Compuerta Basica Logica

UNIVERSIDAD AUTÓNOMA DE GUADALAJARA

LABORATORIO DE INSTRUMENTACIÓN BIOMÉDICA

SEÑAL DE ECG

Práctica Demostrativa

Objective

To familiarize students with the electrocardiogram (ECG) equipment, understand and practically place the 3 ECG leads onto a subject.

Introduction

The heart is a muscle formed in a way that allows it to act as a pump for blood. The heart pumps blood when the muscle cells making up the heart wall contract, generating their action potential. This potential creates electrical currents that spread from the heart throughout the body. The spreading electrical currents create differences in electrical potential between various locations in the body, and these potentials can be detected and recorded through surface electrodes attached to the skin. The waveform produced by these biopotentials is called the electrocardiogram (ECG).

An example of a typical ECG waveform is shown in Fig 2. This particular waveform is typical of a measurement from right arm to left arm. In standard ECG recording there are five electrodes connected to the patient: right arm (RA), left arm (LA), left leg (LL), right leg (RL), and chest (C). These electrodes are connected to the inputs of a differential buffer amplifier through a lead selector switch. The recording obtained across different pairs of electrodes results in different waveform shapes and amplitudes; these different views are called leads. Each lead conveys a certain amount of unique information that is not available in the other leads. The physician is often able to diagnose the type and site of heart disease by examining these different views because waveform anomalies have been correlated with disease conditions in the past.

FIGURE 2. Normal ECG waveform

The electrical connections for the 12 standard leads are shown in Fig 1.12 The ECG unit uses the subject's right leg as the common electrode, and the lead selector switch connects the proper limb or chest electrodes to the differential amplifier input. The bipolar limb leads are those designated lead I, lead II, and lead III and form what is called the Einthoven triangle.

• Lead I - LA is connected to the amplifier's non-inverting input, while RA is connected to the inverting input.

• Lead II - LL is connected to the amplifier's non-inverting input, while RA is connected to the inverting input (LA is shorted to RL).

• Lead III - LL is connected to the amplifier's non-inverting input, while LA is connected to the inverting input (RA is shorted to RL).

The unipolar limb leads, also known as the augmented limb leads, examine the composite potential from all three limbs simultaneously. In all three augmented leads, the signals from two limbs are summed in a resistor network and then applied to the amplifier's inverting input, while the signal from the remaining limb electrode is applied to the non-inverting input.

• Lead aVR - RA is connected to the non-inverting input, while LA and LL are summed at the inverting input.

• Lead aVL - LA is connected to the non-inverting input, while RA and LL are summed at the inverting input (LA is shorted to RL).

• Lead aVF - LL is connected to the non-inverting input, while LA and RA are summed at the inverting input (RA is shorted to RL).

The unipolar chest leads (V1 through V6) are measured with the signals from certain specified locations on the chest applied to the amplifier's non-inverting input, while the RA, LA, and LL signals are summed in a resistor Wilson network at the amplifier's inverting input.

• V1 - Fourth intercostal space, right sternal margin

• V2 - Fourth intercostal space, left sternal margin

• V3 - Midway between V2 and V4

• V4 - Fifth intercostal space, mid-clavicular line

• V5 - Same

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