7. ECG

INTRODUCTION TO ELECTROCARDIOGRAMS

Course Code: MLT 1307

ELECTROCARDIOGRAMS

  • Definition: An Electrocardiogram (ECG) is a test that assesses a patient’s heart rhythm and electrical activity.

  • Location: Conducted within a healthcare facility or outpatient clinic.

  • Purpose: Provides results based on short-term monitoring of a patient’s cardiac status.

ECG FUNCTION

  • Description: An ECG serves as a graphical representation of the heart’s electrical activity.

  • Mechanism:

    • Electrodes are affixed to the patient’s chest and limbs.

    • These electrodes connect by leads (cables) to an ECG machine acting as a voltmeter.

    • The machine detects and records voltage changes caused by depolarization and repolarization of cardiac cells.

    • Results are displayed as distinct waveforms and complexes.

PURPOSES OF ECG

  • Used to:

    • Monitor the patient's heart rate.

    • Assess the effects of disease or injury on heart function.

    • Evaluate pacemaker function.

    • Monitor responses to medications, such as antiarrhythmics.

    • Obtain baseline recordings before, during, and after medical procedures.

    • Evaluate for signs of myocardial ischemia, injury, and infarction.

  • Information Provided by ECG:

    • Orientation of the heart in the chest.

    • Conduction disturbances.

    • Electrical effects of medications and electrolytes.

    • Mass of cardiac muscle.

    • Presence of ischemic damage.

LIMITATIONS OF ECG

  • Mechanical Assessment: The ECG does not provide information regarding the mechanical (contractile) condition of the myocardium.

  • Other Assessments: To evaluate the heart's mechanical activity, assessments such as the patient's pulse and blood pressure must be done.

CARDIAC CYCLE

  • Definition: The cardiac cycle describes the repetitive pumping process that encompasses all events associated with blood flow through the heart.

  • Phases:

    • Each heart chamber undergoes two phases:

    • Systole: The contraction phase where blood is expelled from the chamber.

    • Diastole: The relaxation phase during which chambers fill with blood.

    • Ventricular diastole allows the myocardium to receive fresh oxygenated blood from the coronary arteries.

CARDIAC CELLS

  • Types:

    • Myocardial Cells (Working Cells): Contain contractile filaments and contract when electrically stimulated by pacemaker cells.

    • Pacemaker Cells (Conducting Cells): Specialized cells that generate electrical impulses spontaneously without nerve stimulation.

PROPERTIES OF CARDIAC CELLS

  • Automaticity: The ability of cardiac pacemaker cells to autonomously develop electrical impulses.

  • Influence of Ions:

    • Increased calcium (Ca++) levels enhance automaticity.

    • Decreased potassium (K+) levels diminish automaticity.

  • Main Pacemaker: The sinoatrial (SA) node is recognized as the primary pacemaker, operating at a self-excitation rate faster than other potential pacemaker sites in the heart.

CARDIAC ACTION POTENTIAL

  • Voltage Definition: The measurement of potential energy due to separated electrical charges of opposite polarity.

  • Importance of Electrolytes: Human body fluids consist of electrolytes, which are substances that dissociate into charged particles (ions).

  • Cell Excitability: The electrical imbalance across membranes creates potential energy that renders the cells excitable, crucial for heart activity.

ECG ELECTRODES

  • Definition: An electrode is an adhesive pad with a conductive substance that conducts skin surface voltage changes to a cardiac monitor.

  • Application: Applied at specific positions on the patient's chest and limbs to capture electrical activity from various angles.

  • Preparation: Skin must be prepared to remove oil and dead cells before electrode placement (e.g., using alcohol wipes). Hair may need trimming for effective adhesion.

  • Minimizing Distortion: Avoid placing electrodes over bony areas, ensure conductive jelly is moist, and employ skin functions for effective monitoring.

  • Lead Wires: Each lead wire connects an electrode to the ECG machine, facilitating current return for monitoring. Colors of ECG wires may vary and are not standardized.

