EKG Rhythm Analysis and Characteristics

Catching Your Breath in ECG Interpretation

• Initial Guidelines

  • Don’t feel overwhelmed. Follow these systematic steps to analyze ECG strips:

  1. Regularity

     • Assess if the rhythm is regular.

     • Check if the R to R intervals are equal.

  2. Rate

     • Calculate the heart rate. Determine if it falls within normal ranges.

  3. P Wave

     • Is there a visible P wave?

     • Assess whether it looks normal and if all P waves appear alike.

  4. PR Interval

     • What is it and how is it measured?

     • Check if it is normal.

  5. QRS Interval

     • What is it and how is it measured?

     • Ensure it is normal.

Norms and Measurements

• P Wave

  • Indicates Atrial Depolarization (contraction of the atrium)

  • Amplitude (height): No more than 3 mm

  • Characteristics: No notching or peaking.

• PR Interval

  • Indicates AV Conduction Time

  • Normal Duration: $0.12$ to $0.20$ seconds.

• QRS Complex

  • Represents Ventricular Depolarization (contraction of ventricles).

  • Normal Duration: $0.04$ to $0.10$ seconds.

• T Wave

  • Represents Ventricular Repolarization; typically rounded & asymmetrical.

• ST Segment

  • Indicates early ventricular repolarization; normally, it should not be depressed more than 0.5 mm or elevated more than 1 mm in some leads.

• QT Interval

  • Measured from the Q wave to the end of the T wave; indicates one complete ventricular cycle.

  • Usually less than half of the R-R interval.

Calculating Heart Rate

1. 1500 Method

   • Most precise; best for fast, regular rhythms.

   • Count the number of small boxes between two R waves (R-R interval).

   • Formula: $ext{Heart Rate} = rac{1500}{ ext{Number of Boxes}}$

2. Sequence Method (300 Method or Big Box Method)

   • Less accurate but quick and fairly reliable.

   • Count the number of large boxes between R waves.

   • Formula: $ext{Heart Rate} = rac{300}{ ext{Number of Boxes}}$

3. 6-Second Method

   • Quick, simple, and works for both regular and irregular rhythms.

   • Count the number of QRS complexes in 6 seconds (30 large boxes).

   • Formula: $ext{Heart Rate} = ext{Number of QRS complexes} imes 10$

Interpreting Rhythm

• Measure from the beginning of one wave to the beginning of the next using calipers, paper, or counting boxes:

  • R-R for ventricular rhythm.

  • P-P for atrial rhythm.

• A tracing or 6-10 seconds is necessary for accuracy.

• If measurements are the same, rhythm is REGULAR; if off by more than 1.5, it is IRREGULAR.

Intervals & Waveforms

• Each waveform, interval, and segment has significant meaning on the EKG.

• EKG technicians are not expected to diagnose conditions but must be aware of normal vs. abnormal tracings.

• Analyzing a tracing involves assessing each waveform from the start of the P wave to the end of the T wave.

EKG Paper Interpretation

• Value of each block is essential for interpreting intervals, waves, and segments:

  • Time is measured horizontally; Voltage (amplitude) is measured vertically.

  • Each small block represents 0.04 seconds; there are 5 small blocks between each solid line, and each solid line represents 0.20 seconds.

Waveforms and Intervals

1. P Wave

   • Begins when the SA node fires; it is the first wave in the cardiac cycle.

   • Represents atrial depolarization and the spread of the electrical impulse through the R & L atria, resulting in atrial contraction/asystole.

   • Review Criteria:

     • Consistency in shape.

     • If a P wave precedes each QRS waveform.

     • Normally positively deflected.

     • Variances could indicate conduction pathway abnormalities.

2. R Wave

   • The first positive deflection of the QRS complex; signifies ventricular depolarization.

   • In pacing, refers to the entire QRS complex, indicating an intrinsic ventricular event.

3. QRS Complex

   • Comprises several waveforms (Q wave, R wave & S wave).

   • Represents the spread of impulse through ventricles and ventricular depolarization (atrial repolarization occurs concurrently but is not visible).

   • Normal width: $0.04$ to $0.10$ seconds; abnormal widths indicate ventricular dysfunction.

4. J Point

   • Represents where ventricular polarization stops, and repolarization begins; occurs at the end of the QRS complex.

   • Important for measuring the end of the QRS complex; the J point can elevate or depress during myocardial ischemia.

5. T Wave

   • Represents ventricular repolarization; should follow each QRS complex after a brief pause following the J point, and should be uniform in configuration.

6. U Wave

   • Not always visible; represents repolarization of the Bundle of HIS and Purkinje fibers, appearing as a small upward curve following the T wave.

7. PR Interval

   • Represents time for SA node firing, atrial depolarization, and electricity travel through the AV node.

   • Measured from beginning of the P wave to beginning of the Q wave; normal range: $0.12$ to $0.20$ seconds.

8. P-P & R-R Intervals

   • P-P interval: time between atrial depolarization cycles (P waves); measured from beginning of one P wave to the next; valuable for atrial rate & rhythm analysis.

   • R-R interval: time between ventricular depolarization cycles (R waves); measured similarly, valuable for ventricular rate & rhythm analysis.

9. QT Interval

   • Represents one complete ventricular cycle, from the beginning of the Q wave to the end of the T wave.

10. PR Segment

    • Marks the time between end of atrial contraction (end of the P wave) and beginning of ventricular contraction (beginning of the Q wave); allows ventricles to fill.

11. ST Segment

    • Represents early phase of ventricular repolarization; starts at end of S wave and ends at beginning of T wave; critical when assessing for ischemic patterns.

12. TP Segment

    • Interval between successive PQRST complexes while heart is electrically silent, from end of T wave to next P wave onset.

Sinus Mechanisms - Objectives

1. Describe ECG characteristics of sinus rhythm.

2. Describe ECG characteristics, causes, signs & symptoms, and emergency management of:

   • Sinus bradycardia.

   • Sinus tachycardia.

   • Sinus arrhythmia.

   • Sinoatrial (SA) block.

   • Sinus arrest.

Key Terms

• Sinus Arrhythmia: Dysrhythmia from the SA node; occurs irregularly with breathing; a normal phenomenon.

• Sinus Bradycardia: Dysrhythmia with a ventricular rate of less than 60 bpm originating from SA node.

• Sinus Rhythm: Normal heart rhythm, referred to as regular or normal sinus rhythm (NSR), typically 60-100 bpm.

• Sinus Tachycardia: Dysrhythmia with a ventricular rate of 101-180 bpm from the SA node; some consider the upper limit as 220 bpm minus patient age.

Introduction to Sinus Rhythm

• The normal heartbeat results from an electrical impulse beginning in the SA node, with pacemaker cells functioning at a faster rate than other cardiac cells; the SA node leads the electrical activity.

• Characteristics of a SA node-originating rhythm:

  • Positive P wave preceding each QRS complex.

  • Uniform appearance of P waves.

  • Constant PR interval.

  • Generally regular atrial & ventricular rhythm.

• Impulses can be influenced by:

  • Medications.

  • Conditions affecting heart rhythm (speeding up, slowing down, irregularities).

  • Factors delaying or blocking impulses from the SA node.

Characteristics of Sinus Rhythm

• How to Recognize Sinus Rhythm:

  • Rhythm: R-R and P-P intervals are regular.

  • Rate: 60-100 bpm.

  • P Waves: Positive in lead II, look alike, precede each QRS complex.

  • PR Interval: $0.12 - 0.20$ sec, constant across beats.

  • QRS Duration: $0.11$ sec or less unless abnormally conducted.

Sinus Bradycardia

• Definition: SA node fires slower than normal, resulting in a rate less than 60 bpm; severe sinus bradycardia is when rates fall below 40 bpm.

• Recognition Characteristics:

  • Rhythm: Regular R-R and P-P intervals.

  • Rate: Less than 60 bpm.

  • P Waves: Positive in lead II, identical appearance, precede each QRS complex.

  • PR Interval: $0.12$ to $0.20$ sec and constant.

  • QRS Duration: $0.11$ sec or less unless abnormally conducted.

Causes of Sinus Bradycardia

• Common in adults during sleep and well-conditioned athletes; can occur in some MIs (myocardial infarctions).

• Other causes include prolonged standing, vagus nerve stimulation, disease of the SA node, certain medications, hypokalemia, obstructive sleep apnea, hypothermia, post-heart transplant conditions, hypothyroidism, vagal stimulation, and increased intracranial pressure.

Management of Sinus Bradycardia

• Many tolerate rates of 50-60 bpm; symptomatic if drops below 50 bpm (lightheadedness, dizziness, syncope).

• If asymptomatic, no treatment required.

• For symptomatic bradycardia:

  • Assess O2 saturation and signs of difficulty breathing, provide supplemental oxygen or assist ventilation if necessary.

  • Establish IV access and obtain a 12-lead ECG.

  • First-line drug: Atropine.

  • Continue monitoring and reassess.

