Effect of clenbuterol on arterial oxygen tension in the anaesthetised horse.

In horses in dorsal recumbency, spontaneously breathing oxygen, with halothane at a constant end-tidal concentration, the arterial oxygen tension (PO2) increased from 9.9 +/- 0.3 SEM kPa to 21.7 +/- 4.0 kPa with 0.8 micrograms kg-1 clenbuterol and to 29.1 +/- 3.8 kPa with 2.4 micrograms kg-1 clenbuterol. In horses initially in dorsal recumbency then turned to sternal recumbency the PaO2 rose to 54.0 +/- 3.0 kPa, but this rise was unaffected by clenbuterol administration. The response in dorsal recumbency was consistent with clenbuterol counteracting the factor postulated to direct the pulmonary blood flow caudodorsally, but the response to clenbuterol in sternal recumbency was not. Although these results do not unequivocally characterise the postulated factor, the effect of the clenbuterol itself is of interest because of its potential as a treatment for the low PaO2 often observed in anaesthetised horses.