BEST WAYS TO GET ACCURATE ECG READINGS EVERY TIME

Understanding and Interpreting the Electrocardiogram (ECG)

The Electrocardiogram (ECG or EKG) is a fundamental diagnostic tool in clinical medicine, providing a non-invasive window into the electrical activity of the heart. By recording these electrical signals from the body surface, healthcare professionals can assess cardiac rhythm, identify structural abnormalities, detect myocardial ischemia or infarction, and evaluate the effects of medications or implanted devices.

Mastering ECG interpretation requires understanding the principles behind how the electrical signals are captured, the correct procedures for recording, and the standardized components of an ECG tracing. This guide provides a detailed, step-by-step approach to acquiring these essential skills, focusing on the types of leads used, their placement, the recording process, waveform identification, and basic calculations.

Understanding ECG Leads – Bipolar vs. Unipolar

At the core of ECG recording is the concept of an ‘ECG lead’. Contrary to common usage, a ‘lead’ in this context isn’t just the cable and electrode attached to the patient; it represents a specific view of the heart’s electrical activity from a particular angle. This view is created by measuring the voltage difference between two points on the body surface. ECG leads are categorized into two main types based on how this voltage difference is measured: Bipolar and Unipolar.

Bipolar Leads:

• Principle: Bipolar leads measure the electrical potential difference between two distinct, specific points on the body surface, each connected to a recording electrode.
• Standard Limb Leads: The three standard bipolar limb leads (often referred to as Einthoven’s Triangle) are:
  • Lead I: Measures the potential difference between the Left Arm (LA) and the Right Arm (RA). (LA – RA)
  • Lead II: Measures the potential difference between the Left Leg (LL) and the Right Arm (RA). (LL – RA)
  • Lead III: Measures the potential difference between the Left Leg (LL) and the Left Arm (LA). (LL – LA)
• Reference Point: In bipolar leads, both electrodes contributing to the measurement are considered “active” recording electrodes.
• Perspective: These leads provide views of the heart primarily in the frontal plane. Einthoven’s Law states that Lead II = Lead I + Lead III, demonstrating their interconnectedness within the frontal plane.

Unipolar Leads:

• Principle: Unipolar leads measure the electrical potential difference between a single, specific ‘exploring’ electrode placed at a particular point on the body surface and a theoretical ‘indifferent’ or ‘reference’ electrode. This reference electrode is constructed electronically within the ECG machine by averaging the potentials from two or more other limb electrodes, creating a point with effectively zero or near-zero potential. This allows the exploring electrode to record the absolute electrical potential at its location relative to this ‘neutral’ point.
• Types: Unipolar leads are further divided based on their placement:
  • Unipolar Limb Leads (Augmented Limb Leads): These three leads record from the limbs but use the averaged potential of the other two limbs as the reference point. They are electronically ‘augmented’ (amplified) by the ECG machine to make the deflections larger and easier to analyze.
    • aVR: Measures the potential at the Right Arm (RA) relative to the average of the Left Arm and Left Leg potentials [(LA + LL)/2].
    • aVL: Measures the potential at the Left Arm (LA) relative to the average of the Right Arm and Left Leg potentials [(RA + LL)/2].
    • aVF: Measures the potential at the Left Leg (LL) relative to the average of the Right Arm and Left Arm potentials [(RA + LA)/2].
  • Unipolar Precordial (Chest) Leads: These six leads are placed directly on the chest wall over specific locations to record electrical activity in the horizontal (transverse) plane. The reference point for these leads is typically the central terminal, which is a point formed by averaging the potentials from all three limb electrodes (RA, LA, LL), approximating zero potential in the body’s core.
    • V1, V2, V3, V4, V5, V6: Each records the potential directly under its specific chest electrode relative to the central terminal.
• Reference Point: In unipolar leads, one electrode is the ‘exploring’ or ‘active’ electrode, while the reference point is a computed average potential from other electrodes (the ‘indifferent’ electrode).
• Perspective: Unipolar limb leads (aVR, aVL, aVF) provide additional views in the frontal plane, complementing the bipolar limb leads. Unipolar chest leads (V1-V6) provide views in the horizontal plane, offering critical insights into the anterior, septal, lateral, and posterior aspects of the ventricles.

