The causes of abdominal compartment syndrome are diverse, with many patients having several. In many cases, the cause may be obvious based on the clinical scenario. In other situations, CT scan may be helpful to clarify the cause of abdominal distension.
pressure transduction between different compartments
Although we often think only about the abdominal compartment, there are three compartments as shown below.
The diaphragm is a thin strip of muscle, across which pressure can be transduced in either direction. Thus, there is considerable pressure transmission between the thoracic and abdominal compartments.
Elevated intrathoracic pressure leads to elevated central venous pressure, which ultimately increases the intracranial pressure (ICP).
“abdominal compartment syndrome” is a misnomer
Traditionally, the literature has used the following terminology:
Abdominal compartment syndrome refers to elevated intra-abdominal pressures causing organ failure.
Polycompartment syndrome refers to elevation of multiple compartment pressures, with organ failure.
However, due to pressure transduction between compartments, it's impossible to have truly isolated abdominal compartment syndrome. For example, elevated peak airway pressures are a classic component of “abdominal compartment syndrome.” Thus, to some extent, all patients with abdominal compartment syndrome actually have a polycompartment syndrome.
Appreciating the polycompartment nature of “abdominal compartment syndrome” has important clinical implications, for example:
Reduction of intrathoracic pressure may be beneficial for patients with intra-abdominal compartment syndrome.
Patients with abdominal compartment syndrome are at increased risk of elevated intracranial pressure.(11445709)
Abdominal compartment syndrome can cause failure of numerous (heart, lungs, kidneys, brain). In severe cases, this promotes a vicious spiral of multiorgan failure (e.g., when failure of the heart causes worsening failure of the kidneys). Indeed, it's likely that abdominal compartment syndrome is an occult driver of multiorgan failure among many critically ill patients.
The initial manifestations of abdominal compartment syndrome will vary depending on the patient's underlying physiology.
A patient with tenuous renal function could manifest with renal failure.
A patient with severe, chronic COPD might present with respiratory distress.
Compression of the inferior vena cava reduces preload.
Direct cardiac compression causes reduced cardiac filling and decreased contractility (including reduced compliance of the right ventricle).(30454823)
Filling pressures (e.g., central venous pressure) are elevated, but this doesn't reflect effective cardiac filling (because the pressure gradient between the vena cava and right ventricle is reduced).
Peripheral vascular resistance is elevated to to renal compression (which stimulates the renin-angiotensin-aldosterone system). This causes a maintained systolic blood pressure (despite a reduction in cardiac output).(32004192)
Mesenteric ischemia causes bacterial translocation into the bloodstream, which may cause systemic vasodilation and hypotension (late in the evolution of multiorgan failure).
Abdominal compartment syndrome may confound many approaches to hemodynamic optimization.
The systemic blood pressure is often preserved initially, until multi-organ failure develops.
Pressure on the diaphragm reduces thoracic compliance (e.g., an intra-abdominal pressure of 16 cm may reduce the pulmonary compliance by 50%).(30454823)
Atelectasis may occur due to compression of the lung bases, promoting hypoxemia and hypercapnia.
A non-intubated patient may experience increased work of breathing.
For an intubated patient, this may manifest as increased airway pressures on the ventilator.
Renal failure with reduced urine output is often the first sign of abdominal compartment syndrome (often occurring when intra-abdominal pressure rises >15 cm).(32004192)
Increased pressure may compress the kidney directly and also cause renal congestion (due to elevation of venal vein pressure causing impaired drainage of blood out of the kidney).
Brain hypoperfusion may increase cerebral vasopressin secretion, which also serves to reduce urine output.(33480617)
Oliguric acute renal failure is generally one of the earliest manifestations of abdominal compartment syndrome.
brain: increased intracranial pressure
Increased abdominal pressures will translate into increased intrathoracic pressures and increased central venous pressures. This can actually cause elevated intracranial pressure.
Abdominal pressures can be elevated despite finding a soft abdomen. Palpation is only ~50% sensitive for abdominal compartment syndrome.(31524716)
Examination demonstrating tense abdomen is ~80% specific for abdominal compartment syndrome.(12297912)
abdominal pressure measurement using a Foley catheter
Patient should be fully supine, intubated, and breathing passively on the ventilator (e.g., not coughing or bucking ventilator).
Measured at end-expiration.
Limitations on abdominal pressure:
(1) Pressures may vary across individuals (with obese patients having higher baseline pressure values).
(2) As with all physiological phenomena, pressure gradients are more important than absolute pressure values. Consequently, the intra-abdominal pressure value in isolation is inadequate to define abdominal compartment syndrome.
