Contents
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Segments
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Intervals
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR Interval
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR Interval Abnormalities
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR interval (PRI)
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: QT Interval
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: QT interval as it compares with RR interval
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: The Whole Picture
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Systematic Interpretation
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Verify Regularity
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Use of calipers to determine rhythm regularity
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Use of index card/paper to determine rhythm regularity
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Count the Rate
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Counting heart rate
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Check the P Wave
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Establish the PR Interval
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Evaluate the QRS Complex
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Assess the QT Interval
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Consider the T Wave
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Notice the ST Segment
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Normal Sinus Rhythm
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Normal sinus rhythm
 Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Conclusion
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Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Segments
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
There are several horizontal straight lines on the EKG tracing. These straight lines are called segments and are associated with the wave or complex that precedes it. These segments follow the isoelectric line that is the baseline for the rhythm and represent different periods of time in the cardiac cycle (EKG segments).
 PR Segment: The PR segment is the straight line from the end of the P wave to the beginning of the QRS complex. It corresponds to the end of atrial depolarization and the beginning of ventricular depolarization. Atrial repolarization is also taking place during this segment of time. As the impulse conduction is moving through the cells, this denotes its passage through the AV node, the bundle of His, the right and left bundle branches, and the Purkinje fibers. This impulse transmission is too weak to produce discernible voltage and therefore displays as a straight line. A depression of the PR segment can occur with chronic pulmonary issues and ventricular enlargement that can be produced from a chronic excess of pressure being exerted on the ventricular walls. This then creates a thickening of the heart wall known as hypertrophy.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: EKG segments
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
 ST Segment: The ST segment begins at the end of the QRS complex and completes at the beginning of the T wave. This “end” of the QRS complex is still named ST whether an S wave is present or not (as stated previously it may end with an R wave). This occurs during the early part of repolarization of each of the ventricles and is usually isoelectric. The J point occurs in this segment and is the point at which the QRS complex itself joins to the ST segment. The J point is important because most people start measuring either depression or elevation of the ST segment at this point. Others may begin the measurement of the segment 0.04 to 0.06 seconds after the J point. Some patients with rapid heart rates or high, peaked T waves that may be seen with hyperkalemia do not clearly show the J point. The ST segment is an important factor in determining certain situations. It is of utmost importance in the diagnosis of acute myocardial injury and ischemia. When looking at the ST segment, compare it to the isoelectric line of the PR segment. ST Segment Elevation and Depression shows some of the circumstances that can create either an elevated or depressed ST segment. The ST segment is said to be elevated if it is greater than 1 mm above the isoelectric line. Depression is recognized if it is greater than 0.5 to 1 mm below the isoelectric line.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: ST Segment Elevation and Depression
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Clinical Alert
The ST segment is one of the most important pieces of interpretation of the EKG pattern in the diagnosis of an ST elevation myocardial infarction (STEMI). It is used in conjunction with laboratory values to determine this diagnosis but, when present in an elevated “coved” type of pattern, is usually considered to be an acute injury pattern in that particular view of the heart. This will be covered in more detail in the chapter on myocardial infarction.
 TP Segment: The TP segment begins at the end of the T wave and continues to the beginning of the next cardiac cycle or P wave. This line should be isoelectric and in faster heart rates may be unrecognizable due to the proximity of the T and P waves.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Intervals
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
An interval consists of a combination of a waveform and a segment. Two important intervals exist in the EKG tracing. These are the PR interval and the QT interval.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR Interval
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The PR interval can be abbreviated as PRI. This is measured from the beginning of the P wave to the beginning of the QRS complex and includes the PR segment. The cardiac cycle activity that is represented by the PR interval is that of atrial depolarization to the onset of ventricular depolarization. Normal length for this interval is 0.12 to 0.20 seconds (35 small boxes on the EKG graph paper). A fast heart rate can shorten the PR interval.
