One of the things that I enjoy most about reviewing cardiopulmonary exercise tests is that I always have to reach back into the basics of physiology in order to get a sense of what the results are trying to say. Oftentimes it isn’t so much the absolute or percent predicted value of a given parameter but its relationship with other parameters that is revealing. One of the bits of human physiology that has always struck me as fascinating is the relationship between heart rate and oxygen consumption.
For almost everyone there is a linear relationship between heart rate and oxygen consumption. When you plot them against each other you can put a ruler on the plotted points and see that they form a straight line. Chronotropic Index and O2 pulse are two ways of analyzing this relationship.