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OESO©2015
 
Volume: Primary Motility Disorders of the Esophagus
Chapter: Diffuse esophageal spasms (Corkscrew esophagus)
 

Do recording systems permit correct measurement of the true amplitude of the esophageal waves ?

G. Cargill (Paris)

The classical systems for measuring intra-esophageal pressures use measurement catheters employing two different technologies : water infusion, where the pressure transducers are placed on the outside of the patient, and micro-transducers in situ, where the small electronic receptors are situated in the sound. The collection of pressure variations differs from one system to another, and so does the analysis of the results. Various other parameters play a part in measurement and are essentially linked to the polygraphic systems for collecting the signal. It should be noted, however, that exact measurement of the pressures is impossible.

Any system of measurement, however sophisticated, can only provide an image of what is happening around the receptor, this image being determined by the capacities of the different components of the chain of data gathering and measurement ; and, as in all electronic chains, the exit signal is a direct function of the least effective component of the entire chain.

Perfused systems

These consist of a sound [1] composed of several small capillary tubes which are perfused with distilled, gas-free water at a constant flow-rate (0.5-0.7 ml/min) by means of a pneumohydraulic system of low compliance. They are reliable and easy to use and the external position of the pump makes for easy maintenance.

(In our experience over the last 5 years, 1 capillary tube was changed each year for an annual average of 2448 examinations, using a 4-path Arndorfer pump). The same applies to the receptors, which are rarely subject to breakdown [1] (in our experience no troubles in 5 years for 5 Statham P231D receptors).

This type of perfusion system has progressively replaced perfusers of plunge-syringe type because of the very variable flow in the latter, under the influence of

variations in intraesophageal pressure [2]. In fact, these systems, designed for injection of fluids at low (venous) pressures, have a flow which is a function of the counterpressure exerted at the perfusion site (figure 1) i.e. at the ends of the capillary tubes. This flow becomes zero for the values attained during physiologic

0220F1.JPG

Figure 1. Syringe perfusion system.

Fl = force generated by counterpressure associated with esophageal

motility.

F2 = perfusion pressure generated by plunger syringe.

D = outflow (function of differential pressure).

phenomena in the esophagus, such as contraction of the body of the organ on swallowing or during study of the upper sphincter. In the study of pathologic situations, where the esophagus and/or the lower sphincter exhibit extreme anomalies of tonus, this arrest of perfusion is prejudicial to correct understanding of the disorders. Moreover, because the syringes of these systems are not independent (figure 2), an elevation of pressure in one pathway decreases the perfusion flow

0220F2.JPG

Figure 2. Syringe perfusion system. If the counterpressure exerted by the esophagus on one of the pathways is considerable, the system is no longer perfused.

in the entire system. This has the corollary of disturbing pressure measurement by virtue of the compliance phenomenon.

The principle of the pneumohydraulic pump [2] is based on an autonomous flow for each measurement pathway and on the least variation of flow in relation to the esophageal motor phenomena (figure 3). The perfused water, doubly distilled and degasified, is stored in a pressurized reservoir at a pressure of the order of 15 PSI, adjustable by means of a relief valve. A pipe emerging from this reservoir

0220F3.JPG

Figure 3. Low compliance perfusion system for esophageal manometric studies.

supplies a series of capillary tubes which convey water to the different catheters of the sound. Under these conditions, the flow in each pathway is a function of the diameter and length of the capillary tubes and the difference in pressure between the pipe, i.e. the atmospheric pressure plus 15 PSI (or one atmosphere) and the distal end of the capillary tube connected to the pressure transducer. In normal conditions, the counterpressure generated by esophageal motility varies from -10 cm H20 (≈ -1 KPa) to around 100 cm H20 (≈ 10KPa), with a variation of flow of around 10 p. cent for the highest physiologic pressures observed in the esophagus. This is not the case in motor disorders of the type of diffuse spasm of the esophagus, where the pressures developed may reach much higher values. In this type of pathology, we have observed peristaltic waves with an amplitude of more than 400 cm H20 (≈ 40KPa). In these particular conditions, measurement of the pressure peaks may be impaired, especially if there are very rapid pressure variations.

Analysis of the function dp/dt as an index of muscle contractility is increasingly impaired as the pressures increase, since the flow of fluid lessens. It should be noted

that the use of such systems implies monitoring of the fluid circuit in order to expel any bubbles of air it may contain, which considerably increase compliance. This involves the use of doubly distilled and degasified water, and of a poorly soluble propellant gas : nitrogen or, even better, argon.