The PR interval is an important aspect of the EKG. Increases and decreases in the length are associated with cardiac and other pathophysiologic abnormalities. PR Interval Abnormalities lists several potential reasons for these changes. A shortened PR interval is usually indicative of an impulse generated from an area other than the usual SA node. Conduction defects or delays are characterized by a prolonged PR interval (PR interval (PRI)).
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR Interval Abnormalities
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: PR interval (PRI)
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Clinical Alert
The pediatric patient is unique in that the PR interval is very short at birth (0.070.14 seconds) and reaches a more normal “adult” length by the age of 12 to 16 years (0.090.18 seconds). The shorter interval is due to the faster heart rate and the smaller size of the ventricles.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: QT Interval
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The QT interval starts at the beginning of the QRS complex and ends at the conclusion of the T wave. This is representative of activity within the ventricles inclusive of both ventricular depolarization and repolarization. (The T wave is wider than the QRS indicating that more time is spent in repolarization than depolarization.) Remember that there are times when a Q wave is not present and this complex begins with an R wave. This measurement is still titled “QT Interval”, whether it actually begins with a Q wave or not. This interval is normally less than half of the length of the time from one QRS complex to the next. If it is purely one half the length of the RR interval, time between one QRS and the next QRS, it is considered to be “borderline”. If it is greater than half the of the RR interval, then it is considered to be prolonged. To measure in this way, the rhythm must be regular, meaning that the same number of boxes between QRS complexes is present consistently. It usually varies between 0.36 to 0.44 seconds. Various factors can impact the QT interval such as age, heart rate, and gender. Slower heart rates will prolong the QT interval. Other causes for a shortened QT interval are digitalis toxicity and hypercalcemia. A prolonged QT interval impacts the relative refractory period which is the vulnerable phase of ventricular repolarization. When it is longer than normal, it can permit an ectopic impulse to seize control of conduction (QT interval as it compares with RR interval).
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: QT interval as it compares with RR interval
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Clinical Alert
A prolonged QT interval places patients at risk for a lifethreatening dysrhythmia known as torsades de pointes, a type of ventricular tachycardia. Many different situations can cause this increased threat including: hypokalemia, hypocalcemia, hypothermia, bradycardia, myocardial ischemia, some antiarrhythmic medications, certain antidepressants, specific psychotropic medications, particular antibiotics and antimigraine medications, some antinausea agents, genetic factors (hereditary long QT syndrome), and liquid protein diets.
RR intervals and PP intervals (measured in the same way as RR intervals) can be used to establish rate and regularity versus irregularity in rhythms. Regular rhythms will demonstrate the same length between R waves throughout the rhythm strip. Atrial regularity can be determined in the same way utilizing the PP interval.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: The Whole Picture
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Each piece of the EKG tracing has been discussed. The whole picture is a visual representation of the each of the components of the cardiac cycle as depicted on the EKG strip. This is an important picture to understand as the health care professional begins the process of analyzing and understanding rhythm strips.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: The whole picture
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Systematic Interpretation
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Interpretation of rhythm strips takes time and experience. Each health care professional who works with EKGs will develop his or her own method. It is important to utilize a systematic approach which will then serve as a guideline when reading each patient’s particular rhythm. Normal sinus rhythm will be the most common rhythm encountered, but of course there are many abnormal rhythms as well. Understanding normal sinus rhythm will help to identify the aberrant rhythms. With a strong base upon which to build, the individual attempting to interpret rhythms will be successful in accuracy so that correct interventions can be initiated and evaluated. Each of the waves, complexes, straight lines, and intervals that have been discussed now come together to create a “rhythm”. When reading or interpreting rhythms, a 6second strip is utilized (30 large boxes = 6 seconds). Markings should be present on either the top or bottom of the EKG paper that denotes either 1 or 3 seconds in length to assist with rapid calculations and rhythm analysis.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Verify Regularity
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Several methods can be utilized to verify regularity of the rhythm. Basically this is a confirmation of the RR and PP intervals as described previously. The same number of small or large squares between each R wave and P wave should be present. This can be accomplished by counting these squares, but other methods can be used to make this easier.
