How to Measure PR Interval

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The PR interval is measured using an electrocardiogram, also called ECG or EKG. This is a painless test performed on cardiac patients in order to measure the electrical conductivity of the heart and identify any abnormalities.

Part 1
Part 1 of 3:

Understanding What EKGs are Used For

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  1. To understand how the test is done it is important to understand how the heart works.
    • When the heart beats, electrical conductivity takes place from the top of the heart to the bottom. This enables the heart to contract and pump blood to the rest of the body.
    • The heart pumps blood with each heart beat and therefore electrical conductivity takes place with each heartbeat.
  2. An EKG is usually done to show any abnormalities that may be present in the heart, for example arrhythmias and abnormal heart beats. This is important, so the EKG is the first test that is done on cardiac patients to help in the diagnosis of abnormalities of the heart.
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  3. The EKG helps to show the speed at which the heart is beating and identify whether it is beating slower or faster than it should.
    • A regular heartbeat is between 60 to 100 beats per minute. When the heart beats less than 60 beats per minute, this is an abnormality called bradycardia and when the heart beats at a speed of more than 100 beats per minute, this is an abnormality called tachycardia.
    • The EKG also helps to assess the heart rhythm, as it shows whether the heartbeats are irregular or steady. It also shows the timing and strength of the heart's electrical conductivity as the impulses pass through the heart with every heartbeat.
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Part 2
Part 2 of 3:

Performing the Test

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  1. Once the patient is lying down, small patches called electrodes will be attached to the chest, arms and legs.
    • These areas are cleaned with a special EKG paste that helps in improving the transmission of the electrical impulses. The area may need to be shaved prior to the EKG so that the electrodes stick on the skin.
    • The patches are then connected to the EKG machine by wires and this helps to turn the electrical conductivity of the patient’s heart into wavy lines that are printed on a paper.
  2. The ten electrodes of the EKG machine should be clearly labeled, as this helps the caregiver to know where they should be placed on the patient’s body. The correct labels and corresponding positions on the body are as follows:
    • RA: Positioned on the right arm making sure to avoid the thick muscle
    • LA: Positioned on the same place as RA but on the left arm
    • RL: Placed on the lateral calf muscle of the right leg
    • LL: Positioned on the same place as RL but on the left leg
    • V1: Placed between the fourth and fifth ribs at the intercostals space, at the right side of the sternum (breastbone)
    • V2: Placed between the fourth and fifth ribs at the intercostals space, to the left of the sternum
    • V3: Placed between leads V4 and V2
    • V4: Positioned in the fifth intercostal space between the fifth and sixth ribs, in the middle of the clavicle.
    • V5: Placed horizontally at an even place with V4 in the anterior axillary line on the left
    • V6: Placed horizontally at an even place with V4 and V5 in the middle place of the axillary line.
  3. The patient is advised not to move while the procedure is being done, because any movement will alter the results of the test and prevent a clear diagnosis.
    • The health care provider may also ask the patient to hold their breath while the procedure is being done, and it is important for the patient to follow the instructions that are given.
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Part 3
Part 3 of 3:

Assessing the Results

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  1. The normal cardiac cycle starts with depolarization of the sinus node. This occurs in a specialized tissue that is located in the right atrium (RA).
    • The electrical activity (depolarization) commences from the right atrium through the inter-atrial septum to the left atrium (LA). The atrium and ventricles are separated by the atrioventricular node (AV). The AV node delays the electrical activity for a short while and then it spreads down through the interventricular septum into the right (RV) and left ventricles (LV).
    • The electrical wave passes through the bundle of His and the left bundle branches to reach the RV and the LV. The two ventricles contract concurrently to push blood and this is important to maximize the efficiency of the heart.
  2. The first chamber of the heart to undergo an electrical activity is the right atrium and then it spreads to the left atrium. The first electrical activity initiates in the atrium and it is called the P wave.
    • The EKG usually shows one P wave but it is the sum of the electrical conductivity of the two atria on the left and the right.
    • There is a short delay as the AV node sends the electrical activity to the ventricles and this is the PR interva. The PR interval is the period when there is no electrical activity visible on the EKG.
  3. Depolarization of the lower chambers of the heart then follows. This is the largest part of the signals visible on the EKG and it is called the QRS complex.
    • The Q wave is the initial downward deflection. The R wave follows as the rising deflection and then S wave as the last downward deflection.
    • Electrical activity reflects on the EKG as the depolarization of the myocardium. This occurs as the T wave and ST segment. The T wave appears as an upright deflection of variable duration and amplitude while the ST segment is isoelectric.
  4. The recording of an EKG is done on a standard paper and this allows the electrical activity to be measured, usually in terms of milliseconds.
    • Normal PR intervals range from 120-200 ms, which is equivalent to 3 to 5 small squares on the EKG paper.
    • The standard EKG paper can give an approximate heart rate from the recording of the EKG. The time in seconds is represented by 250mm and this is measured along the horizontal axis.
  5. The number of large squares between each of the QRS complex helps to determine the heart rate.
    • When there are five squares, the heart rate is 60 beats per minute.
    • When there are three squares, the heart rate is 100 beats per minute.
    • When there are two squares, the heart rate is 150 beats per minute.
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      References

      1. http://www.webmd.com/heart-disease/electrocardiogram?page=2
      2. Wilkinson, E. J., Stone, I. K., & Wilkinson, E. J. (2012). Wilkinson & Stone atlas of vulvar disease. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins.
      3. Springhouse Corporation. (2004). ECG interpretation made incredibly easy. Philadelphia, PA: Lippincott Williams & Wilkins.
      4. Rowlands, A., & Sargent, A. (2011). The ECG workbook. Keswick: M&K Publishing.

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