Figure 1.

Cardio-respiratory phase-relations: upper curves show heart rate (HR, solid line) and cardiac beat-to-beat interval (RRI, dotted line), lower curves show respiration. Slow cardiac and respiratory waves are presented in both panels for reasons of comparability. The left panel depicts positive (classical) RSA with inspiration coinciding with increasing HR (decreasing RRI) and expiration coinciding with decreasing HR (increasing RRI). This phase-relation typically occurs in a resting and relaxed state. The right panel depicts negative (inverse) RSA with inspiration coinciding with decreasing HR (increasing RRI) and expiration coinciding with increasing HR (decreasing RRI). This phase-relation occurs rarely. In awake subjects, it has predominantly been observed in situations of elevated anxiety and during slow breathing.