What exactly do you have in this data set?
Is it just values of ECG per time or HR (heart rate) per time?
I'm currently working on an instances set that contains few attributes representing physiological signals such as Blood Pressure, Respiratory Rate, Heart Rate or End Tidal CO2.
I get it as a numeric attributes, however I don't only use classifiers suitable for handling numeric attributes (such as kNN - Weka's ibk), but also the ones appropriate for nominal attributes.
To achieve that I create a Probability Distribution, either by putting data into histogram (i.e. using Weka's Discretize Filter).
With nominal attributes you can use for example Bayesian Classifiers (Weka's Naive Bayes, Bayesian Network, Hidden Naive Bayes etc.).
Here for example you have basic info on using filters: