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Chapter 7

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I. BASIC ELECTROCARDIOGRAPHY

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How to Use This Section

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This chapter includes criteria for the diagnosis of basic electrocardiographic waveforms and cardiac arrhythmias. It is intended for use as a reference and assumes a basic understanding of the electrocardiogram (ECG)*.

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Electrocardiographic interpretation is a “stepwise” procedure, and the first steps are to study and characterize the cardiac rhythm.

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Step One (Rhythm)
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Categorize what you see in the 12-lead ECG or rhythm strip, using the three major parameters that allow for systematic analysis and subsequent diagnosis of the rhythm:

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  1. Mean rate of the QRS complexes (slow, normal, or fast).

  2. Width of the QRS complexes (wide or narrow).

  3. Rhythmicity of the QRS complexes (characterization of spaces between QRS complexes) (regular or irregular).

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*Several parts of this section on electrocardiography are based on the work of G. T. Evans, MD, who was the author of this chapter in the first edition of the book.

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Step Two (Morphology)
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Step 2 consists of examining and characterizing the morphology of the cardiac waveforms.

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  1. Examine for atrial abnormalities and bundle branch blocks (BBBs) (pp. 339–342).

  2. Assess the QRS axis and the causes of axis deviations (pp. 342–343).

  3. Examine for signs of left ventricular hypertrophy (pp. 343–344).

  4. Examine for signs of right ventricular hypertrophy (pp. 344–345).

  5. Examine for signs of myocardial infarction, if present (p. 346).

  6. Bear in mind conditions that may alter the ability of the ECG to diagnose a myocardial infarction (p. 353).

  7. Examine for abnormalities of the ST segment or T wave (pp. 354–357).

  8. Assess the QT interval (pp. 357–358).

  9. Examine for miscellaneous conditions (pp. 359–361).

     

    (bpm = beats per minute, s = second, ms = millisecond, m/s = meters per second, cm/s = centimeters per second)

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STEP ONE: DIAGNOSIS OF THE CARDIAC RHYTHM

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A. Approach to Diagnosis of the Cardiac Rhythm

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Most electrocardiograph machines display 10 seconds of data in a standard tracing. A rhythm is defined as three or more successive P waves or QRS complexes.

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Categorize the patterns seen in the tracing according to a systematic method. This method proceeds in three steps that lead to a diagnosis based on the most likely rhythm producing a particular pattern:

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  1. What is the mean rate of the QRS complexes?

    Slow (<60 bpm): The easiest way to determine this is to count the total number of QRS complexes in a 10-second period. If there are no more than 9, the rate is slow.

    Another method for determining the rate is to count the number of large boxes (0.20 s) between QRS complexes and use the following formula:

    Rate = 300 ÷ (number of large boxes between QRS complexes)

    A slow heart rate (<60 bpm) has more than five large boxes between QRS complexes.

    Normal (60–100 bpm): If there are 10–16 complexes in a 10-second period, the rate is normal.

    In normal heart rates, the QRS complexes are separated by 3 to 5 large boxes.

    Fast (>100 bpm): ...

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