Electrocardiograms of 100 healthy infants aged from 2 weeks to 12 months were recorded and analysed in order to provide standard measurements for comparison with those made in electrocardiograms of infants suffering from idiopathic hypercalcaemia and other disorders of cation balance. The conclusions of this investigation may he summarized as follows:- 1. The heart rate falls as age increases. The rate lay between 207 and 102 per minute in this series. 2. The P wave, PR interval, QRS complex and T wave lengthen as age increases, independently of heart rate. 3. The PR interval, QRS complex and T wave lengthen as the heart rate falls, independently of age. 4. The ST segment shortens as age increases, and lengthens as the heart rate falls. 5. The QT interval lengthens as the heart rate falls. The influence of heart rate is so profound that it is necessary to apply a correction to the measured QT interval to permit comparisons between QT intervals recorded at different heart rates. The corrected QT interval is referred to as the QTc value. 6. Time intervals in this series of healthy infants have the following minimum and maximum values: P wave = 0.04-0.08 second; T wave = 0.09-0.15 second; PR interval = 0.08-0.14 second; ST segment = 0.05-0.11 second; QRS complex = 0.05-0.08 second; QT interval = 0.21-0.29 second; QTc value = 0.36-0.42. Electrocardiograms of 25 patients with idiopathic hypercalcaemia were recorded. Certain features were compared with those of healthy subjects, and with those of 3 newborn infants with hypercalcaemia caused by the injection of calcium gluconate during replacement transfusion. The conclusions of this investigation may be summarized as follows:- 1 . Newborn infants with hypercalcaemia have a shortened ST segment and a decrease in the QTc value. 2. In patients with idiopathic hypercalcaemia, hypercalcaemia is accompanied by a shortened ST segment, but not by a decrease in the QTc value because of the undue length of the T wave. The QTc value cannot therefore be used to give an indication of the serum calcium level. 3 . In idiopathic hypercalcaemia the contour of the ST-T complex is often abnormal with a prominent broad flat-topped T wave, which is not directly related to the level of serum calcium. 4. The abnormal contour of the ST-T complex in idiopathic hypercalcaemia may reflect interference with myocardial activity either by a heart lesion caused by vitamin D, or by the action of a chemical substance associated with it. An animal experiment was conducted in order to gain information concerning heart lesions caused by vitamin D. Vitamin and vitamin D[3] were each given to 9 young rabbits and there were 9 controls. A study was made of the pathological changes in the heart and aorta. The conclusions of this investigation may be summarized as follows:- 1. Aortic lesions, found in every rabbit given vitamin D, are accepted as evidence of the toxicity of the doses given. 2. Of the heart lesions caused by vitamin D, lesions immediately subjacent to the endocardium apparently occur most frequently, focal lesions of the myocardium less frequently and coronary artery lesions least frequently. 3. Vitamin D[2] is at least as toxic to the heart as vitamin D[3] and may be more so. 4. It was not found possible to assess the prognosis of these heart lesions, and their ultimate effect, if any, on heart function, but myocardial and coronary artery lesions such as these could account for the electrocardiographic changes in idiopathic hypercalcaemia. 5. Myocardial and coronary artery lesions are unlikely to be caused by the usual prophylactic doses of vitamin D unless the vitamin were to exert a cumulative effect by such a mechanism as has been postulated in idiopathic hypercalcaemia of infancy. 6. Detailed histological examination of the heart of infants dying in the course of idiopathic hypercalcaemia is an important requirement
