Sir James Mackenzie FRS FRCP (12 April 1853 – 26 January 1925) was a Scottish cardiologist who was a pioneer in the study of cardiac arrhythmias.

[3] After completing his residency in Edinburgh, Mackenzie became a general practitioner in borough of Burnley in Lancashire, England where he continued to practice medicine for more than a quarter of a century.

While he was engaged in a busy practice, he made many original observations, completed his MD degree on hemi-paraplegia spinalis[3][4] (awarded by the University of Edinburgh in 1882) and had many scientific papers published.

In 1890 he discovered premature ventricular contractions and use of the polygraph enabled Mackenzie to make original distinctions between harmless and dangerous types of pulse irregularities (arrhythmias).

Three years later he founded the influential Mackenzie Institute of Clinical Research in St Andrews, which involved local General Practitioners in detailed long-term recording of patients' symptoms and illnesses.