Ileus in critical illness

Introduction

Evidence of ileus, gastroparesis, or small bowel dysmotility is common in the setting of critical illness. Data on the incidence of ileus estimate an occurrence ranging from 50% to 80% in critically ill patients, with an average incidence of 38%. In the postoperative surgical intensive care unit (ICU), evidence of postoperative ileus ranges from 24% to 75%. Ileus can affect the entire gastrointestinal (GI) tract or just a segment, from the proximal gut, to the small bowel, to the colon. Definitions are poorly standardized and can be vague and elusive. Usually, ileus is characterized clinically by hypoactive or absent bowel sounds, abdominal distention, and delayed passage of stool and gas, but the process may involve nausea, vomiting, and abdominal tenderness. Objective measurements are limited mostly to research tools such as measuring gastric emptying by acetaminophen absorption tests or passage of radiolabeled carbon compounds, or small intestinal manometry. When present, ileus in critical illness has been shown to be associated with nutritional deficits, greater risk of aspiration, sepsis, prolonged mechanical ventilation, and increased allocation of healthcare resources. Although similarities exist between the ileus of critical illness and postoperative ileus in the surgical ICU patient, the precipitating factors are different. These entities share similar pathophysiologic patterns, and both are usually self-limited, tending to resolve spontaneously within days.

Many aspects of ileus are counterintuitive. Bowel sounds are not needed or required to initiate nutritional therapy, as recommended by the societal guidelines. A nasogastric tube placed for gastric decompression in postoperative ileus would be expected to benefit the surgical patient, but instead actually worsens outcome by increasing the incidence of pneumonia and atelectasis and leading to slower return of GI function. Surprisingly, gastric feeding is well tolerated in over 90% of critically ill patients. When the level of infusion of formula within the GI tract is diverted from the stomach to the small bowel, there is little change in outcome. Although multiple findings of GI dysfunction do reflect gastroparesis or intestinal dysmotility, other signs and symptoms are often included in the definition, which may or may not be related, such as diarrhea, GI bleeding, or increased abdominal pressure.

What continues to stoke interest in this entity is the correlation between ileus and adverse outcomes. The link to adverse outcome is shown in studies where evidence of GI dysfunction has prognostic value and is associated with increased ICU length of stay, duration of mechanical ventilation, and even mortality. Ileus is also linked to decreased delivery of enteral nutrition (EN), feeding intolerance, or feeding failure. The presence of ileus does identify a patient at high nutritional risk, a patient who may be more difficult to feed, whose risk of complications is increased, and whose delivery of EN should be provided with more caution. Critically ill patients need the therapeutic advantage of early EN; however, resolution of the ileus is important to facilitate further delivery of the nutritional regimen.

Defining gastrointestinal dysfunction

Definitions for ileus and GI dysfunction in critical illness vary over a wide spectrum of signs and symptoms. GI dysfunction usually relates to disordered motility and is most often defined by a constellation of symptoms, including nausea, vomiting, regurgitation, abdominal distention, and hypoactive-to-absent bowel sounds. ^, Feeding intolerance is defined separately as a reduced delivery of EN for whatever reason. ^, Often, the definitions of GI dysfunction have an overreliance on gastric residual volumes (GRVs) to make the definition. Additional factors are added to the definition in a somewhat haphazard manner, such as intraabdominal hypertension, overt GI bleeding, and diarrhea.

There is a difference in the definitions of ileus between a static score of signs and symptoms seen on admission to the ICU and a more graded series of findings suggesting a spectrum of increasing severity. GI dysfunction is more of a static definition and is a greater issue on admission to the ICU. GI dysfunction in the acute and immediate postacute phases of critical illness is often influenced by prolonged bed rest, increasing disease severity, comorbidities, the metabolic state, and use of opioid narcotics. ^, Feeding intolerance, on the other hand, is a more dynamic process and is a greater issue with initiation and advancement of early EN. ^, Feeding intolerance is more likely to be influenced by nursing care, the existence of protocols, the culture or leadership of the ICU, and whether GRVs are used. A component of “grading” implies the continuum of a process over a spectrum from mild to severe degree, a contrived definition where such a spectrum may not exist. ^{, , ,} The designation of increasing grades of GI dysfunction has been defined by a greater number of GI signs and symptoms, by a reduction in the delivery of EN, or by the finding of additional complications such as organ failure (as measured by Sequential Organ Failure Assessment or SOFA score), the presence of overt GI bleeding, or the development of intraabdominal hypertension. ^, Other systems designated an increased grade by whether there was a positive response, a poor response, or no response to therapy.

What sustains interest in GI dysfunction is its correlation with adverse outcomes. Multiple studies have shown that evidence of ileus or GI dysfunction upon admission to the ICU is associated with prolonged duration of mechanical ventilation, increased ICU length of stay, and greater mortality. ^, In those studies describing a spectrum of severity of GI dysfunction, increasing grades were shown to be associated with a demonstrated stepwise reduction in survival. ^{, ,} In one study, increasing grades of an abdominal GI score correlated with greater 28-day and 90-day mortality.

For the critically ill patient receiving early EN in the ICU, use of these signs and symptoms of ileus or GI dysfunction to define “intolerance” is problematic. No standardization of the practice exists. There are over 40 definitions of feeding intolerance in the literature, and 88% of them rely heavily on the use of GRVs. There is large interobserver variability, such that hypoactive bowel sounds had to be removed from the definition in one study because of discrepancies between clinical participants. Clinical conditions can change the interpretation of these signs and symptoms. Abdominal distention, reduced bowel sounds, and cramping may be interpreted differently in a patient with hemodynamic instability on vasosuppressive therapy compared with a patient who is hemodynamically stable and not requiring these agents. A change in symptoms as EN is initiated may be more important than the findings of ileus on admission to the ICU. Homeostatic proteins such as albumin, prealbumin, and transferrin have prognostic value on admission but provide no nutritional information and are not markers of nutritional status or adequacy of nutritional support. Feeding intolerance is often defined by the reduced delivery of EN as a percentage of goal requirements, but this parameter can only be measured retrospectively. Protocols in the ICU may affect the perceived incidence of feeding intolerance, as protocols that direct slow ramp-up and encourage reaching only 80% of goal requirements may be misinterpreted as feeding intolerance.