Intro to Cardiology Module 1 Guide Part 6

Module 4: Basic ECG and Arrhythmia Learning Outcomes: ● Identify structures of the cardiac conduction system. ● Describe lead placement for a standard 12-lead ECG recording. ● State correct paper speeds, ECG grid measurements and calibration standards. ● Describe the basic ECG heart rate measurement techniques. ● Identify and describe the components of a normal ECG. ● List the steps to systematically interpret an ECG. ● Identify and describe ECG changes with myocardial infarction. ● Identify and describe some common arrhythmias. How ECGs Work The cardiac conduction system is composed of highly specialized cells. These specialized cells have unique characteristics and functions that allow the creation and propagation of electrical impulses throughout the cardiac muscle. The cardiac conduction system is composed of: ● ● ● ● ● ● the sinoatrial node (SA node) internodal tracts the atrioventricular node (AV node) the bundle of His right and left bundle branches Purkinje fibers The standard electrocardiogram is the summation of the many electrical potentials generated by the heart: that is, cardiac electrical activity. These electrical potentials are viewed and recorded from specific sites, known as leads. Note that the terms sensor, electrode, and lead are often interchanged. ECG components, the P, QRS, and T waves, represent the electrical activity needed to bring about certain physiological events. The electrical activity is either myocardial depolarization or repolarization. Note that by convention, electrical activity directed upwards is a positive deflection while a downward deflection is negative. Twelve-lead ECG Sensor (Electrode) Placement To record a standard 12-lead ECG, it is necessary to place all six precordial sensors in their assigned positions: ● V1 - 4th intercostal space, 1.3 cm (1/2 inch) to the right of the sternum ● V2 - 4th intercostal space, 1.3 cm (1/2 inch) to the left of the sternum ● V3 - In a straight line, midway between V2 and V4 ● V4 - 5th intercostal space, midclavicular line (always placed under the breast) ● V5 - parallel to V4 at the anterior axillary line ● V6 - parallel to V4 and V5 at the mid axillary line The right and left arm sensors are located at the wrists. The right and left leg sensors are located just above the ankles. The right leg sensor acts as the ground lead. The lead or lead wire is an insulated wire that attaches to the sensor (electrode) with an alligator clip. The lead wires come together to form the main patient cable that connects to the main ECG machine. Specific leads in a standard 12-lead ECG reflect activity in specific parts of the heart. Recording Paper/Speeds/Calibration/Signal filtering/Measurements A standard 12-lead ECG is recorded on a millimeter-grid sheet of paper, where: ● Each millimeter (smallest square) of the horizontal grid equals 0.04 seconds. ● The larger square is 5 x 0.04 = 0.2 seconds. ● The largest squares are 5 x 0.20 = 1 second. It is possible to determine the heart rate on an ECG from these measurements. A standard ECG usually consists of 4 columns and 3 rows. Each ECG tracing is printed on a separate sheet by the machine. A 4th row is sometimes included as a rhythm strip of one specific ECG lead: lead II or V1. ● Each section represents one (lead) and is 2.5 seconds in duration. ● A whole page is 10 seconds. The standard format for a normal ECG with normal columns and rows is shown in the image below. GAIN (or Standard) On a standard ECG, each vertical millimeter of paper equals 0.1 millivolt. The standard calibration for an ECG is 10 mm (i.e., 1 millivolt). Calibrations can be changed to adjust for too large a deflection or too small a deflection. Alternate gains are 1/2 and x2. ● At 1/2 gain, the recording height is reduced by 1/2 of the standard. ● At x2 gain, the recording height is twice the standard. Some ECG recorders will allow for a "split" calibration. A split calibration is usually normal standard for the limb and augmented leads, and 1/2standard for the chest (precordial) leads. Paper speed The standard ECG recording speed is 25 mm/sec. Alternate paper speeds are 12.5 mm/sec and 50 mm/sec. ● At 12.5 mm/sec, the recording is compressed to 1/2 of the standard. ● At 50 mm/sec, the recording is stretched to twice the standard. Signal filtering ECG recording filters for adults, as recommended by the American Heart Association (AHA), are 0.05 to 150 Hz. These are programmed into the ECG recording device. Components and Normal Readings An ECG is made up of a series of waveforms, segments, and intervals that represent a cardiac cycle: P wave, PR segment, PR interval, QRS complex, J point, ST segment, T wave, QT interval, U wave. The U wave is not usually visible. P Wave The P wave is the first waveform in the normal cardiac cycle. In Lead II of the ECG, it is normally smooth, rounded and upright or positive. The P wave is the complete depolarization of the right and left atrium. The P wave: ● duration is normally <0.12 s ● amplitude is <0.2 mV The mean P wave depolarization vector of the two atria occurs from right to left and from superior to inferior. Remember: Electrical activation must precede contraction. Therefore, the P wave represents the electrical activation sequence of the atria. Contraction of the atria follows immediately. PR Segment The PR segment is the period immediately after the P wave, up to the beginning of the QRS complex. This is an electrically silent period when the impulse slowly traverses the AV node. During this time, the atria contract. PR Interval The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It measures the time it takes for the impulse to travel from the atrial myocardium near the sinoatrial node to the ventricular myocardium. The normal PR interval is 0.12 to 0.20 seconds. The PR rate varies with heart rate because the PR segment is the transmission of the impulse across the AV node, which is controlled by the sympathetic and parasympathetic divisions of the autonomic nervous system. It also varies with age. Note the PR interval in the following image. QRS Complex The QRS complex represents ventricular depolarization. Atrial repolarization occurs during this time, but is usually obscured on the ECG by the larger ventricular depolarization. The QRS duration is the time taken for complete ventricular depolarization. Duration is normally in the range of 0.08 to 0.12 sec. Although (by convention) the ventricular depolarization is called the QRS, certain ECG complexes may not have a Q, an R or an S component, depending on the lead being examined. All three waves on a normal ECG can be appreciated in V5 and V6. ● The Q wave is the first downward or negative deflection after a P wave. ● The R wave is the first positive deflection after a P wave. ● The S wave is a negative deflection following an R wave. J Point The J point is the precise point (junction) where the QRS and ST segment meet. This point is relevant to hypothermia, hypercalcemia, stress testing, and the Brugada Syndrome. Later modules will discuss these conditions in relation to the J point in more detail. ST Segment The ST segment of the electrocardiogram typically represents a period when the ventricular myocardium remains in a depolarized state. It is measured from the end of the QRS complex to the beginning of the T wave. Normally, the ST segment is flat (isoelectric). It may be slightly upsloping, downsloping, or horizontally depressed. The ST segment is a very sensitive indicator of myocardial ischemia or infarction (heart attack). By comparing the relationship of the isoelectric PR segment with that of the ST segment, you can determine whether there is significant ST elevation or depression. Elevation or depression of the ST segment by 1 mm (0.1 mV) from the isoelectric baseline is considered "abnormal". T Wave The T wave represents ventricular repolarization. Usually, the T wave is upright and positive (except in aVR), as illustrated in the following image. The duration by itself is not usually measured; rather it is measured within the QT or QTc interval, as defined below. QT Interval The QT interval consists of the QRS, ST segment, and T wave. Together these represent the entire depolarization and repolarization of the ventricles, also known as the electrical activity and recovery. The QT interval is measured from the beginning of the QRS complex to the end of the T wave and varies with heart rate, gender and age. The QT interval is corrected (QTc) for heart rate by a specific formula: Note: the RR interval is in seconds. The QT interval is of interest due to its predisposition for causing ventricular arrhythmias. U Wave