Example 8.3.7 of the textbook shows how the dot product and scalar projections are important in vectorcardiography. For simplicity we restricted attention to a 2-dimensional heart voltage vector but of course heart voltage vectors really exists in 3 dimensions. Additional leads can be attached to a patient to measure this vector in 3 dimensions. In fact, 12-lead electrocardiography is often used to obtain more accurate measurements in 3 dimensions. The book listed below is an excellent reference for more information about vectorcardiography.
References
Macfarlane, P.W. et al. (eds). 2010. Comprehensive electrocardiology (2nd edition). Springer