Measuring Intrabladder Pressure With the Head of the Bed Elevated 30º: Evidence to Support a Change in Practice

The study was reviewed and approved by the institutional review board, and signed informed consent was obtained for all patients.

Patients admitted to any of 6 adult ICUs of Allegheny General Hospital, West Penn Allegheny Health System, in Pittsburgh, Pennsylvania, were screened to participate in the study regardless of the patient’s sex, race, diagnosis, or hospital treatment. Inclusion criteria were age at least 18 years and a clinical need for a urinary bladder drainage catheter as determined by the attending physician. Patients were excluded from the study if they were unable to assume any of the 30º HOBE body positions or if they had a neurogenic bladder, a bladder tumor, or a perforation that would potentially influence the muscular tone of the bladder or if they had hematuria, which would potentially alter the measurement techniques.

The study was prospective, randomized, and experimental. After informed consent was obtained, participants were positioned supine with a 30º HOBE and an IBP measurement was taken with 25 mL of saline instilled into the patient’s bladder (initial study position). Participants were then randomized to have a second IBP measurement taken in 1 of 12 possible combinations of 4 body positions and 3 volumes of instilled saline. The 4 randomized positions were supine and flat with a 0° HOBE (S), supine with a 30° HOBE (S-30), right lateral with a 30° HOBE (RL-30), and left lateral with a 30° HOBE (LL-30). The 3 randomized volumes of saline instilled were 25 mL, 50 mL, and 200 mL.

The positions and instillation volumes chosen for the study were based on clinical relevance, research interest, and prior research data. The S position was chosen because it is the traditional position used to measure IBP and the position recommended by the American Association of Critical-Care Nurses (AACN)³⁰  and the World Society of the Abdominal Compartment Syndrome (WSACS).³  The 30° HOBE position was chosen because it is the recommended position for the prevention of pneumonia in patients who require mechanical ventilation by the Centers for Disease Control¹⁷  and the Canadian Critical Care Trails Group¹⁸  and is the position recommended in the ventilator bundle developed by the Institute for Healthcare Improvement¹⁵  to prevent ventilator-associated pneumonia regardless of concomitant administration of enteral nutrition^20,21  and is of clinical nursing relevance. The RL-30 and LL-30 positions were chosen because they represent alternative positions recommended to prevent complications of bed rest encountered in intensive care such as pressure ulcers.^31,32  A 45º angled wedge positioning pillow (Hausmann Industries Inc, Northvale, New Jersey) was used to support the patient’s back while in the lateral positions. The 25- and 50-mL instillation volumes were chosen to represent the recommendations of WSACS¹³  and of AACN,³⁰  respectively, and a 200-mL volume was chosen to represent the larger instillation volume previously used in research studies.^33,34 

The randomization scheme was prepared by the statistician and delivered to the principal investigator in sealed envelopes that were opened only after signed informed consent was obtained. Patients participated only once per ICU stay. All measurements were taken by the principal investigator (M.H.S.). Once all measurements were obtained, the patients were positioned in the most appropriate and comfortable position on the basis of their medical condition and wishes.

IBP was measured by the method of Kron et al³⁵  with modifications as described.³⁶ 

Data were analyzed by using SPSS statistical software (SPSS-15 SPSS Inc, Chicago, Illinois) and are presented as mean (SD). Normally distributed data were analyzed by using a 1-way or 2-way analysis of variance with Neuman-Kuels test for post hoc comparisons. Data that were not normally distributed were analyzed by using Kruskal-Wallis test with Dunn post hoc analysis or the Wilcoxon signed rank test. IBP measured in the initial study conditions and compared with the conditions recommended by the AACN and the WSACS were analyzed by means of a paired t test.

Between April 2007 and December 2007, 280 adult patients were screened to participate. Sixty-two (22.1%) declined to participate after a detailed explanation of the study was given. Fifty-nine subjects (21.1%) did not meet inclusion criteria; some were less than 18 years of age and others had anuria, hematuria, or neurogenic bladder. Eighteen patients (6.4%) were unable to participate for other reasons, for example, the urinary bladder catheter had been removed or orders for transfer from the ICU were written and adequate time to complete the study could not be guaranteed. Seventeen patients (6.1%) were unable to give informed consent because of sedation, and a surrogate decision maker was not available to give consent. Finally, 3 patients’ families (1.1%) declined participation, and 1 patient (0.4%) was not enrolled for an unknown reason. The remaining 120 subjects (42.9%) agreed to participate in the study and were enrolled after a detailed explanation of the study was provided by the principal investigator and a signed written informed consent was obtained.

