:
  The electrocardiogram detects and prints out the electrical activity of the heart.

      How can we be sure that the electrocardiograph can
accurately measure and print out

      the cardiac electrical activity?

      Is the electrical activity measured and printed correctly?

  Calibration of the electrocardiogram answers this question.

 
Standard calibration of the ECG is 10mm/mV . At this calibration,

      1 miliVolt calibration signal is expected to produce a rectangle of
10 mm height and 5 mm width.

 
If the recording speed of ECG (sweep speed) is adjusted at 50 mm/second,

      1 miliVolt calibration signal is expected to produce a perfect square with a
10 mm height and 10mm width.

  When ECG waves are tall, the R or S waves may extend into the QRS complexes above or below them.

      To prevent this superimposition, the whole ECG may be calibrated at
5mm/mV .

  In some patients,
10-5 mm/mV ( 1-1/2 ) calibration may be chosen to decrease the amplitude of precordial

      lead deflections only. This is done to prevent superimposition of the QRS complexes in precordial leads

  Some ECG recorders detect tall ECG waves and switch from one calibration to another automatically.





ECG 1a. The ECG above resembles atrial fibrillation at first glance: P waves are absent (or seem so), baseline is irregular and
QRS complexes are not regular. The ECG has a standard calibration of 10mm/mV.

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ECG 1b. To see the details more clearly, calibration was readjusted at
20 mm/mV which now showed P waves , thus ruling out
the diagnosis of atrial fibrillation. The patient will not receive unnecessary anticoagulation therapy.

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Figure 1. An example of correct calibration
at 10 mm/mV and speed of 25 mm/sec : The calibration signal is a
rectangle making 90 degrees of angles.



Figure 2. The same patient's ECG, but at a paper speed of
50 mm/sec and 10 mm/mV calibration .
This is also correctly calibrated: The calibration signal is a
square with 90 degrees of angles.





Figure 3. The above ECG is recorded with a faulty calibration. The 1 mV calibration stimulus could not succeed to draw a
rectangle with a width of 5 mm and a height of 10mm. The ascending and descending limbs of the calibration stimulus
are not
linear , thus not making perfect 90 degrees of angle.





ECG 2a. The above ECG shows
delta waves in a patient with hypertrophic cardiomyopathy and WPW syndrome.
When the ECG is recorded at a standard calibration of
10mm/mV , it may be difficult to recognize the delta waves at first glance.

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ECG 2b. The same patient's ECG at a calibration of
20mm/mV shows delta waves clearly.


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ECG 3a. The above ECG is from a hypertensive patient and shows left ventricular hypertrophy pattern in. The voltages in
precordial leads are increased so much that
the R waves in leads C5 and C6 extend into the QRS complexes above them . In
addition,
the deep S wave in lead C3 extends down into the QRS complex below . The ECG has a standard calibration of
10 mm/mV .

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ECG 3b. The same patient's ECG, but at a calibration of
1 - 1/2 ( 10 - 5mm /mV ). At this calibration, limb leads are recorded at
10mm/mV while the chest leads are recorded at 5mm/mV. Since chest leads are recorded at half the previous voltage, the QRS
complexes do not overlap now. When calculating the QRS voltages to detect left ventricular hypertrophy in an ECG calibrated
like this one, do not forget to multiply the chest voltages by 2 to compensate for the decrease of standard voltage.

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ECG 4a. To prevent overlapping of the ECG deflections, some ECG machines automatically adjust the calibration of the ECG.
The ECG above is from a 74 years-old man with mitral stenosis, atrial fibrillation and hypertension.
The ECG machine printed
the limb leads at 10 mm/mV, but the chest leads at 5 mm/mV .
This has prevented the overlapping of ECG deflections in the precordial leads.

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ECG 4b. The ECG summary page of the same patient is seen above.
Now the machine has printed all derivations at a calibration of 10 mm/mV.
The R wave in lead V4 has an amplitude of 64 mm..