It should be noted, too, that, in physical terms, these two techniques do not measure a pressure but a counterpressure (figure 4), since what is being recorded in this way are the effects of contraction of the esophageal wall on flow in the catheters ; the esophageal contraction occludes the orifice of the catheter, thus increasing the pressure within it and in the measurement chamber of the transducer.

0220F4.JPG

Figure 4. Miniature pressure transducer for direct measurement.

In situ microtransducers

Unlike previous techniques, this technology measures the pressure in the esophageal cavity directly (figure 4). Measurement here is relatively directional. The wave-band passing from such microtransducers is a high one (>10KHz), which permits virtually instantaneous measurement of the pressure variations. Since variations of over 2500 mmHg/sec can be dealt with, this means that the problems of compliance encountered with perfused systems can be completely ignored with such possibilities. However, if the measurement is to approximate to reality, the scale of measurement must correspond to the values obtained in the esophagus.

While a scale of -5 KPa/+20 KPa is adequate for study of the motility of a normal esophagus, this is not the case for the study of esophageal spasm where the scale of measurement may be limited. Therefore it is necessary to make a careful choice of the range of measurement and the scale of reproduction of the signal.

Besides this limiting function of choice of microtransducer, it is to be noted that variations in measurement are sometimes observed connected with shunt from the transducer, and may be associated with variations in temperature. This last criticism is becoming increasingly inapplicable to modern generations of transducers.

The use of such technologies allows a much more proximate measurement of the true pressures obtained in the alimentary tract.

Other factors influencing measurement

Parameters other than measurement techniques influence the measurement of pressure. They include the following factors: the presence of air in the esophagus prevents direct application of the force from the esophageal wall to the transducer, which creates a certain degree of intraesophageal compliance not associated with the measurement technique but certainly at the site of measurement and its content. In certain situations where there is salivary stasis in the esophagus, this presence of air may give rise to marked derangements of measurement.

The use of frequency filters may also smooth out the pressure variations and thus serve to limit the pressure maxima, especially if the peak of maximum pressure is reached over a short period.

It should be remembered in this context that the manometric signal has a frequency of 8 to 10 Hz. Thus, to have due regard for the signal, the frequency filter of the recorder must be adjusted for at least 30 Hz (i.e. a value exceeding at least 2.5 times the value to be reproduced : Maxwell's Law).

Finally, the scale of reproduction of the signal on the polygraph must be adjusted to the signal. If the signal exceeds the scale, it will « ceiling out», which prevents correct measurement. In this sense, the development of a polygraph with super-imposable tracks is a step forward, the ideal being the ability to modify the amplification of the signal during measurement if this exceeds the scale of reproduction, and so to adjust the scale of reproduction to the effectively measured signal. Unfortunately, this modification of the scale sometimes leads to modification of the reference zero if the different parameters of the polygraph are not perfectly adjusted and calibrated. Again, the increasingly practised computerized assessment of values does not always allow exact assessment because of digitalisation that is sometimes too low (less than 2.5 times the frequency to be reproduced) and a sometimes ill-adjusted scale.

All these limitations mean that measurement of the true amplitudes of the waves in the pathology of diffuse spasm depends on a flawless technology adjusted to the type of measurement to be carried out.

Conclusions

Thus, the measurement of the true amplitude of the esophageal waves in the disorder of diffuse spasm depends on the technology employed. Measurement does not seem reliable with a perfused system of plunger syringe type. It is greatly facilitated by the use of pneumohydraulic systems of low compliance (except perhaps for waves of very high amplitude), or by means of sounds with in situ microtransducers which measure the pressure virtually instantaneously because of their extremely low intrinsic compliance.

Apart from these technologic precautions, the reproduction factors (frequency filters, signal amplification) must be adjusted to the signal measured. But the real question to be faced in this problem is surely the following : « What is the usefulness of this precise measurement of amplitude, once the measurement is close enough to reality and the diagnosis of diffuse spasm can be made ? » The answer to the question thus raised is more permissive, now that systems perfused by means of low compliance pneumohydraulic infusers permit relatively correct assessment of the motility of the body of the esophagus and of the lower sphincter, factors essential to the diagnosis of diffuse spasm.

References

1. Dalton CB (1987) The esophageal motility laboratory: materials and equipment. In: Esophageal motility testing, Castell DO, Richter JE, Dalton CB Eds, Elsevier, New York p 28-34.

2. Arndorfer RC, Stef JJ, Dodds WJ, Linehan JH, Hogan WJ (1977) Improved infusion system for intraluminal esophageal manometry Gastroenterology, 73 : 23-27


Publication date: May 1991 OESO©2015