Guyton and Hall Physiology Chapter 12 - Vectorial Analysis
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Questions and Answers

What does the vectorcardiogram depict during the cardiac cycle?

  • The blood flow patterns in the ventricles.
  • Changes in vectorial electrical potentials at different times. (correct)
  • The contraction strength of the heart muscle.
  • The location of the heart valves throughout the cycle.

    • What is the mean electrical axis of the ventricles typically measured at?

  • 30 degrees.
  • 59 degrees. (correct)
  • 90 degrees.
  • 45 degrees.

    • Which event occurs first during ventricular depolarization according to the vectorcardiogram?

  • The positive end of the vector leaves the zero reference point. (correct)
  • The ventricles experience rapid contraction.
  • The septum becomes depolarized.
  • The vector extends upward towards the base of the ventricles.

    • Which electrocardiographic leads are used to plot the mean electrical axis of the ventricles?

    <p>Leads I and III.</p> Signup and view all the answers

    What may happen to the direction of the mean electrical axis during pathological conditions of the heart?

    <p>It can change markedly, potentially opposing its normal direction.</p> Signup and view all the answers

    What is the most likely cause of left axis deviation when the mean electrical axis points to −15 degrees?

    <p>Hypertrophy of the left ventricle</p> Signup and view all the answers

    Which condition is NOT associated with left ventricular hypertrophy leading to left axis deviation?

    <p>Inferior wall myocardial infarction</p> Signup and view all the answers

    How does the position of the heart during deep expiration affect the mean electrical axis?

    <p>It causes the axis to shift to the left</p> Signup and view all the answers

    What change occurs in the mean electrical axis when a tall person stands up?

    <p>It shifts to the right side</p> Signup and view all the answers

    Which of the following is a consequence of right ventricular hypertrophy regarding mean electrical axis?

    <p>Electrical axis points to 170 degrees</p> Signup and view all the answers

    Which condition is likely to cause right axis deviation due to hypertrophy of the right ventricle?

    <p>Chronic lung disease</p> Signup and view all the answers

    What physiological change can shift the electrical axis to the left during normal condition?

    <p>Compression of the diaphragm due to abdominal contents</p> Signup and view all the answers

    What characterizes the mean electrical axis during hypertrophy of one ventricle?

    <p>It deviates towards the hypertrophied ventricle</p> Signup and view all the answers

    What does the intersection of the perpendicular lines from the apices of leads I and III represent?

    <p>The apex of the mean QRS vector in the ventricles</p> Signup and view all the answers

    What causes right axis deviation in a patient with congenital pulmonary valve stenosis?

    <p>Hypertrophy of the right ventricle</p> Signup and view all the answers

    What is the average potential generated by the ventricles during depolarization represented by?

    <p>The length of the mean QRS vector</p> Signup and view all the answers

    In a normal heart, the mean electrical axis of the ventricles typically averages about how many degrees?

    <p>59 degrees</p> Signup and view all the answers

    What alteration in the heart's electrical vector is most likely due to greater muscle mass in a hypertrophied ventricle?

    <p>Shift towards the hypertrophied side</p> Signup and view all the answers

    What can cause the mean electrical axis to deviate towards the hypertrophied ventricle?

    <p>Strong positive charge remaining in the hypertrophied side</p> Signup and view all the answers

    Which congenital heart condition can also lead to right axis deviation besides congenital pulmonary valve stenosis?

    <p>Tetralogy of Fallot</p> Signup and view all the answers

    What is indicated by a prolonged QRS complex in an electrocardiogram?

    <p>Blockage in the Purkinje system</p> Signup and view all the answers

    How much can the electrical axis vary even in a normal heart?

    <p>From about 20 degrees to about 100 degrees</p> Signup and view all the answers

    Which leads are primarily referenced to determine the mean QRS vector?

    <p>Leads I and III</p> Signup and view all the answers

    What electrical change is associated with left bundle branch block?

    <p>Left axis deviation</p> Signup and view all the answers

    What happens to the depolarization timing between a normal ventricle and a hypertrophied ventricle?

    <p>The normal ventricle depolarizes earlier</p> Signup and view all the answers

    How does the depolarization wave travel differently in a ventriculary hypertrophied heart?

    <p>More slowly requiring increased time</p> Signup and view all the answers

    What does the direction of the mean vector in the context of depolarization indicate?

    <p>The electrical predominance of one ventricle over the other</p> Signup and view all the answers

    Which of the following factors contributes to the generation of greater electrical potential in a hypertrophied ventricle?

