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Acid-base homeostasis and myocardial and cerebral preservation during and after induced hypothermia


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Huang, YY 1995 , 'Acid-base homeostasis and myocardial and cerebral preservation during and after induced hypothermia', PhD thesis, University of Tasmania.

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Two acid-base strategies, pH-stat and alpha-stat, are used in hypothennic cardiac
pulmonary bypass (CPB) although wide spread practice has not clarified which is the
better (Swan, 1984; Takao, 1991; Aoki et al, 1994). pH-stat exists among the
hibernators, which tends to preserve normothennic pH and PC02 values as body
temperature falls (Malan, 1982). Alpha-stat, a term used to describe the responses of
ectotherms and heterotherms, which tends to maintain an optimal function in all organ
systems at all temperatures with a lower PC02 and a higher pH, but no change in
plasma HC03- or C02 stores (Reeves, 1972). Previous investigations have compared
pH-stat and alpha-stat and postulated that a more alkaline pH would be even more
effective in preserving myocardial and cerebral function during hypothermia
(Buckberg, 1985; Becker et al, 1981).
In this study the effects of pre-alkalinization and pre-acidification on cerebral and
cardiovascular function during and after hypothermia were investigated in 37 fentanylanaesthetised
New Zealand rabbits.
Animals were divided into four groups: a normothennic (Norn, n=6) and 3
hypothermic groups a) hypothennic control (HCo, n=9); b) hypothermic acidosis
(HAc, n=lO); c) hypothennic alkalosis (HAI, n=12). Alkalosis was induced by
injection of carbicarb (0.33 M Na2C03 and 0.33 M NaHC03, 2 ml kg-I, Rhee et al,
1993) and acidosis by NH4Cl (2 M, 2.5 ml kg-I) before reducing body temperature
(Tb). Animals were sedated with fentanyl (4.2 µg kg-I min-I) during surgical
operation and 2.1 µg kg-I min-1 during monitoring and ventilated with 35% Oi in N1.
The experiment was divided into 5 phases: 1. normothennic control; 2. cooling; 3.
profound hypothennia (Tb = 25±1°C for one hour); 4. rewanning and 5.
normothennic recovery. Blood samples were taken for P02, PC02 and pH analysis.
Heart rate (HR), mean aortic pressure (MAP), central venous pressure (CVP), carotid
arterial blood flow (CrdBF), electrocardiograph (ECG) and electroencephalogram
(EEG) were monitored. Cerebral oxygen delivery (D02), oxygen consumption (\! 0 2)
and oxygen extraction (ER) were calculated from oxygen content (CtQi) and CrdBF.
pH increased when Tb was reduced. All pHa and pHv values were higher in HAI than
those in other groups (P<0.01). During the one hour of profound hypothermia (phase
3), pHa was constant within the three hypothermic groups ( HCo: 7.52 ± 0.07 to 7.52
± 0.08; HAI: 7.69 ± 0.06 to 7.67 ± 0.06; HAc: 7.43 ± 0.05 to 7.40 ± 0.05). After
rewarming, only pHa of HAI in hypothennic group was 7 .37±0.09 and not different
from the value of Norn (7.38±0.04). The other two groups were significantly lower,
with values of 7 .17±0.08 for HCo and 7 .06±0.06 for HAc (P < 0.05). ECG records from HAI often showed conduction disturbances during cooling and
hypothermia but returned to normal after rewarming. Conduction disturbances and
arrhythmias were most severe and frequent in HAc during rewarming, and persisted
after the Tb had returned to normal. HR fell during cooling and hypothermia in all
animals but recovered better in HAI after rewarming. Also in HAI, MAP was higher,
and stayed at a relatively constant level during hypothennia and returned to normal
after rewarming.
CrdBF and D02 decreased when Tb was reduced and were slightly higher in HAI than
in HAc. ER was lower in HAI than in HAc during rewarming (P<0.05). Suppression
of EEG started earlier in HAc and HCo than in HAI during cooling and recovered
faster in HAI than in the other two groups. The results showed 1) a normal pH was attained after rewanning in HAI; 2) Prealkalinization
has beneficial effects on cerebral and myocardial performance; 3) the
rabbit provides a suitable small animal model for induced hypothermia, and has been
successfully tested for experiments over 6 or more hours duration.
This work suggests that alkalinization before cooling could reduce or eliminate
anaerobic metabolism and subsequent acidosis, and thus could be the optimal acidbase
strategy during induced hypothermia and during rewarming.

Item Type: Thesis - PhD
Authors/Creators:Huang, YY
Keywords: Hypothermia, Induced, Acid-base equilibrium, Body temperature
Copyright Holders: The Author
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Copyright 1995 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

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Thesis (Ph.D.)--University of Tasmania, 1996.

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