Electrical stimulation to the heart

Early experiments on stimulation of nerves and muscles date back to the late 18th century.
- Notable discussion documented in 1793.
- Such experiments inspired Mary Shelley's Frankenstein.

Cardiomyocytes are excitable cells that behave like axons, transmitting action potentials.
Action potentials require a threshold to trigger depolarization, controlled by calcium and sodium ions.
At rest, the membrane potential is about -70 millivolts, with the outside considered 0 volts.
The inside of the cell must become less negative (more positive) to initiate depolarization.

Gradient Setup: To effectively stimulate a cell, create an electrical potential gradient across it.
- A gradient allows ionic current to flow from a more negative to a more positive area, stimulating action potentials locally.
After stimulation, the membrane returns to resting potential following a characteristic time determined by resistance and capacitance.
Time Constant: The time constant is the product of resistance and capacitance; longer time constants indicate slower return to equilibrium.

Cells behave like resistors and capacitors in electrical circuits.
Depolarization can trigger an action potential if sufficient stimulation is applied over a period of time.
Effective stimulation depends on current density, which is current per cross-sectional area.

Electrode Types: Used for monitoring and stimulating cells include capacitive (dry) and electrolytic (wet) electrodes.
- Capacitive Electrodes: Use electrostatic interactions; electrons accumulate at the electrode, attracting positively charged ions in tissues, useful for recording activity.
- Electrolytic Electrodes: Use electrochemical reactions; ions exchange occur at the electrode which can stimulate tissues effectively.
The basic operation involves conversion between electrons (in metal) and ions (in tissue).

Defibrillators:
- Deliver a large shock (up to 10,000 volts) to halt and reset a fibrillating heart.
- Uses a high-voltage capacitor charged through AC and discharges when the electrodes contact the patient.
Cardioversion: Similar to defibrillation but targets atrial activities, utilizing synchronized timing with the QRS complex to avoid causing ventricular fibrillation.
Artificial Pacemakers: Implanted devices controlling heart rhythm by providing small, consistent electrical stimuli.
- Uses a charging mechanism similar to a defibrillator but operates continuously, typically at 70 beats per minute.
- Modern designs adapt to physiological needs of the patient.

Energy Storage:
- Defibrillators and cardioversion devices store hundreds of joules of energy, use high voltage, and are single-use.
- Artificial pacemakers and implantable defibrillators work continuously with a small amount of energy, designed for ongoing monitoring and support.

Understanding electrical stimulation of cells benefits medical applications, aiding in the development of crucial devices like defibrillators, pacemakers, and electrocardiograms.