Departments of 1 Medicine,
2 Gastrointestinal Medicine, and
5 Radiology, There is currently
no ideal method for concurrently assessing intraluminal pressures and
flows in humans with high temporal resolution. We have developed and
assessed the performance of a novel fiber-optic laser-Doppler
velocimeter, mounted in a multichannel manometric assembly. Velocimeter
recordings were compared with concurrent fluoroscopy and manometry
following 50 barium swallows in healthy subjects. During these
swallows, the velocimeter sensor was situated in either the proximal
(24 swallows) or the distal (26 swallows) esophagus. It signaled
intraluminal flow following 46 of 50 swallows. A greater mean number of
deflections were recorded in the distal compared with the proximal
esophagus (4.3 vs. 2.4, P = 0.001).
The maximal flow velocity recorded did not differ between the proximal
and distal esophagus (76.7 vs. 73.8 mm/s). No velocimeter signals
commenced after fluoroscopic lumen occlusion. The velocimeter signals
were closely temporally related to fluoroscopic barium flow. Upward
catheter movement on swallowing sometimes appeared to cause a
velocimeter signal. Manometrically "normal" swallows were no
different from "abnormal" swallows in the number and velocity of
deflections recorded by the velocimeter. This novel instrument measures
intraluminal flow velocity and pressures concurrently, thus enabling
direct study of pressure-flow relationships. Flow patterns differed
between the proximal and distal esophagus.
intraluminal flow; pressure-flow relationships; laser-Doppler flow
measurement
THE OVERALL ABORAL direction of flow within the gut is
the net result of individual episodes of flow that occur in both
directions (3). Relatively little is known about these patterns of flow and the patterns of luminal pressures that cause them. We are currently
able to monitor intraluminal pressures with a spatial resolution as
close as 1 mm between each of up to 21 recording sideholes. Exact
temporal resolution of these pressures is also possible due to the
capacity of computer-based recording systems for high-frequency data
acquisition. Concurrent recordings of intraluminal flow with a similar
temporal resolution are essential for determining how these individual
pressure events relate to luminal flow. Previously, it has not been
possible to monitor pressures and intraluminal flows simultaneously in
humans with a temporal resolution of <1 s, although suitable methods
for use in animals exist (8). The methods that are currently available for evaluating luminal flows in humans include radiographic contrast studies (5), Doppler ultrasonography (1), radiographic marker studies
(4), scintigraphy (11), and impedance plethysmography (12). These
techniques have substantial limitations with respect to the length of
observation that is permissible, reliability of the measurement, and/or
the spatial-temporal resolution possible.
We have recently developed a laser-Doppler velocimeter suitable for use
in the gut lumen that measures the polarity and velocity of liquid
movement, with a temporal resolution of between 4 and 7 Hz (10). This
device was embedded in a multilumen silicone rubber manometric assembly
(Dentsleeve, Wayville, SA, Australia) to allow concurrent measurement
of intraluminal pressures. The potential advantages of this device
include complete electrical insulation, high frequency detection of
flows, and the absence of radiation exposure.
In this study, we aimed to 1)
perform safety and bench testing of the instrument,
2) validate the laser-Doppler signal
in vivo as an indicator of flow by comparing the velocimeter signal with fluoroscopically observed flow in the human esophagus,
3) define more precisely the
patterns of esophageal luminal flows during swallowing, and
4) if possible, determine the
relationships between intraluminal flows and pressures.
We chose to perform these studies in the esophagus because of the ease
of triggering episodes of bolus flow predictably and the ability to
observe them fluoroscopically. We report here the analyses of
esophageal luminal pressures and flows that occurred during 50 barium
swallows recorded with concurrent video barium fluoroscopy,
laser-Doppler velocimeter, and a water-perfused multichannel manometric device.
Design of the Velocimeter System
Design and system overview.
A schematic of the equipment is illustrated in Fig.
