Review for 10.1186/1471-230X-10-70 (2010)

Reviewer's report
Title:Sensitivity of intra-abdominal, intra-organ and intravascular pressures in
detecting changes induced by external abdominal pressure with and without
PEEP: an experimental study
Version: 2Date:27 September 2009
Reviewer: Alexander Schachtrupp
Reviewer's report:
Major Compulsory Revisions.
In this investigation, 12 pigs were subjected to increased intra-abdominal
pressure (via placement of an external weight of 7 kg) and or increased PEEP of
10 cm H20. Each period lasted for 10 minutes. Resulting pressures as well as
those before and after the intervention were measured in the stomach, the
urinary bladder, the peritoneal cavity, pulmonary artery and inferior vena cava.
The animals then underwent another investigation on hepato-splanchnic blood
flow regulation
There were significant differences between the locations of pressure
measurement at baseline as well as after application of the external weight with
highest readings derived from the inferior vena cava.
When comparing readings derived from one measurement location, the
application of external weight (with or without increase of PEEP) increased the
readings when compared to baseline. This could be observed at every
measurement place excluding the pulmonary artery where only the combination
of PEEP and weight was leading to a significant increase.
The authors that changes in intraabdominal pressure can be accurately
measured either from the urinary bladder, stomach or inferior vena cava.
In the title, the authors refer to the sensitivity, a term that is common for
diagnostic tests. In order to calculate the sensitivity of test, the true result has to
be known using the goldstandard. In pigs however, the goldstandard of IAP
measurement is not defined yet.
Consequently, the sensitivity is not given in this manuscript and should therefore
not be referred to. It rather seems that different tests were compared with each
other which could normally be done using a Bland & Altman analysis with the
calculation of mean difference and limits of agreement. The conclusion, that the
IAP can be accurately measured from either depends on the clinical perspective.
Therefore, limits of agreement should be given.
This basically calls for the use of a parametric analysis of data. It is not explained
why a non- parametric test was chosen. In general, a non-parametric test is not
more precise but is certainly less powerful to detect differences. An ANOVA for
repeated measures is likely to be appropriate. In any case, a global test should
be combined with an adequate post-hoc test. In case of multiple pairwise testing,
the level of significance has to be adjusted regardless if parametric or
non-parametric tests were used. This would be applicable for the underlying
study.
Moreover, the value of this study is to be discussed as only a small increase of
IAP was to be observed. As pointed out by the authors, the pathological level is
reached with a value of 12 mmHg or higher. This however, was only noticed in
the inferior vena cava. Relevant pressures ranging from 12-25 mmHg as seen in
severe hypertension were not induced in this study. It therefore remains unknown
whether the observation made here would have also been made in relevant
pressure ranges.
As the abdomen can be considered a liquid system (Tzelepis 1996), it can be
expected that pressures measured at different points within the system agree
with each other. Several porcine studies have compared different sites of
pressure measurement. Despite the – here undiscussed - anatomical variance of
the porcine bladder which is intraperitoneally most investigators used bladder
pressure measurement to assess intraabdominal hypertension.
Gudmundsson 2002 described, that the pressures recorded in the urinary
bladder, the inferior caval vein, and the femoral vein reflected the pressure
(range 8-20 mmHg) in the abdominal cavity very well. Engum described 2002,
that continuous direct intraabdominal pressure monitoring … correlates well with
indirect bladder or gastric pressure measurement. In 1987, Lacey observed that
vesical and inferior vena caval (IVC) pressures had good statistical correlation
with IAP should be used to be used to evaluate IAP in the clinical setting. The
results of underlying study however have not been compared to the results of
Gudmundsson and Lacey although both are listed in the reference list.
So it is basically already well known, that changes in intraabdominal pressure
can be accurately measured either from the urinary bladder, stomach or inferior
vena cava. This leads to the rationale of this study which remains completely
unclear. A clear hypothesis is not given and it can be argued, that this
investigation is not much more than a spin-off product of a study, aiming to
evaluate hepato-splanchnic blood flow regulation.
Minor comments
Introduction
Prolonged IAH > 25 mmHg with at least one organ failure, defined as abdominal
compartment syndrome (ACS), is known to be associated with a higher risk of
developing renal [14] and multiple organ failure and with increased mortality [15,
16].
Sentence is irritating as ACS is defined by organ failure. So far, there is no data
on the natural course of ACS available. After decompression, De Waele
summarized mortality to be still 50%.
Despite widespread awareness of IAH and ACS, many intensive care units never
measure the intraabdominal pressure
Substitute widespread awareness with “increasing evidence regarding the
relevance of”
Page: 5
The effects of pressure changes on both sides of the diaphragm on lower and
upper intraabdominal pressure, and on intrathoracic and intraabdominal
intravascular pressures, have not been well established.
What is meant by well established and why should this be important? A rationale
for this study or a hypothesis is not given.
Methods Page: 6
A pulmonary artery catheter...
Please explain which technique/ equipment was used
..via the right internal jugular vein..
This vessel is relatively small while the external jugular vein is often larger in
diameter. Why was this vessel used?
Page: 7
..was confirmed by ultrasound.
How was the right position confirmed? Please describe technique and
landmarks.
Additional Ringer’s lactate solution and hydroxyethyl starch were administered in
equal amounts to keep the pulmonary artery occlusion pressure between 6 and
12 mmHg.
As the pulmonary artery occlusion pressure is known to interact with increased
intraabdominal pressure (Malbrain and Wilmer 2007), discuss why this is not of
relevance in this experiment. How much fluid was given in each animal.
The position of the catheters was checked by palpation during surgery and by
direct visualization after the experiments had been completed
How can catheters checked by palpation, please explain.
All pressure transducers were simultaneously zeroed to the level of the heart.
How was this determined. Give landmarks and references.
Page: 11
In patients with acute lung injury, an increase and decrease in PEEP from 8-13
H2O and back did not change total hepato-splanchnic blood flow or the gastric
mucosal-arterial pCO2 gradient [43].
What is the relevance of this statement as the underlying study did not measure
organ perfusion and is not about patients with acute lung injury?
Page: 12
Nevertheless, we believe that with the small changes in pressures produced
(around 5 mm Hg), significant flow changes are unlikely.
This should not be question of believe but a question of literature search. Several
porcine models were described in which organ perfusion was measured.
Accordingly, our results do not support the use of higher filling pressure targets in
situations with borderline IAH.
The transdiaphragmal pressure transduction is variable. It has been shown, that
IAP may lead to increased so-called filling pressures. Consequently, a higher
pulmonary artery occlusion pressure may or may not be the result of increased
IAP. On the other hand, a normal "filling pressure" in the presence of IAH is
considered be an indicator of fluid demand. Nevertheless, intravascular volume
parameters should be measured and this has not been performed in the
underlying study, consequently, this statement bears only little weight. (in this
regards see also Malbrain and Wilmer 2007)
Page: 15
Reference list
54 references are too much while the latest citation is from 2006.
Page: 21
Table 2
The pressure in the inferior vena cava is higher than at other places. Why this is
not discussed regarding the results of others.
Peep alone was not tested for statistical significance, please explain why.
Page: 23
Figures
Figures do not provide more information than the table. Moreover, they could be
confusing and should better be avoided. Additionally, all the relevant data is
already given in the table.
Level of interest: An article whose findings are important to those with closely
related research interests
Quality of written English: Acceptable
Statistical review: Yes, and I have assessed the statistics in my report.
Declaration of competing interests:
I declare that I have no competing interests