Download ELECTROCARDIOGRAPHY

Clinical Support
8500 S.W. Creekside Pl.
Beaverton, OR 97008-7107 U.S.A.
Telephone: 503-526-4200
Toll Free: 800-289-2500
[email protected]
ELECTROCARDIOGRAPHY
Introduction
This article provides a basic introduction to the physiology of the human heart and
the clinical information provided by electrocardiography (ECG), with reference to
ECG monitoring with a Propaq vital signs monitor.
For more information about the use of the Propaq monitor, refer to the Propaq
Directions For Use.
Monitoring HR/PR with the Propaq Monitor
When monitoring a patient using ECG leads, SpO2, and/or CO2, the rate that is
displayed is a true heart rate. If monitoring a patient using NIBP only, what is actually
being displayed is the patient’s pulse rate. This may be important when assessing
your patient’s cardiac status because a patient’s heart rate and pulse rate may vary if
there is any cardiac compromise.
On the Propaq monitor you can set the HR/PR tone loudness to LOW, MEDIUM,
HIGH, or OFF. This does not affect the tone of the alarm if a patient exceeds an
alarm limit setting.
Anatomical Structure of the Heart
The main function of the heart is to pump blood throughout the body to deliver the
oxygen and nutrient demands of the body’s tissues as well as to remove carbon
dioxide (a byproduct of metabolism).
•
The heart is approximately the size of a clenched fist.
•
The heart is positioned in the mediastinum, near the midline.
•
The heart is rotated and positioned on its side.
Welch Allyn Protocol Clinical Support
ECG
•
2/3 of the heart is on the left side of the chest.
•
The base of the heart faces up and to the right. The apex faces down, out,
and to the left. The apex actually comes into contact with the chest wall at the
5th intercostal space in the mid-clavicular line. This is the PMI or Point of
Maximum Intensity. It’s easiest to hear the heart at this area.
•
Of course, people are of all different shapes and sizes. The position of the
heart in the chest will vary slightly with age, weight, and physical conditions.
Four Chambers of the Heart
There are four chambers of the heart – the right atrium and right ventricle, and the left
atrium and left ventricle. The wall of the left ventricle is quite a bit thicker than that of
the right ventricle. Actually, the wall of the right atrium is approximately 3-5 mm thick,
and the right ventricle is about 2-6 mm thick. The wall of the left atrium is also about 26 mm, slightly thicker than the wall of the right atrium, and the left ventricle is the
largest muscle mass of all at 13-15 mm thick. The wall thickness directly affects the
pressure in each of the chambers of the heart.
The function of the right side of the heart is to deliver deoxygenated blood from the
body to the lungs. The function of the left side of the heart is to deliver oxygenated
blood from the lungs to the body.
Systole and Diastole
The two phases of the cardiac cycle are Systole and Diastole. Both the right and left
ventricles go through each phase at the same time.
In Systole, the ventricles are full of blood and begin to contract. The mitral and
tricuspid valves (between the atria and ventricles) close. Blood is ejected through
the pulmonic and aortic valves out to the lungs (RV) and the body (LV). The aortic
and pulmonic valves then close.
2
Welch Allyn Protocol Clinical Support
ECG
During Diastole, blood flows into the
atria and then through the now open
mitral and tricuspid valves into the
ventricles. The ventricles refill, and the
cycle repeats.
NOTE
Atrial systole occurs during
ventricular diastole, and atrial
diastole during ventricular
systole.
Cardiac Conduction System
In order for the heart muscle to contract,
an electrical impulse is necessary. The
electrical conduction system of the heart
is known as the Cardiac Conduction
System.
SA Node
The SA node is often referred to as the
“Pacemaker” of the heart. In a normal
heart, the SA node generates the
electrical impulse and “sets the pace” of
the heart.
The intrinsic rate of a rhythm that begins at the SA node is 60 to 100 beats per
minute. Once generated, the impulse spreads out along tiny nerve fibers called
“internodal tracts” and stimulates the atrial muscle.
AV Node
The AV node is located on the floor of the Right Atrium. It can be thought of as a
“gateway” to the ventricles. The AV node delays the electrical impulse just long
enough to allow the atria to contract and blood to enter the ventricles.
The intrinsic rate of the AV node is slower than that of the SA node, approximately
40 – 60 bpm.
