Clueless on McCluhan?

So I'm rereading Part I in preparation for class tonight and so I can actually blog on it, and I'm confused by his characterization of TV as cool. If a hot medium "extends one single sense in 'high definition'", wouldn't tv fall into this category? A few pages later McLuhan goes onto include an ad in the hot medium category with his example of the insurance commercial showing "Dad in an iron lung" More recently there is this TV ad. "This ad did more to strike terror into the reader than all the warning wisdom in the world", he writes. Doesn't this apply to this graphic PSA? Can mediums shift from being hot to cool, depending on what is being broadcast? I'm confused about this.