Follows 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..., involving the return to a pre-depolarization state; the myocardial cell’s electrical potential returns from +30 mVmilliVolt; the ECG is usually calibrated to display 1mV = 10mm of amplitude. 1. Six Second ECG Guidebook (2012), T Barill, p. 200 to its polarized state of –90 mV; the ions potassium, calcium and sodium are largely involved; note that contraction of the myocardial cell occurs during repolarization.
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... is a representation of ventricular depolarizationThe depolarization of the myocardium is represented on an ECG by a series of waveforms, one for atrial depolarization and soon after a larger waveform for ventricular depolarization. Normal ventricular depolarization begins with the septal fascicle of the left bundle.... The repolarization of the atriaRight and left atria (1/3 volume and muscle mass of the ventricles) pump blood to the ventricles. Chambers and Layers of the Heart 1. Six Second ECG Guidebook (2012), T Barill, p. 190 is also buried in the QRS complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194.
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... is a graphical representation of the repolarization of the ventriclesThe larger chambers of the heart (3 times the volume and muscle thickness than the atria), responsible for the pumping of blood to the lungs and the rest of the body..
The T wave is typically about 0.10 to 0.25 seconds wide with an amplitudeThe height or depth of waves and complexes of an ECG in millimetres; represents millivolts where 10 mm is 1 millivolt with a properly calibrated monitor. 1. Six Second ECG Guidebook (2012), T Barill, p. 190 less than 5 mm. While ventricular depolarization occurs rapidly producing a tall QRS complex, ventricular repolarizationThe T wave is a graphic representation of the repolarization of the ventricle. The T wave is typically about 0.10 to 0.25 seconds wide with an amplitude less than 5 mm. While ventricular depolarization occurs rapidly producing a tall QRS... is spread over a longer 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..., resulting in a shorter and broader T wave.
If ventricular repolarization returns cell membrane voltage back to its predepolarization resting electrical voltage, then shouldn’t the wave produced by ventricular repolarization be opposite that of ventricular depolarization? In other words, should the QRS complex and the T wave face opposite directions, upright and inverted. This is usually not the case.
Ventricular depolarization proceeds from the endocardiumThe smooth innermost layer of the heart covers the inner chambers and the cardiac valves. The mechanical structures of the heart include the heart’s layers, chambers, septum, valves, and the major vessels (including the coronary arteries). Each of these structures... to the epicardiumThe external layer that covers the heart – also called the visceral layer of the heart; between the visceral layer and the outer parietal layer is the pericardial sac; note that the coronary arteries travel along the epicardium before burrowing..., essentially depolarizing the ventricles from the inside out. It follows that repolarization also occurs from the inside out, producing inverted T waves opposite in direction to the QRS complex. Instead, repolarization is delayed in endocardial cells, allowing the epicardium to repolarize first. Repolarization normally proceeds opposite to depolarization, from the outside in. An upright T wave results.
An inverted T wave can point to cardiac ischemiaThe ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac deaths are associated with an ischemic episode. Patient deaths due to an acute myocardial infarction (MI) typically... among other causes. IschemiaInsufficient supply of oxygen to meet the oxygen demands of tissue. Anaerobic metabolism becomes increasingly important during periods of ischemia. Ischemia results from an inadequate blood flow that fails to meet the oxygen demands (energy demands) of tissues. If tissues... to the epicardium prolongs ventricular repolarization to this area. This extended repolarization of the epicardium removes the delay between the repolarization of the endocardium and the repolarization of the epicardium, with repolarization now following the sequence of depolarization. An inverted T wave results.
Occasionally, another wave -the U waveA wave that follows the T wave and precedes the P wave; its significance remains somewhat unknown, but it may be the repolarization of the Purkinje fibres; a U wave taller than 2 mm may suggest hypokalemia or that the... – is recorded immediately following the T wave and before the P waveA wave generated from the depolarization of the atria; the P wave is upright when originating from the SA node. The P wave represents the depolarization of the right and left atria. The P wave begins with the first deviation.... The U wave remains rather mysterious but is thought to represent a final stage of repolarization of unique ventricular cells in the midmyocardium. The U wave will most often orient in the same direction as the T wave with an amplitude less than 2 mm.
An abnormal U wave is inverted or tall with an amplitude of 2 mm or more. An abnormally tall U wave is associated with conditions such as 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..., diabetes, ventricular hypertrophyVentricular hypertrophy is reflected in QRS axis deviation towards the hypertrophied ventricle, increased amplitude in the QRS complex, altered R wave progression, and possibly signs of ventricular strain - ST depression and T wave inversion. Note that ST changes can..., and cardiomyopathy. Cardiac medications such as digoxin and quinidine can also cause a tall U wave.
Figure 4.13 depicts the component parts of the QRS complex. The QRS complex consists of a series of waves, the ‘Q’, ‘R’, and ‘S’ waves. The ‘Q’ wave is the first negative deflection from baselineThe baseline or isoelectric line is a reference point for the waves, intervals and segments. While the PR segment is often used as the baseline, the TP segment - between the end of the T wave and the beginning of.... The ‘R’ wave is the first positive deflection above baseline. The ‘S’ wave follows the ‘R’ wave with a negative deflection. A QRS complex may or may not have all three waveforms. The 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... begins at the J pointA small notch in the QRS where the ST segment begins. The point where the S Wave ends and begins to flatten out. Figure 4.13 The QRS Complex, ST Segment and the T Wave Figure 4.13 depicts the component parts... and continues to the beginning of the T wave.
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. 87, 204