3
Welch Allyn Protocol Clinical Support
ECG
Bundle of His
The Bundle of His is a thick bundle of nerve fibers that carries the electrical impulse
very rapidly down the interventricular septum from the AV node. The bundle
branches out to the right and left, terminating in tiny fibers called Purkinje fibers. These
fibers bring the electrical impulse to the individual heart muscle cells, leading to
ventricular stimulation or depolarization. Depolarization can be simply thought of as
the electrical stimulation of the heart muscle cells.
The resting heart is polarized. Charges are balanced in and out of the cell, and no
electricity is flowing.
The cell at rest is negatively charged. When a stimulus begins, positive ions enter
the cell, changing the charge to positive.
This “depolarization” spreads from cell to cell, causing the heart muscle fibers to
shorten. The shortening of the heart muscle fibers causes contraction of the heart
muscle as a whole.
Repolarization is the return of the heart muscle cells to the polarized, or resting state.
The positive ions are pumped out of the cells, the cells return to their normal shape,
and the heart muscle relaxes.
ECG Tracings
Each wave and interval appears on the ECG display or printout as a result of a
particular electrical
function of the heart.
The isoelectric line,
also referred to as
the “baseline”, is
simply the point from
which each of the
waves of the ECG
departs.
4
Welch Allyn Protocol Clinical Support
ECG
P Wave
The P wave is the wave of atrial depolarization. As the atria depolarize, the P wave
shows up on the ECG.
In a patient with normal
physiology and with the
SA node acting as the
pacemaker of the heart,
the P wave has these
characteristics:
•
Smooth and
rounded
•
<= 3 mm tall
•
upright in leads
I, II, aVF
The P wave at right fulfills
the criteria.
PR Interval
The next component is the PR interval, which includes the P wave and the space up
until the beginning of the QRS complex. The PR interval represents the time it takes
the electrical impulse to travel from the SA node to the ventricles. By the end of the
PR interval, the atria are beginning to repolarize and the ventricles are beginning to
depolarize or become electrically stimulated.
The PR interval is measured from the beginning of the P wave to the beginning of
the QRS complex. The normal PR interval duration is 0.12 to 0.20 seconds or 120
– 200 ms.
QRS Complex
The QRS complex is the wave of ventricular depolarization. We generally call the
wave of ventricular depolarization a “QRS complex” even if not all of the
components (the Q, the R, and the S) are present. Technically, the Q wave is the
first downward stroke. An R wave is the first positive stroke, and an S wave is a
negative stroke that follows a positive upstroke.
The QRS should be at least 5 mm and not more than 20 mm tall. The width of the
QRS is measured from the beginning of the Q wave to the end of the S. Normal
QRS duration is 0.06 to 0.10 seconds, and does not exceed 0.12 seconds.
As discussed earlier, the left ventricular muscle is quite a bit larger than that of the right
ventricle. Because there are more muscle cells to depolarize, the electrical charge of
5
Welch Allyn Protocol Clinical Support
ECG
the left ventricle is significantly greater than that of the right. Therefore, most of what
we see on ECG as the QRS complex is LEFT ventricular depolarization.
ST Segment
The next segment is the ST segment. The ST segment begins at the J point. The J
point is the point at which the QRS complex ends and the ST segment begins.
Measure the ST segment duration from the J point up to the beginning of the T
wave.
The ST segment indicates the period of time between the end of ventricular
depolarization and the beginning of ventricular repolarization. Generally the ST
segment is ISOELECTRIC, or on the “baseline”. A deviation of the ST segment
from the baseline (either a depression or elevation) may be indicative of myocardial
ischemia.
T Wave
The T wave is the wave of ventricular repolarization. The T wave usually deflects in
the same direction as the QRS complex, and should be smooth and rounded.
The period from the beginning of the T wave to nearly the end is called the “relative
refractory period”. At this time, the ventricles are vulnerable. A stronger than normal
stimulus could trigger depolarization. If an R wave (ventricular depolarization) should
occur during this time, a potentially fatal arrhythmia could result.
Summary and Review
• The P wave is the wave of atrial depolarization. The PR interval signifies the
amount of time it takes the electrical impulse to travel from the SA node to the
ventricles.
•
The QRS complex begins to show up as ventricular depolarization begins and
the atria repolarize. The QRS is complete when the ventricles are fully
depolarized.
•
The ST segment occurs on ECG between the end of ventricular depolarization
and the beginning of ventricular repolarization.
•
The T wave begins as the ventricles start to repolarize and is finally complete
when the ventricles have returned to their resting state.