Summary of Differences:

Together, the 6 limb leads (3 bipolar and 3 unipolar augmented) and the 6 precordial leads provide 12 standard views of the heart, offering a comprehensive picture of its electrical activity.

Locating the Position of Different Bipolar and Unipolar Leads

Accurate electrode placement is paramount for obtaining a diagnostic quality ECG recording. Misplacement can lead to significant misinterpretation. The standard 12-lead ECG requires placement of 10 electrodes: 4 on the limbs and 6 on the chest.

Limb Lead Placement:

These electrodes are typically placed on the distal limbs, usually the wrists and ankles, ensuring they are on a flat, fleshy surface away from bones. If limb amputation or casting is present, place the electrodes on the proximal limb or torso in corresponding locations. Clean the skin area, and if necessary, shave hair to ensure good contact. Apply electrode gel if required.

• RA (Right Arm): Anywhere on the right arm, usually the wrist.
• LA (Left Arm): Anywhere on the left arm, usually the wrist.
• LL (Left Leg): Anywhere on the left leg, usually the ankle.
• RL (Right Leg): Anywhere on the right leg, usually the ankle. This serves as the ground electrode, reducing electrical interference but not contributing directly to the recorded signals.

Precordial (Chest) Lead Placement:

These electrodes must be placed precisely on the chest wall. Identify anatomical landmarks first: the sternal notch, clavicles, and ribs (by palpating the intercostal spaces).

• V1: Fourth intercostal space (the space between the 4th and 5th ribs) just to the right of the sternum.
• V2: Fourth intercostal space just to the left of the sternum.
• V4: Fifth intercostal space at the midclavicular line (an imaginary vertical line drawn down from the middle of the clavicle).
• V3: Midway between V2 and V4.
• V5: Fifth intercostal space at the anterior axillary line (an imaginary vertical line drawn down from the front of the armpit). It should be at the same horizontal level as V4.
• V6: Fifth intercostal space at the midaxillary line (an imaginary vertical line drawn down from the middle of the armpit). It should be at the same horizontal level as V4 and V5.

Ensure good skin contact by cleaning the areas and managing hair. Securely attach the electrode cables to the corresponding electrodes.

Becoming Familiar with the ECG Machine and Recording the ECG

Modern ECG machines are automated but require operator input and understanding.

Familiarization with the Machine:

• Power: Locate the power button.
• Patient Data Entry: Understand how to input patient information (Name, ID, Age, Gender) – this is crucial for record-keeping.
• Lead Connectors: Identify where the limb and chest lead cables plug into the machine. Cables are typically color-coded and labelled (RA, LA, LL, RL, V1-V6).
• Controls: Recognize buttons for starting/stopping recording, adjusting paper speed (standard 25 mm/s), adjusting amplitude or gain (standard 10 mm/mV), applying filters (e.g., to reduce muscle tremor or power line interference), and printing.
• Display: Familiarize yourself with the screen displaying the real-time ECG waveforms and indicators for lead connection status.
• Printer: Ensure paper is loaded correctly.

Recording the ECG:

