Electric field perturbations of spiral waves attached to millimeter-size obstacles

Abstract

Reentrant spiral waves can become pinned to small anatomical obstacles in the heart and lead to monomorphic ventricular tachycardia that can degenerate into polymorphic tachycardia and ventricular fibrillation. Electric field-induced secondary source stimulation can excite directly at the obstacle, and may provide a means to terminate the pinned wave or inhibit the transition to more complex arrhythmia. We used confluent monolayers of neonatal rat ventricular myocytes to investigate the use of low intensity electric field stimulation to perturb the spiral wave. A hole 2-4 mm in diameter was created in the center to pin the spiral wave. Monolayers were stained with voltage-sensitive dye di-4-ANEPPS and mapped at 253 sites. Spiral waves were initiated that attached to the hole (n = 10 monolayers). Electric field pulses 1-s in duration were delivered with increasing strength (0.5-5 V/cm) until the wave terminated after detaching from the hole. At subdetachment intensities, cycle length increased with field strength, was sustained for the duration of the pulse, and returned to its original value after termination of the pulse. Mechanistically, conduction velocity near the wave tip decreased with field strength in the region of depolarization at the obstacle. In summary, electric fields cause strength-dependent slowing or detachment of pinned spiral waves. Our results suggest a means to decelerate tachycardia that may help to prevent wave degeneration.

FIGURE 1

Optical maps of wave propagation in cardiac cell monolayers. (A) 0.9 V/cm electric field stimulation across quiescent monolayer with 2.6-mm-diameter obstacle. (B–D) Stimulation at obstacle (3 mm diameter) in front of the pinned wave caused the wave to advance or partially detach. (B) 1.5 V/cm field stimulus caused the wave to advance and remain pinned to the obstacle. (C) 3 V/cm field stimulus advanced the wave and caused the wave to detach, but the wave was able to reattach during the same cycle. (D) 3.5 V/cm field stimulus advanced and unpinned the wave, and the wave drifted to the boundary and terminated. In all maps the electric field is oriented from top to bottom, and the depolarized region is located on the top side of the obstacle depicted by the white circle. The field stimulus was turned on at t = 0 ms and remained on for 1-s. White arrows show differing degrees of detachment of the wave tip from the obstacle. The color bar indicates the normalized transmembrane voltage: blue represents the resting state, and red represents the peak of the action potential.

FIGURE 2

Effect of stimulus pulse-make on pinned spiral wave. (A) Wave-angle definition and field electrode arrangement across monolayer. For each experiment, depending on the direction of the field and chirality of the spiral wave, coordinates and angles were defined in such a way as to give a standardized clockwise-rotating wave with a positive polarity field directed from top to bottom, so that the region of depolarization would be at 0° and the region of hyperpolarization at 180°. The wave angle was measured between the stimulation site and a point along the wavefront 2 mm from the wave tip (black circle). (B) Field strength and timing dependence of wave detachment. Angle of the wavefront at time of pulse-make is shown. Total of 128 pulse-make trials across 10 monolayers are plotted, including 47 detachment events (37%).

FIGURE 3

Voltage maps of a spiral wave pinned to a 3-mm obstacle during field stimulation. (A) Before electric field. (B) During 2.4 V/cm field applied from top to bottom. (C) After electric field. In each row, voltage maps are shown in 40-ms increments and white arrows in the final frame show position of the wave tip 120 ms after the initial frame. The color bar indicates the normalized transmembrane voltage: blue represents the resting state, and red represents the peak of the action potential.

FIGURE 4

Polarization changes produced by the 3 V/cm electric field. A wave was pinned to a 3-mm obstacle, and a 1-s-long field pulse was applied from top to bottom across the monolayer. Action potential recordings are shown from four channels around the obstacle before, during, and after the field pulse. Dashed vertical lines indicate turn-on and turn-off of field pulse. Baseline change was measured from all channels during the pulse to create a polarization map showing the secondary source responses.

FIGURE 5

Action potential recordings before, during, and after a 3 V/cm electric field pulse of 1-s duration. (A) Depolarized region. (B) Hyperpolarized region. Action potentials are normalized in amplitude to control action potentials obtained before field stimulation. The upstroke velocity during the pulse normalized to the upstroke velocity before the pulse was 0.66 in the depolarized region and 0.98 in the hyperpolarized region. Wave was pinned to a 3-mm-diameter obstacle.

FIGURE 6

Variation in cycle length with time and with field intensity. (A) Cycle length (CL) measured before, during, and after 3 V/cm electric field for a wave pinned to a 3-mm-diameter obstacle. CL was measured at a location 2 mm from the top of the obstacle. (B) CL during the field pulse versus field strength for all experimental trials. For each trial, CL was averaged during the field pulse and normalized to its value before stimulation. A linear function was fit to the data (R = 0.86, p < 0.005, n = 118).

FIGURE 7

Regional propagation delay of a pinned wave around an obstacle. During (A) and after (B) 1, 2, and 3 V/cm field stimulation. Delay was measured as the wavefront passed through the four 90° quadrants around the obstacle (defined by the angle of the wavefront measured 2 mm from the obstacle), and normalized to the delay before stimulation for each trial. Data are shown such that depolarization (hyperpolarization) is at the top (bottom) of the obstacle. Data is plotted as mean ± SD. Asterisks indicate a mean significantly different than 1 (p < 0.005).

FIGURE 8

Variation in conduction velocity along the wavefront as the wave propagates across the top of the obstacle (region of depolarization). (A) Change in CV during 2 V/cm field stimulation at increasing distance from the obstacle edge. CV was measured normal to the wavefront at a point on the wavefront a fixed distance (d) from the obstacle edge measured at θ = 0°. For each trial, CV was averaged during the field stimulus at each location and compared with the average prestimulus CV at the same location. Data is plotted as mean ± SD. Asterisk indicates a mean significantly different than 1 (p < 0.005). (B) Wavefront propagation before and during field stimulation at 1, 2, and 3 V/cm for a single experiment. Wavefronts during field stimulation are aligned at a point 1.5 mm from the obstacle edge measured at θ = 0°. During field stimulation, the wavefront flattened near the wave tip, and at 3 V/cm detached from the obstacle.

FIGURE 9

Pulse-break stimulation. (A) Stimulation of quiescent monolayer containing a 2.6-mm-diameter obstacle. A 1.4 V/cm electric field oriented from top to bottom was applied for 1 s and turned off at t = 0 ms. After 50 ms, excited waves appear from regions near the field anode (upper left edge of voltage map) and at the obstacle. (B) Pulse-break stimulation of a spiral wave pinned to a 3-mm-diameter obstacle in a different monolayer, with a 0.9 V/cm field stimulus turned off at t = 0 ms. Stimulation occurred in the region of hyperpolarization causing wave to detach. After stimulation, a new wave was generated that remained pinned to the hole. (C) Field strength and timing dependence of wave unpinning. Angle of the wavefront at time of pulse-break is shown. For each experiment, angles were calculated as defined in Fig. 2. The color bar indicates the normalized transmembrane voltage: blue represents the resting state, and red represents the peak of the action potential.