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Keywords:

  • barostat;
  • rectal compliance;
  • rectum;
  • visceral sensitivity

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

The barostat is a device that maintains a constant pressure within an air-filled polyethylene bag by means of a feedback mechanism. The system measures variations in rectal tone by recording changes in the intrarectal pressure and volume. Different procedures, such as ramp distension or intermittent distension, are used to test visceral sensitivity and rectal wall compliance. It is not quite clear which method is preferable and how the barostat measurements compare with those of the conventional latex balloon. In 28 healthy volunteers (11 males, mean age 36, range 22–67 years) rectal distension was performed in two ways:

1  Pressure-controlled distension, by both intermittent and ramp methods, with measurement on the Visual Analogue Scale (VAS, 0–5) at 8, 12, 16, 20, 24, 28, 32 and 36 mmHg. Hysteresis (comparing area under the curve during deflation and inflation with ramp pressure distension) and compliance were calculated.

2  Volume-controlled distension, with registration of first sensation, urge to defecate and maximal tolerated distension. This procedure was compared to conventional water-filled latex balloon distension.

No differences were found between intermittent and ramp distension comparing VAS scores at the same pressures. Gender or age did not affect the VAS score. Males had larger volumes at the same pressures than females. Females had larger hysteresis than males. Older females had larger hysteresis than younger females. The pressure volume curves were S-shaped. Compliance at maximal tolerated distension (V/p) and maximal dynamic compliance (ΔV/Δp) was higher in males than females. The polyethylene bag had higher MTV and MTP compared to the latex balloon. In conclusion, no differences were found in volumes, compliance or VAS between the intermittent and the ramp pressure-controlled inflation, indicating potential for simplification of the procedure. Males had larger rectal volumes and compliances; females had more pronounced hysteresis. A systemic difference was found between distension with the water-filled latex balloon and with the air-filled polyethylene bag. This should be taken into account when interpreting results.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

Interest in visceral sensitivity of the rectum has increased because sensitivity can be altered in different conditions like irritable bowel syndrome,1, 2 constipation3 and faecal incontinence.4 Drugs,5, 6 a meal7 or hyperglycaemia8 can influence visceral sensitivity. Several methods have been developed to measure rectal sensitivity. Rectal visceral sensitivity can be tested by using an air- or water-filled latex balloon; high-compliance polyethylene bags in conjunction with a barostat; rectal endosonography;9 or impedance planimetry.10 The high-compliance barostat is increasingly used because of its presumed superiority in measuring visceral sensitivity.

The barostat system maintains a constant pressure within an air-filled bag by means of a feedback mechanism. The system measures variations in rectal tone by recording changes in the intrarectal pressure and volume.11 Several studies have shown the reproducibility of distension studies with intermittent and ramp distensions in normal volunteers and IBS patients.6, 12[13]–14

Different procedures, like ramp distension or intermittent distension, have been used to test visceral sensitivity and rectal wall compliance.15 So far, no method has been shown to be preferable and whether these methods are comparable or measure different aspects of rectal sensitivity is not clear. Also, the effect of age and sex has not been studied. Therefore, this study aimed to measure visceral sensitivity in healthy males and females of different ages using the two procedures (ramp and intermittent distension) with the barostat/polyethylene bag apparatus, and compare this method with the conventional latex balloon.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

Healthy volunteers

Twenty-eight healthy volunteers (11 males, 17 females; mean age 36 years, range 26–66; body mass index 23 kg m2; range 17–28) were recruited. They were divided in two age groups (younger: 27 years, range 23–30; older: 42 years, range 30–66). Subjects presented to the unit after an overnight fast and received an enema. After a digital examination, the barostat bag was inserted in the rectum. The bag was unfolded with a pressure of 20 mmHg. The subjects were positioned on their backs, in 15° decline.15

Barostat

An electronic barostat device (Synectics Visceral stimulator; Synectics Medical, Stockholm, Sweden) was used. A flaccid polyethylene bag (maximal capacity 600 mL) was mounted on a catheter (diameter 5 mm) and fixated at both ends. After inflation, the bag became cylindrical with a length of 10 cm. The catheter was connected with the barostat with an inflation port and a pressure port. Maximal airflow was 38 mL per second. With a feedback mechanism, the barostat can regulate pressure or volume in the bag. Inflation can be accomplished by pressure or volume regulated distension. Pressure and volume were continuously registered. Procedures were stopped if the safety value of the maximal volume of 600 mL or the pressure of 50 mmHg was exceeded or if the subject was unable to support the distension.