ECG LEADS

  • Definition: A lead is a tracing of electrical activity reflecting voltage differences between electrodes.

  • Function: Each lead represents average current flow over time from specific regions of the heart.

  • Importance of Placement: The placement of positive electrodes is crucial as it determines the viewed area of the heart.

  • Clarification on Terminology:

    • Leads: Measure electrical activity.

    • Electrodes: Surface stickers that connect to leads.

  • Lead System:

    • Only 10 electrodes are placed, which yield 12 leads. Two extra leads are derived from interpreting the signals triangulated from these electrodes.

FRONTAL PLANE LEADS

  • Types:

    • Standard Limb Leads: Leads I, II, and III.

    • Augmented Limb Leads: Leads aVR, aVL, and aVF.

  • Nature of Leads: Each bipolar lead has a positive and negative electrode, recording the differential electrical potential between two electrodes.

HORIZONTAL PLANE LEADS

  • Identification: Six chest leads as V1 through V6.

  • View: These leads provide a horizontal perspective of the heart, focusing on the front and left side.

PATIENT PREPARATION FOR ECG

  • Identity Confirmation: Always confirm patient identification before proceeding.

  • Explanation of Procedure: Inform the patient that an ECG has been ordered, gain consent, and explain what they can expect during the procedure.

  • Positioning: Patients should lie still, as electrodes may be placed in sensitive areas.

ELECTRODE PLACEMENT – LIMB LEADS

  • Right Leg (RL): Place above the right ankle and below the torso.

  • Left Leg (LL): Place above the left ankle and below the torso.

  • Right Arm (RA): Position between the right shoulder and wrist.

  • Left Arm (LA): Position between the left shoulder and wrist.

  • Symmetry: Limb electrodes should generally be placed symmetrically.

  • Relaxation: Ensure arms and legs are relaxed to minimize muscle tension distortion on the ECG.

ELECTRODE PLACEMENT – CHEST LEADS

  • V1: Located in the 4th intercostal space to the right of the sternum.

  • V2: Located in the 4th intercostal space to the left of the sternum.

  • V4: Positioned in the 5th intercostal space at the midclavicular line.

  • V3: Placed midway between V2 and V4.

  • V5: Located at the left anterior axillary line, same horizontal level as V4.

  • V6: Positioned at the left midaxillary line, at the same level as V4 and V5.

ELECTRODE PLACEMENT SUMMARY

  • Limb Lead Positions:

    • RL, LL, RA, LA

  • Chest Lead Positions:

    • V1, V2, V3, V4, V5, V6

LEAD ATTACHMENT

  • Attach leads corresponding to their respective electrodes, enabling the change in voltage to generate the ECG traces displayed graphically.

HOLTER MONITORS

  • Definition: A battery-powered, continuous electrocardiographic monitor.

  • Usage Duration: Typically worn for 24-72 hours, though newer models can function for 7-14 days.

  • Application: Helpful for capturing infrequent cardiac events that outpatient ECGs might miss.

PATIENT DIARY

  • Diary Importance: Patients are instructed to maintain a diary/log while using a Holter monitor.

  • Contents of Diary: Entries should detail the time, activity performed, and any symptoms experienced.

THE ECG TRACE

  • Processing: The ECG machine captures signals from the skin and produces a graphic representation of the heart's electrical activity.

  • Basic Pattern Logic:

    • Upward deflection: Electrical activity directed towards a lead.

    • Downward deflection: Activity directed away from a lead.

    • Depolarization and repolarization deflections are opposite in direction.

  • Waveforms: The ECG trace predominantly consists of three named waves: P, QRS complex, and T.

ECG TRACE - WAVE DETAILS

  • P Wave: Small deflection indicating atrial depolarization (atria contract, beginning of atrial systole).

  • PR Interval: Time between the initial deflection of the P wave and the first deflection of the QRS complex.