1. Light from a helium-neon laser (633 nm)
is split into two beams, with half the beam transmitted down an optical
fiber that has its distal end embedded in a manometric assembly. The
optical fiber has an external diameter of 125 µm; however, the
transmitted light is unimodal because it is restricted to a 4 µm core
within the center of the fiber. The transmitted light enters the gut
lumen and is reflected and Doppler-shifted by the passing particles it
encounters, according to the Doppler principle. Some of this reflected
light is recaptured by a second optical fiber within the assembly,
which transmits it back to a photodiode in which the frequency of the reflected light is compared with that of the original beam.
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
Schematic of the laser-Doppler velocimeter. For detailed description,
see text.
Laser sensor tip.
Two separate sensor tips were used:
1) a simple design was used for the
bench calibration, consisting merely of the two optical fibers held at
a fixed angle (~3°) to each other with adhesive, and
2) for the concurrent manometry and
velocimetry, the optical fibers were incorporated into a multichannel
manometric assembly (Fig. 2). The fibers
were passed down the shaft of the extrusion within individual channels
until they reached the last few millimeters of their course. At that
point, they entered a common space, formed by drilling a hole
perpendicular to the side of the extrusion. An acrylic plug was molded
around the two optical fibers, which were held in correct position by a
jig. The plug with the embedded optical fibers was then glued into the
trephined hole in the manometric assembly. The optical fibers curved
toward the side of the extrusion at an angle of 30° and were angled
relative to each other (~3°) so that their light
sampling-projecting areas overlapped 1-2 mm from the side of the
assembly (see Fig. 2). This area of overlap determined the zone of
luminal contents within which velocity measurements were made. During
the molding process, the luminal ends of the fibers were made to
protrude ~1 mm from the side wall of the assembly. The tips of the
fibers were encased in a mound of epoxy glue. This mound of epoxy was
ground down with the ends of the optical fibers to optimize the focus
and overlap of the fibers' respective cones of projection. This
process was assessed visually by evaluating the fibers' cones of
projection and directly by verifying the sensor's ability to detect
flow while it was manufactured.
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Software. Information from the spectrum analyzer was processed on-line. The software's major functions were to 1) remove the residual spectral component of the incident laser light, 2) detect the Doppler shift, and 3) calculate the mean velocity.
Briefly, the software acquired in burst the full spectrum supplied by the analog spectrum analyzer, processed all the information comprised in each data set, and, finally, sent the computed velocity to a digital-to-analog converter. Each sweep of the spectrum analyzer, which took 5 ms, was digitized at 5 kHz with a spectral resolution of 10 Hz. This set of data was then subtracted, point by point, from a similar set acquired with zero velocity (null template). Because, in the absence of fluid motion, the only spectral component is the residual spectral component of the incident laser light, the digital subtraction resulted in the removal of this residual spectral component present in the original signal. A shift in the center frequency of the residual spectral component was corrected for by recalculation of the null template every 120 s. The relevance of this repeated subtraction procedure becomes apparent when the relatively infrequent occurrence of intraluminal flow is compared with no flow. At the pyloric level, for example, flow episodes last for only a brief period of each contractile cycle (9). Without this continual correction of the null signal, it would not be possible to distinguish flow episodes from shift in the center frequency over time. When the signal had been processed to the point at which the only frequency component present was the Doppler-shifted frequency, the software identified the position of the maximal and minimal values of the signal. A linear regression was constructed using all the data points present between these two extremes; the midvalue of this regression line represented the actual mean velocity. Once converted from hertz to meters per second, this value was presented to the digital to analog converter and a new acquisition from the spectrum analyzer commenced.Bench Testing: Turntable Validation
A circular disk with three circumferential grooves cut into it at varying distances from the center was mounted on a turntable and rotated at set speeds, generating movements of known velocities in liquid (dilute milk or lipid emulsion) within the grooves relative to the fixed laser sensor. The number of revolutions of the disk was verified by marking a point on its perimeter and counting revolutions within a fixed time interval. The velocity of the movement of the liquid in each groove was thus calculated. The laser sensor tip was held static in a jig and submerged in the center of the liquid, while the disk was rotated, and the laser-Doppler velocimeter signal was recorded. This recorded velocity was then compared with the calculated velocity of the liquid. Measurements were made after the fluid had been rotated for 1-2 min so that the velocity of the liquid within the groove was equal to that of the groove itself.Safety Assessments
