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Multichannel Intraluminal Impedance: General Principles and Technical Issues
Gastrointest Endoscopy Clin N Am 15 (2005) 257 – 264
Multichannel Intraluminal Impedance: General Principles and Technical Issues Radu Tutuian, MD*, Donald O. Castell, MD Division of Gastroenterology/Hepatology, Medical University of South Carolina, 96 Jonathan Lucas Street, 210 CSB, Charleston, SC 29425, USA
Multichannel intraluminal impedance (MII) is a relatively new technique developed in the early 1990s at the Helmholtz Institute in Aachen, Germany. Silny [1] provided the first description of this technique that assesses intraluminal bolus movement by measuring changes in conductivity of the intraluminal content. The basic component of this method is the impedance circuit (Fig. 1). An alternating-current generator is used to apply an electrical potential difference to two metal (steel) rings separated by an isolator. Because current cannot pass through the isolator, the circuit can only be closed through electrical charges (ie, ions) contained in the structures surrounding the catheter. When surrounded by air, there is virtually no current flow between the two rings and, therefore, the impedance (ie, resistance to alternating current) measured between the electrodes is very high. When placed within the esophagus, current flow between the two metal rings is enabled by electrical charges within the esophageal mucosal, submucosal, and muscular layers. Any other material present within the esophagus produces characteristic changes due to the electrical conductivity (directly related to ionic concentration) and the cross-section (ie, the lower the cross-section, the higher the impedance). The electrical impedance, being the opposite of conductivity, decreases from air, to mucosal lining,
* Corresponding author. E-mail address: [email protected] (R. Tutuian). 1052-5157/05/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.giec.2004.10.009 giendo.theclinics.com
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Fig. 1. Schematic representation of why intraesophageal impedance changes during presence of liquid boluses. In an empty esophagus (A), only a few ions are present in the mucosa to conduct current, leading to relative high impedance. After the bolus bridges both impedance rings (B), the ions present in the liquid bolus facilitate the transmission of electrical current, thereby decreasing the resistance to alternating current (ie, impedance).
to saliva/swallowed material, to refluxed gastric contents (lowest impedance) (Fig. 2).
Bolus presence detected by impedance When liquid is present between the two impedance rings, the following changes [2] in impedance can be observed (Fig. 3A): (1) an initial drop in impedance when the liquid bolus enters the impedance-measuring segment
Fig. 2. Intraesophageal constituents arranged according to their relative impedance and conductivity.
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because this enable the flow of electrical current, (2) a rise in impedance as the bolus is cleared from this segment by a peristaltic wave, (3) an ‘‘overshoot’’ in impedance corresponding to decreased luminal cross-section during muscle contraction, and (4) a return to baseline. Current conventions consider the bolus entry point in the esophagus at the 50% drop from baseline to nadir and the bolus exit point in the esophagus at the recovery of impedance to this 50% value [2]. The presence of an air bolus (ie, belch, air swallow) inside the esophagus produces a rapid rise in impedance to high values (usually greater than 5000 V), followed by an equally rapid decrease in impedance after the air bolus clears from this segment (Fig. 3B). Although there are no strict rules defining air bolus entry and exit from a segment, they can be considered to occur at the onset of the rapid rise in impedance and the return to baseline, respectively. Impedance can also detect the presence of mixed (gas–liquid and liquid–gas) boluses. The presence of a mixed gas–liquid bolus between the two rings produces the following changes (Fig. 3C): (1) a rise in impedance produced by the presence of air in the front of the bolus, (2) a rapid drop in impedance when the liquid component of the mixed bolus enables current flow, (3) a rise in impedance as the bolus is being cleared from this segment, (4) an ‘‘overshoot’’ in impedance corresponding to decreased luminal cross-section during contraction, and (5) a return to baseline. A mixed liquid–gas bolus (Fig. 3D) produces (1) a rapid drop in impedance after the liquid component enters the impedancemeasuring segment, (2) a rapid rise in impedance after the gas component reaches the segment and an equally rapid fall in impedance to the bolus value after the gas component exits the segment, (3) a rise in impedance as the bolus is being cleared from this segment, (4) an overshoot in impedance corresponding to decreased luminal cross-section during contraction, and (5) a return to baseline.
