EKG Ch 8

The Cardiac Conduction System and EKG Tracings

1. Difference Between Mechanical and Electrical Activity of the Heart

• Mechanical Function:

  • Refers to the heart muscle, heart valves, blood supply, and blood vessels.

  • Critical for the heart to pump effectively.

• Electrical Function:

  • Involves specialized tissue in the heart that generates and conducts electrical impulses.

  • These impulses act as the "spark" that initiates mechanical contractions.

• Cardiac Output:

  • Defined as the amount of blood the heart pumps per minute (measured in liters).

  • Determined by two factors:

  • Stroke Volume: Amount of blood pumped with each heartbeat.

  • Heart Rate: Number of beats per minute.

  • Necessary for delivering oxygen and nutrients to the body.

• Impact of Abnormalities:

  • Mechanical or electrical problems can affect cardiac output:

  • Rapid electrical impulses reduce the time for chamber filling, leading to lower stroke volume.

Quick Reference of Abnormalities

• Essential Requirements for Effective Heart Pumping:

  • Proper functioning electrical system.

  • Healthy heart muscle and valves.

  • Adequate blood volume and intact vascular system.

2. Electrical Conduction System of the Heart

• Cardiac Conduction System:

  • Pathway for electrical impulses controlling heart contractions.

  • Originates in the upper part of the right atrium.

• Key Components and Intrinsic Rates:

  • Sinoatrial Node (SA Node):

  • Location: Upper right atrial wall.

  • Primary pacemaker of the heart; sets contraction timing.

  • Firing rate: 60-100 BPM; can escalate up to 180 BPM under sympathetic stimulation.

  • Atrioventricular Node (AV Node):

  • Location: Bottom of the right atrium (behind the tricuspid valve).

  • Delays impulse by 0.1 sec, allowing atrial emptying and ventricular filling.

  • Firing rate: 40-60 BPM.

  • Bundle of His (AV Bundle):

  • Located in the interventricular septum.

  • Firing rate: 20-60 BPM.

  • Bundle Branches:

  • Divided into right and left branches to transmit impulses to ventricles.

  • Firing rate: 20-60 BPM.

  • Purkinje Fibers:

  • Final pathway for impulse conduction to the ventricular myocardium.

  • Firing rate: 20-40 BPM.

EKG Tracing

• Records voltage differences caused by heart's electrical activity during depolarization and repolarization.

• Function of EKG:

  • Facilitate monitoring of cardiac functions; interpret potential abnormalities.

• Normal EKG Features:

  • Isoelectric Line (Baseline): The level at which there are no deflections.

  • Waves, Segments, Intervals, Complexes:

  • Wave: Movement away from baseline (positive is upward, negative is downward).

  • Segment: Isoelectric area between two waves.

  • Interval: Combination of wave and segment.

  • Complex: Group of waves measured together.

• Morphology: Describes the shape/direction of waves, complexes, and segments.

3. Features of an EKG Tracing

• Normal Sinus Rhythm (NSR): Varies between individuals and leads but includes all features described.

Normal EKG Complex Features in Lead II

• Waves: Labeled P, Q, R, S, T - do not represent specific words.

Quick Reference Definitions

Correlation of Electrical Changes and Mechanical Actions

• P Wave: Represents atrial depolarization (atria contraction).

• QRS Complex: Represents ventricular depolarization (ventricular contraction).

• T Wave: Represents ventricular repolarization (ventricles relaxing).

4. Important Intervals and Segments on EKG

Key Measurements

• PR Interval: Measure from beginning of P wave to beginning of QRS complex; normal duration: 0.12-0.20 seconds.

• QRS Interval: Measure from initial movement of QRS complex to where S wave returns to baseline; normal duration: < 0.12 seconds.

• R-R Interval: Distance between peaks of consecutive R waves; should be regular.

• ST Segment: Measure from end of QRS to beginning of T wave; should remain at baseline.

QT Interval:

• Starts at the beginning of the Q wave, ends when T wave returns to baseline; normal range varies with gender and heart rate: generally 0.36-0.44 seconds.

• Abnormal QT intervals can indicate cardiac issues.

5. Measurement of Time on EKG

• EKG graph paper comprises small (1 mm) and large (5 mm) blocks:

  • Small block: 0.04 seconds.

  • Large block (5 small blocks): 0.20 seconds.

6. Six Steps Used to Analyze an EKG

1. Determine heart rate in BPM.

2. Check rhythm for regularity.

3. Note presence/absence of identical P waves before each QRS interval.

4. Measure length of PR interval.

5. Measure length of QRS interval.

6. Identify the rhythm.

7. First Step: Determine Heart Rate from EKG

• Methods:

  • 6-Second Method: Count QRS complexes in a 6-sec section, multiply by 10. Effective for irregular rhythms.

  • Large Block Method: Count large blocks between two R waves; each correlating to a specific heart rate.

  • Small Block Method: Count small blocks between R waves, divide 1500 by number of small blocks.

8. Second Step: Examine EKG Tracing for Regularity

• Categories of QRS Complexes:

  • Regular: Consistent pattern, similar timing.

  • Irregular: Variable appearance and timing.

  • Regularly Irregular: Clear, repeating pattern but with abnormal elements.

9. Third Step: Examine P Wave on EKG

• Observation: The P wave must precede each QRS complex, typically rounded in normal rhythms.

10. Fourth Step: Measure PR Interval on EKG

• Measurement: From beginning of P wave to beginning of QRS complex; normal range: 0.12-0.20 seconds.

11. Fifth Step: Measure QRS Complex Duration on EKG

• Measurement: From first wave of QRS to the last wave before flattening; normal duration is < 0.12 seconds.

12. Importance of Following Each Step in EKG Analysis

• Following these steps meticulously ensures accurate identification of heart rhythm and potential abnormalities, aiding in proper diagnosis and treatment actions.