Women and men have similar rates of CHF until the middle of the 9th decade, after which the incidence in women far outpaces that in men. Because incidence is calculated as a population rate, the increased incidence rate cannot simply be the result of known increased survival of women compared to men.

The figure shows annual CHF incidence by age in the Framingham, MA study. Note the exponential increased risk in CHF incidence around age 75. Why should this occur? We believe that this sharp increase is partially the result of age-related changes in the cardiovascular system which compromise cardiac function. As these age-related changes in the cardiovascular system become more prominent and interact with atherosclerotic heart disease, valve stenosis, hypertension, and other cardiac diseases common in the elderly, the incidence of CHF increases sharply.

REMINDER:The incidence of something is the number of instances of illness commencing, or of persons falling ill, during a given period in a specified population. More generally, the number of new events, e.g., new cases of a disease in a defined population, within a specified period of time. The term incidence is properly used to denote incidence rate.

How would you calculate the incidence of CHF in your study area? First, establish the criteria for making the diagnosis of CHF. Then, ensure that you have a method for accurately identifying all patients who develop CHF for the first time over a period of time, say 1 year. Do not count previously diagnosed CHF patients with symptom relapse. They are not new cases. The incidence rate of CHF will be calculated as follows:

When such rates are calculated as a fraction, they are often multiplied by 1000, 10,000 or 100,000 to ease interpretation.