The rapid shallow breathing index is calculated by finding the respiratory frequency and tidal volume of the patient's breathing pattern. The index is expressed as a mathematical ratio such that the frequency is a lowercase "f" and the tidal volume is a capital "VT," meaning that the index equals f / VT.
The respiratory frequency is how fast a person breathes, while the tidal volume is how deeply he takes air into his lungs. To calculate this, specialists measure the rate of breath and the volume a patient takes in per minute.
The higher a person's rapid shallow breathing index is, the less likely he is able to breathe on his own without a mechanical ventilator, Medscape explains. For example, a person who breathes fast, but takes shallow breaths has a high index, making it more difficult for doctors to wean him off of ventilators.
Doctors primarily use the ratio to determine the steps needed to get patients to breathe on their own. Ventilators help patients breathe, but they are not an effective long-term solution, according to Medscape. As specialists treat patients who have breathing problems, they measure the rapid shallow breathing index throughout treatment to determine how effective a treatment is. Once a patient reaches a ratio that is in the normal range, specialists try to take the patient off of the ventilator entirely.