Cardiac rhythms interpretation made easy!

Interpretation of cardiac rhythms begins with the nurse attaching electrodes to your chest which are pre-gelled stickers that a wire then gets attached to your chest on your right and left upper chest and your right and left lower stomach (abdomen). The machine is a monitor designed to take the tiny current your heart generates (milliamps) and increases it so we can see it clearly on the monitor. The rhythm we see on the monitor is called a cardiac rhythm tracing and it shows us a wealth of information.

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The cardiac rhythms we see on the monitor shows us the electrical activity your heart is doing. Therefore we can determine if there is something wrong with the electrical system or indirectly the plumbing. The 12 ECG/EKG is a tool we use to determine if you’re having a heart attack or not. To get a reading for a 12 lead ECG/EKG we place 10 electrodes strategically on your chest. The 10 electrodes then get converted to a 12 lead by the machine 2 leads are calculated by the computer.

When looking at a cardiac rhythms strip we see 7 points of interest. First we need to identify the parts of the rhythm strip. The first bump we see is called the P wave (Red) this wave is caused by the atrial contraction (top part of your heart). Next we see the QRS wave (blue) it is a jagged wave with 3 points the first point is the Q wave then the R wave and finally the last spike is the S wave. These waves are caused by the ventricles contracting. Then we see the final wave the T wave (Green). This wave is caused by the ventricles repolarizing or getting recharged so it can beat again. We don’t see the Atria repolarize because it is hidden under the QRS wave.

When these parts are identified we then study the length of it takes the signal to go from the SA node to the AV node(Brown). The recharging of the heart is represented by the QT interval (purple). Certain conditions will lengthen and shorten these signals depending what is wrong. Then we take note of the rate the heart is beating we use the QRS wave to determine this. Usually this is enough information to make a decision if there is something wrong. Some cardiac rhythms that will be reviewed: Ventricular Tachycardia , Supraventricular Tachycardia, Ventricular Fibrillation, Atrial Fibrillation , Atrial Flutter, Asystole, First degree Heart block , Second Degree Heart block type one and type two , Third Degree Heart Block all problems with conduction.

The 12 lead ECG/EKG we go one step further and actually note the part between the S wave and the T wave called the ST segment. In a heart attack this signal causes what looks like a step here we would then note and call it an acute myocardial infarction (AMI).

The grid

The grid on the cardiac rhythm strip is divided into 1mm2 boxes. the cardiac rhythm strip is set as a default speed of 25mm/sec. The speed of the strip determines how long in seconds each box is. Each box represents 40ms or 0.04 seconds. The grid is further divided into larger boxes with a bold line every 5th little box. Therefore the larger boxes represent 5mm2 or 0.20 seconds. In order to get a more detailed view of the cardiac rhythm by making the complexes longer the speed of the strip is increased to 50mm/sec. The heart rate at this speed is half of what it is as recorded at the 25mm/sec speed. The grid's vertical axis measures the height of a given complex or its amplitude. 10mm or 10 small boxes equals 1mV with standard calibration. The deflection measured on the cardiac rhythm strip can be positive (upward) or negative (downward). The importance of these deflections denotes where the signal was generated. On the monitor sometimes waves appear small and difficult to interpret by increasing the amplitude wave forms are more easily seen. Conversely, sometimes when wave forms are too large the heart rate monitor may wrongly interpret the wave forms as a lethal arrhythmia such as VT and may constantly alarm. It is therefore advisable to reduce the amplitude and have the machine relearn the cardiac rhythm to avoid unnecessary alarms. Alarms are set to alert the practitioner to problems at the bedside and having an alarm go off constantly due to a large wave amplitude may cause the practitioner to ignore the alarms by tuning them out. This of course is a dangerous situation and could lead to problems. Increasing and decreasing the amplitude is necessary to avoid such a problem.

P wave

The first deflection on the rhythm strip that we analyze is the wave just before the QRS complex. The P wave represents atrial depolarization. Both the right and left atria are represented in the P wave. The P wave comes before the QRS complex and is a upward or positive deflection of lower amplitude. The duration of the P wave is generally <0.12 sec (3 small boxes) and its amplitude < 0.25mV (2.5 small boxes). Lay the strip down on a flat surface. While facing the strip, the upslope in the P wave is moving away from you or anteriorly and the downslope is moving towards you or posteriorly. Right atrial depolarization occurs before left atrial depolarization. The upslope of the P wave denotes right atrial depolarization and the downslope represents left atrial depolarization. When the hearts energy is discharged (depolarization) it has to recharge (repolarization). Atrial repolarization occurs simultaneously with ventricle depolarization (QRS complex) therefore the wave is not discernible. On occasion when a heart rate is increased like sinus tachycardia the PR interval is shortened, atrial repolarization maybe observed at the very end of the QRS complex. This alters the J point by causing a J point depression with a rapidly up sloping ST segment, especially during the the first 80msec after the QRS complex. This is a normal occurrence and important to remember especially during ECG interpretation for ischemia.

