The autonomic nervous system modulates atrial activity, notably through acetylcholine (ACh) release. This timedependent action may alter the dynamics of atrial arrhythmia, and in particular its wavelength. Wavelength is a critical factor that determines the vulnerability to reentry. It represents the spatial extent of refractory periods and may be estimated as the product of action potential duration and conduction velocity. When electrophysiological properties vary in space and/or time, and when conduction is anisotropic, that estimate may be inaccurate. We assessed an instantaneous geometric measure of wavelength applicable to monitoring the effect of externally-driven variations in ACh concentration at a shorter time scale than action potential duration. The method was applied to quantify the hysteresis in wavelength caused by release and degradation of ACh.