Medical Applications of Electricity Lecture

Question 1

What is the equation for charging in an RC circuit?

Answer 1

U_C = U_T ( 1 - e^-t/RC)

Uc is charged voltage

UT is source voltage

t is time

RC reistance and capacitance which equals time constant, tau

Question 2

What is the equation for discharging an RC circuit?

Answer 2

U_t = U₀e^-t/RC

Ut is voltage after discharge

U0 is voltage before

t is time

RC is resistance times capacitance (AKA tau, time constant)

Question 3

What is the equation for the time constant of an RC circuit?

Answer 3

Τ = RC

Τ is time constant

R is resistance

C is capacitance

Question 4

What is rheobase?

Answer 4

minimum current amplitude of infinite duration that will cause sensation

• corresponds to the horizontal asymptote of the below graph

Question 5

What is chronaxie?

Answer 5

the minimum time required for a current twice the rheobase strength to cause sensaton

Question 6

What is a high pass filter?

Answer 6

an electronic filter that only passes signals above a certain frequency

• in a circuit, a high pass filter utilizes a capacitor as a shortcut at high frequencies

Question 7

what is a low pass filter?

Answer 7

an electronic filter that allows only signals below a certain frequency to pass

• in a circuit, a low pass filter utilizes a capacitor as a discontinuity at low frequencies

Question 8

Draw a strength duration curve.

Label the two important values associated with it.

Answer 8

rheobase and chronaxie

are the important values

Question 9

Explain the principle of selective electrical stimulus therapy.

Answer 9

Different pulse shapes can be used to selectively stimulate only injured tissues, which are more sensitive to electrical stimulation.

Because different pulse shapes have different minimum stimulating amplitudes at long duration, sawtooth pulses can be used to stimulate only injured tissue.

Question 10

Explain LC circuits.

Answer 10

An LC circuit contains a capacitor and an inducer.

• fully charged capacitor sends its charge in a + to - direction to the inducer
• as capacitor discharges, a magnetic field gathers in the inducer in the same direction as the current
• when the capacitor is fully discharged, the magnetic field is strongest
• inducer now induces current in same direction as the magnetic field, depositing positive charge on the capacitor, opposite its original location
• once the capacitor is fully charged again, the process repeats in the opposite direction

Question 11

Explain ideal vs. real LC oscillating circuits.

Answer 11

Ideal - an LC circuit with “perfect wires” (no resistance, perfect conduction, capacitance etc.) in which the amplitude of the voltage oscillation is constant over time

Real - any LC circuit in reality in which the resistances of the wiring and the losses via EM radiation cause a gradual decrease in the amplitude of voltage oscillation

Question 12

What are the 3 methods of high frequency heat therapy?

Answer 12

1. Capacitor Field Method
2. Coil Field Method
3. Radiation Field Method

Question 13

How is energy put out by capacitor field heat therapy?

What kinds of tissues does it effect?

Answer 13

• energy is outcoupled via the electric field
• effects fat tissues and has long range effects

Question 14

How is energy put out by coil field method heat therapy?

What tissues does it effect?

Answer 14

• energy is outcoupled via the magnetic field
• mostly heats muscle

Question 15

How is energy put out by the radiation field method?

What tissues does it effect?

Answer 15

• energy is outcoupled through the radiation field
• effects both fat and muscle, but to a lesser degree than other methods

Question 16

What method is used for breaking up kidney and bile stones?

How?

Answer 16

Extracorporeal Shockwave Lithotripsy or ESWL

• focused high intensity acoustic waves generated by discharge of a high voltage (20 kV) condenser
• can be paired with simultanous x-ray or ultrasound to determine effectiveness