    <p>More muscle mass on the affected side</p> Signup and view all the answers

    What effect does ventricular hypertrophy have on the QRS complex?

    <p>It prolongs the QRS complex</p> Signup and view all the answers

    What is the primary impact of right ventricular hypertrophy on the electrical axis?

    <p>Rightward shift of the axis</p> Signup and view all the answers

    What is a common electrocardiographic finding in a patient with right axis deviation?

    <p>Intense right axis deviation</p> Signup and view all the answers

    What is the primary purpose of the mean electrical axis in cardiology?

    <p>To evaluate ventricular depolarization direction</p> Signup and view all the answers

    Which of the following conditions would NOT cause hypertrophy of the right ventricle?

    <p>Left-sided heart failure</p> Signup and view all the answers

    What is indicated by the QRS vectorcardiogram during ventricular depolarization?

    <p>It shows a normal heart activity.</p> Signup and view all the answers

    How is the electrical axis of the heart typically estimated?

    <p>Using standard bipolar limb lead ECGs.</p> Signup and view all the answers

    What happens to the net potential in lead III during the QRS complex?

    <p>It can be both positive and negative at different times.</p> Signup and view all the answers

    What does the mean electrical axis represent?

    <p>The average direction of depolarization in the heart.</p> Signup and view all the answers

    During the calculation of the electrical axis, what is done to a negative recording?

    <p>It is subtracted from the positive potential.</p> Signup and view all the answers

    Which lead is primarily associated with the positive recording in this context?

    <p>Lead I.</p> Signup and view all the answers

    What type of figure does the positive ends of the vectors generate?

    <p>An elliptical figure.</p> Signup and view all the answers

    What occurs at point 5 of the QRS vectorcardiogram?

    <p>The vector becomes completely depolarized with zero value.</p> Signup and view all the answers

    Which phenomenon occurs during the ventricular depolarization as seen in the QRS complex?

    <p>Specific directional vectors of electrical activity.</p> Signup and view all the answers

    What can be inferred if the recordings in leads I and III are both negative?

    <p>There may be potential pathology in the electrical axis.</p> Signup and view all the answers

    What direction denotes a vector that extends straight downward during impulse transmission in the heart?

    <p>+90 degrees</p> Signup and view all the answers

    What is the average direction of the mean QRS vector in a normal heart?

    <p>+59 degrees</p> Signup and view all the answers

    Which of the following statements accurately describes the rotation method for vector direction?

    <p>Rotates clockwise from the zero reference point.</p> Signup and view all the answers

    What happens to the contour of the waves in an electrocardiogram due to abnormalities in impulse transmission?

    <p>They can become distorted.</p> Signup and view all the answers

    How is the direction of a vector described if it extends from the person's left to the right?

    <p>+180 degrees</p> Signup and view all the answers

    What does the length of vector A represent in the context of cardiac potential?

    <p>The voltage of the potential</p> Signup and view all the answers

    How is the voltage recorded in lead I determined from vector A?

    <p>By creating a perpendicular line from the tip of vector A to lead I axis</p> Signup and view all the answers

    What is the angle of vector A when the heart is partially depolarized in this context?

    <p>+55 degrees</p> Signup and view all the answers

    What must be understood before grasping the concepts in the chapter regarding leads I, II, and III?

    <p>The axes and their polarities associated with the leads</p> Signup and view all the answers

    What role does lead I play in relation to vector A in the described scenario?

    <p>It acts as an axis for projecting vector A's voltage</p> Signup and view all the answers

    What do augmented limb leads use for analysis instead of standard bipolar limb lead axes?

    <p>Respective axes of the augmented leads</p> Signup and view all the answers

    Which area of the ventricles is depolarized first during the QRS complex?

    <p>Left endocardial surface of the septum</p> Signup and view all the answers

    How does depolarization spread after initially affecting the left endocardial surface of the septum?

    <p>It progresses rapidly to involve both endocardial surfaces of the septum</p> Signup and view all the answers

    What graphical representation is associated with the analysis of potentials in the context of ventricular depolarization?

    <p>A vectorcardiogram</p> Signup and view all the answers

    What is the significance of understanding the projected vector during the analysis of lead axis?

    <p>It represents instantaneous potential in the ventricles</p> Signup and view all the answers

    What do the darker shaded portions represent in the graphical depiction of ventricular depolarization?

    <p>Regions of faster conduction</p> Signup and view all the answers

    In the context of ventricular depolarization, how is the term 'instantaneous potential' best defined?

