Pharmacologic treatment of gastroesophageal reflux disease is based upon gastric acid suppression with proton pump inhibitors (PPIs). In many patients, symptoms persist despite PPI treatment. For some, ongoing symptoms may be due to nonacid reflux. Accurate measurement of nonacid reflux is not possible with conventional ambulatory pH monitoring. Impedance-pH monitoring has advanced the ability to assess gastroesophageal reflux, because, in addition to detecting acid reflux, it enables measurement of nonacid reflux. This article discusses the principles of impedance-pH monitoring, catheter characteristics and placement, interpretation of studies, and clinical uses of this form of reflux monitoring.
Key points
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Impedance-pH monitoring is considered the most accurate and sensitive tool for measuring all types of gastroesophageal reflux, and it enables detailed characterization of reflux episodes.
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Currently available software for automated detection of reflux episodes in impedance-pH tracings is sensitive but not very specific, and manual editing is still recommended (this may change with further software refinement).
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Nonacid reflux can produce symptoms that are indistinguishable from those caused by acid reflux.
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Some of the main clinical uses of impedance-pH monitoring include the assessment of refractory gastroesophageal reflux disease (GERD) symptoms, GERD-related cough, and belching.
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Limitations include patient discomfort, as it is a catheter-based test; limited duration of monitoring (not more than 24 hours); and limited outcomes studies evaluating the treatment of nonacid reflux.
Introduction
Gastroesophageal reflux disease (GERD) is a common clinical problem. The role of acid in the pathogenesis of GERD is well established, and gastric acid suppression with proton pump inhibitors (PPIs) is the mainstay of medical therapy. The usual approach for patients presenting with classic symptoms of GERD, such as heartburn and acid regurgitation, is to prescribe empiric antisecretory therapy with a PPI. In the current era of rising GERD prevalence and frequent PPI use, there are increasing numbers of patients in whom symptoms persist despite treatment. In some of these patients, the persistent symptoms may be caused by reflux of gastric contents with a pH ≥4, commonly referred to as nonacid reflux. Nonacid reflux may be further classified as weakly acidic (pH ≥4 but <7) or weakly alkaline (pH ≥7). In this article, the term nonacid reflux is used to refer to all reflux with pH ≥4. Nonacid reflux may also occur in the absence of acid suppression during the postprandial period when gastric contents are buffered by food, or in patients with decreased gastric acid output caused by atrophic gastritis.
Ambulatory reflux monitoring quantifies gastroesophageal reflux by measuring esophageal acid exposure and the number or reflux episodes, and it also enables an assessment of the temporal relationship between reflux episodes and reported symptoms. Ambulatory reflux monitoring was based upon esophageal pH monitoring for many years, a technique that relies on drops in esophageal pH to <4.0 to detect acid reflux. Accurate measurement of nonacid reflux is not possible with conventional ambulatory pH monitoring. An important development in the last decade has been the adoption of impedance-pH monitoring, a method that enables measurement of acid as well as nonacid reflux. This article discusses the principles of impedance-pH monitoring, catheter characteristics and placement, interpretation of studies, and the clinical applications of this form of reflux monitoring.
Introduction
Gastroesophageal reflux disease (GERD) is a common clinical problem. The role of acid in the pathogenesis of GERD is well established, and gastric acid suppression with proton pump inhibitors (PPIs) is the mainstay of medical therapy. The usual approach for patients presenting with classic symptoms of GERD, such as heartburn and acid regurgitation, is to prescribe empiric antisecretory therapy with a PPI. In the current era of rising GERD prevalence and frequent PPI use, there are increasing numbers of patients in whom symptoms persist despite treatment. In some of these patients, the persistent symptoms may be caused by reflux of gastric contents with a pH ≥4, commonly referred to as nonacid reflux. Nonacid reflux may be further classified as weakly acidic (pH ≥4 but <7) or weakly alkaline (pH ≥7). In this article, the term nonacid reflux is used to refer to all reflux with pH ≥4. Nonacid reflux may also occur in the absence of acid suppression during the postprandial period when gastric contents are buffered by food, or in patients with decreased gastric acid output caused by atrophic gastritis.
