Basic ECG Concepts and Normal ECG

Basic ECG Concepts and the Normal ECG

Chapter 4

Clinical Exercise Electrocardiography

• Authors: Shel Levine, Brian J. Coyne, Lisa Cooper Colvin
• Foreword by Barry Franklin, Past President ACSM and AACVPR
• Slide lecture content prepared by M. Allison Williams, PhD

Introduction

• Cardiac electrical activity originates in the SA node:
  • Key processes include depolarization and repolarization of myocardial cells.
  • This activity is represented by action potentials.
• The Electrocardiogram (ECG) traces the waveforms of cardiac electrical potentials, corresponding to heart rhythm.
  • EKG is the German equivalent of ECG in English.

ECG Paper and Speed

• ECG paper is standardized for accuracy:
  • Small light boxes = 1 mm² (1 mm vertical [.1 millivolt] & 1 mm horizontal [.04 sec time/speed]).
  • Large dark boxes = 25 mm².
• The paper speed of the ECG influences recorded measurements:
  • At 25 mm/s, measurements are actual values.
  • At 50 mm/s, measurements are divided by half.

Calibration of ECG

• Calibration is applied at the start or end of each 12-lead reading.
• Standard calibration consists of:
  • 10 mm high (1 mV) and 0.2 seconds long, corresponding to one large box.
  • Modification includes doubling or halving the calibration for better interpretation.
  • Figure 4-2 demonstrates standard calibration (1 mV = 10 mm) and half calibration (0.5 mV = 5 mm).

ECG Paper Basics

• The isoelectric line indicates the baseline with no electrical activity.
• Electrical activity causes an upward or downward deflection from baseline, referred to as biphasic deflection:
  • Waveform amplitude is measured vertically (mm).
  • The height of the tracing is indicative of muscle depolarization size.
  • Waveform width is measured horizontally (seconds) indicating the speed of depolarization.
• The Mean QRS Vector represents the overall direction of depolarization in the heart.

Standard 12-Lead Printouts

• The 12-lead printout is standardized:
  • 12 seconds in length, including all 12 leads, printed at 25 mm/s.
  • The printout consists of three lines of four leads horizontally across, separated by lead dividers (not used in ECG interpretation).
  • A rhythm strip below the 12-lead printout (usually lead II) is utilized to determine rate and rhythm.
  • Figure 4-3 illustrates the standard 12-lead ECG printout, including limb and chest leads.

Calculation of Heart Rate

Dark Line Method

• The Dark Line Method (for regular heart rates determined with calipers):
  • The user locates the R wave on a dark line:
  • Subsequent R waves on dark lines represent rates:
    • 300 bpm on the 1st dark line, 150 bpm on the 2nd dark line, 100 bpm on the 3rd, 75 bpm on 4th, 60 bpm on 5th, 50 bpm on 6th, 44 bpm on 7th, and 38 bpm on the 8th.
  • Example: in Figure 4-4 (A), the second consecutive R wave falls on the fourth dark line, giving an HR of 75 beats per minute.

1500 Method

• The 1500 Method (most accurate):
  • Count the number of small boxes between two consecutive R waves.
  • Divide 1,500 by the number of small boxes counted.
  • Example: if there are 17 small boxes between two consecutive R waves, then HR = $rac{1500}{17} = 88 ext{ bpm}$.
  • Refer to Figure 4-5 showing the calculation of HR using the 1500 method.

6-Second Method

• The 6-Second Method applies for irregular HR:
  • 6 seconds correspond to 30 large boxes or distance between the three 3-sec marks at the top or bottom of the strip.
  • Count cardiac cycles during this 6-second period and multiply the count by 10 to estimate HR.
  • Example: in Figure 4-6, a 6-second strip shows eight cardiac cycles; thus, the heart rate equals $80 ext{ bpm}$ (8 cycles × 10 seconds).

Identify Rhythm and Heart Rate

Introduction to Cardiac Rhythm Assessment

• Identifying the rhythm and heart rate involves systematic evaluation of each ECG strip to determine normalcy and any abnormalities.

Development of 12-Leads

• Einthoven’s triangle principle states that 10 electrodes produce 12 leads:
  • 6 limb leads and 6 chest leads comprised of:
  • 3 unipolar and 3 bipolar limb leads.
• Placement of 4 electrodes is necessary:
  • (1) Right Arm (RA)
  • (2) Left Arm (LA)
  • (3) Left Leg (LL)
  • (4) Right Leg (RL) serves as a ground electrode and has no electrical movement.
  • Placement can be on extremities (diagnostic) or the torso (functional).
  • Figure 4-7 depicts Einthoven’s triangle.