    <p>The potential recorded at a specific moment</p> Signup and view all the answers

    Which layer of the ventricles does depolarization first affect?

    <p>Endocardial surface</p> Signup and view all the answers

    What occurs after depolarization spreads along the endocardial surfaces of the septum?

    <p>Spreading to the remainder of the ventricles</p> Signup and view all the answers

    What technique is utilized for the determination of potentials in augmented limb leads?

    <p>Vectorial analysis</p> Signup and view all the answers

    Vector B has a large length when it is in the negative direction.

    <p>False</p> Signup and view all the answers

    When the vector in the heart is nearly perpendicular to the axis of the lead, the recorded voltage is very low.

    <p>True</p> Signup and view all the answers

    The voltage recorded in lead I at this moment is approximately 0.3 millivolts.

    <p>False</p> Signup and view all the answers

    In lead III, the voltage recorded is approximately equal to that in the heart.

    <p>False</p> Signup and view all the answers

    Lead I will record a positive voltage when the heart vector is directed negatively.

    <p>False</p> Signup and view all the answers

    The instantaneous mean vector is represented by a long black arrow drawn from the apex toward the base of the ventricles.

    <p>False</p> Signup and view all the answers

    Lead aVF has an axis of about +90 degrees in the context of electrical potential.

    <p>True</p> Signup and view all the answers

    The right arm connects to the torso in the still polarized areas.

    <p>False</p> Signup and view all the answers

    Lead III has an axis of about +120 degrees.

    <p>True</p> Signup and view all the answers

    The hexagonal reference system shows the directions of the axes of various electrocardiographic leads.

    <p>True</p> Signup and view all the answers

    The mean electrical axis of the ventricles typically averages about 75 degrees.

    <p>False</p> Signup and view all the answers

    The point of intersection of the perpendicular lines from the apices of leads I and III represents the apex of the mean QRS vector.

    <p>True</p> Signup and view all the answers

    Vectors from a hypertrophied ventricle are negatively charged.

    <p>False</p> Signup and view all the answers

    The mean electrical axis can swing from about 10 degrees to about 90 degrees in a normal heart.

    <p>False</p> Signup and view all the answers

    The length of the mean QRS vector indicates the average potential generated by the ventricles during depolarization.

    <p>True</p> Signup and view all the answers

    The direction of the mean electrical axis always points away from the hypertrophied ventricle.

    <p>False</p> Signup and view all the answers

    A prolonged QRS complex is a characteristic finding in cases of ventricular hypertrophy.

    <p>True</p> Signup and view all the answers

    The mean QRS vector is represented by the point of intersection of lead I and lead III axes.

    <p>True</p> Signup and view all the answers

    The mean electrical axis can indicate abnormal ventricular conditions that cause axis deviation.

    <p>True</p> Signup and view all the answers

    During depolarization, the normal ventricle becomes depolarized after the hypertrophied ventricle.

    <p>False</p> Signup and view all the answers

    Match the following vector angles with their corresponding degrees:

    <p>Lead I = 0° Lead II = 60° Lead III = 120° Lead aVF = 90°</p> Signup and view all the answers

    Match the following vectors with their characteristics:

    <p>Vector A = Represents mean direction of current flow Projected Vector B = Drawn along lead I axis Vector recorded in lead I = Voltage derived from vector A Perpendicular line to lead I = Determines voltage contribution</p> Signup and view all the answers

    Match the following electrical potentials with their respective measurements:

    <p>Vector A length = 2 millivolts Lead I axis = 0 degrees Depolarization vector direction = +55 degrees Vector B direction = Aligned with lead I</p> Signup and view all the answers

    Match the following types of heart activity with their corresponding descriptions:

    <p>Instantaneous mean direction = Current flow in ventricles Partially depolarized heart = Heart not fully activating Vectorcardiogram = Depicts electrical activity over time Lead configuration = Arrangement of bipolar limb leads</p> Signup and view all the answers

    Match the following leads with their respective axis orientations:

    <p>Lead I = 0° Lead II = 60° Lead III = 120° Lead aVF = 90°</p> Signup and view all the answers

    Match the following leads with their corresponding type:

    <p>Lead I = Bipolar lead Lead II = Bipolar lead Lead III = Bipolar lead aVF = Unipolar lead</p> Signup and view all the answers

    Match the following directions with their related angle:

    <p>60° = Positive direction 120° = Positive direction 90° = Positive direction -60° = Negative direction</p> Signup and view all the answers