1. Patient Preparation:
   • Explain the procedure to the patient.
   • Ensure the patient is comfortable, relaxed, and lying down on their back.
   • Undress the patient sufficiently to expose the chest, wrists, and ankles.
   • Clean electrode sites to remove oils, dirt, or lotions. Shave excessive hair if needed.
2. Electrode Placement: Apply electrodes at the correct anatomical locations as described in Step 2. Ensure good skin contact.
3. Connect Leads: Attach the correctly labelled lead wires to the corresponding electrodes. Check the machine display to confirm all leads are connected and show a reasonable baseline (not flat or wildly erratic).
4. Reduce Artifacts: Ask the patient to lie still, breathe normally, and avoid talking or movement. Minimize environmental electrical interference (e.g., unplug unnecessary equipment nearby). Filters on the machine can help but use them cautiously as they can distort the signal.
5. Enter Patient Data: Input the required patient information into the machine.
6. Start Recording: Press the record button. The machine will typically acquire 10 seconds of data for all 12 leads simultaneously or sequentially.
7. Review Trace Quality: Examine the printed or displayed ECG trace. Look for:
   • Clear, distinct waveforms.
   • A stable baseline (minimal wandering).
   • Absence of excessive noise or artifact (fuzzy lines, jagged disruptions).
   • Check lead labels on the printout match the patient/recording.
8. Repeat if Necessary: If the trace is of poor quality, identify the source of the artifact (e.g., loose electrode, patient movement) and repeat the recording after correction.
9. Finish: Once a satisfactory recording is obtained, stop the recording, remove electrodes carefully from the patient, and clean the skin if necessary. Disconnect and clean the lead wires and machine cables according to protocol.

Identifying Different Waves, Intervals, and Segments of the ECG

A standard ECG trace consists of a series of waves, segments, and intervals representing the sequential electrical events of the cardiac cycle. Understanding these components and their normal appearance is fundamental to interpretation. The ECG paper itself is a grid: small boxes are 1 mm x 1 mm, representing 0.04 seconds horizontally and 0.1 mV vertically (at standard speed and gain). Large boxes (5×5 small boxes) are 5 mm x 5 mm, representing 0.20 seconds horizontally and 0.5 mV vertically.

Here are the key components to identify:

1. P Wave:
   • Represents: Atrial depolarization (electrical activation).
   • Shape: Smooth, rounded, typically upright in most leads (especially Lead II). May be biphasic (partly positive, partly negative) or inverted in other leads (normally inverted in aVR).
   • Amplitude (Height): Normal: < 2.5 mm (0.25 mV).
   • Duration (Width): Normal: < 0.10 seconds (2.5 small boxes).
2. PR Segment:
   • Represents: The delay of electrical conduction through the AV node and bundle of His. This allows the atria time to contract and pump blood into the ventricles before ventricular contraction begins.
   • Shape: Isoelectric (flat line) following the P wave and preceding the QRS complex.
3. PR Interval:
   • Represents: The time taken for atrial depolarization and conduction through the AV node to the ventricles. Includes the P wave and the PR segment.
   • Duration: Normal: 0.12 to 0.20 seconds (3 to 5 small boxes). Measured from the beginning of the P wave to the beginning of the QRS complex.
4. QRS Complex:
   • Represents: Ventricular depolarization (electrical activation of the ventricles).
   • Shape: Consists of up to three waves:
     • Q Wave: First negative deflection after the P wave. Not always present. Pathological Q waves can indicate prior myocardial infarction (deep, wide). Normal Q waves are usually small and narrow.
     • R Wave: First positive deflection after the P wave or Q wave. Represents the main electrical activity of ventricular depolarization.
     • S Wave: Negative deflection following the R wave.
     • Terminology: Uppercase letters (Q, R, S) denote waves of relatively large amplitude; lowercase letters (q, r, s) denote waves of relatively small amplitude. A purely positive complex is an ‘R’ wave, a purely negative complex is a ‘QS’ wave.
   • Amplitude: Highly variable across leads and individuals. Typically largest in leads V4-V5.
   • Duration: Normal: < 0.12 seconds (3 small boxes). Measured from the beginning of the Q wave (or R if no Q) to the end of the S wave.
5. ST Segment:
   • Represents: The period between ventricular depolarization and the start of ventricular repolarization. Corresponds to the plateau phase of the ventricular action potential.
   • Shape: Isoelectric (flat line) following the S wave and preceding the T wave. The point where the S wave ends and the ST segment begins is called the J point.
   • Significance: Elevation or depression of the ST segment relative to the baseline (TP segment or PR segment) is a critical indicator of myocardial ischemia or injury.
6. T Wave:
   • Represents: Ventricular repolarization (electrical recovery).
   • Shape: Usually upright in most leads (concordant with the QRS complex), rounded, and slightly asymmetrical (upslope slower than downslope). Normally inverted in aVR and may be inverted in V1 and Lead III.
   • Amplitude: Variable. Typically < 5 mm in limb leads and < 10 mm in precordial leads.
   • Duration: Variable, inversely related to heart rate.
7. QT Interval:
   • Represents: Total time for ventricular depolarization and repolarization. Includes the QRS complex, ST segment, and T wave.
   • Duration: Measured from the beginning of the QRS complex to the end of the T wave. Highly rate-dependent. Needs correction for heart rate (QTc) using formulas like Bazett’s (QT / √RR interval) or Fridericia’s (QT / ³√RR interval). Normal QTc is typically < 440-460 ms, varying slightly by gender.
8. RR Interval:
   • Represents: Time between two consecutive R waves. Reflects the ventricular rate and regularity.
9. PP Interval:
   • Represents: Time between two consecutive P waves. Reflects the atrial rate and regularity.

