During heavy exercise, dyspnea will appear when dyspneic index will fall just below:

Respiratory minute ventilation (RMV) is the ventilation at rest. It is approx. 6-7 L/min. Maximum voluntary ventilation (MVV), also called maximum breathing capacity (MBC), is the maximum ventilation possible with greatest voluntary effo. It is 125-170 L/min. The difference between MVV and RMV is the reserve for breathing - Breathing reserve (BR) BR = MVV - RMV Dyspneic index = X 100 = X 100 = 95% (at rest). The numerator in the equation is (MVV - RMV). When the person stas the exercise, with increased rate & depth of breathing the RMV will go on increasing. As RMV goes increasing, the value of the numerator (MVV - RMV) will go decreasing. With heavier and heavier breathing, the RMV will increase and the numerator will decrease fuher, and hence the dyspneic index will fall. At one point, dyspnea will appear. This point is when dyspneic index falls below 70%.