1. A computer-based technique is described for measuring and analysing cardiac beat-to-beat intervals.

2. The R-R intervals were recorded at rest, during forced deep breathing and during the Valsalva manoeuvre in 174 healthy subjects aged between 16 and 89 years.

3. The mean resting R-R interval was independent of sex, age and smoking habit. The normal range was 654.6-1141.4 ms with a mean value of 864.7 ms.

4. Variation in interval length at rest and during deep breathing depended on age and the mean interval length, whereas the Valsalva ratio depended on age alone. Normal ranges were calculated from regression analysis of these relationships.

5. It is concluded that the technique described can be used to test cardiac autonomic function at the bedside.

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