Participants were from 18 to 93 years old, with a mean (SD) age of 63.5 (17.1) years. Sixty-six participants (55.0%) were male, and 111 (92.5%) were white. Body mass index (calculated as weight in kilograms divided by height in meters squared) ranged from 15.8 to 54 with a mean (SD) of 29.0 (7.3). The length of stay in the ICU before IBP measurement was from 1 to 50 days, with a mean (SD) of 4.7 (6.8) days. Thirty-eight patients (31.7%) had IBP measured on the first day of admission, 56 (46.7%) within 5 days of admission, and the remaining 26 (21.7%) more than 5 days after admission. A 1-way analysis of variance showed no significant difference in length of stay among the 12 groups, but significant differences were observed for age (F = 2.5, R² = 0.2, P = .007) and body mass index (F = 2.5, R² = 0.2, P = .007). Subjects randomized to the supine 30° HOBE position with 25 mL of saline instilled (see Figure, group 2) were a mean (SD) of 75.8 (6.8) years old and differed in age from the subjects randomized to the supine 0° HOBE position with 50 mL of saline instilled (see Figure, group 5; mean [SD] 51.8 [15.5] years, P = .05). Subjects randomized to the RL-30° HOBE position with 50 mL of saline instilled (see Figure, group 7) had a mean (SD) for body mass index of 35.5 (9.3) and differed from subjects randomized to the RL-30° HOBE position with 200 mL of saline instilled (see Figure, group 11), who had a mean (SD) body mass index of 23.8 (3.2) (P = .01). All other paired comparisons were not significant.

Forty-eight patients (40.0%) were recruited from the trauma unit, 25 (20.8%) from 2 neurosurgical ICUs, 18 (15.0%) from the medical ICU, 15 (12.5%) from the coronary care unit, and 14 (11.7%) from the surgical ICU. Eight patients (6.7%) were receiving mechanical ventilation, and 5 others (4.2 %) were treated with biphasic intermittent positive airway pressure. Twenty-five patients (20.8%) were in negative fluid balance, ranging from −7661 to −73 mL, and 95 patients (79.2%) were in positive fluid balance, ranging from 1 to 35 497 mL.

The IBP measured in the S-30 position with 25 mL of saline instilled yielded values ranging from 1 to 44 mm Hg with a mean (SD) of 11.6 (5.9) mm Hg. One hundred eighteen patients had values between 1 and 25 mm Hg, and 2 patients had values greater than 25 mm Hg. Fifty-three participants (44.2%) had values of 12 mm Hg or greater.

One patient randomized to the supine position with 200 mL of saline instilled was unable to assume the position because of respiratory distress and did not complete the study. The IBP measurements in the initial study position and after randomization for each individual patient according to randomization group are shown in the Figure, and the mean (SD) for IBP for each randomized group is shown in the Table. The IBP measured in patients randomized to the 3 different 30º HOBE positions with 25 or 50 mL of saline instilled yielded results within the range of values previously reported in critically ill patients who had IBP measured in the supine position with 25 or 50 mL of saline instilled. The 200-mL instillation volume also yielded measurements in the expected range but occasionally produced very high values (>40 mm Hg), and a few participants complained of the urge to urinate. As a result, the absolute values and the inter-patient variability of IBP with 200 mL of saline instilled were higher than for the 25- and 50-mL instillation volumes. For the supine and the RL-30 positions, IBP progressively increased as the volume of saline instilled increased. However, for the S-30, IBP was higher when measured with 25 mL rather than 50 mL of saline instilled, whereas the IBP values measured with the patients in the LL-30 position were similar for the 25- and 50-mL instilled volumes (see Table). One-way analysis of variance showed a significant volume effect for each body position. Post hoc analysis via the Dunn comparison test showed that the IBP measurements obtained with 200 mL of saline instilled into the patient’s bladder were significantly different from the measurements obtained after instillation of 25 and 50 mL for each body position, but IBP did not differ significantly when measured with 25 vs 50 mL of saline instilled, regardless of position.

