EKG: Cardiac Monitoring and 12-Lead EKG Basics

EKG Nursing

Cardiac Monitoring and 12-Lead EKG Basics: Paper

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

The EKG machine or cardiac monitor records the voltage (potential difference) that is present between electrodes that are placed on the patient. This voltage is a visible representation of the electrical movement through the myocardial or contractile cells. Depolarization and repolarization is noted as the stylus or needle of the machine creates positive and negative deflections. The final product is a representation of both time, duration of the wave, and voltage, amplitude or height of the wave. Another criterion that is noted is the configuration of the wave, which looks at the contours and final shape of the waveform.

EKG paper is a special type of graph paper made up of small boxes that are 1 mm high and 1 mm wide. Each horizontal 1-mm box represents 0.04 seconds. The large boxes, denoted by the heavier lines on the grid paper, are made up of five of these small boxes and thus represent 0.20 seconds (5 × 0.04 = 0.20). Five large boxes that are comprised of five small boxes each (25 small boxes total) represent 1 second. Fifteen large boxes equal an interval of 3 seconds while 30 large boxes represent 6 seconds. The 6-second strip consisting of 30 large boxes can be used to calculate the patient’s heart rate. This horizontal movement of the machine’s needle is a measurement of time and is labeled as seconds.

The voltage or amplitude of the waveform is measured by the vertical axis of the EKG paper. The voltage is measured in millivolts (mV) and amplitude is measured in millimeters (mm). Each small box represents 1 mm or 0.1 mV and each large box represents 5 mm or 0.5 mV. When measuring amplitude, simply count the number of small boxes from the baseline wave (isoelectric line) to the highest point visible of the wave. This can also be used to determine amplitude of intervals or segments. To confirm the accuracy of this measurement, the machine must be calibrated to validate that a 1-millivolt (mV) electrical signal will produce a wave that equals 10 millimeters (mm) in height. This measurement is expressed in millimeters (EKG paper).

Clinical Alert

The paper moves through the machine at a rate of 25 mm per second. This speed can be altered to increase or decrease. If the patient has a rapid heart rate, increasing the paper speed will make it easier to see potential P waves and can assist in the diagnosis of the rhythm. When looking at these faster or slower rates of paper speed, remember that the complexes will be distorted.

Cardiac Monitoring and 12-Lead EKG Basics: EKG paper

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Cardiac Monitoring and 12-Lead EKG Basics: Leads

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

The word “lead” is used to mean both a physical object, that is, the lead wire and electrode that is attached to the patient and also to indicate a specific recording of electrical currents between one positive pole and one negative pole, or two electrodes. The direction of the current at a specific time in a portion of the heart is recorded as waveforms on the EKG. This direction of the current is known as a vector.

A straight line on the EKG means the current is nonexistent or very weak. This occurs when there is not a true negative or positive field within the cell because the charges are balanced. During depolarization, waveforms deflect downward when the current travels to the negative pole and upward when the current travels towards the positive pole. During repolarization, these waveforms create the opposite effect. Current that flows perpendicular (at a 900 angle) to the lead is expressed by a biphasic (both directions) waveform.

The representation on the EKG is the sum of the electrical currents that are passing through the heart at any given time. This picture of electrical activity of the myocytes will correspond to the largest mass of activity, meaning that some activity is masked if a stronger or larger mass is undertaking similar activity at the same time (Correlation of waveforms and flow of current). Different leads allow the heart’s electrical activity to be viewed in planes, or cross-sectional orientations.

These include the vertical (frontal or coronal) and horizontal (transverse) planes. This provides for a complete look at the heart. Twelve leads allow the health care professional to view this three-dimensional view of the electrical activity and this is accomplished with 10 electrodes strategically placed on the body. The EKG machine or the cardiac monitor changes the polarity between the leads to achieve these 12 views instead of the health care professional having to change the positive and negative leads to different places on the body.

Cardiac Monitoring and 12-Lead EKG Basics: Correlation of wave forms and flow of current

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Cardiac Monitoring and 12-Lead EKG Basics: Frontal Plane

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

The six vertical or frontal plane leads are also known as the limb leads and consist of three bipolar and three unipolar leads. The three bipolar leads form a triangle around the heart to show a frontal plane view. This triangle is historically known as Einthoven’s triangle. Einthoven’s triangle is formed by the axes of the first three limb leads with the heart at the epicenter. The axis of the lead is the imaginary connection between the positive and negative electrodes of a lead (Einthoven’s triangle).

A bipolar lead has both a positive and a negative electrode that records the electrical potential difference between the two electrodes. The bipolar leads (also called the standard limb leads) are leads I, II, and III (Standard Limb Leads).

