Intra-abdominal pressure measurement: validation of intragastric pressure as a measure of intra-abdominal pressure
1 Academic Unit of Anaesthesia, Sheffield University, UK
2 Department of Anaesthesia, Royal Hallamshire Hospital, Sheffield, UK
3 Department of Pulmonary Medicine, Sahlgrenska University Hospital, Sweden
* Corresponding author: Academic Anaesthetic Unit, University of Sheffield, Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, UK. E-mail: cahilturnbull{at}doctors.org.uk
Accepted for publication February 6, 2007.
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Abstract |
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Background: The diagnosis of abdominal compartment syndrome depends upon the demonstration of an elevated intra-abdominal pressure (IAP). Direct measures of IAP are impractical in the critical care unit; intravesical pressure (IVP) and intragastric pressure (IGP) should represent acceptable surrogate measures. IVP is the preferred measure of IAP in critical care. We considered that IGP represents a practical alternative. The objective of this preliminary study was to observe the relationship between IGP and IAP.
Methods: After Institutional Ethics Board approval, 29 patients having elective laparoscopic surgery were recruited. IAP was measured directly via the abdominal trochar. This was compared with IGP measured via a commercial balloon catheter placed into the stomach.
Results: Measured IGP was always more positive than IAP; both showed linear correlation (r2>0.9). When IGP was calibrated against IAP, an estimated difference between the IGP and IAP of ± 2.5 mm Hg for 95% of the measurements was seen.
Conclusions: The study demonstrates the strength of the relationship between IGP and IAP in normal individuals. Application of IGP measurement in the critical care patient is necessary to demonstrate its suitability for continuous IAP assessment.
Keywords: Abdomen, intra-abdominal pressure; complications, abdominal compartment syndrome; monitoring, intra-gastric pressure, intra-abdominal pressure
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Introduction |
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The combination of elevated abdominal pressure and the adverse physiological effects that develop has been termed abdominal compartment syndrome (ACS).1 This condition may occur in the critical care patient because of sepsis,2 pancreatitis,2 3 gross ascites, bowel obstruction, megacolon,4 abdominal trauma,2 5 or retroperitoneal haemorrhage.2 The diagnosis of ACS depends upon the demonstration of an elevated intra-abdominal pressure (IAP).6 As direct measures of IAP are impractical under most circumstances, a surrogate measure of IAP has been chosen. The homogenous transmission of pressure within the abdomen7 allows IAP to be estimated via either the bladder,8 rectum,9 or stomach.9 The technique of intravesical pressure (IVP) measurement8 10 11 has been validated12 and remains the accepted surrogate measure of IAP for clinical use.13 14 Despite modifications of the technique,15 16 IVP measurement is cumbersome to perform. Furthermore, as abdominal pressure is influenced by a number of factors, including patient position, ventilation strategies,17 and bowel function, a continuous real time recording of abdominal pressure is a worthwhile objective. Though continuous vesical pressure is commercially available,18 IAP measurement is still rarely employed.19
Intragastric pressure (IGP) is commonly employed as a surrogate measure of IAP in respiratory research. However, evidence for the relationship between IGP and IAP is conflicting.8 20 Previous attempts to measure the relationship between IGP and IAP in humans have often used the index measurement of IAP as that recorded either by the pressure monitor built within the high flow carbon dioxide insufflator during laparoscopy or IVP when neither measure may reliably reflect IAP.21 22
This study describes the relationship between IGP measured with a single use, commercially available, air-filled balloon catheter and a direct recording of IAP during laparoscopic surgery.
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Methods |
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The Institutional Ethics Committee approved the study. Twenty-nine female subjects, who provided written consent to participation in the study, were recruited. All the patients underwent laparoscopic surgery as part of gynaecological investigations; none reported abdominal pathology that might compromise IGP measurement. All subjects received a general anaesthetic, with muscle paralysis, tracheal intubation, and mechanical ventilation. IGP was measured with an 80 cm long polyvinyl chloride balloon catheter (Ackrad Laboratories, Cranford, NJ, USA) with a 10 cm distal balloon (Fig. 1). The balloon catheter was inserted orally to a depth of 60–70 cm. Previous studies in our department in spontaneously breathing subjects have demonstrated that placement of the balloon catheter to this distance is sufficient to guarantee gastric placement.
