Medtronic, Guidant, and St Jude make up most of the pacer implants, >90%. Medtronic and guidant in particular are very popular. Do some google research, there are some published articles on models names and recalls. And types, look into that too, as there are single lead, 2 lead, etc.
Pacers only activate at too low of a rate, they do nothing over a certain heart rate (usually around 40).