Possible Relation between the Degree of Cardiac Adrenergic Innervation and the Resistance to Hypothermic Ventricular Fibrillation in Young Cats

Kittens weighing 0.5 to 1 kg, corresponding to an age of 5 to 10 weeks, were subjected to immersion hypothermia to 17.4–16.2 o C rectal temperature. Blood pressure, ECG, respiration, and rectal temperature were continuously recorded. Metaraminol was administered intermittently to maintain an adequate blood pressure at lower temperatures, and the animals were respirated mechanically when necessary. Only 1 out of 12 kittens developed ventricular fibrillation; the remaining animals could be re‐warmed without major complications. Under similar conditions, adult animals have previously been found to develop ventricular fibrillation constantly. Fluorescence microscopy revealed a low degree of development of cardiac adrenergic nerves compared with adult animals, in agreement with a lower concentration of cardiac noradrenaline. The possibility is discussed that the not yet fully developed adrenergic innervation of the heart in young animals may account for the higher resistance to reduction of the body temperature as compared with adult animals.