Visualizing excitation waves inside cardiac muscle using transillumination

Voltage-sensitive fluorescent dyes have become powerful tools for the visua lization of excitation propagation in the heart. However, until recently th ey were used exclusively for surface recordings. Here we demonstrate the po ssibility of visualizing the electrical activity from inside cardiac muscle via fluorescence measurements in the transillumination mode (in which the light source and photodetector are on opposite sides of the preparation). T his mode enables the detection of light escaping from layers deep within th e tissue. Experiments were conducted in perfused (8 mm thick) slabs of shee p right ventricular wall stained with the voltage-sensitive dye di-4-ANEPPS . Although the amplitude and signal-to-noise ratio recorded in the transill umination mode were significantly smaller than those recorded in the epi-il lumination mode, they were sufficient to reliably determine the activation sequence. Penetration depths (spatial decay constants) derived from measure ments of light attenuation in cardiac muscle were 0.8 mm for excitation (52 0 +/- 30 nm) and 1.3 mm for emission wavelengths (640 +/- 50 nm). Estimates of emitted fluorescence based on these attenuation values in 8-mm-thick ti ssue suggest that 90% of the transillumination signal originates from a 4-m m-thick layer near the illuminated surface. A 69% fraction of the recorded signal originates from greater than or equal to1 mm below the surface. Tran sillumination recordings may be combined with endocardial and epicardial su rface recordings to obtain information about three-dimensional propagation in the thickness of the myocardial wall. We show an example in which transi llumination reveals an intramural reentry, undetectable in surface recordin gs.