Aa. Olkowski et Hl. Classen, PROGRESSIVE BRADYCARDIA, A POSSIBLE FACTOR IN THE PATHOGENESIS OF ASCITES IN FAST-GROWING BROILER-CHICKENS RAISED AT LOW-ALTITUDE, British Poultry Science, 39(1), 1998, pp. 139-146
1. The first objective was to evaluate baseline heart rate (HR) respon
ses in fast growing broilers fed ad libitum from 1 to 45d or subjected
to a food restriction (85%, 70% and 55% of ad libitum from 7 to 21d)
and a refeeding (22 to 45d) regimen in a normal thermal environment. T
he second was to evaluate HR and haematocrit responses in fast growing
broilers reared at a low environmental temperature and fed ad libitum
(A-L) from 1 to 42d, subjected to food restriction (70% ad libitum) b
etween 7 to 21d and refed thereafter (R-R), or subjected to food restr
iction between 7 to 42d (R), and to follow the changes in normal birds
and those prone to ascites. 2. The baseline HR in the normal thermal
environment at 21 and 45d in birds fed ad libitum was lower (P<0.001)
than at 7d. The food-restricted groups had higher HRs at 21d than at 7
d, with the differences significant (P<0.01) for the 70% and 55% treat
ments. At 21d the HRs in restricted groups were significantly higher (
P<0.01) than in ad libitum-fed birds. After returning to ad libitum fe
eding the HRs in these birds tended to decline, with the differences s
ignificant (P<0.05) for the 70% and 55% treatments. 3. In the birds ex
posed to cold, each feeding regimen produced distinct patterns of chan
ge in HR and haematocrit. Both feeding regimen and age had significant
(P<0.001) effects on the changes of both variables. Relative to 7d, o
n 14 d and 21d the HRs decreased and haematocrits increased in all gro
ups, but the rates of change were highest in the A-L and R-R birds. Th
e first fulminant cases of ascites were observed during the third week
in the A-L group and during the fourth week in the R-R group. None of
the chickens from the R group developed ascites. Altogether, the inci
dence of ascites was 48% in the A-L group and 28% in the R-R group. 4.
Among the birds exposed to cold, the ascitic birds had significantly
(P<0.001) lower HRs and higher haematocrits than normal birds from the
same feeding regimen group. The time trends in the rates of change in
HR and haematocrit were distinctly different (P<0.001) for ascitic an
d normal birds. The change in feeding regimen from restricted to ad li
bitum in the R-R group resulted in a significant (P<0.001) decrease in
HR and increase in haematocrit. 5. In view of the evidence presented,
it appears that hypoxaemia and tissue hypoxia in broiler chickens may
be a result of circulatory insufficiency associated with progressive
bradycardia rather than, as commonly believed, pulmonary insufficiency
. In this situation the pulmonary hypertension may be a secondary symp
tom.