Cardiac Monitoring and 12-Lead EKG Basics: Einthoven’s triangle

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Cardiac Monitoring and 12-Lead EKG Basics: Standard Limb Leads

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

CaptureCapture

 

Bipolar leads include a ground electrode to prevent extraneous electrical interference. When cardiac monitoring is utilized, this ground wire is placed on the chest.

  • Lead I: The positive electrode for this lead is placed on the patient’s left arm or on the left side of the chest while the negative electrode is placed on the right arm. This is because current flows from negative to positive. This allows the EKG to show current moving from right to left.
  • Lead II: The positive electrode is placed on the patient’s left leg while the negative electrode is placed on the right arm. The current travels down to the left in this lead and produces a positive deflection that causes tall P, R, and T waves. Sinus node and atrial arrhythmias are monitored using this lead.
  • Lead III: The positive electrode is placed on the left leg while the negative electrode is placed on the left arm. This lead produces a positive deflection and is used with lead II to view inferior myocardial infarctions.

The last three frontal or vertical leads are unipolar. A unipolar lead has a single positive electrode and a relative negative “electrode” which is the heart itself. The unipolar leads (also called unipolar limb leads or augmented limb leads) are aVR, aVL, and aVF. The small letter “a” used in reference with these leads means that they are “augmented” or enhanced because of their normal small amplitude. The “R”, “L”, and “F” refers to the location of the positive electrode, thus, the positive electrode is placed on the right arm in aVR, on the left arm in aVL, and on the left foot (or leg) in aVF. The “V” stands for vector since each one is looking at the direction of the electrical current from that particular view.

  • aVR is augmented vector right. The positive electrode is placed on the right arm and normally produces a negative deflection since the heart’s electrical activity moves away from the lead. This lead provides views of the atria and great vessels but no view of the heart’s walls.
  • aVL is augmented vector left. The positive electrode is placed on the left arm and produces a positive deflection. This lead shows electrical activity coming from the lateral wall of the left ventricle.
  • aVF is augmented vector foot. The positive electrode is placed on the left leg and produces a positive deflection. This lead shows activity coming from the heart’s inferior wall (Frontal plane leads).

Cardiac Monitoring and 12-Lead EKG Basics: Frontal plane leads

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Cardiac Monitoring and 12-Lead EKG Basics: Horizontal Plane

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

The chest leads provide information about the heart’s horizontal plane and are placed in chronological order across the patient’s chest. The view of the heart produced by these leads is as if the body were sliced in half horizontally, providing a left and front side view of the heart. These leads are also unipolar with the center of the heart, as calculated by the EKG machine, serving as the opposing pole for these leads. These are listed in order as: V1, V2, V3, V4, V5, and V6. In the normal EKG, there is a progression of wave formation. This is depicted in Precordial Leads and Precordial lead placement.

Clinical Alert

If leads need to be placed on the torso, position them as close to the appropriate limb as possible to decrease artifact. It is important that the limb leads be placed on the extremity (location does not matter) over tissue and not bony prominences.

Cardiac Monitoring and 12-Lead EKG Basics: Nonstandard Leads

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Several leads that are not part of the standard 12-lead EKG can provide valuable information regarding specific situations. These are the right chest leads (right-sided EKG), posterior chest leads (posterior EKG), and modified chest leads.

Cardiac Monitoring and 12-Lead EKG Basics: Precordial Leads

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

CaptureCapture

 

Cardiac Monitoring and 12-Lead EKG Basics: Precordial lead placement

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

  • Right chest leads: Right chest leads are utilized to assist in the determination of a suspected right ventricular myocardial infarction. A regular 12-lead EKG looks only at the left ventricle. The leads are placed identical to the standard chest leads but all leads are instead placed on the right side of the chest. The right chest leads are V1R, V2R, V3R, V4R, V5R, and V6R. In this situation, the normal V1 and V2 switch places. Often only V4R to V6R is done to view the right ventricle. This will demonstrate the changes consistent with a myocardial infarction on the right side (Right-sided EKG). Be sure to label this EKG as being right sided. Pediatric patients will have some specific changes on the right-sided EKG that are not consistent with their adult counterparts such as a prominent R wave in V1R and V2R that may be present up to age 8.

Cardiac Monitoring and 12-Lead EKG Basics: Right-sided EKG

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Clinical Alert

If time does not permit the placement of all six of the right chest leads, V4R is the lead of choice.

  • Posterior chest leads: Posterior chest leads are additional leads that are used to specifically view the posterior surface of the heart. These leads are placed on the same horizontal plane as V4 to V6, on the left side of the posterior surface (the patient’s back). Lead V7 is placed at the posterior axillary line; lead V8 is placed at the posterior scapular area (midclavicular); and lead V9 is placed at the border of the spine on the left side. Rarely, a right-sided posterior EKG may be requested by a practitioner. The posterior EKG will help to identify characteristic changes for a posterior wall myocardial infarction. These leads must be identified manually on the EKG printout (Posterior EKG electrode placement).
  • Modified chest leads: Modified chest leads (MCL) are special leads that might be used to assist in detection of bundle branch blocks, premature beats, and supraventricular rhythms. It is often difficult to differentiate between supraventricular tachycardia and ventricular tachycardia. These leads can help in this instance. These modified chest leads are bipolar as opposed to the usual unipolar chest leads. Each lead contains both a positive and negative electrode (Electrode placement for modified chest leads MCL1 and MCL6).

