Learn to estimate the frontal QRS axis by plotting the net QRS deflections in leads I and aVF. The SAM mnemonic turns the limb-lead reference system into a memorable right-angle vector exercise.
By the end of this lesson, you should be able to:
The frontal QRS axis describes the average direction of ventricular depolarization when viewed in the frontal plane. It is assessed from the limb leads. Rather than stopping at a broad label such as “normal”, estimating the angle can help identify a meaningful change in electrical orientation between ECGs.
The SAM teaching method focuses on two perpendicular limb-lead directions: lead I at 0° and lead aVF at +90°. Their net QRS deflections form the horizontal and vertical components of a right triangle. The direction from the origin to the plotted point approximates the frontal QRS axis.
Lead I tells you how far to plot horizontally; aVF tells you how far to plot vertically. The signs determine the direction.
Imagine a person standing in the centre of a circle. Each limb lead points along a different viewing direction. The lead angles used in the supplied SAM reference are arranged in 30° increments around the frontal plane.
Schematic redraw for teaching. Angles follow the supplied SAM article; aVR may also be expressed as +210° or −150°.
| Limb lead | Approximate angle | Direction on SAM |
|---|---|---|
| I | 0° | Horizontal to the right |
| aVL | −30° | Upper right |
| II | +60° | Lower right |
| aVF | +90° | Vertically downward |
| III | +120° | Lower left |
| aVR | −150° (or +210°) | Upper left |
For each selected lead, compare the total positive QRS deflection above the baseline with the total negative deflection below it. Express both in the same units, such as millimetres or small ECG-paper squares, then subtract the negative magnitude from the positive magnitude.
Net QRS = total positive QRS amplitude − total negative QRS amplitude.
A positive result is plotted toward the lead direction; a negative result is plotted in the opposite direction.
If lead I has 7 small squares of upward deflection and 3 squares downward, its net deflection is +4 squares. If aVF has 16 squares upward and 1 square downward, its net deflection is +15 squares.
Use the same amplitude scale for both leads. Do not mix millimetres in one lead with millivolts in the other unless the calibration has been converted consistently.
Schematic waveform only. Measure QRS deflections from the isoelectric baseline; do not include the T wave.
The positive aVF component is plotted downward. The resultant vector points into the lower-right quadrant.
When lead I is the horizontal component (x) and aVF is the vertical component (y), the estimated angle can be expressed as θ = atan2(net aVF, net lead I). Use a calculator in degree mode if you need a numerical angle. The SAM diagram is a visual way to understand the same geometry.
The following examples retain the measurements and teaching points from the supplied article. Values are approximate because the original measurements are read from ECG tracings.
Lead I: 7 squares up and 3 down.
aVF: 16 squares up and 1 down.
Plot +4 to the right and +15 downward. The resultant is in the lower-right quadrant.
≈ +75°Lead I: 5 squares up and 5 down.
aVF: 17 squares up and 2 down.
There is no horizontal component. The resultant follows the aVF direction.
+90°Lead I: 8 squares up and 1 down.
aVF: 1 square up and 11 down.
Plot +7 to the right and −10 upward, away from aVF.
≈ −55°The supplied article's prose states −45°, whereas its own plotted components (+7 and −10) and the angle label in the accompanying diagram indicate approximately −55°. The value shown here is calculated from those components: atan2(−10, +7) ≈ −55°. This is a source inconsistency, not a change to the underlying method.
A lead is described as isoelectric in this teaching context when its positive and negative QRS deflections are approximately equal, leaving a net deflection close to zero. The mean QRS axis is then approximately perpendicular to that lead.
In the source's second worked example, lead I is isoelectric and aVF is strongly positive. The axis is therefore approximately +90°, which is perpendicular to lead I.
A lead has two directions at 90° to it. Use the polarity of the other limb leads to decide which direction is consistent with the ECG. In the illustrated source example, the positive aVF deflection supports the +90° direction.
A positive net aVF value is plotted downward, toward aVF. A negative value is plotted upward, away from aVF. This is a frequent source of plotting errors.
The source highlights aVR/aVL limb-electrode reversal as a common placement error. Such a reversal alters leads I, II, and III and makes the SAM plot unreliable.
In a typical correctly recorded ECG, aVR usually has predominantly negative P waves, QRS complexes, and T waves. The source also notes that aVR and aVL may appear as mirror images.
Compare the two components using a consistent scale. The source uses small ECG squares; equivalent voltage measurements can be used when calibration is consistent.
Confirm limb-electrode placement and review the complete ECG. An unexpected axis or a change from a prior tracing should be interpreted in the clinical context rather than treated as a stand-alone diagnosis.
A broad statement that the axis is “normal” may conceal a meaningful shift within a commonly encountered range. The source gives the example of a change from 0° to +90°: both values may be casually labelled normal, yet the shift itself can be clinically relevant. Comparing serial ECGs can therefore add information that a simple category may miss.
The source specifically cautions that an axis change may be relevant when considering right-heart strain, including in a patient with a significant pulmonary embolism. Axis must still be integrated with symptoms, examination, and the rest of the ECG.
Try to answer each question before opening the explanation.
Both components are positive, so the vector points to the lower-right quadrant. With equal components, the approximate direction is +45°.
It is approximately +90°, perpendicular to lead I and directed toward the positive aVF axis.
Upward, away from aVF, because the net aVF deflection is negative. The approximate angle is −55°.
Swapping the right- and left-arm electrodes changes the calculated limb leads and can invalidate the SAM construction. Review electrode placement and lead polarity before interpreting the axis.
Document key findings and keep a structured record for your own reference.
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