Inspect previous peaks from the last detected qrs peak (if any),
        using a lower threshold

def _backsearch(self):
        """
        Inspect previous peaks from the last detected qrs peak (if any),
        using a lower threshold

        """
        if self.last_qrs_peak_num is not None:
            for peak_num in range(self.last_qrs_peak_num + 1, self.peak_num + 1):
                if self._is_qrs(peak_num=peak_num, backsearch=True):
                    self._update_qrs(peak_num=peak_num, backsearch=True)