In short, the consequences of malnutrition in critical illness are as follows:
- Poor wound healing
- Impaired immune function and increased risk of sepsis
- Muscle wasting due to protein catabolism:
- Decreased ventilatory drive
- Weakness complicating separation from the ventilator
- Increased duration of ventilation, with associated complications (eg. increased risk of VAP)
- Weakness complicating physiotherapy and mobilisation
- Exposure to the complications of immobility, eg. DVT
- Cardiomyopathy as a consequence of atrophy
- Mucosal atropthy and diminished barrier function of the gut
- Apathy and depression
- Increased duration of ICU stay
- Increased in-hospital mortality
The disadvantages of poor nutrition are amplified in the hypercatabolic state of criticall illness. In the ICU, in most circumstances one would rather be fat than skinny. Question 27 from the second paper of 2013 asked about the effects of malnutrition on the critically ill patient. In short, it affects every system adversely, and the objective of this summary is to bring this complicated topic together into an easily remembered point-form list.
Airway-related consequences of malnutrition
- Malnutrition leads to the development of premature age-related changes in the trachea (de Andrade et al, 2012). In other words, you may be thirty with an eighty year old trachea.
- Delayed healing of the tracheostomy wound results from protein malnutrition
- Breakdown of the wound, and the need to up-size the tube as the patient loses weight
- Greater risk of periprocedural complications (eg. puncture of the posterior wall)
- Greater risk of tracheal cartilage damage, as the malnourished trachea is less elastic
Respiratory consequences of malnutrition
There are numerous excellent sources for this. The 1984 article by Rochester and Esau is an excellent primer on the effects of malnutrition on the respiratory system. Unless otherwise noted, the information below comes from this article.
Decreased respiratory muscle mass
- In patients who are at ~ 70% of their ideal body mass, the diaphragmatic muscular mass was reduced by 43%.
- Half of this reduction is due to thinning of the diaphragm, and half is due to loss of its length.
- Intercostal and accessory muscles are also wasted
Decreased capacity to compensate for respiratory disease
- There is a diminished ability to compensate for an acute increase in demand: there is decreased respiratory muscle bulk, and respiratory muscle fatigue occurs with less effort.
- Thus, these patients are more susceptible to respiratory failure.
- This is illustrated in the increased propensity of malnourished COPD patients to undergo mechanical ventilation, as compared to their well-nourished counterparts (Laaban et al, 1993)
- Respiratory muscle loss results in a greater propensity towards hypercapneic respiratory failure, and also in a decreased capacity to compensate for a metabolic acidosis.
Decreased ventilatory drive
- Nutritional depletion affects the CNS respiratory drive centre.
- Nutrient-restricted volunters (on 550kcal/day) had a 58% reduced ventilatory response to hypoxia.
- The vital capacity was reduced by 37% from expected normal values in malnoruished individuals who did not have respiratory disease
- In these people, maximum inspiratory and expiratory pressures were also reduced by 63% when compared to healthy age-matched controls.
Structural changes of lung tissue
- Lungs of rats starved to lose 40% of their total body weight demonstrated emphysema-like changes (Sahebjami et al, 1981)
- There was also a decrease in the surfactant properties of the surfactant
Prolonged ventilator weaning
- Decreased muscle mass and easier fatiue gives rises to poorer response in spontaneous breathing trials; these people tire faster and require either shorter trials, or a slower decremental wean of pressure support.
- Duration of ventilation correlates with the severity of nutritional depletion
Blunted respiratory immune defences
- Impaired regeneration of respiratory epithelium
- Decreased alveolar macrophage activity and number (Moriguchi et al, 1983)
- Decreased levels of IgA in the respiratory secretions
- Thus, increased incidence and severity of respiratory tract infections
Circulatory consequences of malnutrition
Cardiac muscle atrophy and reduced left ventricular function
- Cardiac mass is lost in proportion to body mass (Alden et al, 1987)
- This is mainly due to LV wall thinning
- Intrinsic properties of the myocardium are usually maintained.
- In severe marasmic kwashiorkor, circulatory failure with high vascular resistance is seen (Viart et al, 1977)
Bradycardia, hypotension, and decreased cardiac output
- Decreased whole-body metabolic rate tends to result in decreased demand on the myocardium.
- The energy-conserving adaptations to prolonged starvation include the following cardiac changes:
- Decreased cardiac contractility
- This results in a decreased capacity to compensate for increased demand, eg. septic shock state.
Consequences of micronutrient depletion, eg. Beri Beri
- Key micronutrients involved in cardiac metabolism are coenzyme Q10, l-carnitine, thiamine, and amino acids, including taurine (Soukoulis et al, 2009)
- The depletion of thiamine may give rise to a high output cardiac failure state (wet Beri Beri)
Endocrine and electrolyte consequences of malnutrition
Depression of hypothalamic and pituitary function
- Nonessential metabolic activities are suppressed, eg. gonadotropins and other reproductive hormone secretion is impaired
- In starvation, menstruation is suspended and amenorrhoea develops
- Thyroid function is depressed; the patients trend to hypothermia.
- Adrenal function is depressed, and though not clinically hypoadrenal the patient has diminished adrenal reserve and may more easily develop the relative adrenal insufficiency of critical illness
Electrolyte derangement in response to refeeding
- Refeeding syndrome is discussed in greater detail elsewhere.
- In brief, one can expect the following characteristic abnormalities:
Effects of malnutrition on the function of other organ systems
Increased mortality in malnorished renal failure patients
- Malnourished patients have increased mortality when they develop acute renal failure, particularly when it is in association with septic or cardiogenic shock (Fiaccadori et al, 1999).
- Haemodialysis patients are at risk of malnutrition because they suffer a constant amino acid loss via the circuit
- CRRT is particularly good at removing water-soluble vitamins
Gastrointestinal consequences of malnutrition
- Poor healing of anastomotic joins
- Poor barrier function of the gut
- Increased propensity to gastic ulceration (and decreased rate of gastric ulcer healing)
Haematological consequences of malnutrition
- Decreased haematopoiesis
- Iron deficiency
- Haematinic vitamin deficiency
- Structurally abnormal cells (eg. the macrocytosis of B12 depletion)
- Diminished synthesis of clotting factors
Immunological consequences of malnutrition
- Decreased immune cell number and function
- Decreased wound healing
- Decreased phatgocytosis
- Decreased synthesis of complement proteins
- Decreased synthesis of immunoglobulins