4.1 Correlation between bladder and rectal pressure measurement techniques
Various techniques have been developed to measure IAP, of which the intra-vesical approach is regarded as the gold standard, but it is relatively invasive in an ambulatory setting. There is a need for minimally- or non-invasive techniques of IAP monitoring, especially in obstetrics. Recently different techniques like microwave reflection and transient radar method have been suggested however these are not yet available for clinical use [
20,
21]. IAP estimation via rectal measurements seems, at least from a theoretical or hypothetical point-of-view, appropriate and feasible in the pregnant population because of the low-infection risk and lower risk of trauma compared to vesical measurements. However, taking into account the many disadvantages and limitations we cannot recommend its routine ambulatory use, as will be discussed further.
This validation study found that IAPrect is higher when compared to IAPves, and thus may overestimate the true IAH incidence. Furthermore, the IAP differences and trend evolution after position change, or the application of an external abdominal pressure belt, are not similar to the gold standard technique. Correlation was poor, concordance was low, percentage error was too high, and Bland and Altman analysis showed too large LA, hence the two techniques cannot be used interchangeably. We also observed a very high failure rate in obtaining a reproducible IAPrect measurement.
4.2 Effect of body position
Change in body position has a significant impact on IAP measurement [
22,
23]. We found that HOB elevation increased both the IAP
rect and IAP
ves, (Fig.
3) which is in keeping with results from the literature. Previous studies confirmed that even a slight elevation in HOB results in a clinically apparent increase in IAP measured through the bladder [
2,
22,
23]. Similar investigations were performed to check the impact of body position on IAP measurement not only through the bladder, but also through the stomach (intra-gastric pressure) [
24]. HOB elevation increases the intra-gastric pressure (IGP) as well as the intra-bladder pressure, however, the IGP changes were observed to a smaller degree compared to IAP
ves. In contrast to these studies that have investigated the impact of the upper body position on IAP measurement, the impact of the lower body position on IAP measurement has been assessed recently [
25]. In this experiment, IAP measurement through the vagina and (in some cases) rectum at supine, low lithotomy, and high lithotomy positions were evaluated. Based on the results of this study, there is no clinically remarkable change in IAP when the legs are positioned differently. However, the IAP with the patient’s legs in the supine position were lower compared to the low and high lithotomy positions.
4.3 Strengths and limitations of the study
This study is the first to attempt validation of IAP
rect measurements against the gold standard IAP
ves in an ICU-setting [
9]. The inclusion of sedated patients, in whom confounding variables are lower, and the strict protocol are strengths of this study [
26]. However, the small sample size, the poor description of patient demographics, high drop-out rate and the incompletely performed protocol due to patient or technique related issues are weaknesses and may have underpowered our study results.
The results from this validation study are similar to a study by McCarthy et al., who validated IAP
rect in 12 patients but found excessively high or unreliable values in 4 patients (33.3%) due to abdominal traction and technical difficulties on catheter insertion. They concluded that the rectal catheter should be inserted at least 10 cm deep to prevent pressure changes inside the rectum that may result in overestimated readings [
27].
Significantly higher IAP
rect measurements were observed compared to IAP
ves, even when IAP is within the physiological range, and as a result IAP
rect over-diagnoses IAH. This is in keeping with IAP
rect obtained with a fluid-filled rectal catheter balloon in which residual faecal mass can block the catheter-tip opening leading to overestimation of IAP [
16]. Correcting this overestimation with a correction factor or the use of a different reference range might not be appropriate as there was no significant correlation between supine
1 and supine
2. Also, after re-insertion of the rectal catheter, measurement was not repeatable in more than half of the patients. This is in agreement with the results of Lacey et al., who evaluated different indirect techniques against invasive direct IAP measurement in rabbits [
28]. Regression analysis showed good correlation with measurements performed in the inferior vena cava (R = 0.87) and the urinary bladder (R = 0.85), but not with intrarectal measurements (R = 0.10) [
28]. On the contrary, Shafik et al. found IAP
rect to be similar to direct IAP measurement [
15]. Note that IAP
rect was measured using a fluid filled rectal catheter.
4.4 Limitations of rectal pressure measurement
Several factors may affect IAP
rect measured via an air-filled balloon. First, previous studies showed that body temperature is higher in the rectum compared to the urinary bladder [
29,
30]. In the present study a small amount of air (at ambient temperature) was used to fill the rectal balloon and air is very sensitive to temperature changes. The higher rectal temperature will result in an increased air temperature in the balloon. In relation to the constant volume this may lead to a significantly increase in rectal pressure measured via the balloon-tipped catheter.
Second, the muscles in the rectum are stronger than the muscles in the urinary bladder. Physiologically, each rectal manipulation and filling stimulates the contraction of the rectal muscles. In a similar way, the insertion and filling of the (even small) balloon could trigger this reflex. Additionally, IAP
rect is also affected by the internal anal sphincter tension which contributes about 85% of the pressure in the anal canal [
31]. Studies in healthy volunteers showed a significant increase in the internal anal sphincter tension followed by an increase in IAP
rect after insertion of an artificial manometer for IAP
rect measurement [
31,
32]. Therefore, we can assume that rectal insertion of the T-DOC 7Fr air-filled balloon catheter can increase IAP
rect per se.
Third, the high failure rate experienced was largely due to IAPrect measuring values out of the physiological range, or due to difficulties with rectal catheter insertion. These excessive IAP values may be caused by interference from faecal masses or bowel movements on the catheter-tip opening, or an incorrect catheter position at the level of the rectal sphincter. Measuring pressure at the level of the rectal sphincter is used in anorectal manometry but it does not yield information regarding true IAP.
Fourth, as pregnant women have an increased risk of constipation and haemorrhoids, therefore, this IAP measurement technique is not suitable to perform in a pregnant population. Laxatives might help to overcome the problem of obstructing stool; however, this is not appropriate in an ambulatory setting.
Fifth, although we perceive rectal pressure measurement as being less or even minimal invasive compared to bladder pressure measurement because of the virtual absence of infection risk it must be noted that rectal manipulation can induce parasympathetic hyperactivity with severe bradycardia and cardiac arrhythmias [
33].
Sixth, continuous IAP (CIAP) monitoring is the future [
34,
35] and even if rectal pressures would be accurate and comparable to bladder pressures (which was not the case) continuous IAP
rect monitoring would be difficult because of probe positioning, displacement, and faecal interference.