Do the other variables vary over time? How many datapoints for the heartbeats do you have per patient? It's hard to answer without more information. The duration the heart beats were captured over need to be the same for an adequate comparison. However, here's some thoughts:
- Compute some summary statistics, like mean, median, mode, min and max over the time series.
- Compute moving averages over the time series, such as simple moving average, and exponential moving averages.
- Compute the standard deviation of each datapoint
That will give you multiple numbers for the time series, but that is likely to be better for your model as it more completely describes the time series.