Methods of Calculating the Heart Rate from the Recording ECG

The heart rate represents the number of times the heart beats per minute. An ECG tracing captures the electrical events leading to each heartbeat, allowing us to determine the rate by measuring the time interval between successive cardiac cycles. The most easily identifiable and consistent point in the cycle for this measurement, especially for ventricular rate, is typically the R wave of the QRS complex.

ECG paper has a standard grid system. At a typical recording speed of 25 mm/second:

• Each small square represents 0.04 seconds (40 milliseconds).
• Each large square (consisting of 5 small squares horizontally) represents 0.20 seconds (200 milliseconds).
• Therefore, 5 large squares represent 1 second (5 * 0.20 s = 1 s).
• And 300 large squares represent 1 minute (60 s * 5 large squares/s = 300 large squares).
• Similarly, 1500 small squares represent 1 minute (300 large squares * 5 small squares/large square = 1500 small squares).

These relationships form the basis of the most common heart rate calculation methods.

Method 1: The 300 Method (For Regular Rhythms)

This is a quick and easy method, particularly useful for estimating heart rate when the rhythm is regular (the distance between R waves is consistent or varies only minimally).

• Step 1: Locate a prominent R wave that falls directly on or very near a thick line (the start of a large square). This makes counting easier. If no R wave is exactly on a thick line, choose one clearly before a thick line.
• Step 2: Count the number of large squares between this R wave and the next consecutive R wave.
• Step 3: Divide 300 by the number of large squares counted in Step 2. The result is the approximate heart rate in beats per minute (bpm).

Example:

• If there is 1 large square between R waves: Rate = 300 / 1 = 300 bpm
• If there are 2 large squares between R waves: Rate = 300 / 2 = 150 bpm
• If there are 3 large squares between R waves: Rate = 300 / 3 = 100 bpm
• If there are 4 large squares between R waves: Rate = 300 / 4 = 75 bpm
• If there are 5 large squares between R waves: Rate = 300 / 5 = 60 bpm
• If there are 6 large squares between R waves: Rate = 300 / 6 = 50 bpm

Note: For distances that fall between whole large squares, you can estimate or use the next method for more precision. For example, if the distance is 2.5 large squares, the rate is 300 / 2.5 = 120 bpm.

Method 2: The 1500 Method (For More Precise Calculation in Regular Rhythms)

This method uses small squares and provides a more precise heart rate calculation for regular rhythms, especially when the R-R interval does not align neatly with large square boundaries.

• Step 1: Locate an R wave.
• Step 2: Count the number of small squares between this R wave and the next consecutive R wave.
• Step 3: Divide 1500 by the number of small squares counted in Step 2. The result is the heart rate in bpm.

