Nicholls was the first to describe dissecting aneurysm of the aorta, the patient being King George II, to whom he was physician from 1753-60. Nicholls was also the first to give a correct description of the mode of production of aneurysm. Nicholls' pa;er was illustrated with two folding plates of the heart engraved by J. Mynde and printed in two colors (brown and sanguine). These were probably the first color-printed plates in a major scientific periodical.

This "case was that of a rupture of the right ventricle of the heart showing an effusion of blood into the pericardium and an aneurism of the aorta. The King had complained for some years of frequent distress about the region of the heart. His death was due to tamponade by the extravasated blood from a tear in the myocardium, probably caused by a coronary occlusion. Though complicated by a 'transverse fissure in the trunk of the aorta, one and a half inches long' Nicholls' report may be regarded as a contribution to the history of myocardial infaraction" (Leibowitz, The history of cononary heart disease, 83).

Also published in Gentleman's Magazine (issue of November 1762, 520-523.