If you're nervous about your pet being nervous while he flies, calm down. Sedatives, it turns out, are the number one cause of death of pets who fly in the cargo holds of planes.
Information from a meeting of government agents and vets leads to the suspicion that some people are giving their pets extra doses of sedatives, because the pets seem agitated. Even with a prescribed dose, it's possible that the pressurization in the hold -- usually set at 8,000 feet or more -- may have some deleterious effect on a sedated animal.
The worst scenario is probably over-sedating a pet with a "pushed-in" face, like a boxer or a pug, who is at greater risk flying to begin with, especially in hot or humid weather.
Overall, though, deaths during flight are said to be quite rare. Always contact your veterinarian if you have concerns about your pet flying.