Flow–circuit interaction drives continuous aerosol delivery during pediatric HFNC

Background High-flow nasal cannula (HFNC) is the most frequently used noninvasive respiratory support (NRS) modality for pediatric critical asthma and is often administered with continuous short-acting β-agonists. However, aerosol delivery is inversely related to flow, whereas higher HFNC flows can reduce work of breathing—complicating simultaneous optimization of drug delivery and support. Although infants may benefit from 1.5 to 2.0 L/kg/min, older children may benefit from lower flows (0.5–1.0 L/kg/min). In addition to flow, cannula and circuit caliber may influence delivery. This study quantified continuous aerosol delivery during HFNC across four cannula–circuit configurations at flows of 0.5–1.0 L/kg/min in a simulated pediatric model. Methods A pediatric manikin was connected to a lung simulator (V T 150 mL, rate 28/min, I:E 1:2.4, PIF 22 L/min) via a collecting filter. Albuterol (15 mg/h) was nebulized via a vibrating mesh nebulizer placed upstream of a heated humidifier (37°C). Four cannula–circuit configurations were tested at 11 and 22 L/min (0.5–1.0 L/kg/min). Continuous nebulization ran 3 h per condition with hourly collections (three repeated measurements per condition). Albuterol was quantified by spectrophotometry. One-way ANOVA) with Holm–Sidak tests, one-way RM ANOVA, and multiple linear regression (flow, circuit, cannula, flow × circuit) were used. Results Inhaled dose ranged 0.7–5.4% of nominal. Increasing flow from 0.5 to 1.0 L/kg/min reduced delivery by ∼50–60% across configurations. Adult circuits delivered 2–3 times higher inhaled dose than junior circuits at both flows (e.g., 5.2–5.4% vs 1.6–1.9% at 0.5 L/kg/min; 2.3–2.6% vs 0.7–0.9% at 1.0 L/kg/min, P < .001). Regression identified circuit type as the strongest independent predictor with a significant flow × circuit interaction; cannula-related differences were modest by comparison. Conclusions In this simulated pediatric model, inhaled dose during HFNC was driven primarily by the interaction between gas flow and circuit caliber, with cannula design contributing more modestly. At flows of 0.5 and 1.0 L/kg/min, adult-caliber circuits delivered approximately two- to threefold higher inhaled dose than junior circuits. Pairing an appropriately sized cannula with an adult-caliber circuit may result in substantially higher relative inhaled dose during HFNC therapy.