Musculoskeletal System, Nervous System & Bioelectricity: Electrical Stimulation of the Heart

Comprehensive practice flashcards covering the physical principles of electrical heart stimulation, types of electrodes, and the working principles of defibrillators, pacemakers, and ICDs.

According to the lecture, what is required to evoke electrical activity in an excitable cell like a cardiomyocyte?

An initial depolarization of the membrane in (part of) the cell is needed to start an action potential.

What is the specific resting membrane voltage of a cell as described in the lecture?

$$-70\,mV$$

Why is changing the extracellular potential globally ineffective for membrane depolarization?

The intracellular potential immediately drops as well, so the membrane voltage effectively does not change.

What must be created on the outside of a cell to achieve local membrane depolarization?

A potential gradient.

What is required to generate an extracellular potential gradient?

An ionic current.

What determines the characteristic time $\tau$ after which the intracellular potential mirrors the extracellular gradient?

The intracellular resistance and the membrane capacitance, given by $\tau = R_i \cdot C$.

How does an electrical current sent through tissue create an electrical potential gradient?

It follows Ohm's law ($V = I \cdot R$).

What determines the strength of the electrical stimulation in tissues?

The current density, which is current per cross-sectional area.

Why are longer and thinner cells like axons or myotubes more easily stimulated?

They sense a larger part of the extracellular gradient and have a higher intracellular resistance, making depolarization last longer.

How does a DC current affect cellular activity over time?

It will only evoke a single activity at its onset but not a continued activity while it is maintained.

How is electrical current carried respectively in metals and in tissue?

In metals, it is carried by electrons; in tissue, it is carried by ions.

What are 'capacitive' or 'dry' electrodes?

Electrodes where electrons accumulating at the electrode attract or repel ions, creating an ionic current via electrostatic interaction.

What are 'Faradaic' or 'wet' electrodes?

Electrodes where electrons reacting with the material release an ion via an electrochemical reaction.

Why are capacitive electrodes mainly used for recording rather than stimulation?

The amount of charge they can store is limited, allowing only for small currents for short durations.

In implanted metal electrodes, what acts as a capacitor?

A thin oxide layer that tends to form on the metal.

What is required for wet electrodes to maintain good contact with the skin?

Sweat or a conductive gel.

Give an example of a Faradaic electrode mentioned in the transcript.

Silver/silver chloride electrode.

How can the consumption of Faradaic electrode material by DC currents be partially reversed?

By using charge-neutral pulses.

According to Ohm's law, how is current related to resistance for a given voltage?

Current is inversely proportional to resistance.

Why are higher voltages needed with skin electrodes compared to implanted electrodes?

The skin layer has high electrical resistance, whereas implanted electrodes are in tissue of low resistivity.

How do larger electrode area and distance affect current density?

Current density decreases with larger electrode area and increased distance between electrodes.

What is Ventricular Fibrillation?

The random, chaotic electrical activity of the whole heart resulting in ineffective contractions and stopped blood pumping.

What is the goal of using a defibrillator during Ventricular Fibrillation?

To cause a complete excitation/contraction of all muscle fibres in the heart so it may resume a normal rhythm.

Identify the names and roles of the two paddles used in defibrillation.

The apical paddle (current enters the body) and the sternal paddle (current leaves the body).

What voltage and current levels can a defibrillator reach to generate a sufficient gradient?

Up to $10,000\,V$ and up to $10\,A$.

What component in a defibrillator steps up the input voltage from 120/240 V to 5,000-10,000 V?

A transformer.

What is the purpose of the diode in a defibrillator circuit?

It allows current to flow in only one direction.

How much energy is stored in a fully charged defibrillator capacitor?

~$400\,J$

What is Atrial Fibrillation?

Random, chaotic electrical activity of the atria while ventricles can still contract, leading to irregular heart beats and reduced output.

What is DC cardioversion?

An elective procedure where a short current pulse from a defibrillator causes a forced contraction of the atria to restore normal sinus rhythm.

Why are patients anticoagulated for several weeks before DC cardioversion?

To minimize the risk of dislodging a blood clot, which could cause an embolic stroke.

To what part of the ECG is the delivery of a DC cardioversion shock synchronized?

The QRS complex.

How does the energy level of a DC cardioversion shock compare to ventricular defibrillation?

It is about half the energy.

In what patients are artificial pacemakers typically implanted?

Patients with heart block, bradycardia, or cases where DC cardioversion was not successful.

Where is the pacing device of an artificial pacemaker surgically implanted?

Under the skin at the shoulder.

What is the path of pacing wires from the shoulder to the heart?

Through a vein in the shoulder, advanced through the right atrium, and imbedded in the right atrium and right ventricle.

What controls the charge-up and pulse delivery in an artificial pacemaker?

An integrated circuit (CMOS chip).

What formula defines the output voltage $V_{out}$ in a pacemaker circuit?

$$V_{out} = \frac{C_1}{C_2} \cdot V_{REF}$$

Why do artificial pacemakers require much lower energy levels than defibrillators?

They use implanted electrodes, shorter distances, and controlled excitation.

What does 'ICD' stand for?

Implantable Cardioverter Defibrillator.

What is the primary function of an ICD?

It continuously checks the heartbeat and delivers shocks only when needed to restore regular rhythm in atrial or ventricular fibrillation.

According to the comparison table, what is the energy source for a Defibrillator?

Mains Power

What is the energy source for an Artificial Pacemaker or an ICD?

Battery

What is the typical capacitance and voltage for a Defibrillator according to the summary table?

$500\,\mu F$ and $6000\,V$

What is the voltage range for an Artificial Pacemaker?

$$1-10\,V$$

What is the energy per pulse for a Defibrillator versus an ICD?

Defibrillator: $\ge 360\,J$; ICD: $\le 65\,J$

What is the typical energy per pulse for an Artificial Pacemaker?

$$15\,\mu J$$

What is the typical rhythm rate provided by an Artificial Pacemaker?

~$70\,bpm$

Who is credited in the lecture with electrical stimulation experiments in 1793?

Luigi Galvani

On what date was this lecture on Electrical Stimulation of the Heart given?

28-NOV-2025