Example:

• If there are 15 small squares between R waves: Rate = 1500 / 15 = 100 bpm (Equivalent to 3 large squares)
• If there are 20 small squares between R waves: Rate = 1500 / 20 = 75 bpm (Equivalent to 4 large squares)
• If there are 23 small squares between R waves: Rate = 1500 / 23 ≈ 65 bpm
• If there are 30 small squares between R waves: Rate = 1500 / 30 = 50 bpm (Equivalent to 6 large squares)

This method is more accurate but requires careful counting of small squares.

Method 3: The 6-Second Method (For Irregular Rhythms)

When the rhythm is irregular (the distance between R waves varies significantly, as seen in conditions like atrial fibrillation), the 300 or 1500 methods, which rely on a single R-R interval, will give a misleading average rate. The 6-second method is specifically designed for irregular rhythms to provide a more representative average heart rate over a short interval.

• Step 1: Locate a 6-second strip on the ECG tracing. Standard ECG paper often has markings (usually small vertical lines or arrows at the top) indicating 3-second or 6-second intervals. A 6-second strip corresponds to 30 large squares (6 seconds * 5 large squares/second).
• Step 2: Count the number of QRS complexes that occur within this 6-second interval.
• Step 3: Multiply the number of QRS complexes counted in Step 2 by 10. The result is the estimated heart rate in bpm (since 6 seconds is one-tenth of a minute).

Example:

• If there are 7 QRS complexes in a 6-second strip: Rate = 7 * 10 = 70 bpm
• If there are 10 QRS complexes in a 6-second strip: Rate = 10 * 10 = 100 bpm
• If there are 15 QRS complexes in a 6-second strip: Rate = 15 * 10 = 150 bpm

This method provides an average rate over the 6-second period and is the preferred method for estimating the rate in irregular rhythms.

In summary, choose the method appropriate for the context: 300 or 1500 for quick or precise measurement of regular rhythms, and the 6-second method for irregular rhythms.

Calculating Cardiac Axis from an ECG

The cardiac axis refers to the general direction of the electrical activity (specifically ventricular depolarization, represented by the QRS complex) in the frontal plane. It represents the net sum of all the electrical vectors generated during depolarization. The normal cardiac axis points downwards and to the left, roughly following the anatomical orientation of the heart.

The frontal plane leads (Leads I, II, III, aVR, aVL, aVF) are used to determine the cardiac axis. These leads record electrical activity from different perspectives in the frontal plane surrounding the heart. Imagine these leads arranged around a circle, starting with Lead I at 0 degrees, moving counter-clockwise through aVL (-30°), aVR (-150°), III (+120°), II (+60°), and aVF (+90°).

While more sophisticated methods involve plotting vectors on a hexaxial reference system, a simple and commonly used method for estimating the cardiac axis involves examining the net deflection of the QRS complex in two key leads: Lead I and Lead aVF.

The Quadrant Method (Using Leads I and aVF)

This method allows you to place the cardiac axis into one of four quadrants, which is sufficient for identifying common axis deviations.

