Ruminal,cardiorespiratory and adrenocortical sequelae of Na2EDTA-induced hypocalcaemia in calves

A study was undertaken to provide further information on the ruminal, cardiorespiratory and hypothalamo-pituitary-adrenocortical (HPAC) physiological sequelae of hypocalcaemia in dairy calves.The functional picture observed in standing calves experiencing Na₂EDTA-induced progressive hypocalcaemia showed a biphasic pattern. During the first phase (Ca²⁺ varying between 1.20±0.09 and 0.64±0.15 mmol/L, mean±SD), the animals became dull and lethargic, shifting their weight from one hind limb to the other, with cool extremities and hypersalivation. Their ventilation was slightly increased but their heart rate, thoracoabdominal pressure, pulmonary mechanics, haemoglobin and temperature remained constant. Conversely, their systemic arterial pressure (SAP) and the amplitude of their ruminal contractions (RCA) were severely decreased. During the second phase (Ca²⁺ <0.64±0.15 mmol/L), there was restlessness, tachycardia, hypertension, polycythaemia and, finally, inability to stay upright. It is suggested that the diminished Ca²⁺ availability caused smooth-muscle and myocardial dysfunctions which could explain the RCA and SAP changes recorded during the first phase, whereas neural and/or humoral sympathetic discharge probably accounted for the reversal in SAP and heart rate when Ca²⁺ was decreased further. Serum cortisol increased regularly and remained significantly correlated with Ca²⁺ in each animal. Moreover, regression of cortisol/Ca²⁺ on Ca²⁺/ Na₂EDTA was significant (p0.001).It was concluded that mild asymptomatic hypocalcaemia severely impairs ruminal function, which will progressively worsen the Ca²⁺ deficit; that the inability to maintain posture in hypocalcaemia is not due to hypotension; and that the higher the HPAC response to hypocalcaemia, the higher the resistance to its effects. An asymptomatic periparturient cow with barely detectable ruminal activity may merit preventive calcium borogluconate therapy. Also, the physiological role of hypotension in explaining the clinical picture may be less important than other processes, such as neuromuscular failure. Finally, the present results imply a possible HPAC exhaustion in cows with periparturient paretic hypocalcaemia.Abbreviations A-aDO₂ alveolar-arterial O₂ difference - AVP arginin vasopressin - C _Ldyn dynamic lung compliance - ECG electrocardiogram - EEZF end expiratory zero flow - Hb_TOT total haemoglobin content - HPAC hypothalamo-pituitary-adrenocortical - HR heart rate - Na₂EDTA disodium ethylenediaminetetraacetate - O_2CT total O₂ content - P _aCO₂ arterial CO₂ tension - P _aO₂ arterial O₂ tension - PH periparturient hypocalcaemia - P _pl pleural pressure - P _ru intraruminal pressure - PTH parathyroid hormone - P_TOT plasma total protein - RCA amplitude of ruminal contraction - RCD duration of ruminal contraction - RCF frequency of ruminal contraction - R _L total pulmonary resistance - RR respiratory rate - SAP systemic arterial pressure - T _E expiratory time - T _I inspiratory time - T _p core temperature - V respiratory airflow - V _E minute volume - V _T tidal volume