This is a prospective epidemiologic study performed in a population of exclusive critically ill medical patients following the WSACS guidelines for the assessment of IAH (screening, IAP measurement, definitions, and classification recommendations). Other published studies had been performed in mixed populations (medical and surgical), before the WSACS consensus statement, based on different definitions of IAH or limited by incomplete assessment in terms of screening or classification of the IAH [17
Our study revealed several interesting findings. Firstly, there was a large number of patients with high risk (≥2 CRF) for IAH development, and the incidence of IAH in this group was high; in addition, the mortality in this subgroup was significantly higher as compared with the low risk subgroup. Secondly, there was an association of IAH with illness severity and the number/type of RF reported in the literature. Thirdly, IAH was a predictor of mortality in association with other clinical factors. And finally, non-resolution of IAH was an independent predictive factor of mortality. Our results support the importance of implementing screening and assessment protocols, according to WSACS recommendation, in order to identify a subgroup of high risk patients with high mortality and high incidence of IAH whom should be monitored for IAP.
Screening, risk factors and incidence of IAH
A large number of patients presented ≥2 CRF for IAH development, and the incidence of IAH in this group was high (67.8%) on admission or during the ICU stay. In addition, several high risk patients without IAH might have shown underestimated IAP levels because of the different factors usually present in the critically ill population, such as sedation or gastric/colonic decompression. A high incidence of IAH had also been reported in the literature [17
]; however, when trying to combine our results with previously described findings, three main methodology issues should be accounted for: a) incidence is recorded on admission or during the first week; b) most reports refer to mixed populations (both medical and surgical); c) definitions of IAH differed across the studies (incidence rates can vary depending on the threshold IAP level used as a diagnostic criterion). Unlike previous studies, our study analyzes the incidence of IAH in a high risk population on admission and throughout the entire ICU stay, following WSACS consensus guidelines, and exclusively in critically ill medical patients. Reintam et al. [17
] and Vidal et al. [18
] have also used WSACS definitions for IAH, and they report incidence rates of 37% in a mixed population, and 43% in the medical subgroup of a mixed population, respectively.
Numerous conditions predispose to IAH development, and our results are consistent with those from other studies [20
]. In the high risk group, we found an association between IAH and the CRF proposed by the WSACS, specially the increased abdominal content. Obesity and the number of CRF for IAH during ICU stay were independent predictors of IAH development. Data on fluid balance were only qualitative (positive or negative), and the association between IAH and massive fluid loading in the presence of sepsis and/or capillary leak described in the literature could not be studied.
Severity scores and organ failure in patients with IAH
Increases in IAP have deleterious effects on end-organ system function by compromising perfusion, thus leading to multiorganic dysfunction (MOD) and poor outcome. In our population, IAH was significantly associated with higher severity of illness scores and incidence of organ failure. In addition, the study design allowed us to compare organ impairment during ICU stay between patients with and without IAH (Table ). Previous studies analyzed organ impairment only during the first days of ICU stay, thus making it difficult to study the association between MOD and IAH throughout the entire ICU stay.
Characteristics of IAH
WSACS gives special attention to the IAH classification; this correlates with the patient's profile, namely, whether the patient is medical or surgical [17
]. Classification of IAH as acute and with high IAP grades (III-IV) is typical for surgical patients, whereas classification of IAH as subacute and with low-moderate IAP grades (I-II) is characteristic for medical patients [17
]. Our results in medical patients are consistent with this observation. The IAH classification varies depending on the study; however, no studies include all the classification types currently recommended by WSACS. In summary, a complete classification is important for planning future studies and comparing results, and the differentiation between medical or surgical patients is, in our opinion, necessary for a correct analysis based on the differences with regard to the causes and evolution of IAH in those two types of patient's profile.
Fourteen patients reached grade III of IAH, and five grade IV, but none of them developed ACS because a sustained elevation of IAP > 20 mmHg associated with organ deterioration was not detected. In concordance with this, no patient required surgical abdominal decompression.
Prognostic implications of IAH
Development of IAH has been described as an independent predictor of mortality in mixed populations [20
]. In our study IAH was a non-independent predictor of mortality, and this supports the fact that IAH was a marker of mortality in association with other clinical factors. In accordance with this, patients with IAH had higher severity scores and number of CRF for IAH that can independently increase mortality risk. In fact, the number of CRF during ICU stay was an independent predictor of mortality in the group with IAH.
We found a high rate of resolution of IAH in our medical patients, but non-resolution was an independent predictor of mortality, resulting in a considerably high OR (non-resolution meant that death was 13 times more likely). Treatment of IAH was nonsurgical in all cases and none developed ACS, although no conclusions can be drawn in the absence of clinical intervention data. Management guidelines for IAH have been proposed by the WSACS [24
] although they are based on studies that analyze therapeutic strategies in isolation and in different patient profiles [36
Limitations of the study
IAP was not measured in all cases (only in those with ≥2 CRF), and patients with < 2 CRF and no IAP recorded might have developed IAH during their ICU stay (e.g., patients with mechanical ventilation and therefore, diminished abdominal wall compliance but with no RF in other categories and no IAP measured). Therefore, the study followed the WSACS screening recommendations but was not designed to validate them in order to identify a subgroup of high risk patients for IAH development, as compared with a low risk subgroup. The study was underpowered to analyze each RF of the four categories as predictors of IAH (small sample size) or the IAP level as a predictor of mortality (the IAPmean in the patients with IAH were relatively low to perform comparisons in the outcome). Not all patients who had their IAP measured were sedated, which could have caused a falsely elevated IAP. Although resolution of IAH was a prognostic factor, our study was not designed to focus on therapeutic management of IAH, and no conclusions can be drawn or ascertainments can be made about the strategies that were effective.