Procedures

Intermittent pressure distension

Intermittent rectal distension in a semi random staircase manner was performed at the pressures 8, 12, 20, 16, 32, 24, 36 and 28 mmHg. Pressure distension was continued one minute and followed by one-minute rest. After 30 s distension, the visual analogue scale (VAS) was scored. VAS score range varies from 0 to 5 (0, no feeling; 1, light sensation; 2, definite sensation (beginning of urge to defecate); 3, normal urge to defecate; 4, strong urge to defecate; and 5, maximal toleration of distension).

Continuous pressure distension

Continuous rectal distension was performed by ramp pressure-controlled inflation up to 36 mmHg over 10 min. The VAS score was registered at pressures 8, 12, 16, 20, 24, 28, 32 and 36 mmHg. Next, deflation was performed over 10 min to register hysteresis. Hysteresis was defined as the relative difference between the area under the pressure volume curve (AUC) in the descending and ascending part shown by the equation:16

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Minimal distension pressure, defined as the pressure that was needed to inflate the rectal bag to 10 mL, was registered. Dynamic compliance was calculated as the increase in volume at a pressure step of 4 mmHg (ΔVp).

Continuous volume distension

Continuous rectal distension was performed by volume-controlled inflation at a rate of 30 mL min−1 until the maximal tolerated distension was reached. ‘First sensation’ volume and pressure (FSV, FSP), ‘urge to defecate’ volume and pressure (UV, UP) and ‘maximal toleration’ volume and pressure (MTV, MTP) were registered. Static compliance was measured at maximal tolerated distension (V/p).

Wall tension was calculated as pressure multiplied by the radius of the bowel (Laplace’s law), where the rectum was assumed to be a cylinder.6, 13, 17, 18

Latex balloon technique

A catheter of external diameter 2 mm with a 6-cm latex balloon fixed to the end was used. After filling, the balloon had a spherical shape. A microtip transducer (Hellige type) at the end of the catheter (inside the balloon) was connected with the measuring system (UD-2000; Medical Measurements Systems, Enschede, the Netherlands). The balloon was filled with water by a pump (50 mL min−1). Volume and pressure were continuously registered. First, the intrinsic compliance of the balloon was measured outside the subject. Next, the catheter was introduced into the subject’s rectum and the procedure repeated. FSV, FSP, UV, UP, MTV and MTP were registered. Compliance was calculated at MTV and MTP. Corrections for the intrinsic balloon compliance were made.

Data and statistical analysis

Results are presented as means with 95% confidence interval. Data were analysed in gender groups and in age groups with the 30-year cut-off point (younger and older). Volume pressure curves and VAS-pressure curves were compared using the ANOVA method for repeated measurements in a quadratic model. The unpaired Student’s t-test for normal log distribution was used to compare hysteresis and compliance between gender and age groups. The VAS scores were correlated with pressure, volume and wall tension by Pearson’s correlation (R, correlation coefficient). The latex balloon distension was compared to the polyethylene bag distension by Pearson’s correlation and the method described by Bland and Altmann.19

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

Intermittent and continuous distension showed no differences in VAS scores and volume pressure curves ( Fig. 1). Males had larger volumes at the same pressures than females (P=0.02). VAS-scores were equal at the same pressures in males and females ( Fig. 2). No significant difference was found between the age groups in VAS and volume/pressure ( Fig. 3).

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Figure 1.  Volume-pressure curve with intermittent and continuous distension (mean and standard deviation).

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Figure 2.  Volume-pressure and VAS-pressure curve in ramp distension. 95% confidence interval in males (▮) and females (▴).

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Figure 3.  Volume-pressure and VAS-pressure curve with ramp distension. 95% confidence interval for younger group (▴) and older group (▮).

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Hysteresis

Hysteresis was different between males and females (32%, CI 25–38 vs. 43%, CI 35–52; P=0.05) ( Fig. 4). Hysteresis was different between the younger and the older group in females (34%, 26–42 vs. 48%, CI 36–61; P=0.037). No relationship was found between hysteresis and maximal tolerated volume or pressure.

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Figure 4.  Volume-pressure curve (mean and SEM) during inflation and deflation parts of the continue pressure distension showing hysteresis in males (▮) and females (▴). The difference between the slope of the inflation curve (arrow up) and the deflation curve (arrow down) is shown.

