Nutritional Support and the Surgical Patient

Yvonne Huckleberry


Am J Health Syst Pharm. 2004;61(7) 

In This Article


When PN was first introduced into clinical practice, aggressive amounts of calories and protein were prescribed with the goal of repleting muscle and adipose tissue that had been lost with undernutrition. Case studies describe fatal consequences in patients fed up to 75 kcal and 3.5 g of protein per kilogram daily.[52] In recent years, calorie recommendations for NS have trended steadily downward based on observations of complications associated with aggressive overfeeding. These potential adverse effects include hyperglycemia, hypertriglyceridemia, hepatic steatosis, and hypercapnia prolonging dependence on mechanical ventilation.[53]

Although overfeeding surgical patients should be avoided, prolonged underfeeding may be equally concerning. This can compromise immune function, delay wound healing, exacerbate muscle wasting, and prolong the recovery of nitrogen balance and visceral protein levels. However, short-term hypocaloric feeding with 1-2 g of protein per kilogram per day, particularly in the acute phase of postoperative stress, may reduce metabolic complications while supporting a reduction in negative nitrogen balance.[54,55] Whether this leads to improved patient outcomes requires further investigation.

In an attempt to avoid prolonged underfeeding or overfeeding, the caloric requirements of the surgical patient should be individualized based on the assessment of a number of factors. First, age, sex, height, and weight should be noted. These characteristics are often entered into predictive equations used to assess caloric requirements ( Table ). Second, defining the preoperative nutritional status of the patient influences the estimation of caloric needs. For example, chronically undernourished patients are at increased risk of metabolic complications associated with overfeeding. Therefore, initial estimations of caloric needs should be conservative, and patients should be closely monitored through the first several days of therapy.[60] Third, the underlying pathophysiologic condition prompting surgery should also be noted. For example, large bone fractures or major burns can markedly increase energy requirements compared with chronic disease, in which long-term undernutrition may have slowed energy metabolism. Similarly, the surgical procedure should be included in the assessment. In contrast to minor surgeries, major surgical interventions can induce impressive hypermetabolism and increase the risk of postsurgical complications, such as wound infection, thus increasing energy requirements. These influences may be included as "stress factors," or additional variables in predictive equations used for hospitalized patients.

Clinicians debate whether the caloric estimates for PN should be expressed in terms of total or nonprotein calories. The argument for using nonprotein calories is based on the theory that protein should be spared for anabolism and not used for energy. Opponents argue that protein is not necessarily prioritized solely for anabolism in the presence of adequate calories and that caloric estimations should be consistent among enteral, parenteral, and oral routes.[61,62] There are few data suggesting that either method is superior.

Caloric requirements can be measured indirectly or estimated using traditional equations, as mentioned above. Indirect calorimetry is a noninvasive test that translates gas exchange across the lungs into a patient's daily energy requirements. While data exist that confirm the value of indirect calorimetry in determining caloric needs,[63] this technology is expensive and not available at all institutions. Further, the impact on outcomes of tailoring caloric provisions with indirect calorimetry has not been determined.

In the absence of indirect calorimetry, predictive equations are commonly used to assess patient requirements. According to the A.S.P.E.N. guidelines and Rollins,[37] calculations of caloric needs for catabolic patients usually fall between 25 and 30 total kcal/kg/day. However, some institutions have implemented a hypocaloric nutritional regimen for use during the acute-phase response in critical illness (described above) or for acutely ill obese patients. Protocols for use in obesity are based on small prospective studies demonstrating the achievement of a positive nitrogen balance in obese patients (>130% of IBW) given high protein, hypocaloric PN (e.g., 2 g of protein per kilogram of IBW, 50% of estimated caloric needs).[64,65,66] In contrast to short-term underfeeding during the acute phase of critical illness, hypocaloric feeding prescribed for obese patients is suggested to promote the utilization of endogenous fat stores for energy and can potentially be used for longer periods without compromising nutritional status. This same effect was not consistently observed in patients >60 years of age,[67] however, and the question of impact on patient outcomes is again raised.

One recent retrospective study evaluated the impact of hypocaloric EN on clinical outcomes in obese patients admitted to trauma or surgical intensive care units (ICU).[68] Patients were stratified to receive either a eucaloric tube-feeding regimen or hypocaloric feeding. Only patients receiving NS for at least seven days were included. The eucaloric group (n = 12) received >20 kcal per kilogram of adjusted body weight (ABW) and averaged between 1.3 and 1.8 g of protein per kilogram of IBW daily. ABW was defined as (current body weight - IBW) × 0.25 ± IBW. Twenty-eight patients receiving the hypocaloric regimen received <20 kcal/kg, with an average of 1.1-1.5 g of protein per kilogram of IBW daily. Endpoints of the trial included length of hospital stay, length of ICU stay, days of mechanical ventilation, days on antibiotic therapy, and survival. Results showed patients receiving the hypocaloric regimen experienced a shorter ICU stay (p < 0.03), fewer days on antibiotic therapy (p < 0.03), and a trend toward fewer ventilator days (p = 0.09). However, the impact on other clinical outcomes was not statistically significant. The authors concluded that hypocaloric EN is at least as effective as eucaloric feeding in critically ill obese patients. An adequately powered, prospective RCT is needed to confirm benefit in terms of clinical outcomes for obese surgical patients.


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