Normal Cardiac Rhythms/ Electrocardiographic Analysis Flashcards
What criteria does a normal sinus rhythm need to meet?
Normal sinus rhythm (NSR) is the rhythm originating from the sinoatrial (SA) node (dominant pacemaker) that meets these ECG criteria:
- Rate: Atrial and ventricular rates of 60 to 100 beats/min
- Rhythm: Atrial and ventricular rhythms regular
- P waves: Present, consistent configuration, one P wave before each QRS complex
- PR interval: 0.12 to 0.20 second and constant
- QRS duration: 0.06 to 0.10 second and constant
What are sinus arrhythmias?
Sinus arrhythmia is a variant of NSR. It results from changes in intrathoracic pressure during breathing. In this context, the term arrhythmia does not mean an absence of rhythm, as the term suggests. Instead, the heart rate increases slightly during inspiration and decreases slightly during exhalation. This irregular rhythm is frequently observed in healthy adults.
Sinus arrhythmia has all the characteristics of NSR except for its irregularity. The PP and RR intervals vary, with the difference between the shortest and the longest intervals being greater than 0.12 second (three small blocks):
- Rate: Atrial and ventricular rates between 60 and 100 beats/min
- Rhythm: Atrial and ventricular rhythms irregular, with the shortest PP or RR interval varying at least 0.12 second from the longest PP or RR interval
- P waves: One P wave before each QRS complex; consistent configuration
- PR interval: Normal, constant
- QRS duration: Normal, constant
Sinus arrhythmias are occasionally due to nonrespiratory causes such as digoxin or morphine. These drugs enhance vagal tone and cause decreased heart rate and irregularity unrelated to the respiratory cycle.
What complexes make up a normal ECG?
Complexes that make up a normal ECG consist of a P wave, a QRS complex, a T wave, and possibly a U wave. Segments include the PR segment, ST segment, and TP segment. Intervals include the PR interval, QRS duration, and QT interval
What is the “P wave”?
The P wave is a deflection representing atrial depolarization. The shape of the P wave may be a positive, negative, or biphasic (both positive and negative) deflection, depending on the lead selected. When the electrical impulse is consistently generated from the sinoatrial (SA) node, the P waves have a consistent shape in a given lead. If an impulse is then generated from a different (ectopic) focus, such as atrial tissue, the shape of the P wave changes in that lead, indicating that an ectopic focus has fired.
What is the “PR segment”?
The PR segment is the isoelectric line from the end of the P wave to the beginning of the QRS complex, when the electrical impulse is traveling through the atrioventricular (AV) node, where it is delayed. It then travels through the ventricular conduction system to the Purkinje fibers.
What is the “PR interval”?
The PR interval is measured from the beginning of the P wave to the end of the PR segment. It represents the time required for atrial depolarization, the impulse delay in the AV node, and the travel time to the Purkinje fibers. It normally measures from 0.12 to 0.20 second (five small blocks).
What is the “QRS complex”?
The QRS complex represents ventricular depolarization. The shape of the QRS complex depends on the lead selected. The Q wave is the first negative deflection and is not present in all leads. When present, it is small and represents initial ventricular septal depolarization. When the Q wave is abnormally present in a lead, it represents myocardial necrosis (cell death). The R wave is the first positive deflection. It may be small, large, or absent, depending on the lead. The S wave is a negative deflection following the R wave and is not present in all leads.
What is the “QRS duration”?
The QRS duration represents the time required for depolarization of both ventricles. It is measured from the beginning of the QRS complex to the J point (the junction where the QRS complex ends and the ST segment begins). It normally measures from 0.06 to 0.10 second (1 ½ to 2 ½ small blocks).
What is the “ST segment”?
The ST segment is normally an isoelectric line and represents early ventricular repolarization. It occurs from the J point to the beginning of the T wave. Its length varies with changes in the heart rate, the administration of medications, and electrolyte disturbances.
What is the “T wave”?
The T wave follows the ST segment and represents ventricular repolarization. It is usually positive, rounded, and slightly asymmetric. T waves may become tall and peaked, inverted (negative), or flat as a result of myocardial ischemia, potassium or calcium imbalances, medications, or autonomic nervous system effects.
What is the “U wave”?
The U wave, when present, follows the T wave and may result from slow repolarization of ventricular Purkinje fibers. It is of the same polarity as the T wave, although generally it is smaller. It is not normally seen in all leads and is more common in lead V3. An abnormal U wave may suggest an electrolyte abnormality (particularly hypokalemia) or other disturbance. Correct identification is important so that it is not mistaken for a P wave. If in doubt, notify the primary health care provider and request that a potassium level be obtained.
What is the “QT interval”?
The QT interval represents the total time required for ventricular depolarization and repolarization. The QT interval is measured from the beginning of the Q wave to the end of the T wave. This interval varies with the patient’s age and gender and changes with the heart rate, lengthening with slower heart rates and shortening with faster rates. It may be prolonged by certain medications, electrolyte disturbances, or subarachnoid hemorrhage. A prolonged QT interval may lead to a unique type of ventricular tachycardia called torsades de pointes.
What is an artifact?
Artifact is interference seen on the monitor or rhythm strip, which may look like a wandering or fuzzy baseline. It can be caused by patient movement, loose or defective electrodes, improper grounding, or faulty ECG equipment such as broken wires or cables. Some artifacts can mimic lethal dysrhythmias such as ventricular tachycardia (with toothbrushing) or ventricular fibrillation (with tapping on the electrode). Assess the patient to differentiate an artifact from actual lethal rhythms! Do not rely only on the ECG monitor.
What is the first step in analyzing and ECG rhythm strip?
- Determine the heart rate. The most common method is to count the number of QRS complexes in 6 seconds and multiply that number by 10 to calculate the rate for a full minute. This is called the 6-second strip method and is a quick method to determine the mean or average heart rate. Normal heart rates fall between 60 and 100 beats/min. A rate less than 60 beats/min is called bradycardia. A rate greater than 100 beats/min is called tachycardia. Current monitoring systems will display a continuous heart rate and print it on the ECG strip. Use caution and confirm that the rate is correct by assessing the patient’s heart rate directly. Many factors can incorrectly alter the rate displayed by the monitor.
What is the second step in analyzing an ECG rhythm strip?
- Determine the heart rhythm. Assess for atrial and/or ventricular regularity. Heart rhythms can be either regular or irregular. Irregular rhythms can be regularly irregular, occasionally irregular, or irregularly irregular. Check the regularity of the atrial rhythm by assessing the PP intervals, placing one caliper point on a P wave and the other point on the precise spot on the next P wave. Then move the caliper from P wave to P wave along the entire strip (“walking out” the P waves) to determine the regularity of the rhythm. P waves of a different shape (ectopic waves), if present, create an irregularity and do not walk out with the other P waves. A slight irregularity in the PP intervals, varying no more than three small blocks, is considered essentially regular if the P waves are all of the same shape. This alteration is caused by changes in intrathoracic pressure during the respiratory cycle.
Check the regularity of the ventricular rhythm by assessing the RR intervals, placing one caliper point on a portion of the QRS complex (usually the most prominent portion of the deflection) and the other point on the precise spot of the next QRS complex. Move the caliper from QRS complex to QRS complex along the entire strip (walking out the QRS complexes) to determine the regularity of the rhythm. QRS complexes of a different shape (ectopic QRS complexes), if present, create an irregularity and do not walk out with the other QRS complexes. A slight irregularity of no more than three small blocks between intervals is considered essentially regular if the QRS complexes are all of the same shape.
