The literature I found points to experience, conditions present in the
patient's esophagus, procedures being performed during the endoscopy
and the type of equipment used.
This study finds that inexperience raises perforation rates:
Prospective audit of perforation rates following upper
gastrointestinal endoscopy in two regions of England," Quine MA, Bell
GD, McCloy RF, Matthews HR, Royal College of Surgeons, London, UK.
"After cardiopulmonary complications, perforation is the second most
important cause of complications following flexible upper
gastrointestinal endoscopy. A recent audit of 14,149 procedures
detected a perforation rate of 0.05 per cent (overall mortality rate
0.008 per cent) during diagnostic endoscopy, and a perforation rate of
2.6 per cent (overall mortality rate 1.0 per cent) following
oesophageal intubation or dilatation. The incidence of perforation
following both diagnostic and therapeutic upper gastrointestinal
endoscopy has not changed over the past 10 years. The risk factors are
numerous but this audit demonstrated that inexperience increases the
likelihood of perforation."
The American Society for Gastroinestinal Endoscopy says that
conditions present in the patient's espoghagus can cause problems:
"1) Predisposing factors to perforations include the presence of
anterior cervical osteophytes, Zenker's diverticulum, esophageal
strictures and malignancies."
You can find page, specifically the section on perforations here:
eMedicine Journal indicates that the rate of perforation varies
greatly depending on which, if any procedures were done in conjunction
with the endoscopy:
Medical instrumentation of the esophagus is the most common cause of
perforation, while spontaneous esophageal rupture is rare. In one
series of esophageal rupture, 48% of ruptures were iatrogenic, 33%
were caused by external trauma, and 8% were spontaneous.
Esophagogastroduodenoscopy is the most common procedure instrumenting
the esophagus. Risk of perforation with diagnostic
esophagogastroduodenoscopy is extremely low (0.03%). The risk of
perforation is increased when therapeutic procedures are performed at
the time of endoscopy."
You can find the full explanation here:
Dr. Minocha, Professor of Medicine and Director, Division of Digestive
Diseases at the University of Mississippi Medical Center says,
"Esophageal perforation due to endoscopy is rare and depends on the
type of endoscope and intervention used. Rigid endoscopes scopes have
higher risk (0.1-1.9%) than the flexible endoscopes (0.007-0.15%)."
Hope that gives you what you were looking for. I'd be happy to provide
clarification on my answer, though please bear in mind I am not a