I spoke to our vet today about this topic.
Handbook of Veterinary DrugsGiant breeds, as well as greyhounds, appear quite sensitive to the clinical effects of the drug, yet terrier breeds appear more resistant. Conversely, boxer dogs are predisposed to the hypotensive and bradycardiac effects of the drug.
Psychopharmacology of Animal Behavior DisordersAcepromazine may cause syncope in brachycephalic breeds. Boxers are reportedly very sensitive to the hypotensive and bradycardiac effects of acepromazine and even small doses should be used cautiously in this breed.
Veterinary Drug HandbookGiant breeds and greyhounds may be extremely sensitive to the drug, while terrier breeds are somewhat resistant to its effects. Boxers are reported to be very sensitive to the hypotensive and bradycardiac effects of acepromazine....
Veterinary AnaesthesiaFainting and cardiovascular collapse has been reported to occur occasionally in all species of animal following the use of even low doses of acepromazine. In some cases it may have been due to administration to a hypovolaemic animal but in others it has not been explained. Some strains of Boxers are known for collapsing after a very small dose of acepromazine given by any route, and it has been suggested that this may be due to orthostatic hypotension or to vasovagal syncope.
Veterinary Pharmacology & TherapeuticsAcepromazine can cause syncope associated with high vagal tone and subsequent bradycardia; this occurs in brachycephalic breeds, particularly in Boxers.
Anaesthesia for Veterinary NursesBoxers are known to be unduly susceptible to ACP [acepromazine], and occasionally respond to the drug by collapsing with profound hypotension and bradycardia.
Currently posted on the website of the South African Veterinary CouncilThe side-effects of short term use of acepromazine include the following (Booth, 1988; Dodman,1998):
Hypotension (often accompanied by a compensatory tachycardia)
Syncope, particularly in brachycephalic breeds, the Boxer being especially sensitive
Currently posted on both of these websites (ace is called ACP in the UK):In some dogs, particularly Boxers and other short-nosed breeds, spontaneous fainting or syncope may occur, due to sinoatrial block caused by excessive vagal tone. An attack may be precipitated by acepromazine...