Many individuals equate lower resting heart rates and faster heart rate declines after exercise with improved cardiovascular fitness. In general, these beliefs are true, but many exceptions exist. Recognizing the many factors, both environmental and genetic, that determine resting heart rates and heart rate responses after exercise can go a long way toward alleviating your concerns about cardiovascular health.
Your resting heart rate is best described as the number of times your heart beats per minute when you are in a fully relaxed physical and mental state. For the typical adult (older than 18), this value usually ranges between 60 and 100 times per minute. Younger ages, however, are strongly associated with higher normal resting heart rates. Normal rates can be as high as 160 beats per minute for infants and 120 beats per minute for children. Females also tend to have slightly higher heart rates than males, as do individuals with higher body masses.
Heart rates, particularly during and after exercise, though, are also highly sensitive to a person’s hydration and nutrition status. Dehydration represents a state of diminished water supply in your body. Because circulating blood is composed of approximately 83 percent water, dehydration effectively lowers the amount of blood that your heart is able to circulate with one beat, or contraction. Your heart compensates for this low "water" volume by beating more times per minute.
Occasionally, though, medical conditions and medications must also be considered if, in fact, your resting heart rate is truly high for your current age. High stress and anxiety are the most common conditions. Anemia, overactive thyroid function, diabetes, eating disorders like anorexia nervosa, and any illness causing a fever can also each associate with higher-than-normal resting heart rates.
Certain stimulants that are smoked, ingested in your diet, or taken as dietary supplements or pills can cause significant increases in your heart rate without causing any other symptoms. These include nicotine (tobacco), caffeine, dietary pills such as ephedrine, and over-the-counter cold remedies such as Dristan and Sudafed.