34 The Approach to Vomiting and Regurgitation
The most common differential diagnoses for regurgitation include esophageal obstruction or megaesophagus. Foreign bodies, of which bones are the most common, can cause obstruction. Obstruction can also occur because of tumors or strictures. The most common causes of strictures are previous foreign bodies, anesthesia-related regurgitation, and medications that become trapped in the esophagus. In very young animals a persistent right aortic arch will also cause regurgitation when solid foods are first taken. Megaesophagus can be acquired or congenital in origin (see Chapter 37).
The act of vomiting involves a certain sequence of events. Initially a feeling of nausea is generated. This can be identified in dogs as yawning, salivation, and restlessness. Increased production and swallowing of saliva occurs. The saliva contains bicarbonate, which helps to buffer stomach acid. The lower esophageal sphincter and the cardia then relax. There also is retrograde movement of small intestinal contents into the stomach. The dog then begins to retch, a process in which the diaphragm and other muscles contract while the glottis remains closed. This causes negative intrathoracic pressure, which draws stomach contents into the esophagus. Active vomiting then occurs when abdominal muscles contract. Contraction of these muscles causes pressure to become positive in the abdomen and when the glottis opens, intrathoracic pressure also become positive, resulting in expulsion of esophageal contents.
