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Use of ultrasound in the evaluation and treatment of intraabdominal hypertension and abdominal compartment syndrome


Overview

Intraabdominal hypertension (IAH) and abdominal compartment syndrome (ACS) are well-documented causes of morbidity and mortality in the critically ill. IAH and ACS may affect almost every organ system. The World Society of the Abdominal Compartment Syndrome ( www.wsacs.org ) has set forth a number of definitions based on clinical evidence and expert opinion. Intraabdominal pressure (IAP) is normally 5 to 7 mm Hg in adults. IAH is defined as sustained or repeated pathologic evaluation of IAP greater than or equal to 12 mm Hg. This is further subdivided into four grades based on pressure value. ACS is sustained IAP greater than 20 mm Hg, associated with new organ dysfunction/failure. Usual causes of IAH and ACS include intraabdominal hemorrhage, pneumoperitoneum from perforated viscus, and third spacing of fluid during massive resuscitation ( Figure 46-1 ).

Figure 46-1, Free abdominal fluid in the right upper quadrant.

Pathophysiologic effects

Cardiovascular

Cardiac dysfunction occurs in patients with IAH and ACS because of increased intrathoracic pressure (ITP) resulting from upward diaphragm displacement. This increased ITP causes a significant decrease in venous return, thereby reducing cardiac output. There is a clear association between IAP and ITP. Fifty percent of the IAP is transmitted and affects the ITP. Cheatham et al point out that catheter-based hemodynamic measures, such as pulmonary artery occlusion pressure and central venous pressure, therefore have significant limitations as indexes of volume status in the face of IAH. They proposed the idea of using volumetric measurements, such as end-diastolic volume index, in directing adequate resuscitation in these patients. Anticipating the relationship between IAP and ITP and possibly using volume indexes for estimation of preload could help to prevent inadequate resuscitation and inappropriate use of vasoactive agents in patients with IAH. Several ultrasound (US) methods are used to guide therapeutic strategies in the face of IAH. Resuscitation efforts geared toward a right ventricular, end-diastolic volume index, goal-directed model have been shown to reduce the incidence of multiple organ failure and death.

Respiratory

IAH results in increased ITP by pulmonary parenchymal compression and reduced chest wall compliance. Parenchymal compression leads to pulmonary hypertension. To overcome the alveolar compression and atelectasis, positive end-expiratory pressure (PEEP) is increased or added to maintain oxygenation and ventilation. Aggressive PEEP can result in not only opening up atelectatic areas of the lung but also overdistending normal lung and inhibiting adequate ventilation. ,

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