Introduction

Critically ill small animal patients require a host of supportive measures including nutritional support to ensure successful management and resolution of the underlying disease. Preservation of nutritional status has important implications for immune function, and wound healing required for recovery. Patients receiving early enteral nutritional support (i.e., within 2 days of admission) have lower risks of developing nosocomial infections, require fewer days of mechanical ventilation and have overall better patient outcomes. However, provision of nutrition in this patient population (i.e., critically ill) is particularly challenging and measures must be taken to decrease risk of complications. Aspects of nutritional support that require special consideration include the timing that nutrition should be initiated, the route of feeding, the composition of the diet and other aspects of management that may impact nutritional management. The biggest obstacles for provision of nutrition support in critically ill patients includes intolerance to enteral feeding, difficulties with airway management and the reality that many of these patients may be cardiovascularly unstable, and therefore, priority is given to interventions with immediate patient benefits. Cardiovascular instability is regarded as a relative contraindication to enteral feeding and, therefore, many of these patients are not assessed as ready to receive nutritional support.

During critical illness, patients often develop gastrointestinal dysmotility problems that require a host of medical and nutritional strategies to manage these complications. These include the use of prokinetic agents (Table 1) and adjusted feeding strategies. Human critically ill patients often have gastrointestinal intolerance that is manifested by an increased gastric residual volume (GRV). Published human feeding protocols have tried to define the threshold volume above which feeding should be withheld as well as recommend the frequency of GRV assessment. This issue is of particular interest in mechanically ventilated patients so much of the data is derived from this population. For example, the GRV considered to be excessive in ventilated patients have ranged from 120 to 500 mL per aspiration [1.7 to 7.1 mL/kg assuming a 70 kg person]. A low GRV threshold results in multiple interruptions in enteral nutrition (EN) provision due to frequent assessments of GRV, discarding checked volumes, and withholding feeding or reducing the rate of feeding due to presumed high GRV result in overall decrease in caloric intake, failure to achieve caloric targets, increased use of parenteral nutrition, which may all negatively impact outcome. More recent data have questioned the relationship between increased GRV and higher risk of aspiration pneumonia as well as the need to check GRV more than once daily. In light of these findings, some authors recommend against adjusting enteral feeding unless GRV exceed 500 mL and to reduce checking GRV to only once daily or not at all. Unfortunately, there is virtually no information available in ventilated veterinary patients in regards to GRV and risk of feeding. While some authors recommended checking GRV before every feed, and that GRV should not be allowed to exceed 10 mL/kg, there is currently no data to corroborate whether such guidelines are necessary although such recommendations may be sensible. Holahan et al. (2010) could not relate GRV and gastrointestinal complications in dogs fed via nasogastric tubes and some of these dogs have >200 mL/kg of GRV, however, it is unknown if any of the dogs evaluated were mechanically ventilated. The median (range) GRV in this population of critically ill dogs was 4.5 mL/kg (0 to 213 mL/kg) (Holahan et al. 2010). As current human guidelines (Bankhead et al. 2009) state that enteral feedings should not be held unless GRV are >500 mL (approximately 7 mL/kg based on 70 kg person), it may be prudent to use a similar approach and so animals with GRV exceeding 10 mL/kg should be treated with prokinetic agents (See Table 1) and their feed reduced temporarily.

Table 1. Prokinetic agents used in small animals for management of gastrointestinal dysmotility disorders

Nutritional Management Strategies

In people, the provision of enteral nutrition can be delivered via nasogastric or nasojejunal feeding tubes. Nasojejunal feeding has been recommended as the preferred route in patients with a high risk of aspiration as well as in those patients for whom gastric feeding is not tolerated, or who have repeated high gastric residual volumes (McClave et al. 2009). Unfortunately, there is little veterinary data regarding nutritional support of small animal patients in relation to this problem. Upon reviewing available veterinary clinical studies on mechanical ventilation, as well as veterinary studies on assisted feeding and parenteral nutrition, there are only brief mentions of patients that were ventilated and received some form of nutritional support (King, Hendricks 1994; Reuter et al. 1998; Beal et al. 2001; Chan et al. 2002; Pyle et al. 2004; Lee et al. 2005; Armitage-Chan et al. 2006; Crabb et al. 2006; Hopper et al. 2007; Campbell et al. 2010; Holahan et al. 2010; Hoareau et al. 2011; Rutter et al. 2011; Gajanayake et al. 2013; Queau et al. 2013; Yu et al. 2013; Edwards et al. 2014). From the information available, it is clear that in some studies, only a proportion of ventilated patients (40 to 50%) receive any form of nutritional support, that a high proportion receive parenteral nutrition (up to 38%) and in those that have EN, it was stopped in up to 60% of patients due to regurgitation, high GRV and aspiration (Hopper et al. 2007; Rutter et al. 2011). Therefore, formulation of feeding recommendations in ventilated small animals must be mostly extrapolated from information available in other populations and species.

The first question relates to optimal timing of nutritional support initiation. The urgency of initiating nutritional support will depend on the nutritional assessment and nutritional status of the patient. As in other critically ill small animals, nutritional support should only be considered in cardiovascularly stable patients, but should generally be initiated within 48 to 72 hours of mechanical ventilation. Although avoidance of enteral feeding in mechanically ventilated patients may seem reasonable, parenteral nutrition should only be used if there are contraindications to enteral feedings such as persistent vomiting or regurgitation. Enteral access devices for ventilated patients include nasogastric, oesophageal, gastric, and nasojejunal feeding tubes. Nasogastric may be most appropriate for most ventilated patients, although some may benefit from post-pyloric placement of feeding tubes. Enteral feed may be delivered continuously or intermittently as there appears to be little difference in complication rates (Holahan et al. 2010; Campbell et al. 2010; Yu et al. 2013).

Summary

Critically ill patients are often the most critical and complex cases in any given veterinary hospital and present the veterinary team with a number of challenges. Determining ideal time for initiating nutritional support, the route of delivery, caloric targets, diet composition, and closely monitoring for tolerance of feeding, are essential components of the strategy to manage mechanically ventilated patients.

Keypoints

• Critically ill small animal patients require a host of supportive measures including nutritional support to ensure successful management and resolution of the underlying disease.
• Preservation of nutritional status has important implications for immune function, wound healing and recovery.
• Provision of nutrition in this patient population is particularly challenging and measures such as use of prokinetic agents, nasogastric or nasojejunal feeding tubes may be necessary.
• We do not yet have good data to make firm recommendations about how GRV should alter enteral feeding plans, though a sensible plan may include that GRV should not be allowed to exceed 10 mL/kg.

References

References are available upon request.