Limb Leads

Bipolar Limb Leads

• Function: Measure electrical movement from negative to positive pole in the heart.
  • Lead I: RA (-) to LA (+)
  • Lead II: RA (-) to LL (+)
  • Lead III: LA (-) to LL (+)
• Mean QRS Vector interpretation:
  • If it moves towards a positive designated electrode, there is an upward deflection.
  • If it moves away, the deflection is downward.
  • If perpendicular, a small deflection or biphasic deflection occurs.

Unipolar Limb Leads

• Augmented leads are combinations of leads utilizing one electrode as a positive pole and the other two as negative poles to augment signal strength:
  • aVF: foot as positive (+) and the other two electrodes as negative (-)
  • aVL: left arm as positive (+) and the other two as negative (-)
  • aVR: right arm as positive (+) and the other two as negative (-)

Hex-axial Reference System

• This system aids in determining the heart's electrical axis in the frontal plane.
• It can identify various cardiac issues such as right or left ventricular hypertrophy, infarctions, COPD, and electrical blocks.

Precordial Leads

Description and Placement

• Precordial (or chest) leads consist of 6 electrodes that measure electrical activity in the chest's horizontal plane:
  • V1 to V6 are placed across different points on the chest.
• The negative pole is the imaginary center of the heart, with each electrode serving as a positive pole.
  • Figure 4-10 illustrates the arrangement of precordial leads.

Electrode Placement for 12-Lead ECG

Table 4-1: Electrode Placement

The Normal Cardiac Cycle

1. Assess for normal rhythm (equal spacing between cardiac cycles, identified by R wave).
2. There should be one P wave for every QRS complex and one QRS for every P (1:1 ratio).
3. Inspect if P, QRS, and T waves look consistent across cycles.
4. P wave and QRS upward deflection noted in lead II.
5. P wave downward deflection can be seen in lead aVR.
6. The heart rate must range between 60 and 100 bpm; conditions to consider include bradycardia vs. tachycardia.
7. Normal axis deviation (NAD) should be present.

ECG Strip Interpretation

• Methodology to read ECGs involves step-by-step analysis to ensure normalcy:
  • Identify each wave, interval, and segment explaining the physiological activity in the heart.
  • Normality assessment criteria for waves, segments, and intervals.

The P Wave

• Represents electrical activity from the SA node to the AV node; correlates to atrial depolarization and atrial kick, aiding in increased ventricular filling.
• The P wave is the first positive deflection present in all leads except aVR.
• Normal height of P wave is < 2.5 small boxes, and its length ranges from $0.08 ext{ to } 0.12$ seconds (2 to 3 small boxes) at 25 mm/sec speed.

The PR Interval (PRI)

• Represents the electrical activity from the SA node through the AV node.
• It is measured from the beginning of the P wave to the beginning of the QRS complex, indicating atrial to ventricular depolarization.
• Normal PRI measures between $0.12 ext{ and } 0.20$ seconds (3 to 5 small boxes).

QRS Complex

• Signifies complete ventricular depolarization.
• It is measured from the beginning of the complex (Q or R) to the end of the complex (R or S).
• A normal QRS complex measures between $0.06 ext{ and } 0.10$ seconds (1.5 to 2.5 small boxes).

QRS Complex Nomenclature

• The initial downward deflection after the P wave is referred to as the Q wave.
• An upward deflection following a Q wave is termed the R wave.
• Any subsequent downward deflection following an R wave is classified as the S wave.
• The use of lowercase letters signifies waves measuring 3 mm or less, while uppercase letters denote waves exceeding 3 mm in height.

QRS Complex Representation in Precordial Leads

• In V1, a small upward r wave (septal r) with a downward deflection (S wave) indicates an rS morphology.
• In V6, a small downward q wave (septal q) followed by an upward deflection (R wave) represents qR or qRs morphology.

ST Segment

• The ST segment is located on the isoelectric line or baseline, indicating no electrical activity.
• During this segment, the ventricular volume decreases as the aortic valve opens.
• The J point marks where the QRS complex ends and the ST segment begins.
• Deviations of the ST segment greater than 1 mm signify potential disease and/or infarction.
  • Figure 4-17 suggests ST segment variations from the baseline greater than 1 mm indicate ST elevation or depression.

ST Segment Depression

• A prolonged electrical signal transition through the myocardium suggests potential coronary artery disease or previous myocardial injury.
• During exercise tests, ST segment depression indicates a possible myocardial ischemia.
• It may also be observed in ventricular hypertrophies and bundle branch blocks.

ST Segment Elevation

• Indicates an insufficient blood supply to the myocardium, often suggesting a recent or impending myocardial infarction.
• During exercise testing, ST elevation can cause immediate concern, especially if the patient is symptomatic, potentially indicating myocardial injury or infarction.