    Match the following vector characteristics with their consequences:

    <p>Vector B = Short and negative Vector D = One-third voltage Vector A = Positive recording Lead I = Negative recording when perpendicular</p> Signup and view all the answers

    Match the following vectors with their respective leads:

    <p>Vector A = Lead I Vector B = Lead II Vector D = Lead III aVF = Overall heart direction</p> Signup and view all the answers

    Match the following voltage levels with their descriptions:

    <p>0.3 millivolts = Small negative recording -0.3 millivolts = Lead I recording -1 millivolt = Representing significant negativity 0 millivolts = Baseline in ECG</p> Signup and view all the answers

    Study Notes

    Anatomical Causes of Normal Variations in Axis Deviation

    • Variations in the Purkinje system distribution and the heart muscle itself can influence axis deviation.

    Causes of Abnormal Axis Deviation

    • Change in Heart Position:
      • Left axis deviation occurs when the heart is angled to the left.
      • Examples include deep expiration, lying down, and obesity.
      • Right axis deviation happens when the heart is angled to the right.
      • Examples include deep inspiration, standing up, and tall, lanky individuals.
    • Hypertrophy of One Ventricle:
      • Hypertrophy of the left ventricle leads to left axis deviation.
      • The left ventricle pumps blood against elevated systemic arterial pressure, leading to hypertrophy.
      • This can be caused by hypertension, aortic valvular stenosis, aortic valvular regurgitation, and certain congenital heart conditions.
      • Hypertrophy of the right ventricle results in right axis deviation.
      • This can arise from conditions like congenital pulmonary valve stenosis, tetralogy of Fallot, and interventricular septal defect.

    Vectorial Analysis of Axis Deviation

    • Left Axis Deviation (Figure 12-12):
      • The mean electrical axis points towards -15 degrees.
      • Hypertrophy of the left ventricle causes this deviation.
    • Right Axis Deviation (Figure 12-13):
      • The mean electrical axis points towards 170 degrees, 111 degrees to the right of the normal.
      • This deviation is caused by hypertrophy of the right ventricle.

    Determining Electrical Axis from Standard ECGs

    • The electrical axis is typically estimated from standard bipolar limb lead ECGs.
    • Measure the net potential and polarity of recordings in leads I and III
    • Plot the net potential on the axes of the respective leads, with the base at the point of intersection.
    • The mean ventricular QRS axis in a normal heart is typically 59 degrees.
    • Axis deviation can indicate various pathological conditions.

    Vectorcardiogram

    • The QRS vectorcardiogram represents the electrical activity during ventricular depolarization.
    • It is an elliptical figure generated by the positive ends of the vectors.
    • The mean electrical axis of the ventricles is represented by the direction of the mean vector.
    • The direction of the mean electrical axis can change significantly in various heart conditions.

    Direction of Vector

    • A vector is measured in degrees from 0° to 360° or 0° to -180°
    • 0° represents a horizontal vector pointing to the person's left
    • 90° is a vector pointing straight downward
    • 180° is a vector pointing to the person's right
    • -90° or 270° is a vector pointing straight upward
    • The average QRS vector, which represents the direction of the depolarization wave through the ventricles, is approximately +59 degrees

    Vectorial Analysis of Potentials

    • The length of a vector represents the voltage
    • The direction of a vector represents the direction of electrical flow
    • To determine how much voltage is recorded in a specific lead, draw a perpendicular line from the tip of the vector to the axis of the lead
    • The length of this perpendicular line represents the projected vector and represents the voltage recorded in that lead

    Vectors during Ventricular Depolarization

    • The QRS complex represents the depolarization of the ventricles
    • The first part of the ventricles to become depolarized is the septum
    • Then, the endocardial surfaces of both ventricles become depolarized
    • These stages of depolarization are represented by changes in vector length and direction

    QRS Complex in Myocardial Infarction

    • An infarction (loss of muscle) can alter the shape of the QRS complex
    • A Q wave, which is a downward deflection at the beginning of the QRS complex, can indicate an infarction
    • The location of the infarction can be determined by the leads that show a Q wave
    • For example, a Q wave in leads I, aVL, and V4-6 indicates an anterior wall infarction
    • A Q wave in lead III, aVF, and V1-3 indicates a posterior wall infarction

    Current of Injury in Angina Pectoris

    • Angina pectoris is chest pain caused by ischemia (lack of blood flow) to the heart
    • Ischemia can cause changes in the ECG, similar to those seen in myocardial infarction, but less severe

    Lead Axis and Vector

    • The direction of current flow in a lead is represented by its axis.
    • Each lead has a specific axis:
      • Lead I: +60 degrees
      • Lead III: +120 degrees
      • Lead aVR: +210 degrees
      • Lead aVF: +90 degrees
      • Lead aVL: -30 degrees
    • The mean QRS vector represents the average electrical potential generated by the ventricles during depolarization.
    • The axis of the mean QRS vector in a normal heart is approximately +59 degrees.

