Chapter 268 - Electrocardiography Flashcards
“ECG leads actually display the instantaneous differences in potential between the electrodes.”
True or False?
True.
What is the clinical utility of electrocardiogram (ECG)? Name the main conditions that might be diagnosed using this test.
“The clinical utility of the ECG derives from its immediate availability as a noninvasive, inexpensive, and highly versatile test. In addition to its use in detecting arrhythmias, conduction disturbances, and myocardial ischemia, electrocardiography may reveal findings related to life-threatening metabolic disturbances (e.g., hyperkalemia) or increased susceptibility to sudden cardiac death (e.g., QT prolongation syndromes).”
The ECG only records the depolarization (stimulation) and repolarization (recovery) potentialts generated by the “working” atrial and ventricular myocardium.
True or False?
True.
What is the characteristic feature of sinoatrial cells that explain why they are the main pacemaker cells of the heart?
Automaticity.
In comparison to other cells with this feature, sinoatrial cells have the quickest automaticity and, therefore, they are responsible for heart rhythm in physiological conditions.
Explain the normal depolarization wavefront of the heart.
“The first phase of cardiac electrical activation is the spread of the depolarization wave through the right and left atria, followed by atrial contraction. Next, the impulse stimulates pacemaker and specialized conduction tissues in the atrioventricular (AV) nodal and His-bundle areas; together, these two regions constitute the AV junction. The bundle of His bifurcates into two main branches, the right and left bundles, which rapidly transmit depolarization wavefronts to the right and left ventricular myocardium by way of Purkinje fibers. The main left bundle bifurcates into two primary subdivions: a left anterior fascicle and a left posterior fascicle. The depolarization wavefronts then spread through the ventricular wall, from endocardium to epicardium, triggering ventricular contraction.”
How does one explain the inherent limitations of ECG sensitivity and specificity?
“The ECG records the complex spatial and temporal summation of electrical potnetials from multiple myocardial fibers conducted to the surface of the body. This principle accounts for inherent limitations in both ECG sensitivity (activity from certain cardiac regions may be canceled out or may be too weak to be recorded) and specificity (the same vectorial sum can result from either a selective gain or a loss of forces in opposite directions).”
What is J point in ECG?
J point corresponds to the junction of QRS with the beginning of ST-segment.
How do you identifiy atrial repolarization in ECG?
“Atrial repolarization (STa and Ta) is usually too low in amplitude to be detected, but it may become apparent in conditions such as acute pericarditis and atrial infarction.”
Explain the correlation between the action potential of a single myocardial fiber and the ECG wavefront.
“The QRS-T waveforms of the surface ECG correspond in a general way with the different pahses of simultaneously obtained ventricular action potentials, the intracellular recordings from single myocardial fibers. The rapid upstroke (phase 0) of the action potential corresponds to the onset of QRS. The plateau (phase 2) corresponds to the isoelectric ST segment, and active repolarization (phase 3) corresponds to the inscription of the T-wave.”
Explain the different phases of myocardial cell depolarization as well as the different factors that might impair the normal duration of these phases. Also, explain the correlation between these factors and the alterations in the ECG.
“Factors that decrease the slope of phase 0 [rapid upstroke] by impairing the influx of Na+ (e.g., hyperkalemia and drugs such as flecainide) tend to increase QRS duration. Conditions that prolong phase 2 [plateau] (amiodarone, hypocalcemia) increase the QT interval. In contrast, shortening of ventricular repolarization (phase 2), such as by digitalis administration or hypercalcemia, abbreviates the ST segment.”
How can one radiply ascertain the heart rhythm by looking at an ECG?
“The heart rate (beats per minute) can be computed readily from the interbeat (RR) interval by dividing the number of large (0,20s) time units between consecutive R waves into 300 or the number of small (0,04s) units into 1500.”
What are the normal times for the following intervals: (i) PR segment; (ii) QRS complex; (iii) QTc interval.
(i) 120-200ms
(ii) Less than 100-110ms
(iii) Equal or less than 0,44s “(Some refences give QT upeer normal limits as 0,43s in men and 0,45s in women. Also, a number of different formulas have been proposed, without consensus, for calculating the QTc.)”
How do you calculate “corrected” QT? Therefore, what is the relation between QT interval and heart rate?
“The QT interval includes both ventricular depolarization and repolarization times and varies inversely with heart rate. A rate-related (“corrected”) QT interval, QTc, can be calculated as QT/√RR”
The limb leads record potentials transmitted onto the frontal plane, and the chest leads record potentials transmitted onto the horizontal plane.
True or False?
True.
Where should one place the precordial leads for body surface ECG?
“lead V1, fourth intercostal space, just to the right of the sternum; lead V2, fourth intercostal space, just to the left of the sternum; lead V3, midway between V2 and V4; lead V4, midclavicular line, fifth intercostal space; lead V5, anterior axillary line, same level as V4; and lead V6, midaxillary line, same level as V4 and V5. Additional posterior leads are sometimes placed on the same horizontal place as V4 to facilitate detection of acute posterolateral infarction (V7, midaxillary line; V8 posterior axillary line; and V9, posterior scapular line).”
V7-V9 leads and V3R/V4R have the same significance for ECG interpretation.
True or False?
False.
V7-V9 are used to identify acute posterolateral myocardial infarction (of the left ventricle), while V3R/V4R are used to identify acute right ventricular ischemia.
What are the characteristic of the normal P-wave on ECG? Which signs would you look for if you suspect an ectopic pacemaker rhythm?
