Exam Tidbits: Split Cathodes, Serial and Parallel Circuits

Split Cathodes, Serial and Parallel Circuits

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Split Cathodal Pacing

• Back in 2002, unipolar left ventricular leads were the only option for cardiac resynchronization therapy (CRT).
• In December 2003, an article in PACE discussed "split cathodal pacing" as a parallel circuit.
• Split cathodal pacing uses the tips of the RV and LV leads as the common cathode.
• The total impedance measured in a bipolar split cathodal configuration was consistently greater than predicted by a parallel circuit.
• The total impedance ($R<em>T$) is given by the equation: $\frac{1}{R</em>T} = \frac{1}{R<em>{LV}} + \frac{1}{R</em>{RV}}$.
• Example:
  • LV lead impedance: $R_{LV} = 874 \Omega$
  • RV lead impedance: $R_{RV} = 705 \Omega$
  • Predicted total impedance: $R_T = 390 \Omega$
  • Measured impedance in bipolar split cathodal configuration: $R_T = 516 \Omega$
• Combining two cathodes dramatically changes the size and shape of the effective electrode, influencing impedance and threshold.
• Conclusion of the study:
  • A split cathodal configuration increases the apparent stimulation threshold for the LV and RV.
  • Pacing threshold further increased by programming from unipolar to bipolar split cathodal configuration.
• In exam context:
  • Capture threshold increased.
  • Lead impedance decreased.

Electrical Circuits

Serial Connections

• A serial connection uses a single impedance to determine the output current.
• When one lead, such as the RV lead, delivers the current, it is a serial connection.
• In a bipolar lead, the current path is from lead tip (cathode) to ring (anode).
• In a unipolar connection, the can is the anode.
• Ohm's Law: $mA = \frac{voltage}{impedance}$.
• The single impedance is the sum of the individual impedances.
• Example: If the RV impedance is $500 \Omega$ and the LV impedance is $600 \Omega$, the measured serial impedance will be $1100 \Omega$, which may result in a slightly higher capture threshold.

Parallel Connections

• Parallel connections occur when there are two independent impedances.
• In parallel circuits, the sum of the output current is used to determine the final lead impedance.
• Examples:
  • Example 1:
    • Output voltage: $2.5 V$
    • LV impedance: $700 \Omega$
    • RV impedance: $400 \Omega$
    • LV current: $\frac{2.5V}{700 \Omega} = 3.6 mA$
    • RV current: $\frac{2.5V}{400 \Omega} = 6.3 mA$
    • Total current: $3.6 mA + 6.3 mA = 9.6 mA$
  • Example 2:
    • Output voltage = $5 V$
    • LV impedance = $700 \Omega$
    • RV impedance = $400 \Omega$
    • LV Current: $\frac{5V}{700 \Omega} = 7.1 mA$
    • RV Current: $\frac{5V}{400 \Omega} = 12.5 mA$
    • Total current: $7.1 mA + 12.5 mA = 19.6 mA$

Pulse Droop

• Pulse droop is caused by the inability of a device's output capacitors to handle high current drain efficiently.
• Pulse droop is the difference between the leading edge voltage of the output waveform and the trailing edge voltage.
• When the current is high (impedance low), the trailing edge voltage may be only 1/2 (or less) the amplitude of the leading edge voltage.
• This results in HIGHER capture thresholds since the delivered current will be less as compared to the capture threshold determined by the PSA.
• PSA capacitors are larger than pacemaker capacitors.

Diagrams

• First-generation CRT-P devices used a parallel circuit for connecting the leads to the output stage of the pulse generator.
• Series and parallel circuit diagrams may appear on the exam.