Direction of bolus movement detected by multiple impedance sites The events described in the previous section refer to the presence of bolus at a single impedance-measuring site. Antegrade- and retrograde-moving boluses produce the same changes at a given site in the esophagus. By using multiple impedance-measuring segments (MII), the direction of bolus movement within the esophagus can be determined. Antegrade-moving boluses (ie, swallows) produce the previously mentioned changes in impedance starting proximally and progressing distally as the head of the bolus advances through the esophagus, followed by peristaltic waves that clear the esophagus (Fig. 4A). Retrograde-moving boluses (ie, reflux events) produce the characteristic impedance changes starting distally and progressing proximally as boluses move from the stomach into the esophagus. The clearance of the bolus starts proximally and progresses distally (Fig. 4B).
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Validating the ability of impedance to detect esophageal bolus movement A first validation of MII to detect bolus presence using fluoroscopy as the ‘‘gold standard’’ was reported by Silny [1] in 1991. Subsequently, Blom et al [3] reported on the correlation of changes in intraluminal impedance with bolus movement during combined videofluoroscopy and impedance studies. Most recently, Simren et al [4] validated the ability of impedance to detect bolus movement. Considering bolus entry at a 50% drop in impedance from baseline to nadir and bolus exit at the recovery of impedance to the 50% value, Simren et al [4] found a strong correlation between videofluoroscopy and impedance for measuring the time of esophageal filling (r 2 = 0.89; P b 0.0001) and emptying (r 2 = 0.79; P b 0.0001).
Clinical applications of multichannel intraluminal impedance Currently, MII is used clinically only in combination with manometry or pH. Combining MII with esophageal manometry (MII-EM) or pH (MII-pH) expands the armamentarium of diagnostic tools to evaluate esophageal function in patients with esophageal disorders and to monitor gastroesophageal reflux. During combined MII-EM, data on intraesophageal pressures and bolus movement are collected simultaneously [5,6]. Because this technique does not use radiation or radionuclide-labeled material to evaluate intraesophageal bolus transit, there is no limit on the number of swallows that can be analyzed. Data acquisition for manometry and impedance occurs at the same time and, therefore, does not require additional synchronization devices. From a patient perspective, combined MII-EM testing is similar to conventional manometry because MII rings and pressure sensors are mounted on the same catheter without changing the size of the catheter or the test sequence.
Fig. 3. Impedance changes observed during bolus transit over a single pair of measurement rings separated by 2 cm. (A) Liquid boluses produce a drop in impedance after the liquid part of the bolus reaches the impedance-measuring site, persisting as long as the bolus is present in this segment. Lumen narrowing produced by the contraction transiently increases the impedance above baseline. (B) Air (gas) produces a rapid, short rise and decline in impedance (usually over 5000 V) due to poor electrical conductivity of air. (C) Mixed (air–liquid) boluses produce a rapid rise in impedance when gas traveling in front of the bolus head reaches the impedance-measuring segment, followed by a drop in impedance after the liquid bolus material reaches the measuring site. (D) Mixed liquid–air boluses produce an initial fall in impedance after the bolus reaches the segment, a rapid rise and decline in impedance when the gas component reaches the segment, and a recovery to the preswallow baseline after the liquid bolus exits the segment. In the esophagus, liquid bolus entry is considered at the 50% drop in impedance from baseline relative to nadir and bolus exit at the 50% recovery point from nadir to baseline.
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Combined MII-pH testing represents a shift in the reflux-testing paradigm [7–9]. The presence of the gastroesophageal refluxate within the esophagus is detected by MII. MII can further separate the types of gastroesophageal reflux into liquid, gas, and mixed (gas–liquid or liquid–gas). The information obtained from the pH sensor is used to characterize the chemical composition of the refluxate (ie, acid versus nonacid) based on predefined criteria. Similar to combined MII-EM, addition of impedance rings to a pH catheter does not change its size or the performance of ambulatory monitoring, making combined MII-pH similar to conventional pH testing from a patient perspective. Augmenting esophageal manometry and ambulatory pH monitoring with the addition of MII offers important new information in patient evaluation. The MII technology should not be considered as a replacement for current manometry and pH techniques but as a complementary procedure that expands the diag-
Fig. 4. Using multiple impedance-measuring sites, MII can detect direction of bolus movement. Progression of impedance changes from proximal to distal (A) are indicative of antegrade bolus movement as observed during swallowing, whereas progression of impedance changes from distal to proximal (B) are indicative of retrograde bolus movement as observed in reflux.
