The importance of refluxed gastric contents in the pathogenesis of GERD was emphasized by Winkelstein who introduced the term "peptic esophagitis" and by Bernstein and Baker who reported the symptom of heartburn following instillation of hydrochloric acid in the distal esophagus. Formal measurement of acid in the esophagus was first described in 1960 by Tuttle. He used a glass pH probe to map the gastroesophageal pH gradient, and demonstrated a sharp gradient in normal subjects and a gradual sloping gradient in patients with esophagitis.