Bronchoscopic evaluation of human airway surface liquid pH.

🎯 Background & Rationale

Airway surface liquid (ASL) pH plays a critical role in pulmonary physiology, influencing ciliary function, mucus viscosity, host defense, and airway reactivity. While low ASL pH is implicated in acute and chronic airway diseases (such as asthma exacerbations and cystic fibrosis), directly measuring human ASL in vivo has historically been challenging. This study addresses this clinical gap by systematically evaluating baseline ASL pH bronchoscopically in healthy controls and patients with stable asthma, while mapping the spatial transcriptomic expression of key pH-regulatory genes.

👥 Study Design & Population

This is a prospective, mechanistic bronchoscopic study comparing healthy controls (HC) and participants with stable asthma (SA).

• Population: 39 participants aged 18 to 50 years (16 HCs and 23 SAs) enrolled using criteria adapted from the Severe Asthma Research Program (SARP).
• Intervention/Procedures: Bronchoscopy using standardized protocols to measure ASL pH in proximal (second- to fourth-generation) and distal (~eighth- to tenth-generation) airways via two complementary assays (epithelial brushing and gastroesophageal pH probes), alongside endobronchial biopsies for spatial transcriptomics and gas aspiration for carbon dioxide tension measurement.

📈 Methodology & Rigor

• Data Handling & Assays: The study utilized dual assays (brushing vs. pH probe) across two separate clinical institutions (Cleveland and Indianapolis) to cross-validate measurements. Carbon dioxide tensions were assessed using specialized medical software.
• Transcriptomics: Endobronchial biopsies underwent spatial transcriptomics (Visium high-definition assays) using strict quality control metrics (DV200 >30%).
• Statistical Analysis: Data normality was tested, t-tests with Bonferroni corrections were applied for group comparisons, robust linear regression models assessed associations, and differential gene expression was evaluated using a zero-inflated negative binomial mixed-effects model.

🔬 Key Findings [or Planned Endpoints]

• Proximal Airway pH: In second- to fourth-generation airways across 39 participants (259 measurements), the mean proximal ASL pH was normally distributed at 6.8 ± 0.7, with no significant differences between HCs and SA participants, or between sexes.
• Methodological Comparison: Probe pH values (7.2 ± 0.5) were notably higher than brush pH values (6.5 ± 0.6), and probe measurements demonstrated significantly better intraparticipant reproducibility between left and right lungs (r = 0.898 vs. r = 0.231 for brushes).
• Distal Airway pH & CO2: In the eighth- to tenth-generation airways of SA participants, the mean pH was significantly higher at 7.4 ± 0.5. There was no correlation between distal airway pH and wedged CO₂ levels (mean 6.15%), suggesting distal epithelial buffering capacity against CO₂.
• Gene Expression: Spatial transcriptomics confirmed the abundant epithelial expression and enrichment of eight key pH-regulatory genes, including ATP12A, DUOXA1/2, LDHB, SLC26A4 (pendrin), CA12, GLS, and CFTR.

⚖️ Critical Appraisal

• Strengths: This is a pioneering study providing direct in vivo baseline normative human data for ASL pH, overcoming the limitations of indirect methods like exhaled breath condensate or sputum analysis. The dual-center design, multi-method approach (probes vs. brushes), and integration of spatial transcriptomics add substantial mechanistic depth.
• Limitations: The study highlights a lack of precision between the brushing and probe methods (with probes demonstrating superior reliability). Furthermore, the physical act of bronchoscopic measurement carries a theoretical risk of altering the local ASL environment. The distal airway cohort was also relatively small (n = 9 SA participants for distal pH measurements).

💡 The Clinical Bottom Line

This study establishes important baseline normative data for human airway surface liquid pH—approximately 6.8 in proximal airways and 7.4 in distal airways—and confirms that key pH-regulatory genes are actively expressed in the human airway epithelium. For the interventional pulmonologist and clinician, these findings lay the groundwork for future targeted inhalational therapies (such as carbonic anhydrase inhibitors or pH buffers) designed to modify ASL pH and treat obstructive airway diseases.