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Further confirmation of the position of the distal balloon in the stomach was through an observation of pressure swings with epigastric percussion (Fig. 2B) and the absence of cardiac pulsations (Fig. 2C). The catheter was connected to the recording apparatus via a 100 cm polyvinyl chloride catheter. The distal balloon was initially emptied by opening the three-way tap to the atmosphere during positive pressure ventilation. The balloon was subsequently inflated with 1 ml of air from a 2 ml graduated glass syringe. The patient, initially in the supine position, was placed in the Trendelenburg position to aid surgical visualization of the pelvis. IAP and IGP were recorded in both the supine and Trendelenburg positions.
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IAP was recorded via a 13 mm laparoscopic trochar (Fig. 3). A three-way stopcock was placed on the side port reserved for CO2 insufflation. A 100 cm polyvinyl chloride catheter was attached from the three-way tap to independent pressure monitoring equipment. The three-way tap allowed the intermittent CO2 insufflation and IAP pressure recording. The pressure indicated by the CO2 insufflator was also noted. Bench top testing of this set up had previously indicated that there was no discrepancy between pressure measured at the side port of the trochar and that measured at the distal end of the trochar (Fig. 3).
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Abdominal insufflation pressure was set between 10 and 15 mm Hg according to the requirements of the surgery. The IAP and IGP were continuously recorded, and stored for later analysis. The recording equipment for IAP and IGP consisted of two-piezoresistive pressure transducers (Honeywell, Morriston, NJ, USA), placed with the recording equipment on a portable bench next to the patient. These generated an analogue signal that was relayed via a data acquisition box (National Instruments, SCB 68 series), sampling at 1000 Hz to an analogue/digital converter (National Instruments, NI DAQ series 6). The digitized signal was recorded and viewed with customized software1 (Labview, National Instruments) for real time display on a computer terminal (RM PC520, RM, Abingdon, UK), running Microsoft Windows 98. ‘Labview’ is a graphical programming system that is used to acquire, analyse, and present data from a variety of digital or analogue sources. It presents the data in graphical form for analysis, and instrument control.
The pressure transducers were calibrated before each study. Though both the pressure transducers were placed at a convenient position on a portable bench and not at the level of the symphysis pubis as is classically described,10 the error introduced by the difference in height is small in an air-filled system, unlike that of a fluid-filled system. Furthermore, the error in IGP would be of the same magnitude and in the same direction as that of IAP. Small amplitude pressure swings were noted in both the IAP and IGP traces, these were cardiac in origin and did not interfere with pressure measurement (Fig. 2C). Respiratory pressure swings from positive pressure ventilation were also present in both traces. IAP and IGP were taken at end expiration. High amplitude pressure swings were seen in the IAP trace only, these were a result of intermittent CO2 insufflation and did not interfere with IAP measurement.
For each subject, the difference between IAP and IGP was measured at 10 time intervals. A final total of 266 data sets for IAP and IGP in the Trendelenburg position and 259 in the supine position were available for analysis (Table 1).
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Using the statistical software package (SPSS), a linear regression analysis was used to establish the relationship between the IAP and IGP. The calibration method and Bland–Altman method were considered to demonstrate the strength of the relationship between IAP and IGP.
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Results |
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Twenty-nine female patients were recruited for the study. The mean age of the pateints was 29 yr (range 17–44), and mean BMI was 26 (range 17–39). The balloon catheter was successfully inserted in all the subjects. The basal IGP 9 (3.1) cm H2O was recorded before commencement of surgery. There was a low correlation between IGP and BMI (Pearson's coefficient 0.381, r2 = 0.14). IGP and IAP were recorded in all subjects in the supine position and in Trendelenburg position with 15° head down tilt.
The CO2 insufflation was pulsed and produced sharp swings in IAP (Fig. 4B), which were not recorded by the intragastric balloon catheter. These were assumed to represent local rises in pressure within the trochar during the pulsed CO2 insufflation and were therefore ignored. The correct value for IAP was, therefore, taken when CO2 insufflation was absent and the IAP trace was stable.
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During surgical palpation of the abdomen, higher IAPs were seen. These sharp pressure swings were transient and were excluded from statistical analysis (Fig. 4B). Given the risk of cardiovascular collapse, maintenance of these pressures for the benefit of the study was not considered.