6
Welch Allyn Protocol Clinical Support
ECG
Electrocardiogram
The electrocardiogram, also called an ECG or EKG, is a graph of the electrical activity
of the heart over time. We have been discussing the waveforms you find plotted on
the ECG.
When reading the ECG, be aware of a few basic principles:
•
The standard paper speed is 25mm/ second. This is also called Sweep Speed.
•
The vertical lines on the ECG measure time.
•
The space between two small vertical lines (one small box) is 0.04 seconds or 4
ms. The space between two larger lines (5 small boxes or 1 large box) is 0.20
seconds or 20 ms.
•
The horizontal lines on the ECG measure voltage.
•
The space between two small horizontal lines (one small box) is 1 mm or 0.1
mV.
•
The space between two larger horizontal lines (5 small boxes) is 5 mm or 0.5
mV.
Perhaps the best way to measure time and voltage on the ECG is with calipers, but
you can also use a ruler or a piece of paper.
We can tell something about the direction the electricity in the heart is flowing by
looking at the ECG. If the electrical flow of the heart is TOWARDS a positive
electrode, we will see a positive deflection on the ECG. If the electrical flow is
AWAY FROM the positive electrode, then the wave produced on the ECG will be
negative.
7
Welch Allyn Protocol Clinical Support
ECG
Remember that
depolarization is a wave of
POSITIVE charges flowing
through the heart muscle. If
that wave of depolarization is
flowing towards a POSITIVE
electrode, then the result will
be a POSITIVE upstroke on
the ECG.
8
Welch Allyn Protocol Clinical Support
ECG
Understanding Leads
Everyone has heard of 3-lead or 5-lead ECG monitoring, or ordered a 12-lead
ECG. But what exactly is a LEAD?
Leads, by definition, are positive and negative electrodes attached to a recorder and
used to detect electrical activity of the heart.
A simple way to think of a Lead is as a picture of the electrical activity of the heart.
Imagine that a camera is positioned at the location of the positive electrode in each of
the leads we will discuss. From each individual angle, a unique view of the heart can
be captured.
3 Lead ECG
The 3 lead ECG is one of the most common. Leads I, II, and III are also known as
the Limb Leads. To obtain a 3-lead ECG, electrodes are placed on the Right and
Left arms and on the left leg.
Lead I looks from RA to LA
Lead II looks from RA to LL
Lead III looks from LA to LL
9
Welch Allyn Protocol Clinical Support
ECG
Augmented
Leads
The Augmented leads are the “Other Limb Leads”. With the augmented leads, the
two negative electrodes are combined to form a central negative reference point.
These leads offer a “mixed view”, or a single view between two of the views
offered by the standard limb leads.
For example, in lead AVF, our positive electrode is on the LL, and the central
negative reference point is between the LA and RA leads. This offers a view
between Leads II and III.
The imaginary camera is still at the Left Leg, but it’s positioned at a different angle.
Together with the standard Limb Leads, there are now six intersecting views of the
heart.
V Leads
The V leads are also called the
Chest leads.
These six leads offer
HORIZONTAL views of the heart.
This time, the camera is positioned
on the chest wall, taking pictures
through the chest and the heart itself.
Think of the V leads as spokes of a
HORIZONTAL wheel, with the AV
node being the hub of the wheel.
The negative end of each lead is a
point somewhere on the patient’s
back.
10
Welch Allyn Protocol Clinical Support
ECG
V1 & V2 are positioned over the
Right Ventricle, V3 & V4 over the
septum between the ventricles, and
V5 and V6 over the left ventricle.
Remember the rule of electrical flow. If
the electrical flow of the heart is
TOWARDS a positive electrode,
there is a positive deflection on the
ECG.
Remember that the electrical charge of
the Left Ventricle is greater than that of
the right. Therefore, there is a mostly
negative deflection in V1 (over the
Right Ventricle) – most of the
electricity is going down and to the left,
away from V1.
As the leads get closer to the Left
Ventricle, the ECG of a normal heart in
a normal rhythm will demonstrate “R
wave progression” or progressively
more positive QRS complexes.
5 Lead ECG
With 5-lead monitoring, common in
many hospitals, only one V lead is
used.
The most leads used for routine ECG
monitoring are 5 leads. The common
placement of the ECG leads is as
follows.
Proper placement of the V leads is
very important if the V leads are going
to be used for diagnostic purposes.
Proper chest wall placement of the V
leads are shown below.
11
Welch Allyn Protocol Clinical Support
ECG
Place the V1 lead just to the
right of the sternum in the 4th
intercostal space.
