Compute mean heart rate in beats per minute, from a set of rr
    intervals. Returns 0 if rr is empty.

    Parameters
    ----------
    rr : numpy array
        Array of rr intervals.
    fs : int, or float
        The corresponding signal's sampling frequency. Required if
        'input_time_units' == 'samples'.
    min_rr : float, optional
        The minimum allowed rr interval. Values below this are excluded
        when calculating the heart rate. Units are in `rr_units`.
    max_rr : float, optional
        The maximum allowed rr interval. Values above this are excluded
        when calculating the heart rate. Units are in `rr_units`.
    rr_units : str, optional
        The time units of the input rr intervals. Must be one of:
        'samples', 'seconds'.

    Returns
    -------
    mean_hr : float
        The mean heart rate in beats per minute

def calc_mean_hr(rr, fs=None, min_rr=None, max_rr=None, rr_units='samples'):
    """
    Compute mean heart rate in beats per minute, from a set of rr
    intervals. Returns 0 if rr is empty.

    Parameters
    ----------
    rr : numpy array
        Array of rr intervals.
    fs : int, or float
        The corresponding signal's sampling frequency. Required if
        'input_time_units' == 'samples'.
    min_rr : float, optional
        The minimum allowed rr interval. Values below this are excluded
        when calculating the heart rate. Units are in `rr_units`.
    max_rr : float, optional
        The maximum allowed rr interval. Values above this are excluded
        when calculating the heart rate. Units are in `rr_units`.
    rr_units : str, optional
        The time units of the input rr intervals. Must be one of:
        'samples', 'seconds'.

    Returns
    -------
    mean_hr : float
        The mean heart rate in beats per minute

    """
    if not len(rr):
        return 0

    if min_rr is not None:
        rr = rr[rr > min_rr]

    if max_rr is not None:
        rr = rr[rr < max_rr]

    mean_rr = np.mean(rr)

    mean_hr = 60 / mean_rr

    # Convert to bpm
    if rr_units == 'samples':
        mean_hr = mean_hr * fs

    return mean_hr