From the data, a scatter plot was drawn using IAP and IGP (Fig. 5). The correlation coefficients in the Trendelenberg and supine positions were greater than 0.9. A linear relationship between IAP and IGP was then assumed and a line of best fit or regression line was drawn. The values for the slope of the line and the intercept were calculated, and with these values, a new IGP could be calculated using the equation y = a + bx, where ‘a’ is the intercept, ‘b’ the slope, and ‘x’ the IGP value. The calculated y value was called the corrected IGP (IGPc). A new regression analysis of IAP against IGPc demonstrated the intercept of the regression line at zero. Although not described when performing a calibration method, a Bland and Altman plot was drawn using IAP and IGPc (Fig. 5). The graph demonstrated the expected error in IGPc compared with IAP.
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Discussion |
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All our subjects were females, and this was a practical decision given the large number of investigative laparoscopies performed in our institution. Confirmation of catheter placement in the stomach is normally achieved with subjects awake performing various inspiratory manoeuvres. We felt it unrealistic to expect the patients to consent to passage of the balloon catheters while awake.
The BMI of our population may be greater than average that may lead to an elevated IAP; this is unlikely to influence our results, which was to estimate the difference between IGP and IAP. In line with previous studies, there was a low correlation between BMI and basal IGP in our female population.
An elevation of pressure greater than 30 mm Hg (32–39 cm H2O), within the abdominal cavity will result in compromise of perfusion of visceral organs.14 23 24 Persistence of the elevated pressure will result in bacterial translocation, multi-organ failure and death.25 The management of ACS requires confirmation that the abdominal compartment pressure is elevated. Though IAP measured via the bladder2 5 13 may provide a reliable estimate of abdominal pressure, the failure to provide a continuous measure is a potential flaw. This study demonstrates the relationship between IAP and IGP. When calibrated the discrepancy between IGP will lie within 2.6 mm Hg (2 SD in the supine or Trendelenburg position) of the real value of IAP. Though some studies had suggested that IVP (IAP) reflected IAP,11 12 others21 26 27 have contested these findings. Johna and colleagues21 demonstrated that IVP was always positive, even when the IAP was zero. Gudmundsson and colleagues27 noted that the instillation of 50 ml of saline into the bladder11 may lead to an increase in basal IVP. He proposed the instillation of only 10–15 ml of saline before IVP measurement. Fusco and colleagues12 also noted that the discrepancy between IVP and IAP increased, with intravesical instillation of saline. Over the bladder volumes tested of 0–200 ml, IVP was 3.8 mm Hg greater than IAP on average.
The influence of the gastric mechanical activity,28 intestinal ileus, and enteral feeding may be of concern to clinicians considering using IGP in intensive care patients. The major migratory complex causes temporary elevations of IGP and can be readily differentiated from the basal IGP through observation of the gastric pressure trace. Collard and Romagnoli28 demonstrated that the frequency of gastric contraction increased with enteral feeding, but that these pressure waves were distinguishable from basal IGP. We have found no reports of the influence of intestinal ileus upon IGP. We realize that the application of IGP measurement as a standard surrogate measure of IAP will need testing in the critical care area. Though the operator set the IAP delivered by the CO2 insufflator to be between 10 and 15 cm H2O (7.6–11.4 mm Hg), IAP recorded by our system was frequently higher. This reflects the additional IAP generated during surgical manipulation of the abdomen (Fig. 4). Within a subject, the relationship between IGP and IAP pressure was consistent. This was reflected by the small coefficient of variation. For diagnostic purposes, an IAP in excess of 25 mm Hg (30 cm H2O) is considered diagnostic of ACS.21
This study has demonstrated the close relationship between IGP and IAP. IGP monitoring is easy to perform, and may provide continuous observation of the effects of ventilation, high levels of PEEP, and other therapeutic manoeuvres. However, ileus, the effects of the major migratory complex and enteral feeding may compromise the accuracy of IGP measurement in the critical care patient. Follow-up studies in critical care patients are necessary to validate the accuracy of IGP measurement. Nevertheless, the authors consider that the simplicity of IGP measurement should provide the impetus for establishment of routine IGP measurement in critical care.
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