CARDIAC PHYSIOLOGY AND ECG INTERPRETATION (Last of Two Parts)

II. The Electrocardiogram

• “When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram” (Guyton and Hall, 123).
• Defines the graphic representation of the electrical activity of the heart.
• The printed record of the electrical activity of the heart is called a rhythm strip or an ECG strip.
• Normal electrocardiogram (Figures 1 and 2). Please take note that Guyton and Hall (Fig 1) give a specific P-R interval which is 0.16 sec.
• In other sources, the P-R interval is between 0.12 and 0.20 sec. (Fig 2)

Fig. 1. Normal ECG. (From Guyton and Hall, Textbook of Medical Physiology 11^th Ed.,, 124)

Fig. 2

    Fig. 3.A  block (0.20 sec) and a small square/box (0.04 sec)

        X and Y axes (coordinate system)

Any wave that goes up is a positive deflection or up sloping (+X vertical axis).

Any wave that goes down is a negative deflection or down sloping (-X vertical axis)

III. How are the (normal) waves/intervals/segment of the electrocardiogram produced? (Breakdown of ECG)

      A. Normal waves

• P  = (Pacemaker) SA node fires, sends the electrical impulse outward to stimulate both atria and manifests as a P   wave (Fig.4). “The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins”(Guyton and Hall, 123).  P-Wave is the first positive deflection or the first up sloping which is responsible for atrial depolarization (Martin).

“The P wave represents the spread of the electrical impulse through both atria.The electrical impulse begins in the SA node and depolarizes the right atrium and then the left atrium. Thus, the first part of the P wave reflects right atrial activity, and the late portion of the P wave represents electrical potential generated by the left atrium” (Kahn, 81).

Fig. 4. P Wave

Ø  Wave duration: approximately 0.10 second in length. (But Gerard Martin said its wave duration is less than 0.10 sec, which is approximately 2 small squares, that is, 0.08 sec)

 Ø  Wave amplitude: 1-3mm. So, it is only 0.1 to 0.3mV.

• Q =  Q wave follows the P wave (atrial depolarization or atrial contraction ). It is a negative deflection or the first down sloping after the P wave.
• R =  R wave follows the Q wave. This is the 2^nd positive deflection.
• S =  S wave follows the Q wave.  This is the 2^nd negative deflection.        Three waves combine into one: The QRS Complex(Fig. 5)
  Ø  Ventricular depolarization (contraction)
  “The QRS complex is caused by potentials generated when the ventricles depolarize before contraction, that is, as the depolarization wave spreads through the ventricles” (Guyton and Hall, 123).
  Ø  Impulse from the Bundle of HIS throughout the ventricular muscles
  “The QRS complex represents the spread of electrical activation through the ventricular myocardium; the resultant electrical forces generated from ventricular depolarization is recorded on the ECG as spiky deflection. The sharp, pointed deflections are labeled QRS regardless of whether they are positive (upward) or negative (downward)” (Kahn, 9).
  Ø  Wave duration of ventricular depolarization: 0.06 to 0.10 sec
  ·         Small squares: 11/2 to 21/2  or 1.5 to 2.5
  ·         (This is a review. Remember that a small square has a wave duration of 0.04) 
  Ø  Wave amplitude: 25- 30mm
  ·         2.5 – 3mV.
  ·         5-6 big blocks or 25 to 30 small squares
  ·         (Always remember that a block has wave amplitude of 0.5mV and its vertical length is 5mm.)
  

  

  Fig. 5. QRS Complex

  
  
•  T 



Fig. 6. T Wave 

• T wave follows the QRS complex 
•   It is the 3^rd positive deflection or the 3^rd up sloping.
• The T wave represents repolarization, the recovery period of the ventricles (Kahn, 193).
• No associated activity of the ventricular muscle; the resting phase of the cardiac cycle. 
• “The T wave is caused by potentials generated as the ventricles recover from the state of depolarization. This process normally occurs in ventricular muscle 0.25 to 0.35 second after depolarization, and the T wave is known as a repolarization wave” (Guyton and Hall, 123). 
•  Its wave duration is 0.16 sec ( 4 small squares)
•    Its wave amplitude is 5-10mm  ( 0.5 to 1 mV;  1-2 big blocks or 5-10 small squares)
• Is there such a thing as atrial repolarization? Yes. But it is being overshadowed by tall QRS.
•   Absence of Q on ECG is still normal. Q appears on an ECG to gain momentum (like a pre-lengthening stage) as the electrical current continues to spread. But if the ventricles are healthy, they do not need to gain momentum. And so, on an ECG only P, R, S, T appears. In others words, since healthy ventricles are strong, they can easily contract without going through a pre-lengthening stage, so to speak. Hence, Q is absent.

