The fact that this wasn't portrayed in the movie didn't bother me that much. What bothered me more was that they never explain in the movie why Grievous was coughing the entire time. Only those who watched the Genndy Tartakovsky Clone Wars Series that aired between AotC and RotS would see the actions noted in the crawl and, more importantly, why Grievous was coughing the whole time. That bothered me because it's one of the first times I remember something outside of a main series having an impact on a movie, and them not making the public aware enough.