Intra-abdominal hypertension associated with acute lung injury: effects on intracranial pressure.

Intra-abdominal hypertension (IAH) is defined as intra-abdominal pressure (IAP) above 12 mmHg and may be categorized as Grade I (12-15 mmHg), Grade II (16-20 mmHg), Grade III (21-25 mmHg) or Grade IV (> 25 mmHg). Recurrent or persistent IAP above 20 mmHg, in association with failure of at least one organ, is called Abdominal Compartment Syndrome (ACS). The mortality and morbidity of IAH and ACS are high and may reach 100% for unattended ACS. Deleterious effects of increased intra-abdominal pressure are not limited to the abdomen but finally impact the pressure balances on other organ systems such as the respiratory, cardiovascular and cerebral systems.(1-2) On the chest, IAH displaces the diaphragm cranially, thereby reducing the intra-thoracic volume and increasing the intra-thoracic pressure (ITP); this reduces the compliance of the chest wall, lung and heart cavities, resulting in both respiratory and cardiovascular effects.(3-4) Considering the head, IAH increases the intracranial pressure (ICP) and reduces the cerebral perfusion pressure (CPP).(3) Increased ICP is ascribed to the increased central venous pressure (CVP), reduced brain venous flow and reduced lumbar venous plexus flow, with concomitant imbalance of the intracranial contents, as proposed by Monroe-Kellie.(3-5) The mechanic effects of inferior vena cava compression during IAH reduces lumbar venous plexus flow and may also be responsible for the increased ICP.(5) Not only IAH, but also the increase in ITP due to mechanical ventilation, may determine ICP changes. The use of high inspiratory pressure and positive end-expiratory pressure (PEEP), which results in increased airway pressures, can lead to increased ITP and consequently, to increased central venous pressure, resulting in increased ICP. However, the effects of positive pressure ventilation on ICP are apparently influenced by several factors, including pulmonary and cerebral compliance.(6) In the study entitled “Modulation of intracranial pressure in an experimental model of abdominal hypertension and acute lung injury,” the authors evaluated the association between IAH and pulmonary injury on the ICP. According to their results, this interaction impacted the ICP more significantly than IAH alone. The applicability of these findings is immediate.(7) The authors found that plateau (Pplateau ), Peak (Ppeak ) and Pleural (Ppl) pressures were significantly increased with IAH and acute lung injury (ALI), without significant hemodynamic changes. With respect to the association of IAH, ALI and 27 cmH2O PEEP, significant hemodynamic Antonio Luis Eiras Falcão1, Douglas Guimarães de Oliveira2