Ambulatory reflux monitoring quantifies gastroesophageal reflux by measuring esophageal acid exposure and the number or reflux episodes, and it also enables an assessment of the temporal relationship between reflux episodes and reported symptoms. Ambulatory reflux monitoring was based upon esophageal pH monitoring for many years, a technique that relies on drops in esophageal pH to <4.0 to detect acid reflux. Accurate measurement of nonacid reflux is not possible with conventional ambulatory pH monitoring. An important development in the last decade has been the adoption of impedance-pH monitoring, a method that enables measurement of acid as well as nonacid reflux. This article discusses the principles of impedance-pH monitoring, catheter characteristics and placement, interpretation of studies, and the clinical applications of this form of reflux monitoring.
Principles of impedance-pH monitoring
Multichannel intraluminal impedance measurement as a means to detect flow of liquids in the esophagus was first described by Silny in 1991. Intraesophageal impedance, determined by measuring electrical conductivity across a pair of closely spaced electrodes within the esophageal lumen, is dependent on the conductivity of material through which the current travels. By placing a series of conducting electrodes in a catheter that spans the length of the esophagus, changes in impedance can be recorded in response to movement of intraesophageal material in either antegrade or retrograde direction. Because different bolus materials (ie, swallowed food, air, saliva, or refluxed gastric contents) produce a different change in impedance, the technique enables detailed characterization of gastroesophageal reflux episodes, including composition (air, liquid, or mixed), proximal extent, velocity, and clearance time. During impedance-pH monitoring, impedance detects reflux (retrograde bolus movement), whereas pH changes determine its acidity: acid if pH <4, weakly acidic if pH ≥4 but <7, and weakly alkaline if pH ≥7. Examples of acid and nonacid reflux are shown in Fig. 1 . Currently, impedance-pH monitoring is considered the most accurate and sensitive tool for measuring all types of gastroesophageal reflux. The method has been found to have good reproducibility, and normal values obtained by independent multicenter studies are similar.
Impedance-pH Catheter Characteristics and Placement
Impedance-pH is performed with catheters that incorporate a differing number of impedance-measuring segments and pH electrodes in varying configurations. A typical catheter has a single pH electrode to record pH changes 5 cm above the manometrically determined lower esophageal sphincter (similar to conventional pH testing), along with multiple impedance-measuring segments (each composed of 2 metal ring electrodes, usually spaced 2 cm apart) to detect impedance changes along variable lengths of the esophagus and enable detection of reflux into the distal and proximal esophagus ( Fig. 2 ).
The methodology for catheter placement is similar to that of conventional pH monitoring. After a 4- to 6-hour fasting period, a local anesthetic is applied, and the catheter is passed transnasally to the desired position, based upon the location of the lower esophageal sphincter. The catheter is then taped to the patient’s nose and connected to an impedance-pH monitor that the patient carries on the shoulder or a belt for ambulatory recording. After catheter placement, the patient is discharged with instructions to record meals, body position, and symptoms. The patient returns for catheter removal after 24 hours.
Impedance-pH Interpretation
During assessment of impedance-pH tracings, the impedance channels are used to detect the occurrence of reflux, and pH changes help classify the reflux episodes as acid (pH <4) or nonacid (pH ≥4). The composition of the reflux episode (liquid, gas, or mixed) can be easily ascertained. Liquid-only reflux is defined by sequential drops in impedance beginning in the distal esophagus and progressing upwards toward the proximal esophagus in retrograde direction. Gas reflux is defined by sharp increases in impedance moving in retrograde direction from distal to proximal esophagus. Mixed liquid-gas reflux is defined as gas reflux occurring during or immediately before liquid reflux. Examples of liquid-only and mixed reflux are shown in Fig. 3 . In addition to acidity and composition, reflux episodes can be further characterized in terms of proximal extent and clearance time.
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