How is Morrie going to die? What is his test to see how long he has left?

In the book, Morrie tells Mitch that he will die through suffocation. To see how long he has left, Morrie uses a little test. The test involves inhaling air and then counting as many numbers as he one can while exhaling. 
Morrie explains the test by asking Mitch to inhale a few times and then to count numbers while he exhales. Mitch does this and discovers that he can count to seventy before he has to take another breath. At this point, Morrie proclaims that Mitch has healthy lungs. Then, the professor proceeds to demonstrate how he fares on the test.
During his demonstration, Morrie can only count to eighteen. He tells Mitch that he used to be able to count to twenty-three before his breath was gone. The conversation takes a gloomy turn, but Morrie refuses to allow the inevitability of his impending death to mar his relationship with Mitch.