   B. Normal intervals
         PR Interval (PRI)
        - The P is the atrial depolarization and the R is part of the ventricular depolarization.
        -  P to R is the conduction of the heart.
        -  Time which impulse travels from the SA node to the atria and downward to the ventricles. 
        “The PR interval involves the time required for the electrical impulse to advance from the atria    through the AV node, bundle of His, bundle branches, and Purkinje fibers  until the ventricular muscle begins to depolarize”(Khan, 9)

Fig. 7. PR interval (also see figures 1 and 2)

          -Normal PR interval is 0.12 to 0.20 sec. comprises of 3 to 5 small squares.(see fig. 2). If there is a  conduction interference, the patient will have heart block or AV block.

          -  a wave duration of 0.38 sec. is a prolonged PR interval, which leads to first-degree heart block; only light exercises are allowed.

        QT Interval

         - 

Q is the ventricular depolarization

         - T is also ventricular depolarization

         - So, Q-T is ventricular contraction.

           “Contraction of the ventricle lasts almost from the beginning of the Q wave (or R wave, if the Q wave is absent) to the end of the T wave. This interval is called the Q-T interval and ordinarily is about 0.35 second” (Guyton and Hall, 125-126).

         - Normal QT interval is 0.32 to 0.40 sec (Martin).

   C. Normal segment

        -  ST segment (see figures 1 and 2)

            J point (Junctional point) interconnects S and T. This point is significant. If this point goes up, the ST segment elevates. And if it goes down, ST depression occurs. ST segment wave duration is 0.12 sec. (3 small squares). Its wave amplitude is  – .1/2 to +1mm; -0,05 to +0.1mV.

IV. Determining Heart Rate on an ECG

     A.   Heart rate computation: 6-Second Method: Have a six second strip, count the QRS complexes and multipled by 10.   

Formula: H.R. = Number of QRS complexes in 6 second strip x 10.

                                6 second strip = 30 big blocks

                                                         = 30 x 10 = 300 big blocks (beats per minute)

Example 1:

- Calculate the heart rate using 6 second rule (method)

1.       6 second strip is selected between two arrows

2.       Number of QRS complexes counted in 6 seconds is 13.

3.       Heart rate = 13 x 10 = 130 beats per minute

Example 2: (This is the example Sir Gerard used in his lecture. Please check your notebook. I presume you wrote down the examples and ECG strips he gave during the lecture.)

    Given:

§  There are 3 QRS complexes in 3 second strip

§  Big blocks: 15

§  3 x 20 = 60 beats per minute

or: 3 x 2 x 10 = 60 beats per minute

     3 (QRS complexes) is multiplied by 2 to make it a 6 second strip. It is presumed that the rhythm in the second example is regular.

References

Gabriel Khan, M. Rapid ECG Interpretation, 3^rd Ed. Totowa, New Jersey: Human Press, 2008.

Guyton, Arthur C., and John E. Hall. Text Book of Medical Physiology, 11^th Ed. Philadelphia, Pennsylvania: Elsevier Inc., 2006. 

Martin, Gerard L. “Cardiac Physiology and ECG Interpretation.” Class lecture, SLRC, Sampaloc, Manila, May 6, 2011.

Martin, Gerard L. “Cardiac Physiology and ECG Interpretation.” Class lecture, SLRC, Sampaloc, Manila, May 9, 2011.

“Understanding ECG’s.”  Flight Medic Course (ACLS). http://usasam.amedd.army.mil/_fm_course/Study/UnderstandingECG.pdf (accessed July10, 2011)

rol