Respiratory responses of mature horses to intravenous lobeline bolus

The respiratory stimulant lobeline has been used in equine clinical practic e to increase inspiratory and expiratory airflow rates at rest in order to facilitate investigation of both lower and upper airway function. Some of t he responses to lobeline in the pony have been reported, but the detailed t ime course, effect of dose, possible side effects and reproducibility assoc iated with lobeline administration have not been described in the horse. Re spiratory airflow rates and oesophageal pressure were measured with a Fleis ch No. 5 pneumotachometer and lightweight facemask and a microtip pressure transducer catheter, respectively. The output of the Fleisch pneunotachomet er was calibrated for flow rates up to +/- 70 Ys. Seven mature horses with no clinical signs of respiratory disease were studied. Investigations were conducted to determine: (1) the responses to different doses of lobeline (0 .15, 0.20, 0.25 and 0.30 mg/kg bwt) as a rapid i.v. bolus (6 horses); (2) a rterial blood gases during and after lobeline administration (0.20 mg/kg bw t; 3 horses); and (3) the reproducibility of lobeline-stimulated hyperpnoea (5 horses; 2 doses of 0.20 mg/kg bwt lobeline, 15 min apart). All horses t olerated the lobeline-stimulated hyperpnoea well, although one always cough ed or snorted at the onset. Mild tremor was noted following the highest dos e in several horses. Apnoea of approximately 40 s was common after the hype rpnoea. Both tidal volune (V-T) and frequency (f(R)) increased with lobelin e dose, During peak hyperpnoea at a dose of 0.30 mg/kg bwt, peak inspired n ow rate (PIF), peak expired flow rate (PEF) and minute ventilation ((V)over dot (E)) were mean +/- s.e. 41 +/- 5 Ys, 61 +/- 10 Ys and 920 +/- 99 1/min , respectively. The hyperpnoea also caused marked changes in arterial Pao(2 ), Paco(2) and pHa at 90 s after lobeline (0.20 mg/kg bwt) administration ( mean +/- s.e. 146.0 +/- 6.9 mmHg, 20.6 +/- 0.8 mmHg and 7.707 +/- 0.020, re spectively) compared to at rest (mean tt s.e. 104.0 +/- 4.0 mmHg, 50.6 +/- 2.8 mmHg and 7.432 +/- 0.012). Dynamic lung compliance (Ca,) was unaltered by lobeline administration. The lobeline-induced hyperpnoea was highly repr oducible, with no significant difference in any of the parameters during 2 stimulations 15 min apart. Loobeine induced highly reproducible responses w ithout any apparent adverse effects and may be useful in the investigation of pulmonary function in healthy horses and those with airway disease.