The first technique is to utilize a pair of calipers to measure between two R waves. Calipers have two points that can be pulled apart or pushed closer together determined by the heart rate on a 6second strip. Choose two R waves and measure that distance with the calipers. Then, without adjusting the calipers, rotate the calipers from the second R wave to the third R wave. The point of the calipers should land on the tip of the third R wave. Walk the calipers across the entire 6second strip to determine that each R wave is exactly the same distance apart. If the rhythm is regular, they will match all the way across the EKG tracing. If it is irregular, the points will change and it will become apparent that the R waves are not an equal distance apart across the entire strip (Use of calipers to determine rhythm regularity).
The second technique is to use an index card or a small piece of paper. Hold it against the rhythm and place a mark for each of two R waves. Then use the card or paper in the same fashion as the calipers, checking for distance between each R wave across the entire 6second strip. Each R wave should fall on a mark on the paper or card with each subsequent R wave then matching up with the next mark placed prior from the first two R waves measured. Again, if each R wave matches the marks on the paper across the strip, the rhythm is considered to be regular (Use of index card/paper to determine rhythm regularity).
This same procedure can be used to determine atrial regularity by using the P to P intervals as a guide.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Use of calipers to determine rhythm regularity
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Use of index card/paper to determine rhythm regularity
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Clinical Alert
When assessing for regularity of rhythms, a variance of 0.04 seconds can be considered normal. If the variance is less than 0.16 seconds, the term “essentially regular” may be used. This might happen if ectopic beats are episodically present. If the difference is greater than 0.16 seconds, the rhythm is deemed irregular. Irregular rhythms are then divided into “regularly irregular” and “irregularly irregular”. Regularly irregular rhythms might occur if the patient is experiencing ectopic beats at regular intervals. Irregularly irregular rhythms may arise when there is no pattern to the irregularity, such as with atrial fibrillation.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Count the Rate
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The next step in interpretation of rhythms is to determine the heart rate. This can be accomplished using several different methods. Each of these methods will work with either R waves (ventricular rate) or P waves (atrial rate) (Counting heart rate).
 Counting R waves: The simplest and fastest way to count heart rate is to determine the number of complete QRS complexes (some professionals prefer to use the number of QRS intervals) on a 6second strip. Multiply the number of QRS complexes in a 6second strip by 10 to obtain an estimation of the heart rate. This method can be employed for rate calculations for either regular or irregular rhythms. It is most commonly used for irregular rhythms since the other methods can create difficulty in accuracy with irregular rhythms.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Counting heart rate
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
 Large box method: A more accurate way to determine rate is to count the number of large boxes between two R waves that fall closest to a heavy line denoting a large box and then divide that number into 300. So, if there are 5 large boxes in between two R waves, 300 divided by 5 equals a heart rate of 60. If the rhythm is deemed irregular, this method can still be used, but a range is used. Measure the number of large boxes between two of the most distant R waves and the number between two of the closest R waves. That might provide a rate of “60 to 80” beats per minute. Commercial tables are available that have these numbers configured on them for quick reference.
 Small box method: In this method, count the number of small boxes between two R waves and divide into 1500. One small box is equal to 0.04 seconds which would then be equal to 1 minute of time per 1500 small squares. If the number of small squares between the R waves is 15, 1500 divided by 15 shows a heart rate of 100 beats per minute. This is a precise measurement and is best used for regular rhythms; however, rate ranges could also be applied to this method.
 Sequence method: When utilizing the sequence method, find an R wave (or P wave if atrial rate is desired) that occurs on a heavy black line on the graph paper. Then label the next six heavy black lines with a number from the following sequence starting with the largest number first— 300, 150, 100, 75, 60, 50. The rate is then estimated by finding the point at which the next R wave (or P wave) falls and determining where it lies in relation to the marked dark lines.