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Fig. 4 (continued).
nostic potential of esophageal function testing and reflux monitoring without use of radiation.
References [1] Silny J. Intraluminal multiple electric impedance procedure for measurement of gastrointestinal motility. J Gastrointest Motil 1991;3:151 – 62. [2] Srinivasan R, Vela MF, Katz PO, et al. Esophageal function testing using multichannel intraluminal impedance. Am J Physiol Gastrointest Liver Physiol 2001;280:G457 – 62. [3] Blom D, Mason RJ, Balaji NS, et al. Esophageal bolus transport identified by simultaneous multichannel intraluminal impedance and manofluoroscopy [abstract]. Gastroenterology 2001; 120:P103. [4] Simren M, Silny J, Holloway R, et al. Relevance of ineffective oesophageal motility during oesophageal acid clearance. Gut 2003;52:784 – 90. [5] Tutuian R, Vela MF, Balaji NS, et al. Esophageal function testing using combined multichannel intraluminal impedance and manometry. Multicenter study of 43 healthy volunteers. Clin Gastroenterol Hepatol 2003;1:174 – 82. [6] Frieling T, Hermann S, Kuhlbusch R, et al. Comparison between intraluminal multiple electric impedance measurement and manometry in the human oesophagus. Neurogastroenterol Motil 1996;8:45 – 50. [7] Vela MF, Camacho-Lobato L, Srinivasan R. Intraesophageal impedance and pH measurement of acid and nonacid reflux: effect of omeprazole. Gastroenterology 2001;120:1599 – 606.
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[8] Shay S, Bomeli S, Richter J. Multichannel intraluminal impedance accurately detects fasting, recumbent reflux events and their clearing. Am J Physiol Gastrointest Liver Physiol 2002; 283:G376 – 83. [9] Sifrim D, Silny J, Holloway R, et al. Patterns of gas and liquid reflux during transient lower oesophageal sphincter relaxation: a study using intraluminal electrical impedance. Gut 1999;44:47 – 54.
Multichannel Intraluminal Impedance: General Principles and Technical Issues Radu Tutuian, MD*, Donald O. Castell, MD Division of Gastroenterology/Hepatology, Medical University of South Carolina, 96 Jonathan Lucas Street, 210 CSB, Charleston, SC 29425, USA
Multichannel intraluminal impedance (MII) is a relatively new technique developed in the early 1990s at the Helmholtz Institute in Aachen, Germany. Silny [1] provided the first description of this technique that assesses intraluminal bolus movement by measuring changes in conductivity of the intraluminal content. The basic component of this method is the impedance circuit (Fig. 1). An alternating-current generator is used to apply an electrical potential difference to two metal (steel) rings separated by an isolator. Because current cannot pass through the isolator, the circuit can only be closed through electrical charges (ie, ions) contained in the structures surrounding the catheter. When surrounded by air, there is virtually no current flow between the two rings and, therefore, the impedance (ie, resistance to alternating current) measured between the electrodes is very high. When placed within the esophagus, current flow between the two metal rings is enabled by electrical charges within the esophageal mucosal, submucosal, and muscular layers. Any other material present within the esophagus produces characteristic changes due to the electrical conductivity (directly related to ionic concentration) and the cross-section (ie, the lower the cross-section, the higher the impedance). The electrical impedance, being the opposite of conductivity, decreases from air, to mucosal lining,
* Corresponding author. E-mail address: [email protected] (R. Tutuian). 1052-5157/05/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.giec.2004.10.009 giendo.theclinics.com
258
tutuian
&
castell
Fig. 1. Schematic representation of why intraesophageal impedance changes during presence of liquid boluses. In an empty esophagus (A), only a few ions are present in the mucosa to conduct current, leading to relative high impedance. After the bolus bridges both impedance rings (B), the ions present in the liquid bolus facilitate the transmission of electrical current, thereby decreasing the resistance to alternating current (ie, impedance).
to saliva/swallowed material, to refluxed gastric contents (lowest impedance) (Fig. 2).
