The normal cardiac electrical conduction pathway propagates from the right atrium down towards the left ventricle. In other words, the cardiac conduction axis is normally downward and leftward. Bipolar limb lead I (points to the left) and augmented lead aVF (points straight down) represent the component vectors in the x and y planes, respectively, for the cardiac conduction axis.
By convention, right on the screen is left anatomically, and left on the screen is anatomic right (notice how I labeled the atria and ventricles). We need to also define the positive direction in the x-axis (lead I) and y-axis (lead aVF) based on the normal axis. This means that downward is positive for lead aVF and leftward (rightward on the screen) is positive for lead I.
To figure out the cardiac axis, look at the QRS complex in leads I and aVF on the patient’s EKG. Determine if the QRS is positive or negative in lead I and lead aVF. Then you can compare it to the diagram above and figure out your axis. Keep in mind that a slight leftward deviation (-30 degrees) is still considered normal.
I break the causes of right axis deviation (+90 to +180 degrees) into causes of right heart strain (COPD, pulmonary hypertension, right ventricular hypertrophy, pulmonary embolism, etc.) and conduction abnormalities which favor right directional conduction (ischemic left lateral myocardium, left posterior fascicular block, left-sided WPW accessory pathway). The causes of left axis deviation (-30 to -90 degrees) include left ventricular hypertrophy, left anterior fascicular block, and even an inferior wall myocardial infarction.