function [SD1, SD2, SD1_SD2_ratio] = PoincareMetrics(RRints,flag)
% [SD1, SD2, SD12] = PoincareMetrics(rr)
% OVERVIEW:
% The Poincaré plot analysis is a geometrical and nonlinear method to
% assess the dynamics of HRV.
% Poincaré plot is generated via plotting each RR interval (RR[n])
% against the subsequent RR interval (RR[n+1]).
% This function allows the user to calculate a number of Poincaré
% features, which are detailed below.
%
% INPUTS:
% rr : Row vector of NN-intervals in seconds
% flag : (Optional) 1: show Poincaré plot,
% 0: do not show Poincaré plot
% OUTPUTS:
% SD1 : (ms) standard deviation of projection of the
% PP on the line perpendicular to the line of
% identity (y=-x)
% SD2 : (ms) standard deviation of the projection of the PP
% on the line of identity (y=x)
% SD1_SD2_ratio : SD1/SD2 ratio
%
%
% Written by: Giulia Da Poian <giulia.dap@gmail.com>
% REPO:
% https://github.com/cliffordlab/PhysioNet-Cardiovascular-Signal-Toolbox
% COPYRIGHT (C) 2016
% LICENSE:
% This software is offered freely and without warranty under
% the GNU (v3 or later) public license. See license file for
% more information
%%
if nargin<2
flag = 0; % default do not show plot
end
SDSD = std(diff(RRints));
SDRR = std(RRints);
SD1 = (1 / sqrt(2)) * SDSD; % measures the width of poincare cloud
SD2 = sqrt((2 * SDRR^2) - (0.5 * SDSD^2)); % measures the length of the poincare cloud
SD1_SD2_ratio = SD1/SD2;
% Poincare plot
if flag
ax1 = RRints(1:end-1);
ax2 = RRints(2:end);
scatter(ax1, ax2)
xlabel('RR_n (s)')
ylabel('RR_n+1 (s)')
end
% Convert to ms
SD1 = SD1 * 1000;
SD2 = SD2 * 1000;
SD1_SD2_ratio = SD1_SD2_ratio * 1000;
end
% Brennan, Michael, Marimuthu Palaniswami, and Peter Kamen.
% "Do existing measures of Poincare plot geometry reflect nonlinear
% features of heart rate variability?." IEEE transactions
% on biomedical engineering 48.11 (2001): 1342-1347.