Introduction
Summary
- The notes discuss the basics of electrocardiography (ECG), focusing on the heart's behavior as an electric dipole and the utilization of Einthoven's triangle to measure potential differences across the body. Additionally, they cover the principles of electrostimulation, detailing types of electric pulses and their parameters, such as amplitude, duration, and frequency. Key physiological concepts such as rheobase and chronaxie are explained, which are critical for understanding tissue excitability under electrical stimulation.
Heart as an Electric Dipole
- The heart functions as an electric dipole with positive and negative poles, due to the reallocation of ions.
- Direction of the electric moment in the dipole moves from excited to unexcited parts of the heart, altering the electric field with each heartbeat.
- These changes, though minute in scale (millivolts), alter the surface’s electric potential, which is measured with electrodes.
Linear Electrocardiogram (ECG)
- ECG records the electrical activity of the heart, either between two points on the body's surface or between one body point and a reference electrode.
Einthoven's Triangle
- Utilizes an imaginary triangle formed by two shoulders and the pubis to measure potential differences between any two of its points.
- Each potential difference correlates with the projection of the integral electric vector over the line connecting these points.
Electrostimulation and Electric Impulses
- Electrostimulation studies tissue excitability by modifying electric current and impulse patterns. The teasing impact of the electrical current decreases as frequency increases.
- Types of electric impulses:
- Radio impulses: high-frequency electromagnetic wave packages.
- Video impulses: impulses with uniform polarity; characterized by a rising front, a peak with slight change, and a quick decreasing back.
Electrical Parameters
- Altitude (A): Refers to amplitude, maximal height of an impulse.
- Duration of impulse and its front.
- Speed of change of impulse or steepness in front.
- Impulse electric current, coefficient of filling (Q = Δt/T, where T is the period).
Physiological Basis of Electrodiagnostics
- Rheobase and Chronaxie
- Rheobase: Measurement of the lowest constant electric current that can produce action potential when applied indefinitely.
- Chronaxie: Represents the minimum time required for an electric current at twice the strength of the rheobase to stimulate a muscle or neuron. It is more stable as a measure and indicates excitability under varied temperature conditions. Higher temperatures lower the threshold and vice versa.