    Abnormal Ventricular Conditions and Axis Deviation

    • Left Bundle Branch Block:

      • Depolarization spreads rapidly through the right ventricle, leaving the left ventricle polarized.
      • This generates a strong vector from the right ventricle towards the left ventricle.
      • Results in intense "left axis deviation" of about -50 degrees.

    • Right Bundle Branch Block:

      • Depolarization spreads rapidly through the left ventricle, leaving the right ventricle polarized.
      • This generates a strong vector from the left ventricle towards the right ventricle.
      • Results in intense "right axis deviation" of about +105 degrees.

    QRS Voltage Abnormalities

    • Increased Voltage in Standard Bipolar Limb Leads:

      • Normally, voltage in leads I, II, and III varies between 0.5 and 2.0 millivolts.
      • Increased voltage can indicate hypertrophy, where the heart muscle is thicker than normal.

    • Decreased Voltage in Standard Bipolar Limb Leads:

      • Can indicate loss of muscle mass, such as from myocardial infarction (heart attack).

    Myocardial Infarction and QRS Complex Changes

    • Anterior Wall Infarction:

      • Results in a prominent Q wave in leads V1-V4 and a current of injury (ST elevation) in these leads.

    • Posterior Wall Infarction:

      • Results in a Q wave in lead III and a current of injury in leads II and III.

    • Infarction in other areas can also produce distinctive QRS patterns and voltage changes.

    Other Conditions Affecting the ECG

    • Angina Pectoris:
      • Caused by moderate ischemia (lack of blood flow) to the heart.
      • Can cause ST-segment depression or T-wave inversions on the ECG.

    Vectorial Analysis of Potentials

    • The direction of the vector, shown in Figure 12-4, represents the instantaneous mean direction of current flow in the ventricles - in this case, it is +55 degrees, with the voltage 2 millivolts, representing the length of vector A.
    • To determine the voltage recorded in lead I, a line perpendicular to the axis of lead I is drawn from the tip of vector A.
    • The projected vector (B) is drawn along the lead I axis, indicating that the recording in lead I will be negative (below the zero line), with a voltage of −0.3 millivolts - a small voltage since the vector in the heart is almost perpendicular to the axis of the lead.
    • When the heart vector has almost exactly the same axis as the lead axis, essentially the entire voltage of the vector will be recorded.
    • The voltage recorded in lead III (vector D) is about one-third that of the heart.

    Injury Potential

    • Extreme ischemia of the cardiac muscle occurs after coronary occlusion.
    • Injury potential flows from the infarcted area of the ventricles during the T-P interval between heartbeats.
    • The injury potential in lead V2 in Figure 12-19 is intense and indicates an acute anterior wall infarction.
    • In Figure 12-20, the major diagnostic feature of a posterior wall infarction is the positive potential of the current of injury during the T-P interval in chest lead V2.
    • The injury potential is negative in leads II and III - the resultant vector of the injury potential is about −95 degrees, with the negative end pointing downward and the positive end pointing upward.
    • The injury potential diminishes considerably after 1 week of infarction and is gone after 3 weeks, but the ECG can still change during the next year.
    • The existence of injury potential in some patients with myocardial infarction can indicate that the infarcted area never redevelops adequate coronary blood supply.
    • Bouts of exercise can over-load the heart and create an injury potential.

    Q Waves and Myocardial Infarction

    • Q waves developing at the beginning of the QRS complex in lead I can signal an anterior infarction.
    • Q waves on an ECG represent old myocardial infarction, often occurring 1 year after the acute heart attack.
    • In patients who have experienced a myocardial infarction, collateral coronary blood flow can develop enough to re-establish appropriate nutrition to most of the infarcted area.

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    Guyton and Hall Physiology - Chapter 12 - Vectorial Analysis PDF

    Description

    This quiz explores the anatomical causes of normal and abnormal axis deviations in heart positioning. It covers variations in the Purkinje system, the impact of ventricle hypertrophy, and how various physical conditions affect heart orientation. Test your knowledge on these critical cardiac concepts!

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