At other times, this palpitation is due to the presence of poisons in the blood, either those of infectious disease, or of certain waste products produced in the body in excess, as, for instance, when your digestion is out of order, or your skin, kidneys, and bowels are not working properly; or it is due to tea, coffee, or tobacco.
Effects of Tea and Coffee.

Tea and coffee, if taken in excess, will sometimes produce very uncomfortable palpitation, or rapid over-action of the heart, with restlessness and inability to sleep.

They usually act in this way only when taken in large amounts, or upon a small percentage of persons who are peculiarly affected by them; and this palpitation is seldom serious, and disappears when their excessive use is stopped.
Tobacco and its Dangers to the Heart.

Tobacco has a very injurious effect upon the nerves of the heart in the young, making them so irritable that the heart will beat very rapidly on the least exertion; so that gradually one becomes less and less inclined to attempt exertion of any sort, whether bodily or mental, and falls into a stagnant, stupid sort of condition which seriously interferes with both growth and progress.
In other cases, tobacco dulls and deadens the nerves controlling the heart, as it does the rest of the nervous system and the brain, so that the smoker feels as if nothing were worth while doing very hard, and it becomes difficult for him to fix his mind upon a subject.

At the same time, it dulls the appetite so that one takes less wholesome food; and it checks, or clogs up, the sewer-pipes of the skin, the liver, and the kidneys.
Of course, as you know, all trainers and coaches, even though they be habitual smokers themselves, absolutely forbid tobacco in any form to athletes who are training for a contest, on account of its effects upon the nervous system and the heart.
A certain percentage of individuals are peculiarly susceptible to tobacco, so that it has a special poisonous effect upon the nerves of the heart, causing a rapid pulse and shortness of breath, known as _tobacco heart_.