This diagram shows the physiological parasympathetic and sympathetic effects on the sinus node to increase or decrease, respectively, the R-R interval and their interaction ("accentuated antagonism"). These inputs and their changes with physiological activities (i.e., respiration, exercise) influence the R-R interval, heart rate variability, and heart rate recovery. These characteristics can be quantified through short-term electrocardiogram (ECG) recordings, Holter monitoring, or stress testing. The heart rate, heart rate variability, and heart rate recovery that are measured through these tests provide prognostic information in patients with cardiovascular disease. Figure illustration by Rob Flewell.