Doctors usually classify patients' heart failure according to the severity of their symptoms. The table below describes the most commonly used classification system, the New York Heart Association (NYHA) Functional Classification. It places patients in one of four categories based on how much they are limited during physical activity.
| Class | How Patient Feels During Physical Activity |
| I | No symptoms and no limitation in ordinary physical activity. |
| II | Mild symptoms and slight limitation during ordinary activity. Comfortable at rest. |
| III | Marked limitation in activity due to symptoms, even during less-than-ordinary activity. Comfortable only at rest. |
| IV | Severe limitations. Experiences symptoms even while at rest. |
