Premature Atrial Contractions (PACs)

ECG Interpretation Basics EKG Graph Paper The paper used to record EKG tracings is grid like in nature and has specific markings utilized to mark length in seconds. The paper is divided into large and small boxes. Thicker darker lines separate large boxes; thinner lighter lines separate small boxes. x The dark vertical lines are 0.20 seconds apart. x The lighter vertical lines are 0.04 seconds apart x Each small box is 1mm in size x Each small box represents an electrical current that is equal to 0.1 (millivolt) mV x One mV is equal to two large boxes Normally EKG graph paper has thick dark lines at the top or bottom of the paper. In the strip presented here, the lines are at the bottom of the paper. The space between the lines is equal to 3 seconds. A standard EKG strip is run over 6 seconds. When printing an EKG rhythm strip the paper prints at a standard speed of 25 mm per second. omponents of the Normal EKG Waveform The EKG is a real-time recording of the hearts electrical activity, produced by depolarization and repolarization of the hearts cells. The EKG waveform consists of : x P Wave: Represents atrial depolarization x QRS: Represents ventricular depolarization x T Wave: Represents ventricular repolarization Intervals are described as the length of time between one waveform and the next. Isoelectric Line: Flat line of the EKG tracing represents no electrical activity, and is referred to as the baseline. Deflections above the isoelectic line are positive, and deflections below the isoelectric line are negative. P-Wave: The SA node produces an electrical stimulus the P wave, normally upright, usually no more than 3mm in height. PRI (PR Interval): Is representative of the spread of the atrial depolarization wave, and the time it takes for the impulse to conduct through the AV node and to the ventricles. The PRI is marked from the start of the P-Wave to the beginning of the QRS. The PRI is normally no more than 0.20 seconds in length. QRS Complex: The QRS represents depolarization of the ventricles and ventricular conduction time of the electrical impulse. Typically the QRS is narrow with a conduction time of no more than 0.12 seconds. The QRS is measured from the beginning of the first waveform to the point at which the waveform returns to the isolectric line. The QRS is made up of: x Q-Wave: First negative deflection from baseline (below the isoelectric line), x R-Wave: Positive deflection from the baseline (above the isoelectric line). x S-Wave: Negative deflection following the R-Wave (below the isoelectric line) x R’ (Prime) is a secondary positive wave that may represent abnormal ventricular conduction. Below are examples of the shapes that the QRS complex may take: ST-Segment: Represents early ventricular repolarization, and extends from the end of the QRS to the beginning of the T-Wave, it is normally even with the isoelectric line. A deviation either above or below the isoelectric line represents myocardial injury or ischemia. Basic facts of the ST segment. The ST segment starts at the end of the QRS and ends at the start of the T Wave. The ST segment represents the early beginning of ventricular repolarization. The portion of the EKG tracing where the QRS ends and the ST segment begins is called the J Point. Characteristics: The ST-segment normally remains “Isoelectric” which is the normal baseline of the EKG. Elevation greater than 1mm in two or more reciprocal leads may indicate injury. Depression greater than 1mm in two or more reciprocal leads may indicate myocardial ischemia. T-Wave: Represents ventricular repolarization as the ventricles return to a state of relaxation. The TWave is typically rounded and systematical. The T-Wave is typically upright however, this may vary if myocardial injury or ischemia is present. QT-Interval (QTI): The QTI represents the refractory period of the ventricles (The QT interval represents total ventricular activity), as they depolarize and repolarize. It begins with the first wave in the QRS complex representing ventricular depolarization, and ends when the T Wave returns to baseline at the isoelectric line, representing ventricular repolarization. As rule of thumb, a normal QTI is less than 0.40 seconds. The QTI is directly related to the heart rate. Normal Measurements of the QTI: 0.36-0.40 It is best to measure the QTI in a lead in which the T wave is most pronounced. The normal overall length of the QTI should be equal to or less than ½ of the R-R interval. Summary of Heart Measurements: x P-Wave: Upright in lead II x PRI: < 0.20 Sec x QRS: < 0.12 Sec x ST-Segment: Even with isoelectric line x T-Wave: Upright in lead II x QTI: < 0.40 Sec