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Compliance

All subjects had a S-shaped pressure volume curve. Dynamic compliance was calculated at a mean pressure of 15.4 mmHg (CI 14.1–16.7). Dynamic compliance was higher in males (32 mL mmHg−1, CI 25–39) than females (24 mL mmHg−1, CI 9–28; P=0.03). Static compliance at maximal tolerated distension was also higher in males (10.3 mL mmHg−1, CI 8.5–12.1) than in females (8.1 mL mmHg−1, CI 6.8–9.5) (P=0.045). There were no differences between dynamic and static compliance for the younger group (30 mL mmHg−1, CI 23–36 vs. 9.4 mL mmHg−1, CI 7.4–11.4) and the older group (24 mL mmHg−1, CI 20–29 vs. 8.7 mL mmHg−1, CI 7.4–10.1).

Wall tension

Wall tension was correlated with the VAS (R=0.9). Correlation between VAS and pressure (R=0.9) or volume (R=0.8) were of the same magnitude ( Fig. 5).

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Figure 5.  Correlation between VAS and tension (R=0.9); pressure (R=0.9); and volume (R=0.8).

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Minimal distension pressure

Minimal distension pressure was 8.6 mmHg (7.7–9.4). No relationship was found between minimal distension pressure and sex or age, whereas a weak correlation was found with body mass index (R=0.5). VAS-scores were not influenced by differences in minimal distension pressure.

Continuous volume distension

The results of the continuous volume distension are shown in Table 1.

Table 1.   Continuous volume distension with polyethylene bag [mean (95% confidence interval)] Thumbnail image of

Males had a significant higher FSP than females (P=0.035). There was a tendency for higher UP (P=0.075) and MTP (P=0.18) in males compared to females. FSV, UV and MTV tended to be higher in males than females (P=0.12, 0.14 and 0.6 respectively). No relationship was found between age and pressures or volumes.

Rectal distension with water-filled latex balloon

The results of the water-filled latex balloon distension are shown in Table 2.

Table 2.   Continuous volume distension with latex balloon [mean (95% confidence interval)] Thumbnail image of

No significant differences were found between males and females in this test. However, males tended to tolerate higher volumes (P=0.15). No relationship was found between age and pressures or volumes.

Comparison between polyethylene bag and latex balloon

MTV and MTP were larger with the polyethylene bag compared to the latex balloon (Tables 1 and 2. Correlation coefficients for the volumes were moderate (FSV (R=0.50), UV (R=0.65) and MTV (R=0.64)) and poor to moderate for the pressures (FSP (R=0.20), UP (R=0.36) and MTP (R=0.63)).

The mean difference of the maximal tolerated volume between the balloons was 85.5 mL (range 59–111) and the mean difference of the maximal tolerated pressure was 11.4 mmHg (range 6.8–16) ( Fig. 6).

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Figure 6.  Difference vs. mean maximal tolerated volume (above) and pressure (under) of the polyethylene bag and the latex balloon.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

In this study, we showed that visceral sensitivity is equal in males and females at given pressures, which is in concordance with the study by Malcolm et al.20 In addition, we found that the equality of sensitivity in males and females occurred despite the larger volumes in males.

This discrepancy suggests that pressure is the main parameter in sensitivity. Other studies support this concept. After prolonged distension with a constant pressure, the volume increased but the perception did not.21 When the rectum is relaxed by glucagon, perception was changed in volume-controlled distension, but not in pressure-controlled distension.6 Patients with rectal prolapse, who have lower volumes at distension pressures compared to controls, have equal perception.22

In recently published articles, 6, 17, 18 wall tension is considered as an important parameter of gastric visceral sensitivity because it decreases variability in sensation scores. On comparing the following parameters: pressure; wall tension; or volume, we could not find a decrease in variance in sensitivity score compared to tension. This suggests that wall tension is less important than pressure, which is in concordance with the study of Thumshirn et al.18

In this study, no difference was found between intermittent semirandom (phasic) and continuous (slow ramp) distension to determine visceral sensitivity in volunteers. Other studies in healthy volunteers also could not show differences between the methods of distension.1, 6 In contrast to this, Sun et al.23 found that intermittent distension showed lower thresholds in a volume-controlled design.

