To answer your basic question of how to read in the time values to one variable, and the readings into a second variable, use this:
clear;
clc;
ftoread = 'hr.csv';
fid = fopen(ftoread); %OPENS the CSV file
data = textscan(fid,'%s%f','Headerlines',1,'Delimiter',','); %Reads the file
fclose(fid); % closes the file
x = datenum(data{1}); %Time variables moved to new variable called x
y = data{2}; % Readings moved to variable y
plot(x,y); % plot bp readings vs time
% Create xlabel
xlabel('Time of Reading');
% Create ylabel
ylabel('Blood Pressure Reading');
% Create title
title('Blood Pressure Readings vs Time');
Your data currently does not look like much since the blood pressure readings are essentially all recorded at the same second.
To adjust your x-values so they are relative to the first reading and not the beginning of time, you can do this assuming csv entries come in order:
clear, clc;
ftoread = 'hr.csv';
fid = fopen(ftoread); %Reads the CSV file
data = textscan(fid,'%s%f','Headerlines',1,'Delimiter',',');
fclose(fid); % close
x = datenum(data{1});
x = x - x(1); % Times relative to the first recorded time entry
y = data{2};
plot(x,y);
% Create xlabel
xlabel('Seconds since first reading');
% Create ylabel
ylabel('Blood Pressure Reading');
% Create title
title('Blood Pressure Readings vs Time');