2.1 Electrocardiogram (ECG) Monitors and Physiological Signals

Electrocardiogram (ECG) Monitors and Physiological Signals

1. Introduction to Cardiac Electrophysiology

The electrocardiogram (ECG or EKG) is the clinical gold standard for non-invasive, real-time monitoring of the electrical activity of the heart. For Biomedical Equipment Technicians (BMETs) preparing for the CABT exam, understanding the physiological origin of these electrical signals is essential.

The heart’s mechanical pumping action is preceded and controlled by a highly coordinated electrical conduction system. The cardiac cycle begins at the Sinoatrial (SA) Node, located in the upper wall of the right atrium. Known as the heart's natural pacemaker, the SA node spontaneously generates electrical impulses at a baseline rate of 60 to 100 times per minute.

From the SA node, the electrical wave propagates through the atrial muscle fibers, causing atrial depolarization and subsequent contraction, which pumps blood into the ventricles. The wave then reaches the Atrioventricular (AV) Node, located in the interatrial septum. The AV node introduces a critical delay of approximately 0.1 seconds. This delay ensures that the atria have completely contracted and emptied their blood volume into the ventricles before ventricular contraction begins.

Following the AV nodal delay, the impulse travels rapidly down the Bundle of His, splits into the Right and Left Bundle Branches along the interventricular septum, and terminates in the Purkinje Fibers. The Purkinje fibers distribute the electrical wave throughout the ventricular myocardium, causing rapid, synchronized ventricular depolarization and contraction, forcing blood into the pulmonary artery and aorta. Finally, the ventricular muscle cells undergo ventricular repolarization to recover their resting electrical potential.

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2. The ECG Waveform: P, QRS, and T Waves

As the wavefront of depolarization and repolarization moves through the heart, it creates a moving electrical vector. The body fluids act as conductors, allowing these electrical potentials to be detected by electrodes placed on the skin. The standard ECG waveform consists of three major components:

• P Wave: Represents the depolarization of the atria. It is typically a small, rounded deflection. An abnormal or absent P wave can indicate conditions such as atrial fibrillation or SA nodal block.
• QRS Complex: Represents the rapid depolarization of the ventricles. Because the ventricles have a much larger muscle mass than the atria, the QRS complex is the largest voltage change in the ECG waveform. Its normal duration is between 0.06 and 0.10 seconds. A prolonged QRS duration (greater than 0.12 seconds) indicates a delay in ventricular conduction, such as a bundle branch block or a beat originating from an ectopic pacemaker in the ventricles.
• T Wave: Represents the repolarization of the ventricles. It is generally broader and lower in amplitude than the QRS complex. Abnormally tall, peaked T waves can indicate hyperkalemia (high blood potassium), while flat or inverted T waves may indicate myocardial ischemia or hypokalemia.

[!NOTE] Atrial Repolarization occurs at the same time as ventricular depolarization. However, because the ventricular muscle mass is so much larger, the electrical signal of atrial repolarization is completely masked by the much larger QRS complex on a standard surface ECG.

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3. ECG Lead Configurations: 3-Lead, 5-Lead, and 12-Lead Systems

Biomedical technicians must distinguish between an electrode (the physical adhesive patch attached to the skin) and a lead (the electrical view of the heart's activity between electrodes).

3-Lead System

The 3-lead system is used for basic, continuous heart rate and rhythm monitoring, typically in transport or low-acuity settings. It uses three electrodes: Right Arm (RA), Left Arm (LA), and Left Leg (LL). By measuring the electrical potential differences between these electrodes, the monitor generates three bipolar limb leads, forming Einthoven's Triangle:

• Lead I: Measures the potential difference between LA (+) and RA (-).
• Lead II: Measures the potential difference between LL (+) and RA (-). This is the most common monitoring lead because its axis aligns closely with the heart's anatomical electrical axis, producing a tall, clear QRS complex.
• Lead III: Measures the potential difference between LL (+) and LA (-).

5-Lead System

The 5-lead system is the standard for bedside monitoring in intensive care units (ICUs) and operating rooms. It adds two electrodes to the 3-lead set: Right Leg (RL) and Chest (V).

• The RL Electrode acts as the system ground and reference point, allowing the monitor's differential amplifier to perform common-mode rejection of external noise.
• The Chest Electrode (V) is placed in a precordial position (typically V1 or V5) to monitor ischemia and specific ventricular arrhythmias.
• The 5-lead configuration allows the simultaneous view of Leads I, II, III, the augmented unipolar leads (aVR, aVL, aVF), and one precordial lead.

