When weaning is unsuccessful for a patient who successfully performs an SBT, which of the fol-lowing factors should be assessed? 1. Cardiac factors 2. Nutritional status and respiratory muscle strength 3. Acid-base status 4. Psychological factors

Question

a. 1 and 3 only
b. 2 and 4 only
c. 2, 3, and 4 only
d. 1, 2, 3, and 4

Answer 1

ANS: D
Category Factor Mechanism Clinical Presenta-tion
Cardiac status Acute left-ventricular fail-ure Increased preload because of in-creased venous re-turn and decreased pulmonary capillary compression as in-trathoracic pressure is reduced. Weaning fails, often after patient does well initially for 30-60 minutes; may develop acute res-piratory and/or metabolic acidosis, hypoxemia, hypo-tension, chest pain, and cardiac dysrhythmias.
Acid-base status Acute alkalosis in patient with under-lying carbon diox-ide (CO2) retention Loss of preexisting metabolic compen-sation for hyper-capnia; inability to sustain required and WOB. Patient with COPD or other cause of chronic respiratory acidosis before acute insult fails weaning after sev-eral days of ventila-tion to a PaCO2 lower than the pa-tient's pH-compensated level.
Respiratory alkalo-sis Depression of ven-tilatory drive by hypocapnia and al-kalemia. PaCO2 rises and pH falls during wean-ing attempt; patient is said to fail wean-ing if some arbitrary change in these values (e.g., 10 mm Hg increase in Pa-CO2) is used as a criterion for failure.
Metabolic acidosis Increase in ventila-tory demand to compensate for res-piratory alkalosis. Patient may be una-ble to sustain re-quired increase in and WOB to main-tain a lower PaCO2 to compensate for a lower HCO3−.
Metabolic status Hypophosphatemia and hypomagnese-mia Ventilatory muscle weakness. Patient weaning fails because of rapid shallow breathing, respira-tory distress, and acute respiratory acidosis; maximal inspiratory pressure is decreased.
Hypothyroidism Decreased ventila-tory drive with pos-sible ventilatory muscle weakness. Rare cause of weaning failure that occurs because of acute respiratory acidosis with or without respiratory distress.
Drugs Narcotics, seda-tives, tranquilizers, and hypnotics Depression of ven-tilatory drive. Patient fails wean-ing because of acute respiratory acidosis in the absence of tachypnea and res-piratory distress.
Neuromuscular blocking agents Ventilatory muscle weakness; delayed clearance in patient with renal insuffi-ciency. Patient weaning fails because of rapid shallow breathing, respira-tory distress, and acute respiratory acidosis; maximal inspiratory pressure is reduced.
Ventilatory muscle weakness caused by acute myopathy, especially in pa-tients who have re-ceived high-dose systemic cortico-steroids. Same as above; may have elevated mus-cle enzymes; can last for weeks or months.
Aminoglycosides Neuromuscular blockade. Very rare cause of weaning failure that occurs because of rapid shallow breathing, respira-tory distress, and acute respiratory acidosis; maximal inspiratory force is reduced.
Nutrition Overfeeding Increased CO2 production, espe-cially with exces-sive carbohydrate calories. Patient fails wean-ing because of ex-cessive ventilatory demand (high re-quirement to keep PaCO2 normal); un-usual cause of weaning failure un-less very large ca-loric loads are ad-ministered.
Malnutrition Effects of acute illness; preexisting nutritional deficien-cies. May contribute to ventilatory muscle weakness, de-creased ventilatory drive, impaired immunologic func-tion, fluid retention, depression; distin-guishing this from other factors is dif-ficult.
Psychological status Agitation; "psy-chological ventilator dependence" Anxiety, fear, de-lirium, ICU psycho-sis, or influence of preexisting person-ality factors. Patient becomes agitated and pan-icky during attempt to reduce or discon-tinue ventilatory support; can be said to cause weaning failure when other factors are absent.
Lack of motivation Depression, effects of drugs, organic brain dysfunction, or influence of preexisting person-ality factors. Patient refuses to participate in care (e.g., mobilization, bronchial hygiene, physiological meas-urements); flat af-fect and immobility in bed; considered when other factors are absent.