An electrode is a conductor through which electric current leaves the body.
A lead is the electric potential between two defined electrodes (represented by the tracing).
1 small square translated to what on the Y axis?
One small square translates to what on the X axis?
What is the normal paper speed?
Beginning of P to the beginning of QRS complex
Beginning of Q to end of S
Beginning of QRS complex to end of T wave
What do you check on an ECG?
- Patients name, age and gender.
- Date and time of trace.
- Check paper speed and calibration.
Describe the things we look at when analysing an ECG
- P wave
- PR interval
- QRS complex
- ST segment
- T wave
- (QT interval)
- (U wave)
How do you calaculate rate?
(i) R-R interval method
- 300 divided by the number of large squares between two consecutive R waves.
It can only be used if rhythm is regular.
(ii) Six second method
- The number of cardiac cycles in 6 seconds multiplied by 10
It can be used of rhythm is regular or irregular.
Tachycardias or tachyarrhythmias
- Faster than normal rate (>100bpm)
Bradycardias or bradyarrhythmias
- Slower than normal rate (<60 bpm)
- Every P wave must be followed by a QRS complex
- Every QRS complex must be preceded by a P wave
Other: eg. SVT, VT
- e.g. second degree heart block, ventricular bigeminy/trigeminy etc.
- e.g. AF
Calculating axis via the two-lead method
Examine QRS complexes wave in lead I and aVF.
- If both predominanly positive, axis is normal.
- If I is positive, aVF is negative, left axis deviation.
- If I is negative, aVF is positive, right axis deviation.
- If both leads predominanly negative, extreme right axis deviation.
Calculating axis via the three-lead method
Examine polarity of R wave in leads I, II and III.
- If all three are predominantly positive, axis is normal.
- If I is positive, II and III are negative, left axis deviation.
- If I is negative, II is biphasic or positive and III is positive, right axis deviation.
- If all three leads predominanly negative, extreme right axis deviation.
Duration: 0.04-0.12 sec
Duration: 0.05 - 0.11 sec (approximately 1-3 small squares)
Q wave normal if:
- <25% R wave voltage
Rule of thumb:
- <10mm in limb leads
- <20mm in chest leads
Duration not important.
- ST elevation
- ST depression
- Should not exceed 50% to 1/3 preceding QRS complex
Generally same net deflection as preceding QRS complex.
QT interval values
Varies between males and females.
Needs to be corrected for heart rate.
Normal QTc interval = 350-450ms
Normal in children and young adults.
Prominent U waves:
- Mitral valve prolapse