Unusual observations on hypermagnesemic levels and cardiorespiratory effects in neonatal swine

Mg is being used as a therapeutic agent in different clinical conditions, i.e. preeclampsia or eclampsia, and as a tocolytic agent to prevent premature delivery. For these reasons we decided to systematically investigate the cardiovascular and respiratory effects of excess Mg in 25 piglets less than 1 day to 3 months of age, lightly anesthetized with Saffan, tracheotomized, paralyzed with C-10 and artificially ventilated on 100% O2. A 1.0 M infusion of MgCl2 was given for 15-60 min, and arterial plasma samples were drawn before and at the end of the infusion for the determination of plasma Mg and Ca levels. Phrenic nerve activity was recorded monophasically simultaneously with arterial blood pressure, electrocardiogram, end-tidal CO2 and intratracheal pressure on a dynograph and on analog tape. In a subset of experiments (n = 6), spontaneous ventilation was monitored in piglets of different ages to determine the Mg concentration at which spontaneous ventilation ceased. Blood gases were measured at 45-min intervals, as well as both immediately before and after the MgCl2 infusion, and pCO2 and pH were maintained within normal limits. As a consequence of the high plasma concentration of Mg, mean arterial pressure and heart rate declined, and the inspiratory duration significantly decreased. Our results indicate that young piglets can survive high levels of plasma Mg (greater than 14 mM).