INTRODUCTION TO ECG & IT'S INTERPRETATION

INTRODUCTION

Without a regular rate and rhythm, the heart may not perform efficiently as a pump to circulate oxygenated blood and other life-sustaining nutrients to all of the body's tissues and organs (including the heart itself).

With an irregular or erratic rhythm, the heart is considered to be dysrhythmic (sometimes called arrhythmic). This is a potentially dangerous condition.

INFLUENCES ON HEART RATE AND CONTRACTILITY

• The heart rate is influenced by the autonomic nervous system, which consists of sympathetic and parasympathetic fibers.
• Sympathetic nerve fibers (also referred to as adrenergic fibers) are attached to the heart and arteries as well as several other areas in the body. Stimulation of the sympathetic system –
  • Increases heart rate (positive chronotropy)
  • Increases conduction through the AV node (positive dromotropy)
  • Increases the force of myocardial contraction (positive inotropy)
  • Constricts peripheral blood vessels,
  • Increasing blood pressure
• Parasympathetic nerve fibers are also attached to the heart and arteries. Parasympathetic stimulation-
  • Reduces the heart rate (negative chronotropy),
  • Reduces AV conduction (negative dromotropy),
  • Reduces the force of contraction (negative inotropy)
  • Dilation of arteries,
  • Lowering blood pressure
• Increased sympathetic stimulation (eg, caused by exercise, anxiety, fever, or administration of catecholamines, such as dopamine [Intropin], aminophylline, or dobutamine [Dobutrex]) may increase the incidence of dysrhythmias.
• Decreased sympathetic stimulation (eg, with rest, anxiety-reduction methods such as therapeutic communication or meditation, or administration of beta-adrenergic blocking agents) may decrease the incidence of dysrhythmias.

THE ELECTROCARDIOGRAM (ECG)

• Electrical activity that originates in the heart can be detected on the body's surface through an electrocardiogram (ECG).
• Electrodes are applied to the skin to measure voltage changes in the cells between the electrodes. These voltage changes are amplified and visually displayed on an oscilloscope and graph paper.
• An ECG is a series of waves and deflections recording the heart's electrical activity from a certain "view."
• Many views, each called a lead, monitor voltage changes between electrodes placed in different positions on the body.
• The ECG shows only electrical activity; it tells us nothing about how well the heart is working mechanically. Patients should be treated according to their symptoms, not merely their ECG.

LEADS

• To obtain a 12-lead ECG, four wires are attached to each limb and six wires are attached at different locations on the chest. The total of ten wires provides 12 views (12 leads).

LIMB LEADS

• Electrodes are placed on the right arm (RA), left arm (LA), right leg (RL), and left leg (LL). With only four electrodes, six leads are viewed.
• These leads include –
  • The standard leads––I, II, and III––and
  • The augmented leads––aVR, aVL, and aVF

Standard Limb Leads

• Leads I, II, and III make up the standard leads. If electrodes are placed on the right arm, left arm, and left leg, three leads are formed. If an imaginary line is drawn between each of these electrodes, an axis is formed between each pair of leads. The axes of these three leads form an equilateral triangle with the heart in the center (Einthoven's triangle).

Augmented Limb Leads

• Leads aVR, aVL, and aVF make up the augmented leads.
• Each letter of an augmented lead refers to a specific term:
  • a = augmented;
  • V = voltage;
  • R = right arm;
  • L = left arm;
  • F = foot (the left foot).

CHEST LEADS

• The chest leads are identified as V1, V2, V3, V4, V5, and V6. Each electrode placed in a "V" position is positive.

