Having a good understanding of the physiology of the heart will serve you well as you speak to Dr’s, nurses, and ‘Tech-Support!’
Ivan Joyner A refresher
ECG ADDITIONS A Summary of ECG ©2013
ECG Basics As a biomed you will find a great deal of equipment involves vital signs monitoring. Some of the signals most frequently monitored originate with the heart. The following is a brief overview of the ECG signal and associated monitoring. Actions and Reactions Within the specialized tissue of the heart, chemical reactions inside and outside the cells provide movement of ions through the cellular membrane. The imbalance of these ions across the membrane of a resting cell is effectively a difference of potential; therefore, an electric field and voltage exist. The movement of ions through the cellular membrane is essentially current through a resistance. The effect of the electric potential of a single cell is likened to the effect of a single voice in a crowded stadium; one person contributes to the roar.
The SA Node (sinoatrial) is the normal generator of the heart. That node or mass of specialized cells exists near the top of the rear of the upper chamber of the right side of the heart in the superior region of the posterior portion of the right atrium. This node sends action potentials to the heart setting its rate. Nerve input as well as chemical input from the blood can affect this signal and cause the heart to beat faster or slower. The SA node serves as the heart’s pacemaker. The action of the stimulus from the SA node causes the heart muscle to contract (beat). The beating or depolarization of the heart causes a chain reaction that we can read in the ECG waveform. Propagation moves that signal to the atrial wall and causes contraction. The signal continues to propagate to cause ventricular contraction (QRS waves) and then finally the heart rests (T wave)
Monitoring Electric potentials generated by the heart appear throughout the body and on its surface. We determine potential difference by placing electrodes on the skin and measuring the voltage between them. In Clinical ECG more than one lead must be recorded to fully describe the hearts electric activity. For this reason, it is important to have certain standard positions for ECG lead placement. Willem Einthoven used the principle of an equilateral triangle to standardize basic lead placement in the frontal plane of the body and that continues to this day. A description below includes some detail on the basics of ECG lead placement for the biomed. The fundamental components of the ECG monitoring system are electrodes, lead wires and cable, amplifier, and display. The electrodes obtain the signal, the cable conveys them, the amplifier boosts them, and the recorder records them. Electrodes: The main goal is to provide a safe, electrochemically stable and psychologically acceptable way to obtain signals without introducing or allowing artifact. Electrodes include electrolytes, which are pastes, gels, creams, jellies, semi-solids that are between electrodes and the skin to obtain better contact. A main ingredient in the electrolyte is water with the addition of potassium chloride, sodium chloride as well as fungicides and germicides. Thickeners are added to keep the electrolytes from running when applied. Amplifiers: The fundamental task of the amplifier is to boost the signal to a level that is readable on a recorder or display. The gain in ECG is typically 1000 essentially 10millimeters per milli volt (deflection over horizontal). Amplifier should be linear with @ 100 Hz bandwidth. The typical amplifier is a differential input type, which means that the amplifier responds only to the difference between the signals. If the input signals are equal the output is zero. How it works The electric potentials generated by the heart appear throughout the body and on its surface. We determine the potential differences by placing electrodes on the surface of the body and measuring the voltage between them. Different pairs of electrodes are located at different locations and will generally yield different voltages because of the nature of the electric field of the heart. For this reason it is important to have certain standard positions for the evaluation of ECG.
In ECG measurement more than one lead must be recorded to fully describe the hearts electrical activity. Generally several leads are taken from the frontal plane, and the transverse plane. The three leads that make up the frontal plane are RA, LA, LL. The lead configurations are as follows: Lead I= LA to RA; Lead II= LL to RA; Lead III= LL to LA. The lead vectors that are formed can be approximated as an equilateral triangle known as Einthoven’s triangle. In the modern ECG system the patient is not grounded at all. Instead the right-leg electrode is connected to the output of an auxiliary op amp. The common-mode voltage on the body is sensed by two averaging resisters, inverted, amplified, and fed back to the right leg. The body’s displacement current flows not to ground but to the op-amp output circuit. This reduces the interference and effectively grounds the patient. Additionally this provides some electrical safety as any abnormally high voltage detected between the patient and ground would drive the op-amp into saturation effectively ungrounding the patient. As a biomed you need to have a basic understanding of these processes. It is very difficult to debug or troubleshoot a system if you don’t know what signals the system is trying to acquire or how they are generated. Now you know and knowing is half the battle. Be sure to visit biomedworkshops.com for more biomed tips.
Helpful Hints Lead Wire Color Code RL= Green LL= Red RA= White LA= Black V1-5= Brown There are different tricks used to remember the color code such as: White on the Right (RA is White) Also Smoke over fire (RA is white, LL is red). I use the ‘L’ method to remember lead configurations. Lead I is Right Arm to Left Arm Lead II is Right Arm to Left Leg Lead III is Left Arm to Left Leg The ‘L’ method to remember these locations is simply. Lead I contains 1 L Lead II contains 2 L’s Lead III contains 3 L’s See figure below for basic ECG waveform
Basic ECG waveform with descriptions.