function r = wabp(abp)
% WABP ABP waveform onset detector.
% r = WABP(ABP) obtains the onset time (in samples)
% of each beat in the ABP waveform.
%
% In: ABP (125Hz sampled)
% Out: Onset sample time
%
% Usage:
% - ABP waveform must have units of mmHg
%
% Written by James Sun (xinsun@mit.edu) on Nov 19, 2005. This ABP onset
% detector is adapted from Dr. Wei Zong's wabp.c.
%% Input checks
if nargin ~=1
error('exactly 1 argment needed');
end
if size(abp,2)~=1
error('Input must be a <nx1> vector');
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% scale physiologic ABP
offset = 1600;
scale = 20;
Araw = abp*scale-offset;
% LPF
A = filter([1 0 0 0 0 -2 0 0 0 0 1],[1 -2 1],Araw)/24+30;
A = (A(4:end)+offset)/scale; % Takes care of 4 sample group delay
% Slope-sum function
dypos = diff(A);
dypos(dypos<0) = 0;
ssf = [0; 0; conv(ones(16,1),dypos)];
% Decision rule
avg0 = sum(ssf(1:1000))/1000; % average of 1st 8 seconds (1000 samples) of SSF
Threshold0 = 3*avg0; % initial decision threshold
% ignoring "learning period" for now
lockout = 0; % lockout >0 means we are in refractory
timer = 0;
z = zeros(100000,1);
counter = 0;
for t = 50:length(ssf)-17
lockout = lockout - 1;
timer = timer + 1; % Timer used for counting time after previous ABP pulse
if (lockout<1) & (ssf(t)>avg0+5) % Not in refractory and SSF has exceeded threshold here
timer = 0;
maxSSF = max(ssf(t:t+16)); % Find local max of SSF
minSSF = min(ssf(t-16:t)); % Find local min of SSF
if maxSSF > (minSSF + 10)
onset = 0.01*maxSSF ; % Onset is at the time in which local SSF just exceeds 0.01*maxSSF
tt = t-16:t;
dssf = ssf(tt) - ssf(tt-1);
BeatTime = find(dssf<onset,1,'last')+t-17;
counter = counter+1;
if isempty(BeatTime)
counter = counter-1;
else
z(counter) = BeatTime;
end
Threshold0 = Threshold0 + 0.1*(maxSSF - Threshold0); % adjust threshold
avg0 = Threshold0 / 3; % adjust avg
lockout = 32; % lock so prevent sensing right after detection (refractory period)
end
end
if timer > 312 % Lower threshold if no pulse detection for a while
Threshold0 = Threshold0 - 1;
avg0 = Threshold0/3;
end
end
r = z(find(z))-2;