ECG - DEFINITIONS OF TERM

• Isoelectric line - Between waves and cycles, the ECG records a baseline, which indicates the absence of net electrical activity.
• Wave - A deflection, either positive or negative, away from the baseline (isoelectric line) of the ECG tracing
• Complex - Several waves
• Segment - A straight line between waves or complexes
• Interval - A segment and a wave

COMPONENTS OF AN ECG TRACING

• P Wave
  • First wave seen
  • Small rounded, upright (positive) wave indicating atrial depolarization (and contraction)
• PR Interval
  • Distance between beginning of P wave and beginning of QRS complex
  • Measures time during which a depolarization wave travels from the atria to the ventricles
• QRS Complex
  • Three deflections following P wave
  • Indicates ventricular depolarization (and contraction)
  • Q Wave: First negative deflection
  • R Wave: First positive deflection
  • S wave: First negative deflection after R wave
• ST Segment
  • Distance between S wave and beginning of T wave
  • Measures time between ventricular depolarization and beginning of repolarization
• T Wave
  • Rounded upright (positive) wave following QRS Represents ventricular repolarization
• QT Interval
  • Distance between beginnings of QRS to end of T wave
  • Represents total ventricular activity
• U Wave
  • Small rounded, upright wave following T wave
  • Most easily seen with a slow HR Represents repolarization of Purkinje fibers

METHODS FOR CALCULATING HEART RATE

• Heart rate is the number of times the heart beats per minute (bpm). On an ECG tracing, bpm is usually calculated as the number of QRS complexes.
  • Method 1: Count Large Boxes
  • Method 2: Count Small Boxes
  • Method 3: Six-Second ECG Rhythm Strip

METHOD 1: COUNT LARGE BOXES

• For regular rhythms can be quickly determined by –

• Example –

 

(300 / 5) = 60 bpm

 

METHOD 2: COUNT SMALL BOXES

• The most accurate way to measure a regular rhythm is to count the number of small boxes between two R waves.

• Example –

(1500 / 30) = 50 bpm

METHOD 3: 6-SEC ECG RHYTHM STRIP

• The best method for measuring irregular heart rates with varying R-R intervals is to count the number of R waves in a 6-sec strip (including extra beats such as PVCs, PACs, and PJCs) and multiply by 10. This gives the average number of beats per minute.
• Ex. –

7 x10=~70 bpm

RHYTHM ANALYSIS

• Step 1:    Calculate rate.
• Step 2:    Determine regularity.
• Step 3:    Assess the P waves.
• Step 4:    Determine PR interval.
• Step 5:    Determine QRS duration & Shape.

STEP 1: CALCULATE RATE

• Count the # of R waves in a 6 second rhythm strip (30 large box), then multiply by 10.
• Interpretation? - 9 x 10 = 90 bpm

STEP 2: DETERMINE REGULARITY

• Look at the R-R distances (using a caliper or markings on a pen or paper).
• Regular (are they equidistant apart)? Occasionally irregular? Regularly irregular? Irregularly irregular?
• Interpretation? - Regular

STEP 3: ASSESS THE P WAVES

• Are there P waves?
• Do the P waves all look alike?
• Do the P waves occur at a regular rate?
• Is there one P wave before each QRS? (P:ORS Ratio)
• Interpretation? -Normal P waves with 1 P wave for every QRS

STEP 4: DETERMINE PR INTERVAL

• Normal: 0.12 - 0.20 seconds. (3 - 5 small boxes)
• (1 small boxes = 0.04 Sec)
• Interpretation?
• 4 small box = 0.16 seconds

STEP 5: QRS DURATION & SHAPE

• Normal: 0.04 - 0.12 seconds. (1 - 3 small boxes)
• (1 small boxes = 0.04 Sec)
• Shape : normal
• Interpretation - 2 small box = 0.08 second; normal

RHYTHM SUMMARY

Rate             =     ~90 bpm

Regularity        =    regular

P waves        =    normal

PR interval        =    0.16 sec

QRS duration        =    0.08 sec

Interpretation-Normal Sinus Rhythm

NORMAL SINUS RHYTHM (NSR)

• The electrical impulse is formed in the SA node and conducted normally.
• This is the normal rhythm of the heart; other rhythms that do not conduct via the typical pathway are called arrhythmias or Dysrhythmias .
• NSR Parameters

  Rate        =    60 - 100 bpm

  Regularity    =    Regular

  P waves    =    Normal

  PR interval    =    0.12 - 0.20 s

  QRS duration    =    0.04 - 0.12 s