Etiologies | Investigations | Treatment | |
Respiratory/ventilatory | |||
Increased ventilatory demand | Hypoxemia (eg, atelectasis, morbid obesity, abdominal distension, lung disease, sepsis), elevated dead space (eg, hyperinflation, pulmonary embolism, dehydration), excess carbon dioxide production (eg, fever, infection, overfeeding [refer to nutritional below], metabolic acidosis, or neuropsychiatric factors [eg, delirium, anxiety, pain]). | Clinical examination including neurologic examination, chest radiograph, arterial blood gases, routine chemistries, thyroid function tests, nutrition assessment, and occasionally CT chest and/or abdomen or CT angiography. Rarely, nerve conduction studies or bronchoscopy. |
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Increased resistive load | Bronchoconstriction (eg, COPD, asthma), airway edema (eg, lower respiratory infection), secretions (eg, tracheobronchitis, pneumonia), equipment issues (refer to ventilator circuit below). | ||
Increased elastic load | Dynamic hyperinflation (eg, COPD, asthma, increased minute ventilation), alveolar filling (eg, pulmonary edema), atelectasis, pleural disease (eg, pleural effusion, pneumothorax), chest wall disease, or abdominal distension (eg, morbid obesity, ileus, ascites). | ||
Reduced neuromuscular capacity | Electrolyte abnormalities (eg, hypophosphatemia, hypomagnesemia, hypocalcemia, hypokalemia), medications (eg, steroids, neuromuscular blocking agents), malnutrition (refer to nutritional below), hypothyroidism, systemic inflammation (eg, sepsis), neuropathy (eg, Guillain-Barré syndrome, critical illness polyneuropathy), and myopathy (eg, critical illness myopathy). | ||
Decreased ventilatory drive | Excess sedation, metabolic alkalosis (eg, nasogastric suctioning, volume depletion, diuretics and other causes of chloride depletion), central nervous system disease (eg, stroke, encephalopathy), central sleep apnea, or obesity hypoventilation syndrome. | ||
Cardiac | |||
Weaning may induce myocardial ischemia in susceptible patients. | A continuous multi-lead EKG during SBTs or EKG pre- and post-weaning trial. BNP or N-terminal pro-BNP pre- and post-weaning trial. Transthoracic echocardiography. Rarely, cardiac catheterization. |
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Pulmonary edema may develop in patients with cardiac dysfunction or ischemia. | |||
Fluid overload may present similarly in patients with normal cardiac function. | |||
Psychological | |||
Psychological issues (eg, depression, anxiety, delirium, pain) and oversedation may limit ventilation and impede cooperation with an SBT. | Clinical history and examination including pain assessment. |
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Ventilator circuit | |||
Equipment dead space, circuit compliance, gas compression volume, exhalation valve dysfunction, and increased resistance (eg, ETT luminal narrowing due to inspissated secretions and debris or small-sized ETT). | Examine waveforms (eg, ventilator asynchrony), ventilator pressures (eg, peak inspiratory pressure, plateau pressure), and equipment (eg, blocked exhalation valve, excess condensation). |
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Nutritional | |||
Protein catabolism and underfeeding leading to respiratory muscle weakness. Overfeeding leading to increased carbon dioxide production and increased ventilatory load. | Calculate nutrition needs. |
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