1. Creatinine clearance and urine excretion have been measured in fifty-five patients admitted with suspected myocardial infarction. Patients were grouped on the basis of their radiological report on the first day, irrespective of their diagnosis. The first group consisted of twenty-six patients with normal chest X-rays, the second group of twenty-two patients with radiological evidence of upper zone vessel dilatation and the third group of seven patients with radiological evidence of pulmonary oedema, who were significantly older.

2. The mean creatinine clearance on the first day in patients with upper zone vessel dilatation was significantly higher than the creatinine clearance in patients with normal radiographs. By the third and fourth days the clearances were similar in both groups. The patients with pulmonary oedema had low creatinine clearances on all 4 days.

3. Glomerular filtration rate was measured with 51Cr-labelled EDTA on twenty occasions, and the results correlated well with the creatinine clearance values obtained on the same day.

4. The mean urine volumes on day 1 (2245 ± 286 ml/24 h) and day 2 (2219 ±232 ml/24 h) in patients with upper zone vessel dilatation who had not received diuretics, were significantly higher (P < 001) than the urine volumes on day 1 (1652 ± 145 ml/24 h) and day 2 (1713 ± 118 ml/24 h) in patients with normal radiographs. All the patients with pulmonary oedema had received diuretics during the study and were therefore excluded from this analysis.

5. It is concluded that patients with radiological evidence of upper zone vessel dilatation show an elevation of glomerular filtration rate and an increase in urinary output. It is suggested that these responses are mediated via the stimulation of receptors in the left side of the heart and fulfil a homeostatic function, to unload an overdistended heart.

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