“The normal atrial depolarization vector is oriented downward and toward the subject’s left, reflecting the spread of depolarization from the sinus node to the right and then the left atrial myocardium. Since this vector points toward the positive pole of lead II and toward the negative pole of lead aVR, the normal P wave will be positive in lead II and negative in lead aVR. By contrast, activation of the atria from an ectopic pacemaker in the lower part of either atrium or in the AV junction region may produce retrograde P waves (negative in lead II, positive in lead aVR). The normal O wave in lead V1 may be biphasic with a positive component reflecting right atrial depolarization, followed by a small (less than 1 mm2) negative component
reflecting left atrial depolarization.”
How does one explain the normal QRS in leads V1 and V6 regarding the two phases of myocardial depolarization?
“The first phase is depolarization of the interventricular septum from the left to the right and anteriorly (vector 1). The second results from the simultaneous depolarization of the right and left ventricles; it normally is dominated by the more massive left ventricle, so that vector 2 points leftward and posteriorly. Therefore, a right precordial lead (V1) will record this biphasic depolarization process with a small positive deflection (septal r wave) followed by a large negative deflection (S wave). A left precordial lead, e.g., V6, will record the same sequence with a small negative deflection (septal q wave) followed by a relatively tall positive deflection (R wave).”
Define normal R-wave progression and transition zone concepts.
“Intermediate leads show a relative increase in R-wave amplitude (normal R-wave progression) and a decrease in S-wave amplitude progressing across the chest from right to left. The precordial lead where the R and S waves are of approximately equal amplitude is referred to as the transition zone (usually V3 or V4).”
What is the normal QRS axis range?
-30º to +100º
Name the causes for left axis deviation.
“[it] may occur as a normal variant but is more commonly associated with left ventricular hypertrophy, a block in the anterior fascicle of the left bundle system (left anterior fascicular block or hemiblock), or inferior myocardial infarction.”
Name the causes of right axis deviation.
“Right axis deviation also may occur as normal variant (particularly in children and young adults), as a spurious finding due to reversal of the left and right arm electrodes, or in conditions such as right ventricular overload (acute or chronic), infarction of the lateral wall of the left ventricle, dextrocardia, left pneumothorax, and left posterior fascicular block.”
What is the normal polarity of the T-wave and U-wave? Explain the electrophysiology of T-wave.
“Normally, the mean T-wave vector is oriented roughly concordant with the mean QRS vector (within about 45º in the frontal plane). Since depolarization and repolarization are electrically opposite processes, this normal QRS-T wave vector concordance indicates that repolarization normally must proceed in the reverse direction from depolarization (i.e., from ventricular epicardium to endocardium). The normal U wave is a small, rounded deflection (equal or less than 1mm) that follows the T wave and usually has the same polarity as the T wave.”
Which conditions might be associated with an abnormal U-wave? What are the risks associated with this abrnomality?
“An abnormal increase in U-wave amplitude is most commonly due to drugs (e.g., dofetilide, amiodarone, sotalol, quinidine) or to hypokalemia. Very prominent U waves are a marker of increased susceptibility to the torsades de pointes type of ventricular tachycardia. Inversion of the U wave in the precordial leads is abnormal and may be a subtle sign of ischemia.”
Explain the pathophysiology of pathological P-waves on ECG. Apply the terms “P-pulmonale” and “P-mitrale” to this explanation.
“Right atrial overload (acute or chronic) may lead to an increase in P-wave amplitude (≥2,5mm), sometimes referred to as “P-pulmonale”. Left atrial overload typically produces a biphasic P wave in V1 with a broad negative component or a broad (≥120msw), often notched P wave in one or more limb leads. This pattern, previously referred to as P-mitrale”, may also occur with left atrial conduction delays in the absence of actual atrial enlargement, leading to the more general designation of left atrial abnormality.”
What are the indicators of right ventricular hypertrophy/overload?
“Right ventricular hypertrophy due to a sustained, severe pressure load (e.g., due to tight pulmonic valve stenosis or certain pulmonary artery hypertension sydnromes) is characterized by a relatively tall R wave in lead V1 (R≥S wave), usually with right axis deviation; alternatively, there may be a qR pattern in V1 or V3R. ST depression and T-wave inversion in the right-to-midprecodial leads are also often present. This pattern, formerly called right ventricular “strain”, is attributed to repolarization abnormalities in acutely or chronically overloaded muscle. Prominent S waves may occur in the left lateral precordial leads. Right ventricular hypertrophy due to ostium secundum-type atrial septal defects, with the accompanying right ventricular volume, is commonly associated with an incomplete or complete right bundle branch block pattern with a rightward QRS axis.”
What is the characteristic ECG of acute cor pulmonale due to pulmonary embolism?
None is characteristic, although many features might be indicative.
“Acute cor pulmonale due to pulmonary embolism, for example, may be associated with a normal ECG or a variety of abnormalities. Sinus tachycardia is the most common arrhythmia, although other tachyarrhythmias, such as atrial fibrillation or flutter, may occur. The QRS axis may shift to the right, sometimes in concert with the so-called S1Q3T3 pattern (prominence of the S-wave in lead I and the Q wave in lead III, with T-wave inversion in lead III). Acute right ventricular dilation also may be associated with slow R-wave progression and ST-T abnormalities in V1 to V4 simulating acute anterior infarction. A right ventricular conduction disturbance may appear.”
Chronic cor pulmonale is associated with tall R-waves in the right precordial leads.
True or False?
False.
“Chronic cor pulmonale due to obstructive lung disease usually does not produce the classic ECG patterns of right ventricular hypertrophy noted above. Instead of tal right precordial R waves, chronic lung disease more typically is associated with small R waves in right-to-midprecordial leads (slow R-wave progression) due in part to downward displacement of the diaphragm and the heart. Low-voltage complexes are commonly present, owing to hyperaeration of the lungs.”
Name two groups of patients in which ECG has a lower sensitivity for detecting left ventricular hypertrophy.
Obese and smokers.