Bolus presence detected by impedance When liquid is present between the two impedance rings, the following changes [2] in impedance can be observed (Fig. 3A): (1) an initial drop in impedance when the liquid bolus enters the impedance-measuring segment
Fig. 2. Intraesophageal constituents arranged according to their relative impedance and conductivity.
multichannel intraluminal impedance
259
because this enable the flow of electrical current, (2) a rise in impedance as the bolus is cleared from this segment by a peristaltic wave, (3) an ‘‘overshoot’’ in impedance corresponding to decreased luminal cross-section during muscle contraction, and (4) a return to baseline. Current conventions consider the bolus entry point in the esophagus at the 50% drop from baseline to nadir and the bolus exit point in the esophagus at the recovery of impedance to this 50% value [2]. The presence of an air bolus (ie, belch, air swallow) inside the esophagus produces a rapid rise in impedance to high values (usually greater than 5000 V), followed by an equally rapid decrease in impedance after the air bolus clears from this segment (Fig. 3B). Although there are no strict rules defining air bolus entry and exit from a segment, they can be considered to occur at the onset of the rapid rise in impedance and the return to baseline, respectively. Impedance can also detect the presence of mixed (gas–liquid and liquid–gas) boluses. The presence of a mixed gas–liquid bolus between the two rings produces the following changes (Fig. 3C): (1) a rise in impedance produced by the presence of air in the front of the bolus, (2) a rapid drop in impedance when the liquid component of the mixed bolus enables current flow, (3) a rise in impedance as the bolus is being cleared from this segment, (4) an ‘‘overshoot’’ in impedance corresponding to decreased luminal cross-section during contraction, and (5) a return to baseline. A mixed liquid–gas bolus (Fig. 3D) produces (1) a rapid drop in impedance after the liquid component enters the impedancemeasuring segment, (2) a rapid rise in impedance after the gas component reaches the segment and an equally rapid fall in impedance to the bolus value after the gas component exits the segment, (3) a rise in impedance as the bolus is being cleared from this segment, (4) an overshoot in impedance corresponding to decreased luminal cross-section during contraction, and (5) a return to baseline.
Direction of bolus movement detected by multiple impedance sites The events described in the previous section refer to the presence of bolus at a single impedance-measuring site. Antegrade- and retrograde-moving boluses produce the same changes at a given site in the esophagus. By using multiple impedance-measuring segments (MII), the direction of bolus movement within the esophagus can be determined. Antegrade-moving boluses (ie, swallows) produce the previously mentioned changes in impedance starting proximally and progressing distally as the head of the bolus advances through the esophagus, followed by peristaltic waves that clear the esophagus (Fig. 4A). Retrograde-moving boluses (ie, reflux events) produce the characteristic impedance changes starting distally and progressing proximally as boluses move from the stomach into the esophagus. The clearance of the bolus starts proximally and progresses distally (Fig. 4B).
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castell
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Validating the ability of impedance to detect esophageal bolus movement A first validation of MII to detect bolus presence using fluoroscopy as the ‘‘gold standard’’ was reported by Silny [1] in 1991. Subsequently, Blom et al [3] reported on the correlation of changes in intraluminal impedance with bolus movement during combined videofluoroscopy and impedance studies. Most recently, Simren et al [4] validated the ability of impedance to detect bolus movement. Considering bolus entry at a 50% drop in impedance from baseline to nadir and bolus exit at the recovery of impedance to the 50% value, Simren et al [4] found a strong correlation between videofluoroscopy and impedance for measuring the time of esophageal filling (r 2 = 0.89; P b 0.0001) and emptying (r 2 = 0.79; P b 0.0001).
Clinical applications of multichannel intraluminal impedance Currently, MII is used clinically only in combination with manometry or pH. Combining MII with esophageal manometry (MII-EM) or pH (MII-pH) expands the armamentarium of diagnostic tools to evaluate esophageal function in patients with esophageal disorders and to monitor gastroesophageal reflux. During combined MII-EM, data on intraesophageal pressures and bolus movement are collected simultaneously [5,6]. Because this technique does not use radiation or radionuclide-labeled material to evaluate intraesophageal bolus transit, there is no limit on the number of swallows that can be analyzed. Data acquisition for manometry and impedance occurs at the same time and, therefore, does not require additional synchronization devices. From a patient perspective, combined MII-EM testing is similar to conventional manometry because MII rings and pressure sensors are mounted on the same catheter without changing the size of the catheter or the test sequence.
