def current_heart_rate(self):
        """Return current heart rate for in-progress session."""
        try:
            rates = self.intervals[0]['timeseries']['heartRate']
            num_rates = len(rates)

            if num_rates == 0:
                return None

            rate = rates[num_rates-1][1]
        except KeyError:
            rate = None
        return rate
Return current heart rate for in-progress session.