Electrocardiogram (ECG) Concepts

The study of electrocardiograms (ECGs) involves understanding the electrical activity of the heart, which is crucial for diagnosing various cardiac conditions. Below are the key concepts and mechanisms involved in ECGs.

Myocardial Cells and Functional Syncytium

Myocardial Cells: Striated muscle cells interconnected by gap junctions, forming intercalated discs.
Functional Syncytium: Myocardial cells act as a single unit (myocardium) due to electrical coupling, allowing for coordinated contractions.
Differences from Skeletal Muscle: Myocardium contracts fully with each depolarization, unlike skeletal muscle which produces graded contractions.
Separation of Atria and Ventricles: The fibrous skeleton of the heart electrically separates the atria and ventricles, enabling independent contractions.

Pacemaker Potentials and Electrical Conduction

Autorhythmic Cells: Located in the sinoatrial (SA) node, these cells initiate action potentials that regulate heartbeats autonomously, influenced by the autonomic nervous system (ANS).
Pacemaker Potential: The unstable resting membrane potential of SA node cells leads to spontaneous depolarization to threshold, typically from -60 mV to -40 mV.
Ion Channels: Hyperpolarization-activated cyclic nucleotide channels (HCN) open in response to hyperpolarization, allowing Na⁺ and K⁺ to diffuse, causing gradual depolarization.
Action Potential Phases:
- Threshold opens Ca²⁺ voltage-gated channels, leading to depolarization and myocardial contraction.
- Repolarization occurs via K⁺ channels, allowing the heart to relax.

Intrinsic Conduction System

Components: The conduction system includes the SA node, internodal tracts, atrioventricular (AV) node, AV bundle (bundle of His), bundle branches, and Purkinje fibers.
SA Node: The primary pacemaker discharges 70-80 times per minute, establishing the sinus rhythm.
AV Node: Acts as a secondary pacemaker (40-60 beats per minute) and introduces an A-V delay, allowing full atrial contraction before ventricular contraction.
Purkinje Fibers: Serve as tertiary pacemakers (30-40 beats per minute) and are less influenced by neural or hormonal activity.

Cardiac Cycle and Heart Rate

Cardiac Cycle: The sequence of electrical conduction and mechanical contraction (systole) followed by relaxation (diastole).
Heart Rate:
- Bradycardia: Ventricular rate slower than 60 beats per minute.
- Tachycardia: Ventricular rate faster than 100 beats per minute.

Electrocardiogram (ECG) Recording

ECG Definition: A graphic representation of the electrical activity during the cardiac cycle, recorded at intervals of 0.04 seconds.
Waves on ECG:
- P Wave: Represents atrial depolarization.
- QRS Complex: Represents ventricular depolarization.
- T Wave: Represents ventricular repolarization.
Atrial Repolarization: Not visible on ECG due to masking by the QRS complex.
Critical Measurements:
- P-R Interval: Less than 0.2 seconds, indicating atrial depolarization and conduction through the AV node.
- S-T Segment: Represents the plateau phase of the action potential.
- Q-T Interval: 0.31 - 0.43 seconds, from ventricular depolarization to repolarization.

ECG Leads and Recording Techniques

Lead Types:
- Bipolar Limb Leads: Measure voltage differences between limbs (Lead I, II, III).
- Unipolar Leads: Include AVR, AVL, AVF for limbs and V1-V6 for chest leads, providing various perspectives of heart activity.
Standard ECG Leads: A total of 12 leads used for comprehensive monitoring of heart electrical activity.

Abnormal ECG Patterns

Arrhythmias: Irregular heart rhythms resulting from defects in the conduction system.
Ectopic Pacemakers: Abnormal pacemaker activity can lead to conditions like atrial flutter and fibrillation.
- Atrial Flutter: Characterized by rapid atrial waves with a sawtooth appearance.
- Atrial Fibrillation: Absence of P waves, replaced by a wavy baseline.
Ventricular Tachycardia: Rapid discharge from ectopic foci in the ventricles, resulting in a distorted QRS complex.
Ventricular Fibrillation: A serious arrhythmia leading to ineffective pumping of blood, with no definable waves.
Defibrillation: A technique used to restore normal sinus rhythm by depolarizing the myocardium.
Premature Ventricular Contractions (PVCs): Extra contractions of the ventricles, visible as a QRS complex without a preceding P wave.
AV Blocks: Delays in conduction from atria to ventricles.
- First Degree AV Block: Normal ECG but prolonged P-R interval (>0.20 seconds).
- Second Degree AV Block: Multiple P waves before a QRS complex, resembling a dropped beat.
- Third Degree AV Block: Complete dissociation between atrial and ventricular activity, with a slow ventricular rate (20-40 beats per minute).

Laboratory Exercises for ECG Recording

This section outlines the procedure for recording a baseline ECG, which can be used for comparison under different conditions.

Connect the power cord and electrode leads to the ECG machine (Welch Allyn CP 150).
Power on the machine and wait for it to warm up.
If a patient information screen appears, navigate to the main menu.
Attach ECG electrode adapter clips to the designated leads (LL, RL, RA, LA).
Place disposable electrodes on the left and right ankles and wrists.
Ensure the subject is seated comfortably and attach the leads to the electrodes, ensuring no crossed legs.
Press the STAT key to verify lead placement and proceed to record.
Review the ECG preview and print if satisfactory; otherwise, retest.
Exit the program and analyze data using the last line from the printout (bipolar lead II).