Calculations. The energy output of the laser is 15 mW (in free space); however, due to the beam splitter and coupling losses, the maximum power that can be emitted at the fiber tip is 1.5 mW (given the beam diameter of 4 µm). With regard to eye safety, the beat was focused at a distance of 100 mm from the eye (the beam cannot be focused any closer); at this distance, the beam diameter spreads out to 15 mm and the intensity of energy falls well within the limit of 25 W/m2 for a class 3A laser product (13). With regard to the likelihood of tissue burn, the maximum permitted exposure, under this standard, for a duration of 10 s or more is 2 kW/m2. If direct (continuous) tissue contact with the sensor tip is assumed, the velocimeter delivers energy with a calculated intensity of 120 kW/m2. However, at a distance of only 3.25 mm in air or 6 mm in water from the sensor tip, the intensity falls under the 2 kW/m2 limit. It should be emphasized that two other factors mitigate against the likelihood of tissue burn with prolonged use of the velocimeter: 1) the known ability of tissues and local blood flow to act as a heat sink and 2) the narrowness of the 4-µm beam, which makes it highly unlikely that it will remain in direct contact with the exact same patch of tissue for any substantial period of time, in a living organism, given the mobility of tissue.
Direct tissue assessment. Because of the theoretical possibility of tissue burns resulting from use of the velocimeter, the process was evaluated in vivo in anesthetized rats, according to a protocol approved by the Animal Ethics Committee of the Institute of Medical and Veterinary Science (Adelaide). The sensor tip was inserted via a gastrotomy and placed in contact with the proximal intestinal mucosa for varying periods (30, 60, and 120 min). Between one and three observations were made per animal (four animals were used). The sensor was held stationary by use of a jig, and its position could be clearly recognized by the light visible through the intestinal wall. The position of the light was marked by a suture on the serosal surface. Three animals were killed immediately following the experiment, yielding two observations for each time period. A further animal was allowed to recover for 3 days and then killed. Tissue was examined macroscopically, and three circumferential sections at 2- to 5-mm intervals were taken at each level marked by the sutures. Sections were fixed and stained with hematoxylin and eosin before light microscopy, and histological examination was performed by a qualified pathologist.
Human Study
Subjects. Eight healthy subjects (6 males, 2 females) from 19 to 39 yr old were recruited by advertisement. The subjects had no upper gastrointestinal symptoms and were not taking any regular medication.
Protocol. The study was approved by the Human Research Ethics Committee of the Royal Adelaide Hospital, and all subjects gave written informed consent.
The subjects reported to the Radiology Department after a 6-h fast. The recording assembly was introduced transnasally after local anesthetic spray had been applied. The volunteers then stood upright in front of the fluoroscopy tube, and the velocimeter sensor was positioned according to the pressure patterns recorded by the array of sideholes that straddled it. Initially, the sensor was positioned ~5 cm above the lower esophageal sphincter and later withdrawn to lie ~5 cm below the upper esophageal sphincter. With the assembly in each of these two positions, the volunteers swallowed 15 ml of half-strength liquid barium (Polibar, E-Z-EM, New York, NY) on command and had up to a maximum of 5 swallows per site. During each swallow, concurrent recordings of video fluoroscopy, velocimetry, and manometry were made. Radiation exposure was strictly monitored and limited to a total of 70 s. Between barium swallows, volunteers drank water to clear the sensor and esophagus of residual contrasts.Manometry. The recording assembly was a 23-lumen silicone rubber extrusion, with an external diameter of 4.2 mm (Dentsleeve, Wayville, SA, Australia). Two lumens were used to carry the transmitting and receiving optical fibers that led back up to the light source and fiber coupler, respectively (Fig. 2). A chain of 20 sideholes at 1.5-cm intervals was used for recording pressures via 0.4-mm-diameter lumens. The laser-Doppler sensor tip was installed 7.5 cm proximal to the most distal manometric sidehole. Manometric channels were each perfused at 0.15 ml/min, giving pressure rise rates of at least 160 mmHg/s. Data were acquired at 50 Hz in a custom-written program (HAD, G. S. Hebbard, Dept. of Gastrointestinal Medicine, Royal Adelaide Hospital) in Labview 3.1.1 (National Instruments) and were averaged to 10 Hz, digitized, and logged on the master computer (Power Macintosh 7100/80).
