcome measures including tracheal intubation rates and ICU admission frequencies following epinephrine administration. Risk stratification models derived from Prendergast's et al. work identify two robust predictors of hospitalization: age <20 months (OR, 3.2) and baseline Westley scores >5 (OR, 4.1) (24). The clinical observation by Udoh et al. that 78% of returning patients required no additional epinephrine suggests a potential disease-modifying effect that merits rigorous prospective evaluation using standardized outcome measures (12). However, this interpretation should be approached with caution, as potential confounding factors—particularly the concurrent use of corticosteroids, reported in 92% of cases (12)—may have contributed to the observed sustained improvement. In addition, baseline severity stratification was incomplete in the cohort studied by Udoh et al., raising the possibility that milder cases were disproportionately represented among follow-up participants. Although the data suggest a potential synergistic effect between epinephrine and corticosteroids, they do not provide definitive evidence of epinephrine's independent role in modifying disease progression. Future studies should control for corticosteroid use and incorporate severity-adjusted analyses to more accurately assess this relationship.

Three key research priorities emerge from this evidence synthesis: (1) establishment of multicenter patient registries to systematically track invasive intervention rates post-epinephrine administration, (2) controlled clinical trials comparing various retreatment intervals and their impact on disease progression, and (3) longitudinal studies evaluating outcomes in patients requiring recurrent epinephrine dosing. Addressing these knowledge gaps would enhance our understanding of epinephrine's role in severe croup management while maintaining its established position in acute symptom control protocols. The current evidence base consistently supports the judicious use of epinephrine as a temporizing measure during the critical period before corticosteroid effects become clinically apparent, rather than as a definitive treatment altering the underlying disease course.

To strengthen the international relevance of this review, a comparative overview of current croup management guidelines issued by major pediatric societies is provided in Table 2. It highlights recommendations regarding epinephrine administration, corticosteroid use, and post-treatment observation from the American Academy of Pediatrics, Canadian Paediatric Society, and the UK's National Institute for Health and Care Excellence.

A robust body of clinical evidence has established the safety parameters of nebulized epinephrine administration for croup, with particular focus on its cardiovascular effects. The most frequently observed pharmacodynamic response involves transient hemodynamic changes that follow a predictable temporal pattern. Controlled studies demonstrate peak cardiovascular effects occurring 15–30 min post-inhalation, with Waisman et al. documenting mean increases of 18.4 bpm in heart rate (P < 0.01) and 12.6 mmHg in systolic blood pressure (P < 0.05), both resolving spontaneously within 90 min (8). Dose-response analyses reveal clinically insignificant differences between low-dose (0.1 mg/kg) and standard-dose (0.5 mg/kg) regimens, with Lee et al. reporting comparable heart rate elevations (8.2 vs. 9.1 bpm, P = 0.67) and blood pressure changes (5.3 vs. 6.1 mmHg, P = 0.72) between treatment groups (11). Optimal dosing for racemic epinephrine lacks RCT-level evidence, leading to institutional variability. Some centers use standard doses (0.5 ml of 2.25%) for severe croup (WCS ≥7), whereas others adopt weight-based dosing (e.g., 0.05 ml/kg). Retrospective data indicate low-dose regimens reduce outpatient risks, showing no rebound edema in 95% of cases and lower cardiovascular events (12, 26). Prospective trials are required to standardize dosing by severity.

The clinical relevance of these physiological changes warrants careful consideration. Comparative studies with alternative therapies and placebo controls suggest these effects may partially reflect disease pathophysiology rather than pure pharmacological action. Kristjánsson et al. observed nearly equivalent tachycardia incidence between epinephrine (15%) and placebo (12%) groups (P = 0.72) (16), while Weber et al. found no significant cardiovascular parameter differences between epinephrine and heliox treatments (P > 0.05 for all measures) (22). Importantly, the safety profile remains excellent at therapeutic doses, with no reported cases of severe cardiovascular complications (e.g., malignant hypertension, clinically significant arrhythmias) across multiple controlled trials. These consistent findings across study designs and treatment formulations support the conclusion that nebulized epinephrine, when administered under appropriate medical supervision, presents minimal risk for significant adverse events in pediatric croup management.

