H.A. Ramsey

Altered Concentrations of Aldosterone in Neonatal Calves During Chronic Hypoxia and the Subsequent Recovery Period

Significant changes occur in both blood composition and plasma volume of the calf within the first few hour after parturition (McEwan et al., 1968). These changes require sensitive and reliable regulatory mechanisms to maintain electrolyte balance and prevent fluid dyshomeostasis. Although the time required for maturation of the renal system in calves is much shorter than in most experimental animals (Dalton, 1968), significant electrolyte imbalances are a common clinical finding in neonatal calves.

Development of an In Vivo Perfusion System for Bovine Fetal Small Intestine

Previous work at this institution has focused on the role of oxygen availability on the development of bovine small intestine during the perinatal period. In particular, the change in arterial oxygen tension associated with the conversion from placental to pulmonary respiration at birth was hypothesized to initiate an alteration in intestinal macromolecular permeability during the first 24 hours of life that is characteristic of the bovine neonate. The first model used to test this hypothesis was the hypoxic postnatal calf (Tyler and Ramsey, 1989). By providing the newborn calf with a 90:10 mixture of N2:O2, arterial PO2 was maintained at a level similar to that of the fetal calf. The results of this study were inconclusive, however, which may have been due to other changes occurring at birth that influence intestinal development.

Metabolic and Developmental Responses of the Calf to a Chronic Hypoxic Episode in the Immediate Newborn Period

While there is an extensive bank of literature on hypoxia and anoxia in the neonate, the primary focus of previous research has been pulmonary responses to acute hypoxia. Furthermore, no research on the metabolic effects of chronic hypoxia in the immediate newborn period has been published to our knowledge. Given the dynamic state of the newborn in the first hours of life, the relevance of previous observations to the results of the present study is questionable. Previous studies on calves subjected to periods of acute hypoxia have reported decreased PO2, decreased PCO2 and increased pH (Reeves and Leathers, 1964). Thus, it is clear that newborn calves respond differently to chronic hypoxia than they do to acute hypoxia. Much of this difference is due to the biphasic ventilatory response previously discussed. Past reports of acid-base changes in newborn calves utilized venous blood (Moore, 1969; Schlerka et al., 1979; Eigenmann et al., 1981; Maurer-Schweitzer et al., 1977) and are therefore subject to criticism given the variability inherent in venous blood values. Waizenhoffer and Mulling (1978) compared arterial and venous blood gases, but only drew a limited number of arterial samples at 12 and 24 hours. These values were comparable to 12- and 24-hour values in the present experiment. In conclusion, the metabolic effects of a 24-hour hypoxic episode in the newborn calf are relatively mild, consisting primarily of the development of a primary metabolic acidosis due in large part to accumulation of lactate. A relatively large base deficit was incurred concurrently. Most values returned to normal following return of PO2 to normal levels at 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)