Cardiac Monitoring and 12-Lead EKG Basics: Posterior EKG electrode placement

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

  • MCL1: This lead is a variation of chest lead V1 on the 12-lead EKG where the negative electrode is placed below the left clavicle close to the left shoulder. The positive electrode is placed in the fourth intercostal space to the right of the sternum while the ground is placed on the right upper chest area immediately below the clavicle. Although it is similar to lead V1, it is a different view of the heart due to the placement of the positive and negative electrodes. In V1 the heart is the “negative electrode”. In MCL1 the negative electrode is placed below the left clavicle. This lead views the ventricular septum and the QRS complex that is recorded appears negative. Abnormal or ectopic beats will appear as positive deflections. Ventricular and supraventricular tachycardia, bundle-branch defects, and P-wave changes are detected by MCL1. This lead is also used to confirm pacemaker wire placement.
  • MCL6: This lead is a variation of chest lead V6 where the negative electrode is placed toward the left shoulder and below the left clavicle. The positive electrode is placed in the fifth intercostal space at the left midaxillary line (similar to lead V6) while the ground is placed below the right shoulder. This lead may be used as an alternative to MCL1 and views the low lateral wall of the left ventricle while monitoring ventricular conduction changes.
[sociallocker]

Cardiac Monitoring and 12-Lead EKG Basics: Electrode placement for modified chest leads MCL1 and MCL6

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Capture

 

Cardiac Monitoring and 12-Lead EKG Basics: Conclusion

Focus topic: Cardiac Monitoring and 12-Lead EKG Basics

Having a clear understanding of the process of applying electrodes and managing the cardiac monitor and EKG machine are important aspects of patient care. There are many different types of machines on the market and each health care professional must become familiar and comfortable with the products utilized in their institutions. The basics of leads including Einthoven’s Triangle and the limb and chest leads are essential in understanding how these machines enhance the care of patients through proper diagnosis and observations. Note these key points about cardiac monitoring and the 12-lead EKG;

  • The EKG measures the heart’s electrical activity by showing the precise sequence of electrical events during depolarization and repolarization.
  • EKGs do not measure contractile strength.
  • Monitoring systems include both hardwire and telemetry units.
  • Electrodes are made of paper, plastic, or metal (or any combination of the three) and contain conductive material that allows the recording of the heart’s electrical currents.
  • The right arm, left arm, and left leg are the electrode positions in the three-electrode system. The electrode positions in the five-electrode system are right arm, left arm, right leg, left leg, and chest. The EASI system is a specialized system with particular electrode placements.
  • Electrodes are traditionally color coded but the health care professional must check at each institution to verify the color coding.
  • Always talk to the patient prior to application of leads and lead wires. Allay any fears he or she may have.
  • Artifacts can be caused by many different things and must be alleviated before a true reading of the rhythm or the 12 lead can be accomplished.
  • Some common causes of problems with the readings from cardiac monitor or EKG machine include: malfunctioning equipment, improper electrode placement, disconnected lead wires, dry electrode gel, frayed or damaged wires, incorrect positioning of either the electrode or the lead wire, movement of the patient, other equipment in the room, bad contact with the skin, false alarms, and weak signals.
  • Always check the machine against the patient condition, that is, treat the patient, not the machine.
  • Make sure all charting is concurrent regarding times and interventions when dealing with rhythm problems.
  • EKG paper measures both time and voltage.
  • EKG paper is comprised of small boxes that are 1 mm high and 1 mm wide. Each 1 mm box represents 0.04 seconds.
  • Large boxes on the EKG paper measure 0.20 seconds in length.
  • Volume or amplitude of the waveform may need to be readjusted for proper readings.
  • The term “lead” refers both to the actual physical lead that is placed on the patient and also is indicative of a specific recording of electrical currents between a positive and a negative pole.
  • The six frontal plane leads consist of three bipolar leads and three unipolar leads.
  • A bipolar lead has a distinct positive and negative lead.
  • A unipolar lead has a single positive electrode and a relative negative point.
  • The three unipolar limb leads are called augmented leads.
  •  The chest leads measure the horizontal plane and are unipolar.
  • Nonstandard leads include a right-sided EKG, posterior EKG, and the use of modified chest leads, MCL1 and MCL6.
[/sociallocker]

FURTHER READING/STUDY:

Resources:

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.