Fig. 3. Impedance changes observed during bolus transit over a single pair of measurement rings separated by 2 cm. (A) Liquid boluses produce a drop in impedance after the liquid part of the bolus reaches the impedance-measuring site, persisting as long as the bolus is present in this segment. Lumen narrowing produced by the contraction transiently increases the impedance above baseline. (B) Air (gas) produces a rapid, short rise and decline in impedance (usually over 5000 V) due to poor electrical conductivity of air. (C) Mixed (air–liquid) boluses produce a rapid rise in impedance when gas traveling in front of the bolus head reaches the impedance-measuring segment, followed by a drop in impedance after the liquid bolus material reaches the measuring site. (D) Mixed liquid–air boluses produce an initial fall in impedance after the bolus reaches the segment, a rapid rise and decline in impedance when the gas component reaches the segment, and a recovery to the preswallow baseline after the liquid bolus exits the segment. In the esophagus, liquid bolus entry is considered at the 50% drop in impedance from baseline relative to nadir and bolus exit at the 50% recovery point from nadir to baseline.
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tutuian
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castell
Combined MII-pH testing represents a shift in the reflux-testing paradigm [7–9]. The presence of the gastroesophageal refluxate within the esophagus is detected by MII. MII can further separate the types of gastroesophageal reflux into liquid, gas, and mixed (gas–liquid or liquid–gas). The information obtained from the pH sensor is used to characterize the chemical composition of the refluxate (ie, acid versus nonacid) based on predefined criteria. Similar to combined MII-EM, addition of impedance rings to a pH catheter does not change its size or the performance of ambulatory monitoring, making combined MII-pH similar to conventional pH testing from a patient perspective. Augmenting esophageal manometry and ambulatory pH monitoring with the addition of MII offers important new information in patient evaluation. The MII technology should not be considered as a replacement for current manometry and pH techniques but as a complementary procedure that expands the diag-
Fig. 4. Using multiple impedance-measuring sites, MII can detect direction of bolus movement. Progression of impedance changes from proximal to distal (A) are indicative of antegrade bolus movement as observed during swallowing, whereas progression of impedance changes from distal to proximal (B) are indicative of retrograde bolus movement as observed in reflux.
multichannel intraluminal impedance
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Fig. 4 (continued).
nostic potential of esophageal function testing and reflux monitoring without use of radiation.
References [1] Silny J. Intraluminal multiple electric impedance procedure for measurement of gastrointestinal motility. J Gastrointest Motil 1991;3:151 – 62. [2] Srinivasan R, Vela MF, Katz PO, et al. Esophageal function testing using multichannel intraluminal impedance. Am J Physiol Gastrointest Liver Physiol 2001;280:G457 – 62. [3] Blom D, Mason RJ, Balaji NS, et al. Esophageal bolus transport identified by simultaneous multichannel intraluminal impedance and manofluoroscopy [abstract]. Gastroenterology 2001; 120:P103. [4] Simren M, Silny J, Holloway R, et al. Relevance of ineffective oesophageal motility during oesophageal acid clearance. Gut 2003;52:784 – 90. [5] Tutuian R, Vela MF, Balaji NS, et al. Esophageal function testing using combined multichannel intraluminal impedance and manometry. Multicenter study of 43 healthy volunteers. Clin Gastroenterol Hepatol 2003;1:174 – 82. [6] Frieling T, Hermann S, Kuhlbusch R, et al. Comparison between intraluminal multiple electric impedance measurement and manometry in the human oesophagus. Neurogastroenterol Motil 1996;8:45 – 50. [7] Vela MF, Camacho-Lobato L, Srinivasan R. Intraesophageal impedance and pH measurement of acid and nonacid reflux: effect of omeprazole. Gastroenterology 2001;120:1599 – 606.
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[8] Shay S, Bomeli S, Richter J. Multichannel intraluminal impedance accurately detects fasting, recumbent reflux events and their clearing. Am J Physiol Gastrointest Liver Physiol 2002; 283:G376 – 83. [9] Sifrim D, Silny J, Holloway R, et al. Patterns of gas and liquid reflux during transient lower oesophageal sphincter relaxation: a study using intraluminal electrical impedance. Gut 1999;44:47 – 54.