Place V2 just to the LEFT of
the sternum in the 4th
intercostal space.
Place V4 in the left
midclavicular line in the 5th
intercostal space.
Place V3 between V2 and
V4.
Place V5 in the anterior
axillary line in the 5th
intercostal space.
Place V6 in the mid-axillary
line in the 5th intercostal space.
ECG Lead Skin Preparation
Good lead preparation is very important also. The ECG can tell us many things, but
artifact can hinder the accuracy of the ECG. To avoid artifact, be sure to prepare the
skin properly.
Before applying electrodes, skin should be free of hair, clean, and dry. For best
results, attach electrodes to the leads before placing the leads on the patient.
Electrodes should have plenty of gel and should be replaced if they become soiled
or wet.
Place electrodes as close as possible to the recommended areas, but make an
effort to keep them out of the way of areas of large muscle movement. Flat bony
surfaces are the best location.
12
Welch Allyn Protocol Clinical Support
ECG
Rhythm Analysis
Normal sinus rhythm is the rhythm that most of you are probably in right now. The
rhythm is regular.
Sinus tachycardia is really a fast normal sinus rhythm. The SA node still generates the
impulse, but it will be generated at a higher rate.
The rhythm should still be regular.
However, the very high rate can cause strain on the heart, especially in a patient with
CAD. Possible causes include caffeine, stress, nicotine, alcohol, pain, fever,
congestive heart failure, hypovolemia, hyperthyroidism, dig toxicity, and some
medications.
13
Welch Allyn Protocol Clinical Support
ECG
Sinus bradycardia is a slow sinus rhythm. Again, this rhythm is initiated by the SA
node. The rhythm is regular.
Sinus bradycardia can be normal in some people, especially the very athletic. It can
also be caused by sedation, increased intracranial pressure, medications such as
beta blockers, vagal stimulation as with straining or vomiting, hypothyroidism, and
hyperkalemia.
Treatment is based on symptoms. Some of the symptoms may include decreased
urine output, dizziness, weakness, and hypotension. Treatment may include
administration of Atropine or Dopamine, or placement of an external pacemaker.
NOTE
For additional lessons on ECG Rhythm analysis, contact Welch Allyn
Protocol Clinical Support to find out about our AACN-approved CEU
offering: ECG Interpretation and Basic Arrhythmia Analysis.
14
Welch Allyn Protocol Clinical Support
ECG
References
1. Barash, P., Cullen, B., and Stoelting, R. (1992). Clinical Anesthesia. 2nd Edition.
J.B. Lippincott Company. Philadelphia, PA.
2. Guyton, A. (1991). Textbook of Medical Physiology. W.B. Saunders Co.
Philadelphia, PA.
3. Loeb, S. (1993). Monitoring Clinical Functions. Advanced Skills. Springhouse
Corp. Springhouse, Pennsylvania.
4. Thomas, C. Taber’s Cyclopedic Medical Dictionary. 16th Edition. F. A. Davis Co.
Philadelphia, PA. 1989.
5. Dubin D. Rapid Interpretation of EKG’s. Tampa, FL: Cover; 1993.
6. Flynn JM, Bruce NP. Introduction to Critical Care Skills. St. Louis, MO: Mosby;
1993.
7. Grauer K, Cavallaro D. Arrhythmia Interpretation: ACLS Preparation and Clinical
Approach. St. Louis, MO: Mosby; 1997.
8. Lipman B, Cascio T. ECG Assessment and Interpretation. Philadelphia, PA:
FA Davis Co.; 1994.
9. Huszar RJ. Basic Dysrhythmias: Interpretation and Management. St.Louis, MO:
Mosby; 1994.
10. Marler CA. Introduction to ECG. Dallas, TX: Parkland Health & Hospital
Systems; 1993.
11. Marriott H, Conover M. Advanced Concepts in Arrhythmia. St. Louis, MO:
Mosby, 1983.
12. Metzgar ED, Polfus PM. A Study and Learning Tool: Health Assessment
(Second Edition). Springhouse PA: Springhouse Corporation; 1994.
13. Smith- Huddleston S, Ferguson SG. Critical Care and Emergency Nursing: 2nd
Edition. Springhouse, PA: Springhouse Corporation; 1994.
14. Cummins, RO. Textbook of Advanced Cardiac Life Support. American Heart
Association, 1994.
15. Walraven G. Basic Arrhythmias (3rd. Edition). Englewood Cliffs, NY: Brady;
1992.
15