|Author(s)||I.A.C. van der Bilt, MD|
|Moderator||I.A.C. van der Bilt, MD|
|some notes about authorship|
- 1 How do I begin to read an ECG?
- 2 What does the ECG register?
- 3 The ECG represents the sum of the action potentials of millions of cardiomyocytes
- 4 The electric discharge of the heart
- 5 The different ECG waves
- 6 The history of the ECG
- 7 The ECG electrodes
- 8 ECG variants
- 9 Color coding of the ECG leads
- 10 Special Leads
- 11 Ladder diagram
- 12 Technical Problems
- 13 References
How do I begin to read an ECG?
- On the top left are the patient's information, name, sex and date of birth
- At the right of that are below each other the Frequency, the conduction times (PQ,QRS,QT/QTc), and the heart axis (P-top axis, QRS axis and T-top axis)
- Farther to the right is the interpretation of the ECG written (this may be missing in a 'fresh' ECG, but later the interpretation of the cardiologist or computer will be added)
- Down left is the 'paper speed' (25 mm/s on the horizontal axis), the sensitivity (10mm/mV) and the filter's frequency (40Hz, filters noise from eg. lights).
- There is a calibration. At the beginning of every lead is a vertical block that shows with what amplitude a 1 mV signal is drawn. So the height and depth of these signals are a measurement for the voltage. If this is not set at 10 mm, there is something wrong with the machine setting.
- Finally we have the ECG leads themselves.These will be discussed below.
What does the ECG register?
- The electrocardiogram
- An electrocardiogram (ECG or EKG) is a register of the heart's electrical activity.
The ECG represents the sum of the action potentials of millions of cardiomyocytes
|This movie shows the contraction of a single (rabbit) heart cell. The glass electrode measures the electrical current in the heart cell (with thepatch-clamp method). The electrical signal is written in blue and shows the action potential. Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands.|
The electric discharge of the heart
The different ECG waves
- Q: the first negative deflection after the p-wave. If the first deflection is not negative, the Q is absent.
- R: the positive deflection
- S: the negative deflection after the R-wave
- Small print letters (q, r, s) are used to describe deflections of small amplitude. For example: qRS = small q, tall R, deep S.
- R`: is used to describe a second R-wave (as in a right bundle branch block)
The history of the ECG
The ECG electrodes
- The four extremity electrodes:
- LA - left arm
- RA - right arm
- N - neutral, on the right leg (= electrical earth, or point zero, to which the electrical current is measured)
- F - foot, on the left leg
- The six chest electrodes:
- V1 - placed in the 4th intercostal space, right of the sternum
- V2 - placed in the 4th intercostal space, left of the sternum
- V3 - placed between V2 and V4
- V4 - placed 5th intercostal space in the nipple line. Official recommendations are to place V4 under the breast in women.
- V5 - placed between V4 and V6
- V6 - placed in the midaxillary line on the same height as V4 (horizontal line from V4, so not necessarily in the 5th intercostal space)
The Extremity Leads
- I from the right to the left arm
- II from the right arm to the left leg
- III from the left arm to the left leg
- AVL points to the left arm
- AVR points to the right arm
- AVF points to the feet
The Chest Leads
- The 3 channel ECG uses 3 or 4 ECG electrodes. Red is on the right, yellow on the left arm, green on the left leg ('sun shines on the grass') and black on the right leg. These basic leads yield enough information for rhythm-monitoring. For determination of ST elevation, these basic leads are inadequate as there is no lead that gives (ST) information about the anterior wall. ST changes registered during 3-4 channel ECG monitoring should prompt acquisition of a 12 lead ECG.
- The 5 channel ECG uses 4 extremitiy leads and 1 precordial lead. This improves ST segment accuracy, but is still inferior to a 12 lead ECG. 
- In vector electrocardiography the movement of electrical acitivity of the P, QRS and T wave is described. Additional X,Y and Z leads are recorded. Vector electrocardiography is rarely used nowadays, but is sometimes useful in a research setting.
- In body surface mapping several arrays are used to accurately map the cardiac electrical wavefront as it moves over de body surface. With this information the electrical acitivity of the heart can be calculated. This is sometimes used in a research setting.
Color coding of the ECG leads
|AHA (American Heart Association)||IEC (International Electrotechnical Commission)|
- Leads to improve diagnosis in right ventricular en posterior infarction:
- 1. On a right-sided ECG, V1 and V2 remain on the same place. V3 to V6 are placed on the same place but mirrored on the chest. So V4 is in the middle of the right clavicle. The ECG should be marked as a Right-sided ECG. V4R (V4 but right sided) is a sensitive lead for diagnosing right ventricular infarctions.
- 2. Leads V7-V8-V9 can be used to diagnose a posterior infarct. After V6, leads are placed towards the back. See the chapter Ischemia for other ways of diagnosing posterior infarction.
- Leads to improve detection of atrial rhyhtm:
- In wide complex tachycardia, good detection of atrial rhythm and atrio-ventricular dissociation can be very helpful in the diagnosis process. An esophagal ECG electrode placed close to the atria can be helpful. Another, less invasive, method is the Lewis Lead. This is recorded by changing the limb electrodes, placing the right arm electrode in the second intercostal space and the left arm electrode in the fourth intercostal space, both to the right of the sternum. Furthermore gain is increased to 20mm/mV and paper speed to 50mm/sec.ß
- Lead positioning to enhance detection of Brugada syndrome