Periodicity Index

The Periodicity Index is computed by applying Fast Fourier Transform (FFT) spectral analysis to minute ventilation data, looking for power concentrated in the 0.01 to 0.03 Hz frequency band. This band corresponds to breathing cycles of 30 to 100 seconds, which are characteristic of periodic breathing or Cheyne-Stokes respiration.

In periodic breathing, ventilation waxes and wanes in a regular cyclical pattern. The person breathes progressively deeper, then progressively shallower, sometimes with brief pauses between cycles. This pattern is associated with central sleep apnea, heart failure, high-altitude breathing, and certain neurological conditions.

A high Periodicity Index does not necessarily indicate a specific medical condition but warrants clinical discussion. On PAP therapy, periodic breathing may indicate treatment-emergent central sleep apnea (central events appearing after obstructive events are treated) or inadequate pressure settings. Some ASV (Adaptive Servo-Ventilation) modes are specifically designed to counteract periodic breathing.