ECG

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  • DEFINITION :

An electrocardiogram ( ECG or EKG ) is a method of monitoring the health of your heart. Each heart beatis triggered by an electrical impulse produced from the special cells located at your heart's upper right chamber. Your doctor uses the ECG to search for patterns among the heart rhythms and heartbeats to diagnose a multitude of heart conditions.

  • RATE :
At usual speed (25mm/s) each 'big square' is 0.2s;each 'small square' is 0.04s to calculate the rate, divide 300 by the number of big square per R- R interval 

  • RHYTHM :
If circles are not clearly regular, use the 'card method' : lay a card along ECG, marking positions of 3 successive R waves. Slide the card to and fro to check that all intervals are equal. If not, note if: slight but regular lengthening and then shortening ( with respiration ) - sinus arrhythmia common in the young; are different rates multiples of each other - varying block; or it is 100% irregular - atrial fibrillation or ventricular fibrillation . Sinus rhythm is characterized by a P wave followed by a QRS complex. AF has no discernible P waves and QRS complexes are irregularly irregular. Atrial flutter has a 'sawtooth' baseline of atrial depolarization (~300/min) and regular QRS complexes. Nodal rhythm has a normal QRS complex but P waves are absent or occur just before or within QRS complexes. Ventricular rhythm has QRS complexes >0.12s with P waves following them (ECG 6,p100). 
  • AXIS : 
The mean frontal axis is the sum of all the ventricular forces during ventricular depolarization. The axis lies at 90° to the isoelectric complex (the one in which positive and negative deflections are equal). Normal axis is between -30° and +90°. 

  • P wave : 
The deflection is the P wave associated with right and left atrial repolarization is invisible because of low amplitude. Normal P wave is no more than 2.5 mm tall and less than 120ms in width in any lead.

  • QRS complex : 
The second wave is the QRS complex. Typically this complex has a series of 3 deflections that reflect the current associated with right and left ventricular depolarization. By convention the first deflection in the complex, if it is negative, is called a Q wave. The first positive deflection in the complex is called an R wave. A negative deflection after an R wave is called an S wave. A second positive deflection after the S wave, if there is one, is called the R wave. Some QRS complexes do not have all three deflections. But irrespective of the number of waves present they are all QRS complexes 

  • ST segment : 
Following the QRS complex is the ST segment, extending from where the QRS ends to where the T wave begins. The junction between the end of the QRS and the beginning of the ST segment is called the J point.

  • T wave : 
The T wave represents the current of rapid phase 3 ventricular repolarization. The polarity of this wave normally follows that of the main QRS deflection in any lead. The ventricles  are electrically unstable during that period of repolarization extending from the peak of the T wave to its initial downslope. A stimulus falling on this vulnerable period has the potential to precipitate ventricular fibrillation : the so call R-on- T phenomenon.

  • PR interval : 
The PR interval extends from the beginning of the P wave to the beginning of the QRS, whatever the first wave of this complex may be. The interval measures the time from the initial depolarization of the ventricles and reflects a physiological delay in AV conduction imposed by the AV node. Normal range is 120 - 200 ms and no longer.

  • QT interval : 
The QT interval is measure from the beginning of the QRS to the end of the T wave. It represents the time in which the ventricles depolarize and repolarize and is a measure of ventricular action potential (AP) duration. This interval should be determined in the ECG lead where it is longest.

  • ECG purpose : 
An ECG is a noninvasive,painless procedure used to diagnose a variety of conmon heart conditions. Your doctor may recommend ECG used to detect a previous heart attack, a heart attack in emergency situations, blocked or narrowed heart arteries, heart valve problems, heart defects, and arrhythmias.



  • ECG results : 
Your doctor will search for an even and consistent heart rate and rhythm ...around 50 to 100 beats per minute,to be exact. Having an irregular ,slower or faster heartbeat will provide him hints regarding your heart health, including heart rhythm ,heart rate, structural abnormalities and the risk for heart attack. 




References : 

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