  • In MI scenarios, bradycardia can be transient and beneficial to reduce heart's oxygen demand.

Sinus Tachycardia

• Definition: SA node fires faster than normal, resulting in higher rates based on the patient's age; tachycardia begins and ends gradually.

• Recognition Characteristics:

  • Rhythm: Regular R-R and P-P intervals.

  • Rate: Typically between 101-180 bpm; alternatively, upper limit may be calculated as 220 bpm minus the patient’s age.

  • P Waves: Positive in lead II, appear identical and precede each QRS complex.

  • PR Interval: $0.12 - 0.20$ sec (constant).

  • QRS Duration: $0.11$ sec or less unless abnormally conducted.

Causes of Sinus Tachycardia

• It can be a normal response to physiological demands for increased oxygen. Other causes include:

  • Coronary Artery Disease (CAD), acute MI, hypoxia, infection, caffeine, dehydration, medications (epinephrine, atropine, dopamine), exercise, illicit drugs (cocaine, amphetamines), anxiety, pain, pulmonary embolism, fever, heart failure, shock, and hyperthyroidism.

• Sinus tachycardia may indicate early warning signs in acute MI settings.

Management of Sinus Tachycardia

• Treatment focuses on correcting the underlying cause.

  • In acute MI cases, medications may be used to lower heart rate and decrease myocardial oxygen demand (e.g. beta-blockers), provided there are no signs of heart failure.

  • Some dysrhythmias with very rapid ventricular rates (above 150 bpm) may require pharmacological intervention or cardioversion; note that shocking sinus tachycardia is inappropriate.

Sinus Arrhythmia

• Definition: SA node fires irregularly, often linked to the phases of breathing (respiratory sinus arrhythmia). Nonrespiratory sinus arrhythmia is seen in older individuals or with heart disease.

• Recognition Characteristics:

  • Rhythm: Irregular & often phasically varying with breathing; HR increases during inspiration (R-R intervals shorten) and decreases with expiration (R-R intervals lengthen).

  • Rate: Usually 60-100 bpm.

  • P Waves: Positive in lead II, look alike, one precedes each QRS complex.

  • PR Interval: $0.12 - 0.20$ sec (constant).

  • QRS Duration: $0.11 sec$ or less unless abnormally conducted.

Causes of Sinus Arrhythmia

• Respiratory sinus arrhythmia is normal; nonrespiratory forms are more likely in individuals with heart diseases or post-acute inferior MI.

• Changes in rhythm can occur due to medications or carotid sinus pressure.

Management of Sinus Arrhythmia

• Sinus arrhythmia typically does not require treatment unless accompanied by bradycardia causing hemodynamic compromise; in such cases, IV atropine can be indicated.

Sinoatrial Block

• Definition: SA Block entails failure of impulse conduction as it exits the SA node, leading to temporarily absent PQRST complexes.

• Recognition Characteristics:

  • Represents a missed beat (absent P wave, QRS complex, T wave).

  • Rhythm is irregular due to pauses correlated to underlying P-P intervals; typically normal rate with interruptions.

  • P waves, when present, are positive in lead II and similar; PR interval maintains constant interval.

Causes of Sinoatrial Block

• Uncommon, may occur due to:

  • Hypoxia, CAD, myocarditis, acute MI, carotid sinus sensitivity, vagal tone increase, or medications.

Management of Sinoatrial Block

• Monitoring for transient episodes is adequate when there are no significant symptoms; if symptomatic, withholding offending medications is advised.

• More frequent episodes may warrant IV atropine, temporary pacing, or permanent pacemaker intervention.

Sinus Arrest

• Definition: Also known as sinus pause; an impulse formation disorder. It entails failure of SA node cells to initiate impulses, with resulting PQRST complexes missing.

• Recognition Characteristics:

  • Rhythm is irregular; length of pauses varies; multiple PQRST complexes can be absent, differing from other intervals.

  • Normal rates but variations occur due to pauses; positive P waves when present, identical, and preceding each QRS.

Causes of Sinus Arrest

• Common origins include damage from CAD, acute MI, carotid pressure, increased parasympathetic activity, sleep apnea, hypothermia, or responses to certain medications.

Management of Sinus Arrest

• Monitor symptoms of hemodynamic compromise such as weakness or dizziness during arrest episodes.

• Observational management is preferred for asymptomatic cases; IV atropine may be indicated if hemodynamic symptoms appear.

• Frequent and prolonged pauses may necessitate permanent pacing.

Summary of Sinus Mechanisms Characteristics