What is acid reflux?

Esophageal pH Monitoring

Ambulatory intraesophageal pH monitoring is now the standard for establishing pathological reflux. The test is performed with a pH probe passed nasally and positioned 5 cm above the manometrically determined LES. The probe is connected to a battery-powered data logger capable of collecting pH values every 4 to 6 seconds. An event marker is activated by the subject in response to symptoms, meals, and body position changes. Patients are encouraged to eat normally and to pursue regular daily activities. Monitoring is carried out usually for 18 to 24 hours. Reflux episodes are detected by a drop in pH to less than 4. Commonly measured parameters include the percentage of total time that the pH is less than 4, the percentage of time upright and supine that the pH is less than 4, the total number of reflux episodes, the duration of longest reflux episode, and the number of episodes longer than 5 minutes. The total percentage of time that the pH is less than 4 is the most reproducible measurement for GERD, with reported upper limits of normal values ranging from 4% to 5.5%. Ambulatory pH testing can discern positional variations in GER, meals, and sleep-related episodes and helps to relate symptoms to reflux events. As the result of its reliability for measuring GER across normal activities, ambulatory pH testing has replaced other older studies, such as the standard acid reflux (Tuttle) test and radionuclide scintigraphy.

One important problem with esophageal pH monitoring is that there exists no absolute threshold value that reliably identifies pathological GER. Validation studies comparing the presence of esophagitis with abnormal pH test report sensitivities ranging from 77% to 100% with specificities from 85% to 100%. However, these patients rarely need pH testing; rather, the patients with normal endoscopic findings and suspected reflux symptoms should benefit most from ambulatory pH monitoring. Unfortunately, the data are much less conclusive in this group, with considerable overlap between controls and patients with nonerosive reflux. Other drawbacks of pH testing include possible equipment failure, the pH probe’s missing a reflux event because it is buried in a mucosal fold, and false-negative studies resulting from dietary or activity limitations from poor tolerability of the nasal probe.

An important advantage of ambulatory esophageal pH monitoring is its ability to record and correlate symptoms with reflux episodes over extended periods. For this indication, it has essentially replaced the shorter acid perfusion (Bernstein) test. Because only about 10% to 20% of reflux episodes are associated with reported symptoms, different statistical analyses have evolved attempting to define a significant association between these two variables including the symptom index, symptom sensitivity index, and symptom association probability. Unfortunately, no studies to date have defined the accuracy of any of these symptom scores in predicting response to therapy. Therefore, pH testing and symptom correlation can define an association between complaints and GER, but only treatment trials address the true definition of a causal relationship.

Definite clinical indications for ambulatory pH monitoring have been established. Before fundoplication, pH testing should be performed in patients with normal endoscopic findings to identify the presence of pathological reflux. If esophagitis is present, pH testing is not necessary because the disease has been established. After antireflux surgery, persistent or recurrent symptoms warrant repeat pH testing. In these situations, pH monitoring is performed with the patient discontinuing all antireflux medications (PPIs for 1 week, H 2RAs for 2 days). Esophageal pH testing is particularly helpful in the evaluation of patients with reflux symptoms resistant to treatment with normal or equivocal endoscopic findings. For this indication, pH testing is usually done in patients receiving therapy to define two populations: those with and those without continued abnormal esophageal acid exposure times. The group with persistent GER needs intensification of the medical regimen, whereas those patients with symptoms and adequate acid control have another cause of their complaints. Finally, ambulatory pH testing may help in defining patients with extraesophageal manifestations of GERD. In this situation, pH testing is usually done with additional pH probes placed in the proximal esophagus or pharynx. Initially, most of these studies were done when patients were not taking antireflux medications, to confirm the coexistence of GERD; however, this does not guarantee symptom causality. Therefore, the current approach is to treat the patients aggressively with PPIs first and to reserve pH testing only for those patients not responding after 4 to 12 weeks of therapy.