Figure 2. Coexisting forms of cardio-respiratory coupling: RSA and phase synchronization (CRPS) represent different and independent forms of coupling.

While RSA leads to periodic modulation of the heart rate within each breathing cycle (sinusoidal line), CRPS leads to clustering of heartbeats (ovals) at certain phases ϕ_r of the breathing cycle. Shown are consecutive heartbeats over a period of 200 sec. The x-axis indicates the phases ϕ_r of the breathing cycle where heartbeats occur. Data are selected from the same healthy subject during the same deep sleep episode as in Fig. 1, but represent a different segment of the episode where both RSA and CRPS coexist.