Whole Lot of Pressure

The maintenance of mean arterial pressure to prevent tissue dysoxia and conserve organ function is central to the management of the critically ill. In patients with increased capillary permeability this is often achieved by administration of large volumes of IV fluids. However, resuscitation volumes of >5 litres in the first 24 hours are associated with raised intra-abdominal pressure (IAP). Although Conall & Nate have mentioned it before, a recent review of intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS) by Mohmand & Goldfarb presents an excellent opportunity to review the topic.

IAH is defined as sustained or repeated elevation of intra-abdominal pressure >12mmHg, and ACS as an IAP >20mmHg associated with new organ dysfunction. IAP is easy to measure, either through the transduction of pressure in the bladder via an indwelling urinary catheter, or using an NG tube in patients whom bladder pressure measurement is not feasible.

Estimates of the prevalence of IAH and ACS suggest figures of approx. 60 and 10% respectively in non-selected ICU populations. As well as large volume fluid resuscitation, risk factors for IAH/ACS include trauma, abdominal surgery, mechanical ventilation, increased abdominal contents due to ileus or ascites, and increased capillary leak secondary to sepsis, pancreatitis, coagulopathy etc.

The review of Mohmand & Goldfarb collates a number of studies showing that IAH is an independent predictor of both mortality and the development of AKI. Indeed, they report on one study, which found that IAH was the best single predictor of the development of AKI after shock.

As regards management; severe ACS requires abdominal decompression by laparotomy, whereas medical strategies to reduce IAH include drainage of intra or extra-luminal contents, reduction of capillary leak and improvement of abdominal wall compliance. Given the link with volume expansion it is tempting to suggest a role for renal replacement therapy and ultrafiltration. However, good quality data is currently in short supply and probably all that can be surmised from existing studies is that aggressive continuous venovenous haemodiafiltration can be used to reduce IAP.

Clearly, IAP is extremely important to bear in mind when approaching AKI in the ICU. However, whether renal replacement therapy will be able to offer improvements in outcomes requires significant further study.