Velocimetry. Velocimetry measurements were made at 4-7 Hz and logged concurrently with the manometric recordings on the master computer.
Radiology. All views were appropriately coned and shielded. The level of the laser sensor tip was rendered radiologically opaque by tantalum wires inserted within channels not being used for manometry at that level. A lateral oblique projection was used for swallows with the probe in the lower esophagus, which aimed to include the lower esophageal sphincter and diaphragmatic hiatus. When the sensor was in the upper esophagus, a slightly oblique posteroanterior projection was used, with the neck and thoracic inlet in view. The swallows were recorded directly to videotape at 30 frames/s. No still frames were used.
Synchronization. A timing device (TD-100-S, Provideo Systems, Adelaide, Australia) generated and simultaneously sent a number, at 10 Hz, to both the video tape (fluoroscopy) and the master computer (which logged both manometry and velocimetry data). This numerical code was then visible on review of both the video and combined manometric and velocity data, enabling corresponding events to be correlated to within 0.1 s.
Data definitions and analysis. The recordings of pressures and velocities were translated into AcqKnowledge (Biopac, Goleta, CA) for display and analysis. Video fluoroscopic images of the barium flow patterns were analyzed separately from the manometric and velocity data. Two observers each independently recorded the timing of 12 items for each swallow (4 from fluoroscopy, 5 from velocimetry, and 3 from manometry).
The fluoroscopic items scored were as follows: 1) initial upward movement of the velocimeter sensor on swallowing (taken as the swallow reference time), 2) first appearance of contrast at the level of the velocimeter sensor, 3) departure of the trailing edge of the column of barium from the velocimeter sensor, and 4) lumen occlusion at the level of the velocimeter sensor. The items scored during velocimetry were the onsets of 1) the initial and 2) the major velocimeter signals associated with each swallow, 3) the duration of the major signal, 4) the peak velocity of the major signal, and 5) the number of signals associated with each swallow. By virtue of the signal processing on-line, the velocimeter signal in the absence of flow was steady at 0 m/s; deflections were thus easily recognized and were defined as a clear departure of the velocimeter signal from the baseline by 10 m/s or more for 0.5 s or longer. The major signal was defined as the deflection during which the greatest velocity was measured by the velocimeter, and, if the same peak velocity was recorded in more than one deflection for a given swallow, the deflection with the longest duration was judged to be the major signal. The items scored during manometry were 1) the onset of the swallow-induced pharyngeal pressure wave, 2) the onset of the esophageal body common cavity pressure rise that occurs with entry of liquid boluses into the esophagus before the onset of the peristaltic pressure wave, and 3) the onset of the major upstroke of the esophageal body peristaltic pressure wave at the level of the velocimeter sensor. Mean velocities and numbers of deflections were compared using an unpaired Z-test for comparisons of means. Differences in proportions were assessed with a 2 × 2 contingency table and calculation of ![]() |
RESULTS |
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Bench Testing
Turntable validation.
A linear relationship was found between the known velocity of liquid on
the turntable and the measured velocity with the laser-Doppler velocimeter for bidirectional fluid movements (Fig.
3).
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Tissue safety assessment. No evidence of tissue damage or perforation could be identified macroscopically or on serial histological sections.
Human Study
The protocol was well tolerated by all subjects, and no adverse effects were detected. Fifty swallows were recorded with concurrent velocimetry, fluoroscopy, and manometry in the eight subjects, 24 with the sensor situated in the proximal esophagus and 26 with it in the distal esophagus. Each subject contributed between 5 and 10 swallows, from 2 to 5 with the sensor in the proximal and 3 to 5 with the sensor in the distal esophagus.There was excellent interobserver agreement in the assignment of times to the fluoroscopic, velocimetric, and manometric variables, with 89% concurrence to within ±0.2 s and 96% concurrence to within ±0.4 s.
