Objective: To compare breathing patterns and transdiaphragmatic pressure during total intravenous (TIVA) and isoflurane anaesthesia in ponies. Study design: Experimental, cross-over study. Animals: Six healthy ponies weighing 286 (233-388)61kg, age 13 (9-16)3years. Methods: Following premedication with romifidine [80gkg(-1) intravenously (IV)], general anaesthesia was induced with midazolam (0.06mgkg(-1) IV) and ketamine (2.5mgkg(-1) IV) and maintained with either isoflurane (FeIso=1.1%) (T-ISO) or an IV combination of romifidine (120gkg(-1) per hour), midazolam (0.09mgkg(-1) hour(-1)) and ketamine (3.3mgkg(-1) hour(-1)) (T-TIVA), while breathing 60% oxygen (FIO2). The circumference changes of the rib cage (RC) and abdominal compartment (ABD) were recorded using respiratory ultrasonic plethysmography (RUP). Balloon tipped catheters were placed in the distal oesophagus and the stomach and maximal transdiaphragmatic pressure (P-di max) was calculated during Mueller's manoeuvre. Results: The breathing pattern T-ISO was more regular and respiratory rate significantly lower compared with T-TIVA. Ponies in T-TIVA showed regularly appearing sighs, which were never observed in T-ISO. Different contribution of the RC and ABD compartments to the breathing pattern was observed with a smaller participation of the RC to the total volume change during T-ISO. Transdiaphragmatic pressures (mean 13.7 +/- SD 8.61 versus 23.4 +/- 7.27 cmH(2)O, p