PR interval

The PR interval is the time from the beginning of atrial depolarization through the HIS-purjinke system to the beginning of ventricle depolarization. It is measured from the beginning of the P wave includes the PR segment which constitutes the AV node and HIS-purjinke system to the R wave of the QRS complex. Normally the time it takes to complete is 0.12 to 0.20 seconds. The PR interval is shorter with aster heart rates and longer with slower heart rates

QRS complex

The entire complex represents ventricular depolarization. If at the beginning of the complex there is a negative deflection it is called the Q wave. It is normal to have small Q waves in leads 1 aVL and V4 to V6 and represents initial septal depolarization. The first positive deflection of the QRS complex is the R wave. It represents left ventricular depolarization. The right ventricular depolarization is obscured due the larger muscle mass of the left ventricle. R waves should progress in size in leads V1-V6 (precordial leads). The R wave amplitude should increase in size until V4-V6 since more ventricular forces are seen. If the R waves progress in size then it is termed R wave progression. After the R wave is a negative deflection called the S wave and represents the end of depolarization in the high lateral wall. If the whole QRS complex has a negative deflection it is called a QS wave. The length of time it takes for ventricular depolarization to be completed is 0.06 to 0.10 seconds and does not change with heart rate.

QRS Axis

The only way to determine axis is on the ECG. The axis of the heart refers to the direction of the electrical signal. The AV node is located in the top right atrium. The heart is slightly tilted to the left with the apex of the heart pointing down and to the left. This is the normal axis of the heart. As we grow up the axis changes as the heart grows and changes. Sometimes with certain diseases the electrical signal can be shifted or deviated. This is the importance of determining axis and most ECG machines will do that calculation for you.

Left axis deviation refers to the signal being directed upwards and to the left.
Causes of left axis deviation:

Normal variation due to age
Mechanical such as expiration high diaphragm as in pregnancy, ascites, abdominal tumor.
Left ventricular hypertrophy
Left bundle branch block
Left anterior fascicular block
Congenital heart disease (primum atrial septal defect, endocardial cushion defect)
Ventricular ectopic cardiac rhythms
Pre-excitation syndromes (Wolff-Parkinson-White)
Inferior wall myocardial infarction

Right axis deviation refers to the signal being directed downwards and to the right.
Causes of right axis deviation:

Normal variation
Mechanical shifts such as inspiration or emphysema
Right ventricular hypertrophy
Right bundle branch block
Left posterior fascicular block
Ventricular ectopic beats
Pre-excitation syndrome (Wolff-parkinson-white)
Lateral wall myocardial infarction
Secundum atrial septal defect

Determining axis

The easiest way to determine the QRS axis is by reviewing leads I, lead II and AvF only on the ECG.

If the QRS complex is positive in both leads I and II the axis is normal
If the QRS complex is positive in lead I but negative in lead II then the axis is to the left.
If the QRS complex is negative in lead I and positive in AvF, then the axis is to the right.
If the complexes are negative in leads I and II, then the axis is extreme.

ST segment

The St segment is a time of no activity in the heart of electrocardiographic silence. It section of time from the end of ventricular depolarization and the beginning of ventricular repolarization. The very beginning of the ST segment is termed the J point. The ST segment is usually isoelectric (zero electrical potential) and with a slight upward concavity. With different diseased states the ST segment may take on a different configuration, flattened, depressed with upsloping or downsloping, be elevated in a concave or convex direction. In some normal cases the J point is depressed and the ST segment rapidly upsloping becoming isoelectric within 0.08 seconds. The configuration almost looks like a smiley face.

T wave

The T wave is the time of ventricular repolarization (recharging). Recharging of the ventricles takes longer than discharging therefore the T wave is broader over a longer period of time. Usually the T wave is not uniform it is asymmetrical, it has a slow positive deflection and a rapid negative deflection. If there is an irregularity on the T wave a slight bump or notch then a superimposed P wave must be considered. Depolarization of the ventricles begins in the endocardium and ends in the epicardium, while the repolarization of the ventricles begins in the epicardium and ends in the endocardium. The direction of electrical signals is in the opposite direction, but on the rhythm strip the T wave has the same deflection as the major section of the QRS complex. In other words health care practitioners would say that the QRS and T wave axes are concordant. Various diseased states can cause discordance with the QRS and T wave.

