The sham operation group had a pacemaker implanted, with no pacing; the HF group was tachypaced at 50 times/min for 37 d to establish the (chronic or congestive) HF model; Vagal stimulation group were implanted with a pacemaker at 250 times/min rapid pacing for 30 d, then the right vagus nerve was stimulated for 7 d (frequency: 20 Hz).
Source: wiktionary