The electrical axis of the heart, i.e. where the summative vector of electrical conduction points to, can be determined in many ways. Can you tell the QRS axis from the tracing below?
To quickly assess QRS axis you can either use pattern recognition or a stepwise approach.
For pattern recognition take a look at leads I, II and III. The lead with the highest R-wave resembles the main vector of the electrical impulse. So basically, if the highest R-wave is in lead I the axis points towards left. If lead II shows the tallest R-wave the QRS axis points towards the left lower quadrant of Cabreras` cycle (normal axis). If the R-wave is tallest in lead III the axis points towards right.
For a stepwise approach, first look at leads I and aVF. The overall highest QRS amplitude in these leads will tell you whether the axis lies in the upper (aVF negative) or lower quadrant (aVF positive) of Cabreras´ Cycle. Lead I determines whether the axis points towards the left (lead I positive) or right (lead I negative) quadrants. Now you know in which quadrant of Cabreras´ cycle to look for the exact QRS axis.
At this point, take a look at all the six limp leads (I, II, III, aVR, aVL, aVF) and search for the lead with the overall most equiphasic QRS complex (i.e. the QRS complex where positive and negative deflections equal each other out). If the overall QRS potential equals zero, the QRS axis would be perpendicular to this lead.
In the example below:
Take a look at leads I and aVF. The R-wave is positive in both leads. So, lead I points us towards the left, and aVF points us towards the lower quadrant of Cabreras´ cycle. So, the QRS axis should be found somewhere in the left lower quadrant (0° to +90°).
Looking at all the limp leads now, the QRS in aVL would be the most equiphasic. So the axis would be perpendicular to aVL (+60°). However, aVL is slightly negative. Therefore the axis must be corrected towards the right. The true QRS axis therefore is about +75°.