QT interval

The QT interval encompasses the QRS complex, J point, ST segment and T wave. The QT is used primarily as a measurement of ventricular repolaration. The logical and accurate thing to do is use the JT segment but clinically the QT segment is used, which includes the QRS complex. The downfall of this method is that if a QRS complex is widened for any reason it will through off the QT interval causing a false QT prolongation and a wrong assumption. The QT interval is dependent on the heart rate. A faster heart rate will cause the QT interval to be shorter and a slower heart rate will cause the QT to be longer. Therefore it is necessary to correct the heart rate and be able to compare apples to apples. The formula used is Bazetts formula:

QTc = QT interval + square root of the RR interval

Therefore the normal corrected QT interval in men is < or = 0.44 seconds, women it is < or = 0.45 to 0.46 seconds

U wave

A U wave may be seen on a ECG on the V2 to V4 leads (precordial leads). It is unknown what this wave represents, but there have been some speculation. It has been speculated that the U wave represents the repolarization of the HIS-purjinke system, but recently it has been debated that the U wave is due to late polarization of the mid myocardial M cells especially in slow heart rates. The amplitude of the U wave is less than 0.2 mV and is separated clearly from the T wave. In cases of hypokalemia (low potassium) and bradycardia (slow heart rate) it is more pronounced. If the QT interval is prolonged the U wave may merge with the T wave creating a QT-U wave. Conversely, with a shortened QT interval the U wave may become more pronounced as with taking Digoxin or hypercalcemia (high calcium)

Approach to rhythm interpretation

Rate: normal rate is between 60 bpm and 100 bpm. Rates under 60 is bradycardia and over 100 is tachycardia. If the cardiac rhythm is regular that is if the QRS complexes are equally spaced then the interval between QRS complexes can be used using the grid to determine heart rate.
By dividing 300 by the number of large boxes on the grid determines heart rate. If 2 successive complexes are 1 large box apart then the rate is 300 bpm. (300/1=300) if the interval is 2 large boxes then the rate is 150 bpm (300/2=150) and so on.

Another method of calculating heart rate is measuring the time between QRS complexes in seconds. This number can be divided into 60 to get the heart rate. If the time between 2 QRS complexes is 0.75 seconds then the heart rate is 80 bpm (60seconds/minute/ 0.75 = 80 bpm).
Calculate an irregular rate by counting the number of complexes and dividing by 6 since the standard rhythm strip prints out 10 seconds of time.

Cardiac Rhythm: determine if any P waves are present. Is there a P wave before each QRS? Are the P waves and QRS complexes regular? Is the PR interval constant?
1. Locate the P wave

2. Establish the relationship the P wave and the QRS complex

3. Analyze the QRS complex

Are he complexes of normal duration? (<0.12 seconds) if so then the signal originates above the ventricles.
If the complexes are >0.12 seconds then the signal is either above the ventricles with aberrant conduction, pre-excitation or ventricular origin.

4. Search for patterns

Breaks in the rhythm or other irregularities may be clues of a rhythm disturbance.
Do the QRS complexes occur with regular intervals or are they irregular?
If the complexes are irregular is there a pattern that repeats or is there no pattern just completely irregular.

5. Interpret Cardiac Rhythm

Getting a detailed history and review of medications will help to determine the overall cardiac rhythm. Is the patient on digoxin? Or some other rate control drug? In a severe bradycardia digoxin toxicity could be the culprit or a potential overdose.

P wave: what is the shape of the P wave is it positive or negative (upward stroke or downward stroke) a positive P wave originates form the SA node. Amplitude and duration could be analyzed to determine left or right atrial enlargement.

QRS complex is the complex wide? Reviewing the shape and size of the complex could determine right or left bundle branch block or a Pre-excitation. Perhaps an increased voltage would indicate a left or right ventricular hypertrophy. Q waves present could indicate an old MI or an evolving MI.

ST-segment-T wave taking note of this segment could determine if the person is having a heart attack. Is there a depression or elevation? Are the T waves inverted?

Overall interpretation can only be achieved if the steps are followed. By approaching cardiac rhythm analysis scientifically every aspect of the rhythm will be utilized and proper treatment can begin sooner rather than later. Becoming adept at cardiac rhythm analysis takes time withmlots of review and practice.

The cardiac rhythm we see brings back a lot of information. You may see a lot bumps on a computer screen we see a 3D heart beating, alive. It takes time to properly interpret a rhythm with all its nuances and subtleties.

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