One-way analysis of variance demonstrated a significant position effect with the 25- and 50-mL instilled volumes but not with the 200-mL volume. The variability in IBP within the 200-mL BIV group was too great to appreciate any significance related to position. Post hoc Newman-Keuls comparison test showed a significant difference between the supine and all three 30º HOBE positions with 25 mL of saline instilled (P <.01 for all 3 comparisons). For the 50-mL instillation volume, the only significant difference was between the S-30 and RL-30 position (P < .05). For the 200-mL volume, no significant differences were found between positions (P >.05).

To determine if body position and volume of saline instilled are independent determinants of IBP, we performed a 2-way analysis of variance. Because the 4 groups who had IBP measurements taken with 200 mL of saline instilled exhibited massive inter-patient variability relative to the values taken with 25 or 50 mL of saline instilled, model criteria were not met and the data from those 40 patients were omitted from this analysis. The 8 remaining groups (80 patients) were found to have similar variability by using a Brown-Forsythe test and therefore were included in the analysis. Statistical significance was found for both position (P = .007), and volume of saline instilled (P = .05) as well as the interaction of position and volume of saline instilled (P = .004).

To further explore the relationship between body position and volume of saline instilled and IBP measurements, patients randomized to the 2 groups that reflected the current recommendations of the AACN and WSACS for IBP measurement were compared with the initial study position (S-30 with 25 mL of saline instilled). The mean of the differences between the initial study position and that recommended by the AACN (ie, supine with 50 mL of saline instilled) was 0.9 mm Hg (95% confidence interval, −2.5 to 4.3, t = 0.6, P = .02; see Figure, group 5). The mean of the differences between the initial study position and that of the WSACS (ie, supine with 25 mL of saline instilled) was 5.2 mm Hg (95% confidence interval, 3.9 to 6.4, t = 9.8, P = .002; see Figure, group 1).

The definition of intra-abdominal pressure and ACS is based on IBP measurement obtained with the patient supine.³  Patients who are unable to assume this position either forgo IBP measurement or are measured supine with some degree of HOBE, but the interpretation of the results is difficult because the degree of HOBE lacks standardization. Furthermore, positioning a patient supine from a more comfortable HOBE position solely for the purpose of measuring IBP places the patient at risk for aspiration pneumonia developing, increases the risk of equipment dislodgment, is inconvenient, and increases the nurse’s workload. Measuring IBP in a position that is rarely used by nurses when caring for critically ill patients truly does not reflect IBP when the head of the bed is elevated, and the IBP taken when the patient is lying supine for a few minutes does not reflect the abdominal pressure when the head of the bed is elevated. Would it not make more sense to measure IBP with the patient in a safer, more comfortable and widely used position rather than in a position that is rarely used?

The advantages of measuring IBP with the patient supine are standardization, familiarity, and the current universal acceptance of the definitions of intra-abdominal pressure and ACS. The present study demonstrates that it is feasible to measure IBP with a 30° HOBE and to obtain results with similar inter-patient variability and absolute IBP values similar to values obtained with patients positioned supine. The mean values of the IBP measurements obtained in the HOBE positions are within the range of values obtained in previous studies that have measured IBP in mixed ICU populations with patients supine and 50 mL of saline instilled.³⁷  Furthermore, when 50 mL of saline was instilled in the patient’s bladder in the present study, the values obtained with patients supine did not differ significantly from the values obtained in any of the study positions with a 30° HOBE (see Table).

The experimental design used in the present study to a major extent reflects contemporary nursing practice. That is, IBP was first measured in the S-30 position and then patients were moved to the randomized positions. Transition from the S-30 position to supine flat resulted in a mean decrease of 5.2 mm Hg when 25 mL of saline was instilled (see Figure, group 1). This finding is consistent with other studies^25–27  that have shown increases of 4 to 6 mm Hg when patients were moved in the opposite direction, that is, from supine to 30° HOBE positions. The higher IBP values in HOBE positions are to be expected because of the tension of the abdominal wall musculature and the gravitational effects of the abdominal viscera on the bladder.^38,39  Therefore, the difference in absolute IBP values noted in HOBE positions is due to the inherent physiological properties of the abdominal compartment and reflects the sensitivity of IBP to detect changes in intra-abdominal pressure with position changes and should not be construed as a reason to avoid HOBE positions when measuring IBP in critically ill patients.

