Abstract
The intravenous Na2EDTA infusion technique allows effective specific chelation of circulating Ca2+ leading to a progressive hypocalcaemia. Methods previously used were not described in detail and results obtained by monitoring total and free ionic calcium were not comparable due to differences in sampling and analysis. This paper describes a standardized EDTA infusion technique that allowed comparison of the response of calcium, phosphorus and magnesium between 2 groups of experimental cows. The concentration of the Na2EDTA solution was 0.134 mol/l and the flow rate was standardized at 1.2 ml/kg per hour. Involuntary recumbency occurred when ionised calcium dropped to 0.39 – 0.52 mmol/l due to chelation. An initial fast drop of ionized calcium was observed during the first 20 min of infusion followed by a fluctuation leading to a further drop until recumbency. Pre-infusion [Ca2+] between tests does not correlate with the amount of EDTA required to induce involuntary recumbence. Total calcium concentration measured by atomic absorption remained almost constant during the first 100 min of infusion but declined gradually when the infusion was prolonged. The concentration of inorganic phosphate declined gradually in a fluctuating manner until recumbency. Magnesium concentration remained constant during infusion. Such electrolyte responses during infusion were comparable to those in spontaneous milk fever. The standardized infusion technique might be useful in future experimental studies.
Highlights
Induction of hypocalcaemia by means of infusion with EDTA has been performed in experimental veterinary medicine and physiology for over 36 years (Smith & Brown 1963) primarily as a model for spontaneous cases of milk fever and subclinical hypocalcaemia in dairy cows
In other studies the ionized and total plasma magnesium concentration remained constant throughout the infusion process suggesting that Na2EDTA administration does not influence Mg2+ bioavailability (Desmecht et al 1995)
Van Mosel et al (1993) in studies with 2 groups of cows fed either a negative or positive dietary cation-anion difference (DCAD) observed constant plasma magnesium concentration in EDTA induced hypocalcaemia and no significant differences were observed in plasma inorganic phosphate concentration due to the dietary treatments
Summary
Induction of hypocalcaemia by means of infusion with EDTA has been performed in experimental veterinary medicine and physiology for over 36 years (Smith & Brown 1963) primarily as a model for spontaneous cases of milk fever and subclinical hypocalcaemia in dairy cows. Berger & Gerber (1977), Desmecht et al (1995) and van de Braak et al (1997) all reported a triphasic pattern of calcium decay with an initial fast drop followed by a plateau, and a relatively fast drop again Factors such as cow’s response to the gradually developing hypocalcaemia, the dietary calcium and its solubility might influence Ca2+ decay during EDTA infusions. A disagreement between blood [Ca2+] and clinical signs at an infusion speed above 2 mg/kg per minute has been recorded by the authors (unpublished) by cow side monitoring of Ca2+ This has probably resulted from differences between vascular and tissue Ca2+ concentrations during the fast infusions (Mellau et al 1999). It was meant to monitor clinical parameters during infusion as well as the response of plasma total calcium, inorganic phosphate and magnesium in cows
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