Figure 4 shows the typical combined
manometric and velocimeter recordings from swallows with the sensor in
the two sites. The velocimeter gave a signal between the time of
initiation of a swallow and the time of lumen occlusion at the level of
the sensor in 46 of the 50 swallows (92% for both proximal and distal sensor sites). In the remaining four swallows (3 subjects), there was
no velocimeter signal (2 each with the sensor in the proximal and
distal esophagus). In two of these subjects, the velocimeter subsequently registered a signal after the subjects drank 50-200 ml of water. The third subject was extubated directly following the
swallow, which did not register a velocimeter signal, as his X-ray
exposure time did not allow further evaluation and the sensor was found
to be heavily coated with barium.
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The velocimeter signal had between 1 and 13 deflections during each swallow, with a greater number of deflections when the sensor was located in the distal compared with the proximal esophagus (4.3 vs. 2.4, P = 0.001). No velocimeter flow signal had its onset after fluoroscopic lumen occlusion or after the onset of the major upstroke of the peristaltic pressure wave at the level of the sensor. Maximal flow rates recorded with the velocimeter did not differ between the proximal and distal esophagus (76.7 vs. 73.8 mm/s, P = 0.4).
Comparisons between fluoroscopy and velocimetry.
In half the swallows available for examination (22/44), the initial
velocimeter signal commenced within ±0.2 s of the upward movement
of the assembly, before barium arrival at the sensor level (12 proximal, 10 distal). In the other half, the initial velocimeter signal
occurred when contrast was at the level of the sensor (8 proximal, 14 distal). The relationship of the initial velocimeter signal to barium
arrival at the sensor level could not be determined in two swallows
because the fluoroscopic recording commenced after barium had arrived
at the level of the sensor. Figure 5 gives
a more detailed depiction of the temporal relationship between barium
arrival at the level of the sensor and the initial velocimeter signal.
The relationship between initial upward movement of the sensor and the
initial velocimeter signal is shown in Fig. 6. In the proximal esophagus, there was a
greater proportion of swallows in which the timings of the initial
velocimeter signal and barium arrival at the level of the sensor were
closely temporally associated (±0.5 s) compared with the distal
esophagus, although this did not quite reach statistical significance
(81 vs. 54%, P = 0.057).
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Comparisons between manometry and velocimetry. The initial velocimeter signal had an onset within ±0.5 s of the onset of the esophageal common cavity pressure rise at the level of the velocimeter sensor in 28 of 46 swallows (16 proximal, 12 distal). The onset of the major velocimeter signal occurred during or within ±0.5 s of the onset of the esophageal body common cavity pressure rise at the level of the velocimeter sensor in 36 of 46 swallows (16 proximal, 20 distal). No velocimeter signals had their onset after the onset of the major upstroke of the esophageal peristaltic pressure wave at the level of the sensor.
When the manometric recordings of esophageal body peristalsis were classified as normal (n = 27) or abnormal (n = 19) by established criteria (7), there was no difference in the number of velocimeter deflections between the normal (3.22 deflections) and the abnormal (3.58 deflections) swallows. Even when divided by sensor site, no difference in number of deflections was shown between the normal (2.62 proximal vs. 3.79 distal deflections) and abnormal (2.11 proximal vs. 4.9 distal deflections) swallows (P > 0.05 for all). There was also no difference in the maximal velocity recorded during manometrically normal (69 mm/s for proximal, 55.9 mm/s for distal) and abnormal (80.1 mm/s for proximal, 63.4 mm/s for distal) swallows (P > 0.05 for both). ![]() |
DISCUSSION |
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The main aim in developing the instrument described in this report was to be able to recognize individual episodes of luminal flow in the human gut, a capability that is not adequately provided by any established method. The development of a more complete understanding of the relationships between flow pulses and spatiotemporal patterns of intraluminal pressures depends on being able to recognize and monitor luminal flow episodes with high time resolution. In the present study, this laser-Doppler device has been shown to be safe and well tolerated and to be able to accurately detect flow with a temporal resolution of <1 s. The in vivo study provides strong direct evidence of the ability of the device to recognize intraluminal flow episodes under expected conditions of use, with a high detection rate (92%) for barium flow in the esophagus compared with fluoroscopy, and, moreover, differences in flow patterns between the proximal and distal esophagus that were consistent with those seen on fluoroscopy were detected.