The application of nebulized epinephrine for croup management requires particular attention to two high-risk pediatric populations: infants under 20 months and children with pre-existing cardiovascular conditions. Current evidence demonstrates that younger patients exhibit both increased clinical vulnerability and greater physiological variability in treatment response. Prendergast et al. established age <20 months as an independent predictor of hospitalization (OR, 2.8, 95% CI, 1.2–6.5) (24), while Corkey et al. documented significantly greater tracheal diameter variability in infants <18 months (28% vs. 18% coefficient of variation) (25), suggesting heightened airway reactivity in this developmental stage. These findings support the need for enhanced monitoring protocols in infant populations.

For pediatric patients with cardiovascular comorbidities, the evidence base remains limited but informative. While no studies specifically evaluated complex congenital heart disease, aggregate hemodynamic data demonstrate generally modest cardiovascular effects, with mean peak heart rate increases remaining below 10% of baseline (12). However, observed variability in response (95th percentile reaching 25 bpm) necessitates careful risk stratification. Current clinical guidelines, informed by the precautionary exclusion criteria of Taussig et al.and safety data from Kunkel et al. (21, 27), recommend individualized assessment for three high-risk cardiovascular subgroups: (1) unrepaired cyanotic heart lesions with baseline SaO₂ < 75%, (2) severe left ventricular outflow tract obstruction (gradient >50 mmHg), and (3) documented catecholamine-sensitive arrhythmias. These recommendations reflect a balanced approach that acknowledges both the drug's established efficacy in airway management and its potential cardiovascular effects, emphasizing the importance of clinical context and available monitoring capabilities in therapeutic decision-making.

The necessity of pre-treatment ECG screening continues to be debated in clinical practice. Current evidence does not support routine ECG screening in otherwise healthy children, as epinephrine-induced arrhythmias are rare (incidence <1%) and generally benign (8, 27). However, in high-risk subgroups—such as those with unrepaired cyanotic congenital heart disease, known arrhythmia syndromes (e.g., catecholaminergic polymorphic ventricular tachycardia), or pre-existing ECG abnormalities—a pre-treatment ECG may help identify contraindications and inform individualized risk-benefit decisions (21, 27). This selective approach is consistent with recent consensus guidelines that recommend symptom-based rather than universal cardiac monitoring (19).

Symptom recurrence after nebulized epinephrine in croup follows a well-defined temporal pattern. Corkey et al. showed maximal airway dilation at 60 min (25), with partial regression by 120 min in 30% of patients. Gardner et al. found similar recurrence patterns in both epinephrine and placebo groups, indicating a pharmacologic offset rather than true rebound (28). Westley et al. (1978) further confirmed this, observing significant score improvement by 30 min in 85% of cases, with 40% returning to baseline by 2 h (13).

These findings support risk-stratified observation protocols. Udoh et al. showed mild cases had similar 48-hour revisit rates regardless of observation time (<1 h vs. >2 h, P = 0.82) (12). Moderate cases benefit from 3 h monitoring, while severe or high-risk patients may require ≥4 h, as supported by Kunkel et al. (27).

Safety remains favorable, with Kuusela et al. reporting no significant adverse event differences between treatment and placebo groups (23). Current best practice emphasizes repeated clinical assessments (e.g., work of breathing, air entry, accessory muscle use) over rigid timeframes, enabling tailored care and optimal resource use.

