The Field Recovery Critical Point

The field-recovery critical point was observed during an attempt to record the entire strength-interval curve from a single electrical stimulus.18 Classically, the strength-interval curve is determined by sequentially giving shocks of different strengths at a number of different coupling intervals to determine the excitation threshold throughout the relative refractory period (Fig. 4). To determine the entire strength-interval relationship from a single stimulus, Frazier et al.18 recorded simultaneously from 117 electrodes covering approximately 10 cm2 of the right ventricular epicardium of dogs. They launched a linear activation wavefront during S1 pacing by stimulating simultaneously from a row of pacing electrodes on the right side of the array of recording electrodes (Fig. 5a). The recovery times to a local 2 mA 3 ms stimulus following the S1 stimulus were determined at 24-44 recordings o

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Recovery Interval (ms)

Figure 4: Strength-interval curve (dark line) expressed in terms of the extracellular potential gradient and of the degree of refractoriness of the tissue as indicated by the recovery interval from the previous excitation until the time of the stimulus. Three types of responses are indicated. The region to the right of the strength-interval curve is directly excited by the stimulus (clear region). The region below and some of the region to the left of the strength-interval curve is not directly excited by the stimulus (shaded region). The region immediately to the left of the upper portion of the strength-interval curve undergoes a graded response to the stimulus (hatched region).

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Recovery Interval (ms)

Figure 4: Strength-interval curve (dark line) expressed in terms of the extracellular potential gradient and of the degree of refractoriness of the tissue as indicated by the recovery interval from the previous excitation until the time of the stimulus. Three types of responses are indicated. The region to the right of the strength-interval curve is directly excited by the stimulus (clear region). The region below and some of the region to the left of the strength-interval curve is not directly excited by the stimulus (shaded region). The region immediately to the left of the upper portion of the strength-interval curve undergoes a graded response to the stimulus (hatched region).

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