In IBS patients, studies have been performed in which threshold levels of intermittent and continuous distension were compared. Some studies showed lower pain thresholds in intermittent distension compared to ramp distension despite similar compliance curves of both methods of distension.3, 12, 24 Other authors could not find this difference between the distension methods13, 25 or suggested that perceptual alteration observed during the ascending series of phasic distension is more related to hypervigilance of the subject towards aversive and potentially noxious visceral stimuli, which could explain the lower thresholds of IBS patients.26

A possible explanation for the discrepancy in visceral sensitivity between the intermittent and continuous distensions may be found in the different tension receptors. Slow rectal distension is perceived by mucosal receptors of the sacral afferents, whereas rapid phasic distension may stimulate preferentially splanchnic afferents with receptive fields in deep muscular layers, serosa and mesentery, and projecting to the lumbar spinal cord.12, 25, 27, 28 In healthy volunteers, and with the methods we used, it is not possible to test these tension receptors separately.

Compliance can be defined as the capacity of the rectum to stretch (expand) in response to an imposed force.15 In this study compliance was calculated at two levels. Dynamic compliance was calculated as the largest volume augmentation at a pressure step. Static compliance was calculated as the maximal tolerated volume related to the maximal tolerated pressure. Males had a larger compliance at both levels, such differences being caused by the larger volumes in males. In the first part of the compliance curve, the distension represents the muscular tone (active stretch) while the latter part of the curve represents the connective tissue and muscle (passive stretch).23, 26 Drugs such as clonidine influence dynamic compliance or active stretch but not passive stretch or static compliance.29 This model is supported by the mathematical model that Rao et al.30 constructed using the rectal response to rapid and ramp inflation; the responses corresponding to submucous collagen, circular muscle and longitudinal muscle layers. It supports the perception of rectal sensations at lower strain levels and lower rates of inflation through the response of a mechanoreceptor to circular muscle relaxation. First sensation is an anal canal sensation; urge distension occurs with the drop in tension of the circular smooth muscle; and maximal tolerated pressure is a physiological protection against damage. In our study, as in others, no difference in compliance could be found in respect to intermittent or slow ramp distension.24 The clinical relevance of the compliance calculation is still doubtful.

Hysteresis was measured as the relative difference in the pressure-volume curve between the deflating and inflating areas and may be a parameter for adaptive properties of the rectal wall (smooth muscle relaxation).16 In all subjects a hysteresis phenomena could be observed. Older females had a larger hysteresis than younger females. In males, this difference in age could not be found. It could be possible that a history of childbirth or a difference in pelvic anatomy attributes to this discrepancy in hysteresis, but a clear pathophysiological explanation is to be explored.

The best balloon to measure rectal compliance and sensitivity is still controversial. 31 A latex balloon can be filled with air or water and a correction for the compliance of the balloon must be made. The barostat system with the polyethylene bag is filled with air and can provide a constant pressure and volume regulation which seems more sophisticated. In vitro the balloons are tested in ‘artificial guts’. The latex balloon can expand under high pressures in a rigid tube whereas a polyethylene bag cannot because of the attachment. In addition, a latex balloon first distends in volume without a rise in pressure and has a more spherical shape. Both methods had difficulty measuring compliance in in vitro pig gut. This was due partly to heterogenous elastic properties along the length of the gut, and partly to internal compliance of the inflation apparatus. 32 In the study of Sun et al., no difference could be shown in volume or sensation by using a latex balloon distended with water or with gas.23 The threshold of maximal toleration distension has a wide intrasubject range in repeated measures with the latex balloon.1 Our study shows that with the flaccid air-filled polyethylene bag higher maximal tolerated distension can be reached. One explanation could be that the sensation of a water-filled balloon is different from that of an air-filled balloon, because of the weight. Another explanation could be that the latex balloon has a more spherical shape on distension while the barostat balloon is more cylindrical. These differences could also count for the different thresholds between the balloon and bag.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

Visceral sensitivity, measured with intermittent semirandom pressure distension and continuous pressure distension, is equal in healthy volunteers. Gender does not influence visceral sensitivity when determined by pressure distension, however, males have larger volumes. Age does not influence the sensitivity thresholds. Pressure-volume curves are different in males and females due to the larger volumes in males, compliance is therefore larger in males. Older females have a larger hysteresis than males and younger females. The air-filled high compliant polyethylene bag and the conventional water-filled latex balloon study have a systemic difference. These conclusions should be taken into account when interpreting results.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References

Financial support by Janssen-Cilag B.V. is greatly acknowledged.

References

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. References
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