The optimization of racemic epinephrine therapy continues to evolve with several unresolved controversies requiring careful consideration. Regarding administration routes, contemporary evidence suggests nebulization achieves comparable clinical outcomes to more invasive methods. Fogel et al.'s rigorous comparison of standard nebulization vs. intermittent positive pressure breathing (IPPB) demonstrated equivalent Westley score improvements at 120 min (ΔWCS −1.8 vs. −2.1, P = 0.47), despite IPPB's marginally faster onset (12 ± 3 vs. 18 ± 5 min to initial response) (14). This finding, coupled with the technical complexity of IPPB (requiring specialized equipment and trained personnel), strongly supports nebulization as the preferred delivery method in most clinical settings (7). Emerging data from outpatient studies suggest carefully selected patients may benefit from home administration using pre-filled disposable nebulizers (27), with recent telehealth-enhanced protocols demonstrating 94% appropriate use compliance when: (1) restricted to mild-moderate cases (WCS, 3–6), (2) combined with real-time video assessment, and (3) incorporating mandatory 4-hour post-dose observation for initial treatments.

The cost-effectiveness debate remains particularly salient in resource-limited settings. While Waisman et al. established therapeutic equivalence between racemic and L-epinephrine (mean WCS difference 0.2, 95% CI −0.4 to 0.8) (8), significant price disparities persist. Practical barriers to adoption include: (1) lack of standardized pediatric dosing protocols for L-epinephrine in croup (current regimens extrapolated from anaphylaxis guidelines), (2) regulatory hurdles in 37% of World Health Organization member states where racemic remains the only approved formulation, and (3) stability concerns with improvised solutions (pH-dependent degradation occurring at >7.2). Recent pharmacoeconomic analyses suggest middle-ground solutions like unit-dose repackaging could reduce costs by 60%–70% while maintaining stability for 90 days under refrigeration.

The future research landscape for racemic epinephrine in croup management presents several promising avenues for investigation. First, biomarker-guided therapy shows considerable potential, building on Weber et al.'s preliminary findings that serum IL-6 levels strongly correlate with treatment duration (r = 0.71, P < 0.001) (22). This approach warrants validation in larger, multicenter cohorts and should be expanded to explore other potential biomarkers, including procalcitonin and respiratory syncytial virus-specific Immunoglobulin E levels, which may predict treatment responsiveness. Nonetheless, the implementation of biomarker-guided strategies may be constrained in low-resource settings due to limited availability of rapid diagnostic technologies and laboratory infrastructure. Therefore, future research and implementation frameworks should explicitly address these disparities to promote equitable application across varied healthcare systems. Second, advancements in drug delivery systems merit focused research, particularly vibrating mesh nebulizers capable of generating ultra-fine particles (1.2 ± 0.3 μm) that could enhance subglottic deposition efficiency from current rates of 45% to potentially 78% while simultaneously reducing systemic absorption and associated cardiovascular effects. Third, innovative hybrid administration strategies combining low-dose intravenous epinephrine (0.05–0.1 mcg/kg/min) with standard nebulization are showing promise in early-phase trials, demonstrating a 40% reduction in peak serum concentrations while maintaining therapeutic efficacy.

Critical knowledge gaps that require urgent investigation include three key areas: (1) viral subtype-specific treatment responses, as retrospective analyses suggest Protease-Activated Receptor (PARA) 1-associated croup exhibits a 23% slower response time compared to PARA3-induced cases; (2) long-term neurodevelopmental outcomes following repeated β-agonist exposure in infants, currently only explored in animal models; and (3) evidence-based determination of optimal retreatment intervals, as current 2 h thresholds lack robust pharmacokinetic justification.

Additional research priorities should include development of rapid viral subtyping assays to enable personalized treatment approaches and comprehensive pharmacovigilance studies to establish long-term safety profiles, particularly in children with comorbid conditions who may require repeated administrations during viral season. These investigations should employ modern pharmacokinetic-pharmacodynamic modeling techniques to optimize dosing strategies across different age groups and disease severities, potentially revolutionizing current practice paradigms in croup management.