ECG Interpretation: Analyzing PR, QRS, QT Intervals and Heart Hypertrophy in 12-Lead ECGs, Slides of Cardiology

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ECG Rhythm Interpretation

Module VII b

Reading 12-Lead ECG’s

Course Objective

• To systematically analyze a 12-lead

ECG.

Reading 12-Lead ECGs

In Module VII a we introduced a 6 step approach for analyzing a 12-lead ECG and covered the first 3 steps. In this module we will cover the last 3 steps.

1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY
6. Look for evidence of INFARCTION

• Intervals refers to the length of the PR and QT intervals and the width of the QRS complexes. You should have already determined the PR and QRS during the “rhythm” step, but if not, do so in this step.
• In the following few slides we’ll review what is a normal and abnormal PR, QRS and QT interval. Also listed are a few common causes of abnormal intervals.

QRS complex

< 0.10 s 0.10-0.12 s > 0.12 s

Normal Incomplete bundlebranch block

Bundle branch block PVC Ventricular rhythm

Remember: If you have a BBB determine if it is a right or left BBB. If you need a refresher see Module VI.

3 rd^ degree AV block with ventricular escape rhythm

Incomplete bundle branch block

QT interval

The duration of the QT interval is proportionate to the heart rate. The faster the heart beats, the faster the ventricles repolarize so the shorter the QT interval. Therefore what is a “normal” QT varies with the heart rate. For each heart rate you need to calculate an adjusted QT interval, called the “corrected QT” (QTc):

QTc = QT / square root of RR interval

PR interval? QRS width? QTc interval? 0.16 seconds 0.08 seconds 0.49 seconds

QT = 0.40 s RR = 0.68 s Square root of RR = 0. QTc = 0.40/0. = 0.49 s

Interpretation of intervals? Normal PR and QRS, long QT

Tip: Instead of calculating the QTc, a quick way to estimate if the QT interval long is to use the following rule:

A QT > half of the RR interval is probably long.

Normal QT Long QT

QT

RR

10 boxes

23 boxes 17 boxes

13 boxes

• In Module VI we introduced the concept of left ventricular hypertrophy. As you remember the QRS voltage increases with LVH and is characterized by tall QRS complexes in certain leads. Similarly for right ventricular hypertrophy we look at the QRS complexes for changes in voltage patterns.
• With right and left atrial enlargement we analyze the P wave (since the P wave represents atrial depolarization). Here we also look for changes in voltage patterns.
• Note: as mentioned in Module VI criteria exists to diagnose LVH, the same goes for RAE, LAE and RVH. In the following slides we will be presenting criteria you can use. However other criteria exists and as a reference you might find it useful to carry a copy of Tom Evans’ ECG Interpretation Cribsheet.

Right atrial enlargement

• Take a look at this ECG. What do you notice about the P waves?

The P waves are tall, especially in leads II, III and avF. Ouch! They would hurt to sit on!!

Left atrial enlargement

• Take a look at this ECG. What do you notice about the P waves?

The P waves in lead II are notched and in lead V1 they have a deep and wide negative component.

Notched

Negative deflection

Left atrial enlargement

• To diagnose LAE you can use the following criteria:
  • II > 0.04 s (1 box) between notched peaks, or
  • V1 Neg. deflection > 1 box wide x 1 box deep

Normal LAE A common cause of LAE is LVH from hypertension.

Right ventricular hypertrophy

• Compare the R waves in V1, V2 from a normal ECG and one from a person with RVH.
• Notice the R wave is normally small in V1, V2 because the right ventricle does not have a lot of muscle mass.
• But in the hypertrophied right ventricle the R wave is tall in V1, V2.

Normal RVH

Right ventricular hypertrophy

• To diagnose RVH you can use the following criteria:
  • Right axis deviation, and
  • V1 R wave > 7mm tall

A common cause of RVH is left heart failure.