(3) Pressure measurements may be inaccurate in the context of pelvic pathology (e.g., hematoma directly compressing the bladder).
(4) Interpretation among patients who are not intubated and breathing passively on mechanical ventilation is challenging. If analgesia/sedation causes a normalization of the intra-abdominal pressure, this makes abdominal compartment syndrome unlikely.(32204721)
Some rough benchmark numbers for intra-abdominal pressure:
2-7 mm Hg: Normal for a non-obese person.
>12 mm Hg: Defined as intra-abdominal hypertension.
>15-20 mm Hg: Can cause organ failure.
>20 mm Hg: Cutoff used to define abdominal compartment syndrome.
>25-30 mm Hg: Usually causes organ failure, may require emergent decompression.
diagnostic criteria for abdominal compartment syndrome
Diagnosis requires two components:
(1) Sustained intra-abdominal pressure >20 mm.
(2) Organ failure attributable to elevated intra-abdominal pressure.
This diagnosis requires clinical judgement, since critically ill patients invariably have other causes of organ failure.
Sorting out whether organ failure is caused by abdominal compartment syndrome versus other causes can be murky and subjective.
💡 The kidneys are one of the most sensitive organs to increased abdominal pressure. If the urine output is adequate, it's considerably less likely that the patient has abdominal compartment syndrome.
There are many treatments of abdominal compartment syndrome other than surgical decompression of the abdomen. Thus, the inability to perform surgery should not lead to a sense of cynicism about the treatment of this disorder.
Abdominal perfusion pressure is the pressure gradient between the MAP and the abdominal compartment. This is the pressure that drives perfusion of all intra-abdominal organs (e.g., the kidney).
It is probably best to maintain an abdominal perfusion pressure >60mm.(12297912) Abdominal perfusion pressure <60 mm predicts the need for surgical decompression.(30454823) Consequently, the target MAP might be 60 mm plus the abdominal compartment syndrome, as shown below.
Hemodynamic interventions may be tailored to the particular patient. This will generally require vasopressors, because additional crystalloid may merely aggravate abdominal tissue edema.(12799335)
Target MAP > (60 mm + Abdominal Compartment Pressure)
volume removal if possible
Theoretically, volume removal is beneficial:
In many patients, compartment syndrome may be promoted by volume overload.
Efforts to remove volume (e.g., diuresis or dialysis) may be helpful.(25421925)
Unfortunately, in established abdominal compartment syndrome, the patient is often intravascularly volume depleted (despite tissue edema). This may make it difficult or impossible to remove fluid without worsening hemodynamics.
A more realistic fluid target might be to achieve a net even fluid balance.
(1) Abdominal compartment syndrome may compress the inferior vena cava (IVC), making it look empty!
(2) Avoid fluid administration. This may help temporarily, but fluid will often rapidly transudate into the tissues – which worsens swelling and increases intra-abdominal pressure (generating a futile cycle, as shown below).
Adequate analgesia and sedation may be helpful, perhaps to a slightly deeper level than the average ICU patient.
Paralysis may have various effects on intra-abdominal pressure:
(1) Relaxation of muscles in the abdominal wall will improve the abdominal compliance, thereby reducing the intra-abdominal pressure.
(2) Some patients are performing a substantial amount of the work of breathing, which tends to decrease their intrathoracic pressures. In this situation, paralysis will cause a transition to purely passive, positive-pressure ventilation – increasing their intrathoracic pressure, and thereby increasing the intra-abdominal pressure.(32204721)
Short-term paralysis may be trialed, but this doesn't appear to cause persistent improvement. Paralysis may be used temporarily as a bridge to another intervention (e.g., laparotomy).(27016163)
To keep this page small and fast, questions & discussion about this post can be found on another page here.
Adopting the mindset that the only treatment for abdominal compartment syndrome is laparotomy. This leads practitioners to ignore the diagnosis (“well, surgery isn't going to operate on them anyway…”). However, there are numerous non-operative therapies which may be quite effective.
Failure to consider abdominal compartment syndrome (this is a common phenomenon in all types of critically ill patients – not just surgical patients).
Over-interpretation of bladder pressure obtained in patients who aren't supine and breathing passively.
Abdominal compartment syndrome compresses the inferior vena cava (IVC), making it look empty. This may lead to erroneous decisions regarding fluid administration.
Avoid intubation of patients with borderline abdominal compartment syndrome if possible (pressurization of the thorax may worsen intra-abdominal pressure).
Epic lecture by Thomas Scalea on poly-compartment syndrome:
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