Q waves are a normal phenomenon when they are narrow (less than 1 mm in width) and shallow (less than 25% the height of the R waveFollowing the depolarization of the interventricular septum, ventricular depolarization then progresses from the endocardium through to the epicardium across both ventricles producing an R wave and an S wave. An R wave is the first positive deflection of the QRS...). Q waves that exceed any of these criteria are evidence of prior ST elevationThe upward placement of the ST segment greater than 1 mm from the isoelectric line; suggestive of cardiac infarction or ischemia. Between the QRS complex and the T wave, lies the ST segment. The ST segment usually follows the isoelectric... myocardial infarctionThe necrosis or death of myocardial tissue due to insufficient supply of oxygen to the infarcted region. The ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac... – recent to as much as several years ago. The location of the deep and/or wide Q waveA Q wave is the first negative deflection of the QRS complex that is not preceded by a R wave. A normal Q wave is narrow and small in amplitude. Note that a wide and/or deep Q wave may signify... identifies the region of the original infarctionThe necrosis of tissue; acute myocardial infarction involves the acute death of myocardial cells. 1. Six Second ECG Guidebook (2012), T Barill, p. 198 i.e. pathological Q wave in lead II points to damage to the inferior region.
a normal Q wave represents a depolarizationThe rapid influx of positive ions (sodium and/or calcium) into a cell – depolarization is necessary for contraction to occur. A depolarizing wave moves through the myocardium on average along a trajectory or vector. A vector is a force moving... of the ventricular septum, which usually travels from left to right, towards the right ventricleThe right ventricle ejects blood through the main branches of the left and right pulmonary arteries to the lungs. The chambers of the heart are the main drivers within an intricate pathway, delivering blood to the lungs for gas exchange.... When present, a Q wave is the first downward deflection of the QRS complexThe electrical representation of ventricular depolarization; the atrial repolarization is also a part of the QRS. ECG interpretation relies heavily on the QRS complex. The QRS complex represents the depolarization of the ventricles. The repolarization of the atria is also.... While ST segmentSegment (line) between the end of the QRS complex and the beginning of the T wave; the end of the QRS is marked by the J point. Between the QRS complex and the T wave, lies the ST segment. The... deviation is a sign of present events, a prominent Q wave points to an MI that has already occurred, recently to some time ago. A prominent Q wave is like a tattoo – once you have one, it’s pretty much yours for good.
A normal Q wave is usually no deeper than 2 mm and less than 1 small square in width (<0.04 seconds). An abnormal Q wave tends to get the most attention. A Q wave that is wider than 1 small square or at least 1/4 the height of the R wave is a significant marker of a myocardial infarction. In Figure 4.16, the Q wave is about 31% the height of the R wave (4/13 = 31%), making the Q wave prominent. The width of the Q wave is also significant with a width of 0.06 seconds. This Q wave is typical of an MI.
The QT intervalThe QT interval represents a complete ventricular cycle of depolarization and repolarization. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A QT interval should be less than 1/2 the... represents a complete ventricular cycle of depolarization and repolarizationFollows depolarization, involving the return to a pre-depolarization state; the myocardial cell’s electrical potential returns from +30 mV to its polarized state of –90 mV; the ions potassium, calcium and sodium are largely involved; note that contraction of the myocardial.... The QT intervalA period measured on rhythm strip paper that measures a wave and a segment; the distance measured is equal to time taken as an ECG is voltage over time; a PR interval for example is measured from the beginning of... is measured from the beginning of the QRS complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194 to the end of the T waveThe wave that arrives after the QRS; is a graphical presentation of ventricular repolarization. Expect a T wave to follow every QRS complex. The T wave is a graphic representation of the repolarization of the ventricle. The T wave is.... A QT interval should be less than 1/2 the R-R intervalThe distance between the top of the R waves. Figure 4.15 The Normal Q Wave and QT Interval Figure 4.15 illustrates the Q wave, QT interval and the R-R interval. A normal QT interval should be less than half the....
A long QT intervalA long QT interval, wider than 1/2 the R-R interval, is a significant risk factor for developing hemodynamically unstable dysrhythmias such as ventricular tachycardia and torsades de pointes. A prolonged QT interval is also associated with a higher incidence of... wider than 1/2 the R-R interval is a significant risk factor for developing hemodynamically unstable dysrhythmiasUsed interchangeably with arrhythmia, refers to any abnormal rhythm – not normal sinus rhythm or sinus tachycardia. 1. Six Second ECG Guidebook (2012), T Barill, p. 196 such as ventricular tachycardiaIs a cardiac rhythm characterized by rapidly occurring wide QRS complexes with absent or infrequent P waves evident; the absence of atrial kick, the lack of filling time, and the tendency for ventricular tachycardia to change to ventricular fibrillation makes... and torsades de pointesMeans the twisting of the points, a polymorphic ventricular tachycardia characterized by periods of increasing QRS amplitude followed with a period of reduced QRS amplitude – then the pattern repeats again; often associated with effects of cardiac drugs or with.... A prolonged QT interval is also associated with a higher incidence of sudden death.
The concern around a longer QT interval centers around the possibility of the next QRS coming at the tail end of the T wave, called an R-on-T phenomenon. This phenomenon can potentially cause dangerous dysrhythmias such as torsades de pointes. Causes of prolonged QT intervals include long QT syndrome, antiarrythmics such as quinidine and procainamide, tricyclic antidepressants, and hypokalemiaBlood potassium level lower than normal limits. Table 6.5 Normal and Abnormal Characteristics of ECG Waveforms Table 6.5 provides a non-exhaustive list of cardiac conditions that are associated with a variety of abnormal ECG waveforms. Arriving at a useful interpretation....
The Q waves of Figure 4.16 are abnormal in both depth and width, findings that point to a previous myocardial infarction.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 86, 145, 147, 150