The duration of intra-abdominal hypertension strongly predicts outcomes for the critically ill surgical patients: a prospective observational study

Introduction Intra-abdominal hypertension (IAH) is associated with morbidity and mortality in critically ill patients. The present study analyzed the clinical significance of IAH in surgical patients with severe sepsis. Methods This was a prospective study carried out in the surgical intensive care unit (SICU). Intra-abdominal pressure (IAP) was measured three times a day via a urinary catheter filled with 25 mL of saline. IAH was defined as an IAP ≥ 12 mmHg, and the peak IAP was recorded as the IAP for the day. Data were analyzed in terms of IAH development and the IAH duration. Results Of the 46 patients enrolled in the study, 42 developed IAH while in the SICU. The development of IAH aggravated the clinical outcomes; such as longer SICU stay, requirement of ventilator support, and delayed initiation of enteral feeding (EF). The IAH duration showed a significant correlation with pulmonary, renal, and cardiovascular function, and enteral feeding. The IAH duration was an independent predictor of 60-day mortality (odds ratio: 1.196; p = 0.014). Conclusions The duration of IAH is a more important prognostic factor than the development of IAH; thus every effort should be made to reduce the IAH duration in critically ill patients. Trial registration NCT01784458


Introduction
Intra-abdominal pressure (IAP) is defined as the steady-state pressure within the abdominal cavity bounded by the abdominal muscles and diaphragm [1]. It is affected by body weight, posture, tension of abdominal muscles, and movement of the diaphragm [2][3][4]. The World Society of the Abdominal Compartment Syndrome (WSACS) has published a grading system for intra-abdominal hypertension (IAH), with IAH defined as an IAP ≥12 mmHg, and abdominal compartment syndrome as an IAP ≥ 20 mmHg combined with the failure of more than one organ [1,5]. Ever since then, WSACS have updated consensus definitions and clinical practice guidelines for the patients with IAH [6]. The prevalence of IAH on admission to the intensive care unit (ICU) ranges from 31 to 58.8 % [4,7,8], and the incidence increases with the length of ICU stay. Clinical conditions that increase IAH include blood and ascites in the peritoneal cavity, bowel distension and edema [4,9], high-volume resuscitation and massive transfusion, damage control surgery in traumatic patients, excessive tension after abdominal closure, postoperative ileus, echar in burn patients [10,11], and hemodilution [12]. IAH causes not only abdominal organ dysfunction by decreasing the abdominal perfusion pressure (APP) [13][14][15] but also cardiopulmonary dysfunction [16], which increases both morbidity and mortality [17].
IAH has been increasingly recognized in the critically ill patients. However, the duration of IAH has not been under consideration. The aim of the present study was to investigate the influence of IAH development and its duration on the clinical course and outcome of critically ill surgical patients with severe sepsis.

Study design and patients
The study was a prospective observational study in surgical ICU of an academic tertiary care hospital. Patients at least 18

Definitions
Severe sepsis was defined as a sepsis with a failure of more than one organ due to sepsis, an arterial blood lactate concentration of at least 4 mmol/L, or hypotension (with a systolic blood pressure < 90 mmHg).

Measurements and treatment
IAP was measured using a urinary catheter at the level of the mid-axillary line on the iliac crest with the patient supine. IAP was expressed as mmHg. A 3-lumen urinary catheter was inserted into the bladder. After the urinary drainage lumen was clamped, 25 ml of saline was injected through the irrigation lumen to prevent contamination. IAP was measured three times per day while the patient was in the ICU, with the highest reading recorded as the value for that day. The Acute Physiology and Chronic Health Evaluation (APACHE II) score was recorded every 24 h. Resuscitation was performed according to goaldirected guidelines [18,19].

Statistical analysis
Statistical analyses were performed using SPSS 21 for windows (SPSS Inc. Chicago, IL). The Chi-square test or Fisher's exact test were used to compare categorical variables and the Mann-Whitney U test was used to compare continuous variables. Correlation analysis was performed using Spearman's rank correlation coefficient. Statistical significance was set at p < 0.05.

Clinical effect of the duration of intra-abdominal hypertension
The duration of IAH had more impact on outcomes than the development of IAH. Distribution of IAH duration is demonstrated on Fig. 1. There were significant increases in terms of the length of ICU stay (r = 0.860, p < 0.001), duration of mechanical ventilation (r = 0.840, p < 0.001), duration of RRT (r = 0.603, p < 0.001), and initiation of EF (r = 0.330, p = 0.029) according to increase of IAH duration (Fig. 2).

Comparison of survivors and non-survivors
There were no significant differences in any of the study variables in terms of 30-day mortality. However, univariate analysis showed that the duration of IAH (p = 0.001), the initial APACHE II score (p = 0.021), and peritonitis (p = 0.010) were significantly associated with 60-day mortality (Table 3). Multivariate logistic regression analysis identified the duration of IAH as an independent predictor of 60 day-mortality (odds ratio: 1.196; 95 % confidence interval: 1.037-1.380; p = 0.014) ( Table 4).

Discussion
IAP has been measured since the 19th century. Ever since then, its importance has been recognized recently.
Since the mid-1990s it was known that IAH could develop without abdominal trauma [20] and numerous studies have measured IAP, examined its clinical outcomes, and classifications. The prevalence of IAH depends on the patient population. IAH was present in 54.4 % of medical ICU and 65 % of surgical ICU patients [7]. We found a quite high prevalence of IAH (91.3 %) in critically ill surgical patients. The previous study demonstrated that sepsis was the predominant cause of IAH [21], and the population of this study composed of the patients with severe sepsis might make such deviation. Two patients in the IAH group (who did not try enteral feeding) were excluded