Electrocardiography Procedures and Analysis

Recording ECG with CP 150

Ensure chest leads (V1 - V6) are properly attached before proceeding.
Press the record button; the CP 150 will wait for 10 seconds of quality data before recording.
If more than 10 seconds elapse, cancel the recording and adjust the lead and electrode placements.
If the same message appears after adjustments, press override to continue.
After recording, a preview of the ECG will be displayed. If satisfied, press next to print the ECG; if not, press retest and repeat from the previous step.
Press the exit button after printing. If an extra copy is needed, select "print this test."

Data Analysis

Analyze the ECG data using the last line of the printout (bipolar lead II).
The ECG will also display bipolar leads I and III, along with augmented leads AVR, AVL, and AVF.
Note that chest leads will not appear in the recording.
Attach the ECG recording to your lab report, ensuring all waves and segments are identified.
Perform necessary calculations using lead II ECG data. Remember, at a speed of 25 mm/sec, each millimeter equals 0.04 seconds.

Exercise Procedure

Have the subject exercise for 3 minutes by running in place.
Immediately post-exercise, have the subject sit down and record an ECG for lead II using the manual key on the CP 150, following the same steps as for baseline.
Enter the collected data into the lab report table.

Arrhythmia Analysis

Obtain an ECG tracing from the instructor, which will show a specific type of arrhythmia.
Based on conceptual readings and interval calculations, determine the type of arrhythmia present and answer related questions in the lab report.

PhysioEx Computer Simulation

Utilize laboratory computers or complete as homework, working individually or in groups to finish the cardiovascular physiology computer simulation.
Answer all questions and complete the lab report based on the simulation results.

Data from Exercise 9.1 (Baseline)

Heart Rate Calculation: Measure the distance from one QRS complex to the next (RR interval = 1 heartbeat).
Example: 18 mm x 0.04 sec/mm = 0.72 sec/beat; thus, heart rate = 1 ÷ 0.72 sec/beat x 60 sec/min = 83.3 beats/min.
QRS Interval Measurement: QRS interval = 0.08 sec; normal range is 0.06 - 0.10 sec.
P-R Interval Measurement: P-R interval = 0.16 sec; normal range is 0.12 - 0.20 sec.
Q-T Interval Measurement: Q-T interval = 0.6 sec; normal range is 0.35 - 0.45 sec.
Ventricular Diastole Measurement: TP segment = 0.2 sec; normal ventricular diastole is about 0.5 sec.
Confirm if computed values fall within normal limits; for instance, a heart rate of 60 to 100 beats/min is considered normal.

Data from Exercise 9.2 (Post-Exercise)

Immediate Post-Exercise Data:
- Cardiac Rate: 115 beats/min
- P-R Interval: 0.12 sec
- Q-T Interval: 0.2 sec
- Ventricular Diastole: 0.16 sec
Interval Comparisons: The P-R interval is shorter in the exercise recording.
Heart Rate Recovery: After 20 minutes of relaxation, the heart rate should decrease and return to diastole.

Data from Exercise 9.3 (Arrhythmia Identification)

Identified Arrhythmia: Third-degree AV block (complete block).
Reasoning: P waves and QRS waves are not synchronized and do not trigger each other.
Causes and Treatments:
- Arrhythmias are typically caused by issues with the heart's electrical signals, which control heart rate.
- Potential causes include nerve cell dysfunction, age, genetics, medications, and lifestyle factors such as smoking.
- Management may involve avoiding triggers, treating underlying health conditions, or surgical interventions if necessary.

Review Questions

Definitions:
- Fibrillation: An irregular heartbeat due to rapid electrical signals in the atria.
- Bradycardia: A slow heart rate resulting from disruptions in the heart's electrical system.
- Pacemaker Potential: The gradual positive voltage increase across a cell's membrane between action potentials.
- ECG: A graphic recording of the electrical currents generated by the cardiac cycle.
- Unipolar Lead: Exploratory electrodes that utilize a built-in ground in the electrocardiograph machine.
Comparison of Fibrillation Types: Ventricular fibrillation is more serious than atrial fibrillation, as it can lead to sudden cardiac arrest and death within minutes if untreated.
Intrinsic Conduction System: Comprises nodal tissues that coordinate impulses for controlled atrial and ventricular contractions.
ECG Waves and Cardiac Cycle:
- P Wave: Atrial depolarization followed by atrial systole.
- QRS Complex: Ventricular depolarization followed by ventricular systole.
- T Wave: Ventricular repolarization.
Types of AV Blocks:
- First-degree AV block: Normal ECG with a P-R interval > 0.20 seconds.
- Second-degree AV block: Two or more P waves before a QRS complex, resembling a "dropped" beat.
- Third-degree AV block: No signals from atria reach ventricles; P waves are normal but unrelated to QRS complexes.

Understanding ECG: Concepts, Procedures, and Analysis

Understanding ECG: Concepts, Procedures, and Analysis