Is manometric pressure likely to assess esophageal propulsive force ?
C.O.H. Russel, N. Bright, G. Whelan (Melbourne)
In patients where a peristaltic wave has been demonstrated within the esophagus in response to at least 90 p. cent of swallows the characteristics of that pressure wave may provide further quantitative information regarding esophageal function.
Absolute values of pressure velocity and duration of the wave can be assessed at manometry. Normal function, however, occurs within a wide range of these values  and abnormal esophageal function can be demonstrated in patients who have apparently normal manometric pressure recordings [2, 3].
In an attempt to assess the value of absolute measurements of manometric pressure amplitude, we have developed a method of measuring propulsive force (PF) within the esophagus and ascertained the correlation between PF and manometric pressure (MP), measured simultaneously in normal subjects.
This study was performed using a solid capsule 9 mm in diameter by 14 mm long containing a linear strain gauge formed of a silastic loop filled with saline.
A manometric port at the upper border of the capsule allowed simultaneous measurement of manometric pressure (figure 1). This assembly was connected by manometric tubing and electrical wires to an alternating current impedance monitor and three Hewlett Packard Pressure transducers. An alternating current generated by the impedance monitor, which has a maximum output of 65 microamps at 6000 Hz and 1 volt was passed through the saline filled silastic loop.
Figure 1. Diagram of linear strain gauge transducer (see text for functional description).
The gauge was calibrated in a warming oven at 37°C by suspending known weights from the vertically supported gauge and recording the output of the impedance monitor at a series of suitably chosen gain settings. A series of calibration curves for weight ranges from 0-15 gm to 0-200 gm were constructed (figure 2).
Manometric recordings were made in 30 normal volunteers at the level of the capsule and at 4 and 8 cm above the capsule using a standard low compliance perfused catheter system which is an integral part of the catheter assembly. In 10 of the subjects all force and pressure recordings were recorded and stored for later analysis on a Macintosh SE computer via a Maclab digital analogue converter. In all other subjects all recordings were made on heat sensitive paper using a Hewlett Packard chart recorder.
Thirty normal, asymptomatic volunteers aged 23 to 38 were studied. Each subject swallowed the capsule assembly into the stomach where it was allowed to equilibrate for at least ten minutes before being pulled back through the lower esophageal sphincter, identified using a standard pull through technique. The capsule was then
Figure 2. Calibration curve for linear strain gauge transducer over a range of 0 —> 100 gms.
positioned 5 cm above the sphincter and allowed to stabilise for a further ten minutes. The response to 5 X dry swallows, 5 X 5 ml swallows and 5 X 10 ml swallows was measured. The capsule was then positioned at 10 and 15 cm above the sphincter and the response to the same swallows recorded. In 10 subjects the response to 5 X 15 ml swallows was also recorded at all sites.
The recordings (figure 3) were analysed using the Maclab programme and by standard measurement techniques from paper recordings. The peak manometric pressure, as measured at the level of the capsule and the peak propulsive force were ascertained. The duration of the pressure and force waves from the start of the sharp upstroke to the end of the sharp downstroke.
The relationship between peak PF and peak MP was assessed by Spearman's rank correlation coefficient. The coefficient of variation was used to assess the variability.
Figure 3 shows a simultaneous recording of MP and PF. MP was greatest in the distal esophagus and in response to wet swallows (vs. dry). Increasing bolus volume had little effect on MP (figure 4).
PF was also greatest in the distal esophagus and increased with bolus volume, especially in the lower esophagus (figure 5).
There was no strong correlation between MP and PF at any level or in response to any bolus volume.
This study indicates that the PF and MP waves have a simultaneous onset. The poor correlation between values indicates that MP does not predict PF.
Figure 3. Typical recording as seen on computer screen. The upper tracing is manometry pressure as recorded from the capsule and the lower channel is propulsive force.
Figure 4. Mean manometric pressure values ± SEM from all volunteers at all sites and for all bolus volumes.
Figure 5. Mean propulsive force values ± SEM from all volunteers at all sites and for all bolus volumes.