Introduction

Dogs and cats receiving mechanical ventilation represent one of the most critically ill and intense patient populations with regards to their nursing and supportive care needs. One of the challenges frequently encountered in critically ill patients is gastrointestinal (GI) dysfunction and many factors have been found to contribute to the development of GI complications in this patient population. Most commonly, they constitute a combination of underlying illness factors and therapeutic interventions, such as positive pressure ventilation.

In critically ill people, the presence of GI dysfunction has been associated with worse outcomes and it remains possible that this is, at least in part, due to its interference with the provision of nutritional support. Adequate nutritional support is especially important in the critically ill and mechanically ventilated patient to prevent malnutrition and associated negative effects. Respiratory muscle weakness, decreased wound healing, and impaired immune function have all been described as a result of malnutrition and they can lead to lower rates of successful ventilator weaning, increased patient morbidity, and mortality.

Definition and Pathophysiology

Even though intact GI function is an important factor in the outcome of critically ill patients, objective definitions of dysfunction have long been lacking, hampering both research in the area and the development of evidence-based management strategies. In 2012, the European Society of Intensive Care Medicine (ESICM) suggested the term acute gastrointestinal injury (AGI) to describe gastrointestinal digestive dysfunction in people and defined it as “malfunctioning of the GI tract in critically ill patients due to their acute illness.” A grading scheme for AGI severity was also provided, defining AGI grade I as risk of developing GI dysfunction or failure, AGI grade II as gastrointestinal dysfunction where the GI tract is not able to perform adequate digestion and nutrient absorption, AGI grade III as GI failure with complete loss of GI function, and AGI grade IV as GI failure with severe impact on distant organ function. Other than the digestive function, the GI tract also serves important barrier functions and roles as an endocrine and immune organ, all of which can be diminished in critical illness. Definitions, as well as standardized diagnostic and therapeutic approaches for dysfunctions of those aspects continue to be lacking.

Clinically, GI dysfunction can have various manifestations, such as stress related mucosal disease (SRMD) including mild to severe mucosal haemorrhage, GI hypomotility, vomiting, regurgitation, abdominal pain and or distension, diarrhoea, constipation, and enteral feeding intolerance.

The pathophysiology of GI dysfunction in the critically ill, mechanically ventilated patient is not completely understood, but several factors are thought to be contributing. Positive pressure ventilation increases pleural pressures and during the application of positive end-expiratory pressure (PEEP) intrathoracic pressures remain positive throughout the whole respiratory cycle. Reduced venous return and preload will result in decreased cardiac output and diminished splanchnic perfusion, reducing GI function. Systemic hypoperfusion can further lead to an upregulation of the renin-angiotensin-aldosterone axis and to an increase in endogenous catecholamines, both of which can exacerbate splanchnic hypoperfusion. Furthermore, the splanchnic vasoconstriction due to neurohormonal changes can continue to exist even after patients have been haemodynamically stabilized. Systemic inflammatory conditions and reperfusion injury of the GI tract both lead to cytokine release that can contribute to continued splanchnic hypoperfusion and impair intestinal smooth muscle motility. Additionally, medications that are commonly used in critically ill patients also exhibit a transient effect on gastrointestinal motility and barrier function and include the administration of opioid drugs, alpha adrenergic agonists, anticholinergic drugs, benzodiazepines, and other sedative drugs.

Incidence and Implications

Gastrointestinal complications are encountered commonly in the critically ill patient with up to 62% of human patients displaying at least one sign of GI dysfunction during at least one day of an intensive care unit stay. Examining the veterinary literature available makes it clear that, in contrast to human medicine, a smaller proportion of veterinary patients receive gastrointestinal function assessment and nutritional support throughout the course of mechanical ventilation.

In the few studies describing enteral nutrition in mechanically ventilated veterinary patients, nutritional support had to be interrupted in up to 60% of patients due to some form of GI dysfunction (e.g., regurgitation, high gastric residual volumes [GRV], and aspiration), putting patients at risk of experiencing ill effects from a negative energy balance.

Diagnosis and Management

The majority of symptoms of GI dysfunction (such as vomiting, diarrhoea, and macroscopic gastrointestinal bleeding) is well defined and can be diagnosed clinically. GI dysmotility and reduced gastric emptying leading to intolerance of gastric feeding is more difficult to recognize and is frequently indirectly assessed by measuring gastric residual volumes (GRV).

In human as well as veterinary medicine, there is no scientific evidence to define normal GRV. In a veterinary study comparing the tolerance of intermittent versus continuous enteral nutrition in critically ill dogs, no difference was found in the tolerance and GRV between the two modes of nutritional support and no association between GRV and vomiting or regurgitation frequency or aspiration of GI contents could be shown. Therefore, the use of GRV driven nutritional support remains controversial and should be applied with caution. Alternatively, GI motility can be assessed by contrast radiography or bedside ultrasonography, but these tools also only allow for its qualitative or semiquantitative assessment.

It is currently recommended by the ESICM to initiate enteral nutrition in patients suffering from AGI grade I within 24–48 h and to limit drugs impairing GI motility further. Patients suffering from AGI grade II should be treated with prokinetic drugs, while parenteral nutrition within 7 days of ICU admission should be considered in patients with AGI grade III. Emergency surgical interventions are usually required for patients suffering from AGI grade IV in which GI dysfunction has a severe negative impact on distant organ function.

In veterinary medicine, no evidence-based management guidelines for the treatment of GI dysfunction exist. The mainstays of treatment include haemodynamic stabilization of the mechanically ventilated patient to optimize splanchnic perfusion, treatment of the underlying disease and metabolic optimization of the patient, multimodal analgesia, early nutritional intervention, and pharmacological intervention using prokinetic drugs. A review of available prokinetic drugs is beyond the scope of this lecture but due to a lack of evidence for a single prokinetic drug of choice in small animal medicine, a multimodal approach to prokinetic pharmacotherapy is recommended.

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