In this study, we generated a large database of IBP measurements taken by a single nurse when ICU patients were in the S-30 position, which is of clinical importance because this position is the most common position assumed by critically ill patients. Furthermore, S-30 is the position recommended by critical care experts on the basis of evidence obtained in ICU and reflects current practice guidelines and consensus standards published by professional organizations to prevent ventilator-associated pneumonia and aspiration pneumonia in critically ill patients.^{15,17,18,20,21} 

When the present study was designed, no report of a study that used the S-30 position had been published. Since then, 3 studies^25–27  have been reported that show data obtained with patients in this position. In all 3 studies, researchers found that the mean IBP is higher when measured with the patient in a 30° HOBE position than with the patient supine. A major difference between the present study and the previous studies assessing the effect of HOBE on IBP is the position and leveling of the pressure transducer at the time of measurement. Vasquez et al²⁶  and others did not describe the anatomic landmark used to level the transducer, McBeth et al²⁵  and Cheatham et al²⁷  leveled the transducer at the intersection of the iliac crest and the midaxillary line, and in this study the transducer was leveled at the symphysis pubis. In addition, McBeth et al²⁵  and Vasquez et al²⁶  did not relevel the transducer after each position change. Conversely, in the study by Cheatham et al²⁷  and in the present study, the pressure transducer was releveled and rezeroed with each position change. These differences in experimental design are important because elevating the head of the bed may change the relationship of the transducer to the anatomic reference landmark for zeroing and could alter the IBP measurement.

Results of the present study add to the knowledge regarding IBP measurement in ICU patients because this study included an evaluation of 2 lateral positions with a 30° HOBE that had not received adequate attention in earlier studies. These lateral positions were chosen for investigation because after the S-30 position they are the most common positions assumed by critically ill patients and these positions are recommended to prevent the complications of immobility such as pressure ulcers.^32,40  The RL-30 and LL-30 positions yielded higher values than did the supine position when 25 mL of saline was instilled into the patient’s bladder but not when 50 mL of saline was instilled (see Figure and Table). Interestingly, the LL-30 position was less sensitive to the volume of saline instilled than all other positions studied. Finally, IBP did not differ significantly between the RL-30 and LL-30, regardless of the volume of saline instilled. As of this writing, researchers in only 1 other study⁴¹  have reported IBP data when patients were in lateral positions. De Keulenaer et al⁴¹  measured IBP in 10 ICU patients in a lateral position with no HOBE and then again immediately after the patients were positioned supine. In that study, the measurements were taken 3 times in a single day: morning, afternoon, and evening. As in the present study, De Keulenaer et al⁴¹  found that IBP measured with patients in the lateral positions with 20 mL of saline instilled was higher than IBP measured with patients supine and that IBP measurements did not differ between the right and left lateral positions.

The independent effect of the volume of saline instilled noted in this study is in agreement with results of 2 studies^28,42  of the effect of volume of saline instilled on IBP measurements but differed from previous studies because the effect of saline volume instilled was assessed with patients in positions other than supine. The present study demonstrated that regardless of patients’ position, instillation of 25, 50 and 200 mL of saline yielded IBP measurements in a range that was clinically expected (see Figure). However, instillation of 200 mL of saline was associated with larger interpatient variability and produced a higher mean IBP value than did instillation of 25 or 50 mL of saline. Furthermore, IBP measurements did not differ significantly when 25 vs 50 mL of saline was instilled. These results are consistent with results of previously reported studies^28,42  that showed very little differences in IBP when less than 150 mL of saline was instilled. The high variability in IBP observed in selected patients in the present study when 200 mL of saline was used (see Figure, groups 9–12) is most likely due to a lower bladder compliance and/or high residual bladder volume at the start of the study. Thus, to reduce variability in IBP measurements between patients, high volumes of saline should not be instilled. Based on these considerations, instillation of either 25 or 50 mL of saline appears to be appropriate for IBP measurement, supporting the recommendations of the WSACS and the AACN, respectively.