• Step 1: Examine the QRS complex in Lead I. Determine if the net deflection of the QRS complex is positive (mostly above the baseline), negative (mostly below the baseline), or equiphasic (positive and negative deflections are roughly equal in magnitude, resulting in a net deflection near zero).
  • A net positive deflection in Lead I indicates the electrical activity is generally moving towards the positive pole of Lead I (towards the left side of the patient).
  • A net negative deflection in Lead I indicates the electrical activity is generally moving away from the positive pole of Lead I (towards the right side of the patient).
  • An equiphasic QRS in Lead I means the axis is approximately perpendicular to Lead I (either +90° or -90°). Lead I is at 0°.
• Step 2: Examine the QRS complex in Lead aVF. Determine if the net deflection of the QRS complex is positive, negative, or equiphasic.
  • A net positive deflection in Lead aVF indicates the electrical activity is generally moving towards the positive pole of Lead aVF (downwards towards the patient’s feet).
  • A net negative deflection in Lead aVF indicates the electrical activity is generally moving away from the positive pole of Lead aVF (upwards towards the patient’s head).
  • An equiphasic QRS in Lead aVF means the axis is approximately perpendicular to Lead aVF (either 0° or ±180°). Lead aVF is at +90°.
• Step 3: Use the findings from Leads I and aVF to determine the quadrant. Imagine a coordinate system where Lead I represents the horizontal axis (positive to the left, negative to the right) and Lead aVF represents the vertical axis (positive downwards, negative upwards).
  • Normal Axis: Lead I is positive AND Lead aVF is positive. This places the axis in the lower left quadrant of the hexaxial system (between 0° and +90°). This is the normal range.
  • Left Axis Deviation (LAD): Lead I is positive AND Lead aVF is negative. This places the axis in the upper left quadrant (between 0° and -90°).
  • Right Axis Deviation (RAD): Lead I is negative AND Lead aVF is positive. This places the axis in the lower right quadrant (between +90° and +180°).
  • Extreme Axis Deviation (EAD) / Northwest Axis: Lead I is negative AND Lead aVF is negative. This places the axis in the upper right quadrant (between -90° and -180°). This is rare and often associated with significant underlying pathology.

Note: Lead II is also often checked. A normal axis typically has a positive QRS in Lead II (Lead II is at +60°, well within the normal 0° to +90° range). If Lead I is positive and aVF is negative (suggesting LAD), checking Lead II can help refine further: if Lead II is also negative, it strongly supports LAD (axis is more positive than -30° but more negative than +60°).

The Equiphasic Lead Method (For a More Precise Estimate)

A slightly more specific method involves finding the frontal plane lead where the QRS complex is most equiphasic (net deflection is closest to zero).

• Step 1: Identify the frontal plane lead (I, II, III, aVR, aVL, aVF) with the most equiphasic QRS complex. This is the lead where the positive and negative deflections of the QRS are approximately equal.
• Step 2: The cardiac axis is approximately perpendicular to the lead identified in Step 1.
• Step 3: To determine which direction along the perpendicular axis the actual axis lies, look at the QRS complex in the lead that is 90 degrees away from the equiphasic lead. The axis will point towards the positive pole of this perpendicular lead if its QRS is positive, or towards the negative pole if its QRS is negative.

Example:

• If aVL (at -30°) is the most equiphasic lead, the axis is perpendicular to aVL, meaning it’s approximately at +60° or -120°. To know which, look at Lead II (at +60°), which is perpendicular to aVL. If the QRS in Lead II is positive, the axis is close to +60°. If it’s negative, the axis is closer to -120°.
• If Lead I (at 0°) is the most equiphasic lead, the axis is perpendicular to Lead I, meaning it’s approximately at +90° or -90°. Look at Lead aVF (at +90°), which is perpendicular to Lead I. If QRS in aVF is positive, axis is close to +90° (borderline RAD). If negative, axis is close to -90° (borderline LAD).

This method provides a more targeted estimate than just the quadrant, but requires familiarity with the hexaxial system and angles. For routine interpretation, the quadrant method is often sufficient to identify significant deviations.

Conclusion

Mastering the calculation of heart rate and cardiac axis from an ECG tracing is a fundamental skill for anyone involved in cardiovascular care or ECG interpretation. Accurate determination of these parameters provides vital clues about the heart’s electrical function and can help identify normal findings or suggest potential underlying conditions such as arrhythmias, hypertrophy, or conduction defects.

By applying the step-by-step methods outlined above – choosing the appropriate heart rate calculation based on rhythm regularity and using the quadrant method (or more advanced techniques) for cardiac axis – you can systematically derive these essential quantitative measurements from any standard 12-lead ECG. Remember that practice and correlation with the clinical context are key to proficient ECG interpretation. These calculations are tools for analysis, contributing to a comprehensive understanding of the patient’s cardiac status.