12-Lead System

The 12-lead ECG is the diagnostic standard. It uses 10 physical electrodes: 4 limb electrodes (RA, LA, RL, LL) and 6 precordial chest electrodes (V1 through V6) placed in precise anatomical locations. From these 10 electrodes, the system mathematically derives 12 distinct electrical views:

1. 3 Bipolar Limb Leads: Leads I, II, and III.
2. 3 Augmented Unipolar Limb Leads: aVR, aVL, and aVF. These leads use Wilson's Central Terminal (an average of RA, LA, and LL) as the virtual reference point.
3. 6 Unipolar Precordial Leads: V1 through V6, which view the heart in the horizontal plane from right to left across the chest.

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4. Electrode Skin Contact and Impedance Control

The ECG signal picked up at the skin surface is incredibly small, typically ranging from 0.5 mV to 2.0 mV. The interface that converts the body's ionic current (flowing via ions in body fluids) into electronic current (flowing via electrons in the copper lead wires) is the silver/silver chloride (Ag/AgCl) electrode.

To ensure a high-fidelity signal, the electrical impedance between the skin and the electrode must be minimized, ideally to less than 5 kΩ. High skin-electrode impedance leads to signal attenuation, susceptibility to external noise, and false alarms.

Skin Preparation Protocol

1. Hair Removal: Excess hair at the electrode site creates air gaps, preventing proper gel contact. Hair should be clipped or shaved.
2. Skin Cleaning: Wipe the site with mild soap and water or an alcohol prep pad to remove skin lipids, sweat, and dirt. If alcohol is used, it must dry completely; trapped alcohol dries the skin and degrades the electrode gel.
3. Mild Skin Abrasion: This is the most critical step. Using a dedicated abrasive prep pad, a fine sandpaper ring, or dry gauze, gently abrade the skin site. This removes the stratum corneum (the outer layer of dead, cornified skin cells), which acts as a major electrical insulator.
4. Electrode Inspection: Ensure the conductive gel on the electrode is moist. Never use expired or dried-out electrodes.

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5. Common Noise, Artifacts, and Technical Troubleshooting

ECG signals are easily corrupted by electrical and physical noise. BMETs must quickly identify these artifacts to resolve monitor issues.

60 Hz Electromagnetic Interference (Power Line Noise)

• Cause: Capacitive or inductive coupling from nearby 120V AC power lines, power strips, or line-powered medical equipment (such as hyperthermia blankets or motorized beds) into the patient or lead wires.
• Appearance: A thick, dense, fuzzy baseline of uniform frequency (60 Hz in the US, 50 Hz in Europe) that obscures the P and T waves.
• Troubleshooting:
  1. Verify that the monitor's power cord is plugged into a properly grounded hospital-grade outlet and that the ground pin is intact.
  2. Ensure that patient lead wires are bundled together and routed away from power cords.
  3. Inspect lead wires for cracked insulation or broken internal shielding.
  4. Confirm that the monitor's internal notch filter (60 Hz/50 Hz) is enabled.

Muscle Tremor (Somatic / EMG Artifact)

• Cause: Skeletal muscle electrical activity (electromyogram) caused by patient movement, shivering due to cold, anxiety, or pathological tremors (e.g., Parkinson's disease).
• Appearance: Irregular, jagged, high-frequency spikes of varying amplitude superimposed across the entire ECG trace.
• Troubleshooting:
  1. Ensure the patient is comfortable and warm (provide warm blankets to stop shivering).
  2. Reassure the patient and ask them to remain still.
  3. Change the monitor's filter settings from "Diagnostic" mode (typically 0.05 – 150 Hz) to "Monitoring" or "Filter" mode (typically 0.5 – 40 Hz), which utilizes a low-pass filter to attenuate high-frequency EMG noise.

Wandering Baseline

• Cause: Low-frequency changes in skin-electrode impedance, primarily caused by respiratory chest movement, patient breathing, or loose electrodes.
• Appearance: The entire ECG waveform slowly drifts upward and downward across the screen, making it difficult to measure ST segments.
• Troubleshooting:
  1. Perform thorough skin abrasion to remove the stratum corneum and apply fresh electrodes.
  2. Loop the lead wires and secure them to the patient's gown or skin with tape (strain relief loop) to prevent physical tugging on the electrodes.
  3. Ensure the monitor's high-pass filter is enabled (usually set to 0.5 Hz for monitoring).