Respiratory responses of mature horses to intravenous lobeline bolus.
Authors: Marlin D J, Roberts C A, Schroter R C, Lekeux P
Journal: Equine veterinary journal
Summary
Lobeline, a respiratory stimulant administered as an intravenous bolus, has clinical utility in equine practice for enhancing airflow during respiratory investigations, yet its detailed dose–response characteristics and reproducibility in horses remained poorly characterised. Marlin and colleagues measured respiratory mechanics and arterial blood gases in seven healthy mature horses receiving lobeline at doses ranging from 0.15 to 0.30 mg/kg, with repeated dosing protocols to assess reproducibility. Both tidal volume and breathing frequency increased with dose in a predictable manner; at the highest dose (0.30 mg/kg), peak inspiratory flow reached 41 l/s and peak expiratory flow 61 l/s, generating minute ventilation of 920 l/min alongside substantial arterial hyperoxygenation (PaO₂ 146 mmHg) and hypocapnia (PaCO₂ 20.6 mmHg). Responses proved highly reproducible with repeated administrations 15 minutes apart, and all horses tolerated the procedure well, though mild tremor occurred at higher doses and apnoea of approximately 40 seconds commonly followed the hyperpnoeic period. For practitioners utilising lobeline as a diagnostic aid—whether investigating suspected airway obstruction, assessing dynamic lung compliance, or performing endoscopic examinations during increased respiratory drive—this work confirms the drug's reliable and reproducible effect profile without significant adverse consequences in clinically healthy horses.
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Practical Takeaways
- •Lobeline is a safe, reproducible respiratory stimulant for functional airway testing in horses, useful for investigating both upper and lower airway disease without apparent adverse effects
- •Doses of 0.20-0.25 mg/kg bwt appear optimal for clinical investigation, producing substantial increases in airflow rates while minimizing side effects
- •The high reproducibility of response supports use in longitudinal assessments of airway function during treatment or disease monitoring
Key Findings
- •Lobeline doses of 0.15-0.30 mg/kg bwt produced dose-dependent increases in tidal volume and respiratory frequency, with peak minute ventilation of 920±99 l/min at 0.30 mg/kg bwt
- •Lobeline-induced hyperpnoea caused marked arterial blood gas changes within 90 seconds, including PaO2 increase from 104.0 to 146.0 mmHg and PaCO2 decrease from 50.6 to 20.6 mmHg
- •Lobeline-stimulated responses were highly reproducible with no significant variation between two administrations 15 minutes apart
- •All horses tolerated lobeline well with only mild side effects (tremor at highest dose, transient apnoea ~40 seconds post-hyperpnoea) and no adverse clinical effects