Clinical Alert
When counting heart rate utilizing a rhythm strip, be aware of the fact that just because there is a QRS complex on the rhythm strip, it does not always correlate to the patient’s actual pulse rate. There are situations in which the electrical activity is present, but the heart is not actually beating. This is called pulseless electrical activity and is mentioned in Chapter 3. Also, if a patient is experiencing ectopic beats, those beats may not actually be creating a “pulsed beat” that is, it is not perfusing the body with oxygenated blood. Therefore, the underlying rhythm may be one factor and the extra beats are a secondary factor in the rhythm. So a reading may state, “normal sinus rhythm with premature ventricular contractions”. Always check the patient’s pulse against what is seen on the rhythm strip or cardiac monitor. The monitor may be reading a heart rate of 60 when every other beat is a premature ventricular contraction (PVC), but the patient’s actual pulse rate is 30. Apical and radial pulses are important tools in the health care professional’s inventory of assessment parameters. Never forget to actually touch the patient. Do not rely solely on machines.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Check the P Wave
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The next step is to determine that each QRS complex is preceded by a P wave. These P waves should look similar and correlate to the normal parameters for the P wave. Clearly distinguishable upright P waves come from the SA node (depending on the view from the heart). P waves that are absent or inverted in leads that should show positive P waves, means that the conduction probably started in the AV junction.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Establish the PR Interval
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Make sure that the PR interval is within normal range. Count the number of small squares that comprise the PR interval and multiply that by 0.04. The PR interval should fall between 0.12 to 0.20 seconds. If the PR interval is not consistent, check for a pattern of inconsistency. Heart blocks are determined by clarifying PR interval changes.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Evaluate the QRS Complex
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Measure the duration of the QRS complexes to determine that they are within the normal range of 0.06 to 0.10 seconds. Also check the configuration of these complexes. Make sure they are the same across the 6second strip.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Assess the QT Interval
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Utilize the small squares as a basis to measure the QT interval. It should fall within the normal range of 0.36 to 0.44 seconds. This can be difficult to perform with fast heart rates. Also, remember that, in general, if it is less than half of the RR interval, it is considered to be normal. A corrected QT interval can be accomplished using a formula based on a heart rate of 60. This is known as the QTc.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Consider the T Wave
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Check the T wave for configuration and size. If the QRS and the T wave are in opposite directions, it is most likely an ectopic beat. Another consideration with the T wave includes inversion of the T wave which can signify ischemia to the area of the heart which the lead is viewing. Hyperkalemia (high potassium) can be seen in high peaked T waves.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Notice the ST Segment
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The ST segment should be analyzed for either elevation or depression. The normal ST segment should be on the same line as the PR and TP segment. If it is above or below this line, it can carry significance for a potential acute myocardial infarction.
Clinical Alert
Senior members of the patient population may have normal changes to these aspects of EKG tracings. Prolonged PR, widened QRS, and lengthened QT intervals can be normal variances for this age group.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Normal Sinus Rhythm
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
The most common rhythm to be assessed is Normal Sinus Rhythm, abbreviated NSR. This indicates a heart rhythm that originated in the SA node and traveled correctly through the heart (SA node → atria → AV node → Bundle of His → Bundle Branches → Purkinje Fibers). Every other rhythm is compared to NSR. Remember that sometimes the underlying rhythm is NSR with ectopic beats or short bursts of other rhythms entering the picture.
Normal sinus rhythm will have the following distinctive characteristics: (Normal sinus rhythm)
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Normal sinus rhythm
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
 Both atrial and ventricular rates are regular.
 Rate will be between 60 and 100 beats per minute (rates greater than 100 beats per minute are named tachycardias and rates less than 60 beats per minute are called bradycardias).
 A normal P wave precedes each QRS.
 PR interval is within normal limits.