For the initial testing in humans of the laser-Doppler velocimeter, we chose to use the esophagus because we could reliably trigger and image movements of luminal contents fluoroscopically within acceptable radiation limits. Along with its ability to detect flow, the velocimeter also revealed apparent differences in flow patterns between the proximal and distal esophagus. In particular, the flow pattern signaled by the velocimeter is more simple in the proximal esophagus, with fewer flow pulses (deflections) per swallow. Fluoroscopic observation of the barium movement showed good agreement with the velocimeter recording, with straight-forward, unidirectional flow in the proximal esophagus occurring soon after swallowing. Our observations of barium flow in the distal esophagus are consistent with those of Biancani and Behar (2) who describe liquid flow occurring in two distinct phases: 1) during filling against a closed lower esophageal sphincter and 2) during emptying of the esophagus after the opening of the sphincter. Moreover, there is a variable time interval, from swallow to swallow (and between subjects), between the filling and emptying flow phases in the distal esophagus, during which to and fro movement of the column of barium can sometimes be seen. These fluoroscopically observed flow patterns in the distal esophagus are likely to account for the greater number of velocimeter signal deflections that occurred over a longer time interval in the distal esophagus.
In a number of swallows (22 of 44), the onset of the initial velocimeter signal appeared to precede fluoroscopic flow, particularly when the sensor was in the proximal esophagus. On fluoroscopy, these "premature" flow signals were closely related to the rapid upward movement of the recording assembly on swallowing, although timing uncertainty (see below) precludes a definite conclusion. Initial, rapid upward movement (shortening) of the pharynx and esophagus occurs on swallowing (6), and, based on our observations, rapid upward movement of the recording assembly also occurs, although these two upward movements (of the probe and the esophagus) are not necessarily synchronous or equal, as the assembly is not fixed to the esophagus. Because the velocimeter simply records movement of particles relative to the sensor, movement of either the recording assembly or the mucosa or an unequal movement between them will generate a signal. The velocity of the upward movement of the sensor seen fluoroscopically is certainly sufficient to fall within the resolution of measurement of the velocimeter (>30 mm/s). Formal evaluation under standardized conditions is warranted to clarify the issue of signals arising from relative movement between sensor and mucosa.
The differences between the two methods used to sense flow in this study are important, as they have influenced the data obtained. Fluoroscopy gives an indication of volumes flowing and the movement of the leading and trailing edges of a bolus, whereas the velocimeter gives information on timing and velocity of flow in the immediate region of the sensor but does not signal volume. Fluoroscopy of barium swallows, by outlining the mucosa, may also allow lumen occlusion to be visualized if sufficient air contrast is present. Fluoroscopy is limited by the need for visual interpretation and the density of barium used. In this study, the barium used was half strength (to reduce sensor clogging), increasing the likelihood that the temporal resolution of the fluoroscopy was decreased. In contrast, interpretation of the laser-Doppler velocimeter's recording is less subjective. When these differences are considered, the substantial concurrence of flow detected by the two methods in this study strongly supports the velocimeter's performance.
The analysis of our in vivo data required somewhat subjective definitions of events. To counter this, the process was highly structured, with two observers independently evaluating the data sets. The high concordance between the analyses of the two observers strongly supports the validity of this approach.
We found no overall difference between manometrically "normal" and "abnormal" swallows in maximum velocity or number of flow episodes recorded by the velocimeter, although there was a wide range of velocities and flow patterns recorded, attesting to the fact that flow patterns vary from one swallow to the next and between individuals. It was somewhat surprising that maximal flow velocities, as measured by the velocimeter, were no different between proximal and distal sites; when the passage of contrast radiologically was viewed, the rate of advance of the bolus front appeared to be faster in the proximal esophagus. It is likely that the shorter time from swallow to contrast appearance at the sensor level (0.2-0.9 s proximally vs. 0.4-2.5 s distally) influenced our visual judgment of this. The lack of difference in velocities between the two sites may also partially relate to the differences between fluoroscopic and velocimetric measurement of flow (see above).
