Page 76 - Clinical Application of Mechanical Ventilation

42 Chapter 2

and infectious states causing increased metabolic rate. Most mechanically ventilated
patients who are fed enterally do not receive their energy requirement mainly because of
frequent interruptions in enteral feedings (O’Leary-Kelley et al., 2005).
Inadequate nutrition leads to protein catabolism and a loss of muscle performance
(MacIntyre, 2001). Undernutrition in patients is found to deplete their stores of glyco-
gen and protein in the diaphragm. In addition, COPD patients who have nutritional
deficits may develop muscular dysfunction and peripheral muscle waste (Ambrosino
et al., 2004). Since the diaphragm is the major respiratory muscle, loss of muscle mass
in the diaphragm may reduce the efficiency of spontaneous ventilation.
Energy requirements for critically ill patients are normally computed by using
the Harris-Benedict equation. This equation estimates the resting energy expendi-
ture (REE) based on weight, height, age, and gender. The REE is not equivalent to
the total number of calories required by a given patient. Rather, the patient’s total
calorie needs are estimated by multiplying the REE by an appropriate activity or
stress factor. In ventilator patients who are hypermetabolic or hypercatabolic (i.e.,
infection, trauma, burns), a correction factor is included to allow for additional
metabolic needs. It ranges from 1.2 to 1.6 times the REE for severe infection and
1.5 to 2.1 times the REE for burns. For most hospitalized patients under mild to
moderate stress, the calorie requirement may be estimated by using 25 to 35 cal/Kg
(Parrish et al., 2003).

Nutrition and the Work of Breathing

Total parenteral nutrition (TPN) or hyperalimentation is a complete nutritional
total parenteral nutrition
(TPN): Complete nutritional program provided to patients by any method (usually intravenous) other than the
support provided to the patient by intestinal route. It is often used to support and supplement a patient’s nutritional
any method (usually intravenous)
other than the intestinal route. needs with a hypertonic solution consisting of amino acids, glucose, vitamins, elec-
trolytes, and fat emulsion. When TPN is used, it is essential to keep the amount
of dextrose (a carbohydrate) to a minimum, as it can cause lipogenesis and increase
O consumption and CO production. Contribution to the total caloric needs by
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glucose should be in the range of 40% to 60% (Brown et al., 1984).
Since hydrous dextrose generates 3.4 kcal/g and fat emulsion provides 9.1 kcal/g,
Since fat emulsion fat is the ideal source of energy for patients who have restricted fluid intake. Fat also
provides 9.1 kcal/g, it provides
maximum caloric intake with reduces CO production, a by-product of glucose metabolism, thus reducing the
2
minimum CO 2 production. work of breathing (Brown et al., 1983). A fat-based TPN should be considered for
patients with significant or persistent CO retention, as fat emulsion may provide
2
maximum caloric intake with minimum CO production.
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It is important to note that the work of breathing is significantly increased in
patients receiving high caloric intake by means of TPN. The increase in work of
breathing is primarily due to increases in oxygen consumption and carbon dioxide
production during TPN (van den Berg & Stam, 1988). Mechanical ventilation and
weaning strategies must take this condition into account in order to provide ad-
equate ventilatory support.
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The increase in VCO causes a rise in PaCO , resulting in respiratory acido-
2
2
sis. Ventilatory failure can occur if the patient is unable to increase ventilation in

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