 QRS complexes are normal in configuration and width.
 QT interval is within normal limits.
 T wave is normal in configuration.
 ST segment lies on the isoelectric line.
 No ectopic beats are present.
Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting: Conclusion
Focus topic: Waves, Complexes, Straight Lines, and Intervals/Labeling and Interpreting
Understanding the different parameters of the electrocardiograph or cardiac monitor strip is essential in caring for patients. Cardiac abnormalities must be discovered and attended to in order to maintain optimal health for those who are entrusted to health care professionals. In order to recognize abnormalities, normals must be comprehended. A standard systematic method of interpretation will assist the health care provider in assessing rhythm strips easily and quickly, especially when an emergent or urgent situation arises. Key points of this chapter include:
 Each waveform, segment, interval, and complex has its own identifying characteristics and normals.
 The letters PQRST are used to denote each waveform.
 Isoelectric lines are known as segments.
 A grouping of waveforms is a complex.
 The P wave is the first waveform and is representative of the depolarization of the atria.
 A P wave is normal if it is above the isoelectric line (positive deflection) in lead II.
 P waves can have a biphasic configuration.
 Some leads will normally have a negative or biphasic P wave.
 Normal width for a P wave is 0.12 seconds or less.
 One small square on the EKG graph paper is equal to 0.04 seconds in width and height.
 An ectopic P wave is one which originates from another source other than the SA node.
 Some disease processes can cause abnormalities to occur with the P wave.
 Ventricular depolarization is noted on the EKG tracing in the QRS complex.
 Not all QRS complexes have a Q, R, and S wave.
 The R wave is always a positive deflection. The Q and S waves are negative deflections.
 An R or S prime is the presence of an extra R or S wave. This is written as R1 or S1.
 A notched R wave is different than an R prime. This happens when the R wave simply changes directions and does not come back to the isoelectric line before the second R wave occurs.
 R wave progression should occur with the lowest amplitude in V1 and the highest amplitude in V6.
 Normal duration for a QRS complex is 0.06 to 0.10 seconds.
 A widened QRS may be indicative of delays in ventricular conduction, bundle branch blocks, premature ventricular contractions, or aberrant atrial impulses.
 Deep or wide Q waves can be indicative of a myocardial infarction.
 The normal T wave is positive in lead II.
 The refractory periods occur during the T wave.
 Many disease processes can cause abnormal T waves.
 The T wave should normally have the same deflection as the QRS.
 Patients who are experiencing a subarachnoid hemorrhage may have “giant” T wave inversion.
 A U wave may or may not be present.
 Tall U waves can be caused by a variety of disease processes and medications.
 Segments include the PR segment, the ST segment, and the TP segment.
 The ST segment is very important as a diagnostic tool with myocardial infarction.
 The PR interval (PRI) is important in the diagnosis of conduction defects or delays.
 The normal length for a PR interval is 0.12 to 0.20 seconds.
 Both shortened and prolonged PR intervals are important in certain disease processes.
 The QT interval normal length is between 0.36 to 0.44 seconds.
 The QT interval should be less than the RR interval to be normal.
 Heart rate can impact the QT interval.
 A prolonged QT interval can influence the relative refractory period and cause a life threatening dysrhythmia known as torsades de pointes, a type of ventricular tachycardia.
 When interpreting an EKG rhythm, it is best to have a systematic way to perform this.
 The steps in interpretation of an EKG rhythm include: verification of regularity, counting the rate, checking the P wave, establishing the PR interval, evaluating the QRS complex, assessing the QT interval, considering the T wave, and noticing the ST segment.
 There are two ways to verify regularity—the use of calipers or the use of a file card or piece of paper.
 There are four ways to count the rate—counting R waves, the large box method, the small box method, and the sequence method.
 Every QRS should have a P wave to be normal.
 The most common rhythm to identify is normal sinus rhythm and is the benchmark to which all other rhythms are evaluated.
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