From our data, by both velocimetric and fluoroscopic assessment, esophageal luminal flow was largely completed before the arrival of the peristaltic pressure wave at any given point. This is consistent with previous observations of liquid flow in the upright position (2), as gravity accounts for a large component of the flow. The spatiotemporal patterning of pressures in the esophagus is, however, important in transport of solids regardless of posture. The causal relationship between pressures and flows is likely to be better understood when regions in which gravity plays a lesser role are evaluated, such as in the pylorus and proximal small intestine. At the time of lumen occlusion judged fluoroscopically, some fluid movement was still being signaled by the velocimeter, although no velocimeter signals commenced after this time. This is likely because of flow in the thin layer of liquid left coating the mucosa as it is squeezed during lumen occlusion; if so, this is real flow, although of a small volume, emphasizing the different nature of flow monitoring between the two techniques as discussed earlier.
The characteristics of the instrumentation used unfortunately resulted in some timing uncertainty. The manometric data were recorded to disk at 10 Hz, while the velocimeter gave measurements at 4-7 Hz due to limitations of the components processing the signal. Consequently, there was an unavoidable mismatch of the two signals that reduced the temporal resolution. In addition, the internal processing of the velocimeter signal, from measurement of the Doppler shift to logging with the manometry, created a time lag between the velocimeter signal and the manometric data. Compensation for this time delay was complicated by the fact that it was not constant but varied slightly from one swallow to the next (between ~0.3 and 0.6 s). Because it was not possible to retrospectively define this delay precisely for each swallow, an average delay of 0.4 s was taken from 10 standardized (video recorded) movements performed in vitro before analysis of the data. This figure was then used as a best estimate to correct for these uncertainties in temporal resolution between the velocimetric and manometric recordings. From these considerations, events within ±0.2 s of each other may in fact be simultaneous. This uncertainty in timing can only be resolved by modifying the system so that the velocimeter measures at the same frequency as the manometric device (10 Hz) and eliminating or standardizing the internal delay in signal processing. These technical aspects are currently being addressed in further development of the instrument.
If the box containing the spectrum analyzer was bumped or exposed to radio frequency-emitting devices, the velocimeter system used in this study was noted to give signals when no flow was occurring at the sensor. These artifactual signals demanded special care to be taken during use of the device. This limitation could be addressed by measures that will increase stability and shielding from external electromagnetic signals. The clogging of the sensor with barium is more of a nuisance than a serious technical problem; use of other contrast strategies during instrument development could help to avoid this problem. When the velocimeter is sufficiently validated to be used alone, a number of other solutions that are less likely to coat the sensor, such as dilute milk and lipid emulsions, are suitable alternatives, as the only absolute requirement is the presence of particles in the liquid to reflect the laser beam.
The novel instrument described here is capable of making a significant contribution to an improved understanding of human gastrointestinal mechanics; it is safe, well tolerated, and performs intraluminal flow measurements with a temporal resolution of better than 1 s. The initial experience described in this report indicates the need for enhancement of some aspects of its function. Once these enhancements have been achieved, the velocimeter may enable us to monitor and thus evaluate pressure-flow relationships in the human upper gut.
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ACKNOWLEDGEMENTS |
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This work was supported by project grant funds from the National Health and Medical Research Council (NHMRC) of Australia, the Special Purposes Fund at the Royal Adelaide Hospital, and Astra (Sweden). J. M. Andrews was in receipt of an NHMRC Medical Postgraduate Scholarship. C. K. Rayner was in receipt of a Dawes Scholarship from the Royal Adelaide Hospital.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: J. M. Andrews, Dept. of Medicine, Royal Adelaide Hospital, North Terrace, Adelaide, SA 5000, Australia (E-mail: jandrews{at}medicine.adelaide.edu.au).
Received 25 August 1998; accepted in final form 5 December 1998.
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