BRIEFLY: THE BASIC ECG FORM & MEASUREMENTS



P wave - the spread of electrical activation (depolarisation) through the atrial myocardium.

 QRS complex - the spread of electrical activation through the ventricular myocardium.

 T wave - electrical recovery (repolarisation) of the ventricular myocardium. 

U wave may be due to slow replolarisation of the papillary muscles.  Some causes include: Bradycardia, hypokalaemia and digoxin.


Measurements




  • Heart rate: Rule of 300 - Provided that the rhythm is regular, heart rate can be estimated by dividing 300 by the number of large boxes in the R-R interval. 
    i.e. Heart Rate = 300/ no. of large boxes in the R-R interval
  • PR interval: 0.12 - 0.20 s 
  • QRS duration: 0.06 - 0.10 s 
  • Corrected QT interval (QTc): normal QTc = 0.40 s
    Bazett's formula: QTc = QT / RR1/2 (in seconds)  

  • Determining of Heart Axis. The fastest, non-specific method to determine the QRS axis is to find the major direction of the QRS complex (positive or negative) in leads I and aVF.

     1)   If the QRS complex is upright (positive) in both lead I and lead aVF, then the axis is normal.



     2)  If the QRS is upright in lead I (positive) and downward in lead aVF (negative), then the axis is between 0 and -90 degrees. However recall that left axis deviation is defined as between -30 and -90 and thus this scenario is not always technically left axis deviation. In this scenario, the QRS axis could fall between 0 and -30 which is within normal limits. To further distinguish normal from left axis deviation in this setting, look at lead II. If lead II is downward (negative), then the axis is more towards -120 and left axis deviation is present. If the QRS complex in lead II is upright (positive), then the axis is more towards +60 degrees and the QRS axis is normal.



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