Practice Quiz : BP, ECG

Electrical Axis of the Heart

One can calculate the electrical position of the heart in the chest. This requires some geometry, a pair of compasses, and a protractor.

Measure the R waves from Leads I, II, and III. (This is from the baseline to the top of the R wave. It should be measured to the nearest 1/2 mm. The illustration below measures the Q and R waves and then subtracts the Q wave).

Construct an Isosceles triangle. The size is not important just so it is large enough with which to work. On each side of the triangle, construct a 90^o or perpendicular line at the center of the side. These perpendicular lines will meet at the center of the triangle.

On the side that represents Lead I, measure off the length of the R wave for Lead I (to the right of the center line if positive and to the left if negative). Construct a new perpendicular line at this new point.

Repeat this process on the sides that represent Leads II and III. These new perpendicular lines should also meet at a point within the triangle. (If they do not find the center of the new triangle that is formed and put a point there.)

Draw a line that is defined by the points of intersection of the two sets of perpendicular lines and extend the line to cross the side that represents Lead I.

Measure the angle between the Lead I side and the newly constructed line. This is the electrical axis of the heart.

1. Suppose that on an ECG you measure the R waves of Leads I, II, and II and get the following measurements in order -7,9, 16. What is the electrical axis of this heart?

Ans: I didn't have a protractor, but my best guess is somewhere around 118^o.

2. With a standard set up, it is 14.7 mm between R waves in an ECG. What is the heart rate?

Ans: 102 beats/min

3. Explain how a blood pressure cuff works.