An important finding of the present study is that both position of the patient and volume of saline instilled at the time of IBP measurement independently affect IBP, but of greater interest is the identification of an interaction between patients’ position and volume of saline instilled that was not previously recognized. This observation was made possible because of the unique study design that allowed these variables to be investigated simultaneously and interactively. By instilling 3 different volumes of saline into the bladder with patients in 4 different positions, we are able to confirm the effect of volume of saline instilled on IBP with patients supine, but additionally found that the effect of volume of saline instilled on IBP is not as pronounced when the head of the patient’s bed is elevated.

These findings clearly illustrate the volume-position interaction and show that either position or volume of saline instilled can affect IBP measurement and further illustrate the need for both accurate positioning of patients and instillation of a consistent volume of saline, both of which are paramount in obtaining interpretable IBP measurements. These findings also highlight the need for all investigators to clearly define the position and the volume of saline instilled into the patient’s bladder when reporting IBP measurements.

The AACN and the WSACS recommend that IBP be measured with patients supine only, but neither of these organizations has provided evidence to support this recommendation. To provide such validation, studies must be conducted where intra-abdominal pressure is measured directly in patients by placement of intraperitoneal catheters.^43,44  These techniques are not standard practices in the ICU, and their use would significantly increase the risk to patients and cannot be ethically justified. The lack of direct measurement of intra-abdominal pressure is a limitation of the present study, as with all other clinical studies assessing the effects of the volume of saline instilled into the patient’s bladder and position on IBP.^{26,27,37,42,45}  Because of this limitation, recommendations may need to be made on the basis of the quality of research data obtained with indirect IBP measurements.

The present study demonstrated that IBP measured with patients in 3 positions with 30º HOBE yielded results that are within the expected range of measurements obtained with patients supine. Furthermore, our results indicate that repositioning patients from the S-30 to the supine position solely to measure IBP yields IBP values that are significantly lower, by approximately 5.2 mm Hg (see Figure, group 1). No significant changes in IBP occurred when the patients were repositioned from the S-30 to LL-30 or RL-30 position. In addition, the supine position is more sensitive to changes in the volume of saline instilled than are the other HOBE positions studied, as shown by progressive increases in IBP values as the volume of saline instilled was increased. In contrast, when measurements were made with patients in the S-30 and the LL-30 positions, progressive increases in IBP were not observed (see Table).

Finally, although this point was not directly assessed in this study, keeping patients in the S-30 position for IBP measurement is theoretically expected to decrease nurses’ workload, reduce the risk of aspiration pneumonia developing and equipment being dislodged, and indirectly increase patients’ comfort and safety. If current guidelines are followed for prevention of ventilator-associated pneumonia and aspiration pneumonia related to enteral nutrition, most patients are expected to have a 30º HOBE or greater while positioned supine or lateral and would need to be repositioned supine and flat for the sole purpose of measuring IBP. After IBP measurement, patients would be repositioned to an HOBE of at least 30º, and it is unclear what the value is of measuring IBP with patients in a position that is rarely assumed in the ICU. When the technical aspects of IBP measurement such as interobserver and intraobserver variability are considered in tandem with nurses’ workload and patients’ safety and comfort, the S-30 appears to be a more desirable position to measure IBP than the supine position.

Serious consideration needs to be given to nursing research that is devoted to developing protocols where the S-30 becomes the standard position for measuring IBP or at least an acceptable alternative standard position for patients who are unable to assume the supine and flat position. Because the diagnosis and prognosis of intra-abdominal hypertension and ACS are all based on IBP values measured with patients supine and flat, accepting the S-30 position for IBP measurement will require more research to redefine the intra-abdominal pressure value that correlates with intra-abdominal hypertension and ACS, thereby requiring a revision of the definitions of intra-abdominal hypertension and ACS.

Further studies are needed that compare the positions of supine and flat and S-30 with outcome variables rather than the absolute differences in IBP, which has been the only outcome variable assessed. For instance, it would be of interest to determine if IBP when measured in the S-30 position is associated with less nursing time, less disruption of care, and better quality of life for critically ill patients as compared with the supine position. In addition, it would be important to investigate the IBP that defines intra-abdominal hypertension in the 30º HOBE position and if this position correlates better with other important outcomes such as mortality and need for abdominal decompression than does the supine flat position.