Schematic of the diode bridge and the other high-voltage components of the stimulator. The discharge signal comes from the NI 6503 card. Bridge controls A–D are connected to the bridge. The terminals at the right are the two sets of high-voltage outputs.
Intermediate logic stage between the digital computer outputs and the diode bridge. This circuit is meant to protect the bridge from programming and I/O errors. Fire bridge (external) is the external TTL pulse from the timing and data acquisition computer. Fire bridge (internal) comes from the stimulator control computer and can be used for testing. Return signal is a TTL output on the front panel that can be used to monitor the firing of the device for testing.
Overview of experimental setup illustrating the data flow.
A block diagram of the stimulator. Arrows indicate the direction of data flow. Dark black connections are for the high voltage. Optical isolation is not pictured.
Complete schematic of the medical-grade capacitor bank. The low-voltage, operator-controlled rotary switch pictured can select 150, 300, 450, 600, or of capacitance. Each individual capacitor is rated up to at least . There are eight and one capacitors. The capacitors are arranged into three units rated up to that can be either activated or discharged. By using different combinations of the three blocks ( and ), each of the five settings can be achieved.
A cartoon timing diagram illustrating the proper functioning of the device running in single-output (S2 only) mode. In this mode, the external state signal is used to envelop the charging, with rising edges representing the beginning of charging and falling edges representing the end of charging. The device state is a software state drawn as a three-level line diagram with diagonal lines representing software delays. Note that this is not a voltage trace. In this example, changing to the charging state is a two-step process: first the external state signal triggers the software to transition to the charging state, and some finite time later, the software changes the physical lines to put the device in the charging state. In contrast, the external fire signal has an almost immediate impact on the stimulator itself, allowing synchronized timing between the cameras and the stimulation. The proper fire counter increments when the CCS end of charge is high and when the external fire signal has a falling edge. Dashed lines represent concurrent events.
An example equivalent to Fig. 6 except that the CCS has not been provided with sufficient time to fully charge the capacitors to the requested voltage. The end of charge never becomes high, causing the rising edge of the external fire signal to trigger the improper fire counter.
A timing diagram for the two-output (S1/S2) mode. Unlike the S2 only mode, the external state signal causes a switch between charging up to two different voltages (S1 and S2). If the new voltage is equal or greater to the previously requested voltage, the capacitors are not discharged in between two pulses. However, if the voltage requested is less than the previous one, the capacitors are discharged to reset them, as can be seen in the first S1 after the S2.
(Color) An example of the spatiotemporal response of an isolated rabbit heart to strong field stimulation of short duration, as obtained with fluorescence imaging of a dye that is sensitive to the transmembrane potential following a , shock. In (A), the electric field is applied from right to left, and in (B), the field is applied from left to right. The numbers below frame indicate the time in ms since the shock onset. Images were acquired with a CCD camera (Redshirt, at ) (from Ref. 8, with permission).
Temporal voltage traces from the two movies in Fig. 9. The top of each figure is the S2 TTL pulse, the middle is the voltage trace, and the bottom is the current trace. The voltage trace is scaled from the actual voltage because the measuring apparatus involved two electrodes at an arbitrary distance apart placed on either side of the heart in the bath. In reality, the maximum voltage should be within 3% of . The current in (C) and (F) was measured with a dc current probe (Tektronix TCP305) that encircled one of the leads between the defibrillator and the heart. The voltage in (B) and (E) was measured with a high-voltage differential probe (Tektronix P5205) connected to Ag–AgCl electrodes in the bath and adjacent to the heart. Each curve has data points recorded with a digital oscilloscope (Tektronix TDS5034B).
Voltage and current data from the same heart, but for a longer duration shock. While stimulation strength after is less than the maximum due to the voltage decay of the capacitors, it is still significant at almost 30% of the maximum. This was made possible by the large capacitance of the device . Panel (D) is the ratio of part (C) to (B), clipped to only the active firing region. The curve is also Gaussian filtered with a sigma of 50 points. The fast upstroke of this curve (first ) is due to the response time of the current probe, and the slower upstroke between 1.5 and is probably the result of electrode polarization. The final plateau is consistent with a constant load resistance.
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