  • QRS Complex: Indicates ventricular depolarization (ventricles contract; beginning of ventricular systole):

    • R Wave: An upward deflection following P, largest wave representing the main mass of ventricles.

    • Q Wave: A downward deflection; can be small or indicate old myocardial infarction if large and wide.

    • S Wave: Final depolarization at the heart's base.

  • ST Segment: Measured between the end of the QRS complex and the start of the T wave, reflecting zero potential between depolarization and repolarization.

  • T Wave: Represents ventricular repolarization; atrial repolarization is not visible due to the dominant QRS complex.

ECG WAVE DIRECTION AND SIZE

  • Influence of Lead Orientation: The ECG trace reflects net electrical activity and the predominant direction of depolarization from the ventricles, which account for the overall size of the recorded waves.

SINUS MECHANISMS

  • Sinus Rhythm: Characterized by a normal heartbeat originating from the SA node.

  • Features:

    • Positive P wave precedes QRS complex.

    • P waves maintain consistency and consistent distance from QRS.

    • Constant PR interval with no ST elevation.

ECGINTERPRETATIONS - SINUS RHYTHM

  • Heart Rate: 60-100 BPM denotes normal sinus rhythm.

TACHYCARDIA

  • Definition: A heart rate exceeding 100 BPM.

  • Presentation: If the SA node fires too quickly, the condition is termed sinus tachycardia; this may be a normal response to increased oxygen demands.

  • Diagnostic Challenge: Differential identification between P waves and T waves may be complicated.

BRADYCARDIA

  • Definition: Heart rate is less than 60 BPM.

  • Causes: Can occur during sleep in adults and is prevalent in well-conditioned athletes.

MYOCARDIAL INFARCTION

  • ST-Elevation Myocardial Infarction (STEMI):

    • Condition where the SA node fails to initiate an impulse.

  • Measurement: Requires assessment for ST elevation on the ECG trace.

SINUS ARREST

  • Definition: Also known as sinus pause or sinoatrial arrest; occurs when the SA node fails to initiate impulses, leading to periods of non-electrical activity.

  • Causes: Can be attributed to coronary artery disease (CAD), acute myocardial infarction (MI), among others.

SINUS ARRHYTHMIA

  • Characteristics: Absence of P waves and irregular firings of the SA node.

  • Common Variation: Respiratory sinus arrhythmia, a phenomenon where heart rate increases during inspiration and decreases during expiration is typically benign.

ATRIAL FIBRILLATION

  • Description: Characterized by the absence of identifiable P waves and an erratic baseline.

  • Prevalence: Most commonly treated dysrhythmia in clinical practice.

  • Mechanism: Caused by abnormal automaticity leading to excessive firing (300-600 BPM) from the atrial areas, resulting in quivering.

  • Risks: High risk of thromboembolism due to blood pooling in atria, which can lead to stroke.

VENTRICULAR FIBRILLATION

  • Characteristics: No observable waveforms with chaotic rhythm initiation in the ventricles, representing disorganized depolarization.

  • Implications: This results in lack of effective myocardial contraction or pulse.

  • Causes: Associated with acute coronary syndromes, electrolyte imbalances, environmental issues, or severe heart failure.

  • Emergency Response: Patients typically lose responsiveness and pulse, requiring immediate use of Automated External Defibrillator (AED) or manual paddles in hospital settings.

ECG ARTIFACT

  • Definition: Distortion of ECG tracings from electrical activity originating from sources other than cardiac in nature.

  • Significance: Artifacts can mimic various cardiac issues; thus, evaluating the patient before treatment is crucial.

  • Sources of Artifacts:

    • External: Faulty equipment, loose electrodes, broken leads; proper maintenance of ECG machines is required.

    • Internal: Patient movement (shivering, muscle tremors) can also cause artifacting.

  • Preparation Guidelines: Proper skin preparation and evaluation of equipment are necessary to minimize artifacts during ECG monitoring.