T Wave

• Represents an upward deflection following the S wave, aligning directionally with the QRS complex.
  • Notably, the downward QRS and T wave can appear in lead aVR.
• When ventricles are in a state of isoelectric inactivity, they repolarize during diastole.
• Normal T waves are asymmetrical; significant clinical importance is ascribed to inverted T waves when observed in multiple leads.

T Wave Variants and Clinical Significance

• Normal T-waves:
  • Characterized by a smooth transition from ST-segment to T wave.
  • Usually asymmetrical with a steeper downslope.
• Large, asymmetric T waves:
  • Observed in conjunction with slight J point elevation, particularly in leads V2-V4.
• Hyperkalemia:
  • Presents as large, symmetric, pointed T waves with a short base.
• Hyperacute T waves:
  • Identified in transmural ischemia.
  • May yield high, broad-based T waves, often in conjunction with ST-segment elevation.
• Inverted T waves:
  • Can occur post-ischemia and vary from flat to deeply inverted T waves.
  • Inversions may appear after ischemic events without implying ongoing ischemia.
  • Note: Acute ischemia correlates with T wave inversions coupled with ST-segment deviations, most commonly ST-depression.
• Cerebrovascular insult pattern:
  • Deep (gigantic) T wave inversions primarily in chest leads, reported in up to 30% of patients suffering from intracerebral hemorrhage.
• Hypertrophic cardiomyopathy:
  • Characterized by deep, symmetric T wave inversions in leads V1-V3, often coinciding with substantial R waves and potential ST-segment depression.

Q-T Interval

• The Q-T interval signifies the duration from the beginning of the QRS complex to the end of the T wave.
• This interval can be corrected for heart rate variances:
  • Normal Q-T range: $0.36 ext{ to } 0.44$ seconds (9 to 11 small boxes).
• Elongated Q-T intervals are associated with an increased risk for sudden death, particularly among patients with a history of ventricular tachyarrhythmia.

U Wave

• Represented as a small, rounded upward deflection in lead II; it signifies the last stages of ventricular repolarization.
• Typically, U waves are not visible on a normal ECG and may fuse with the preceding T wave.
• Note: U waves are beyond the scope of common exams/quizzes.

Axis Deviation

• The determination of the mean QRS vector can indicate underlying cardiac disease or prior events:
  • Calculation involves the hex-axial reference system, leveraging limb leads (I, II, III, aVR, aVL, aVF).
• Two-lead method for axis determination used:
  • Normal Axis Deviation (NAD): ranging from 0° to 90° (±15°; -15° to 105°).

Axis Deviation Representation

• Normal electrical axis lies between -30° to 90°.
  • Right Axis Deviation (RAD): occurs between 105° and 180°.
  • Left Axis Deviation (LAD): occurs between -30° and -90°.
  • Indeterminate Axis falls between -90° and 180°.
  • Refer to Table 4-3 for the two-lead axis determination method.

Equiphasic Method for Mean QRS Vector

• The equiphasic limb lead identifies the vector moving perpendicular to that lead.
• Subsequently, it determines two leads perpendicular to the equiphasic lead to ascertain the direction of deflection (upward or downward).
  • Upward indicates movement toward the positive electrode, while downward shows movement away from the positive electrode.

Left Axis Deviation (LAD)

• Non-pathologic LAD is defined as ranging from 0 to -30 degrees, often seen in athletic individuals or those who are endurance-trained.
• Pathologic LAD from -30 to -90 degrees may indicate left ventricular hypertrophy, right or inferior myocardial infarction, left anterior hemiblock, hyperkalemia, or Wolff-Parkinson-White syndrome.

Right Axis Deviation (RAD)

• Defined as occurring between 105° and 180°, RAD can result from right ventricular hypertrophy, COPD, anterior lateral myocardial infarction, left posterior hemiblock, or pulmonary embolism.
• This deviation is typically normal in children and tall, thin adults.

Indeterminate Axis

• Ranges from -90° to 180°, classified as "No man’s land."
• May be caused by conditions like emphysema, hyperkalemia, or the presence of a pacemaker.

Review Questions and Assessments

1. Not normal - Arrhythmia, illustrated as a long pause indicates SA node failure.
2. Not normal - Sinus tachycardia demonstrated by HR of 150; all P waves are consistent from the SA node.
3. Not normal - Variability in P wave appearance.
4. Right axial deviation observed as 180° implicating big axis deviation, often due to right ventricular hypertrophy.
5. Not normal - Variability in identified patterns.
6. Normal.
7. Not normal - Any structural irregularities noted.
8. Not normal.
9. Right axial deviation recorded at 60°.
10. Normal rhythm identified.