Guyton and Hall Physiology Chapter 12 - Vectorial Analysis

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.