Patients who are susceptible to chronic critical illness are as heterogeneous as the general ICU population (Table 18-2). Patients with postoperative complications from cardiac or abdominal surgery are at risk, and trauma patients are common as well. Medical patients with acute lung disease with or without multiorgan failure and patients with chronic lung disease (especially COPD) or neurologic diseases as their primary diagnoses are also susceptible. Critically ill patients admitted to the ICU with significant comorbidities are at higher risk, especially those with underlying heart disease, chronic obstructive pulmonary disease (COPD), and kidney disease. For surgical patients, preoperative instability, COPD, prolonged operation, and in the case of cardiac surgery patients, increased bypass time are important risk factors for prolonged mechanical ventilation.5 Development of nosocomial pneumonia, aspiration events, and failed extubations are additional proven risk factors for prolonged mechanical ventilation.6 In one predictive model, primary disease, Acute Physiology and Chronic Health Evaluation survey (APACHE III) score, age, COPD, prior functional limitations, and length of hospital stay prior to ICU admission were significant risk factors for mechanical ventilation for greater than 7 days.1 The acute physiology score and primary reason for ICU admission accounted for 0.66 of the explanatory power for the model. Of the variables in the acute physiology score, pH, PaCO2, PaO2/FiO2 ratio, albumin level, and respiratory rate were significant. Further development of clinically useful prediction models for prolonged mechanical ventilation would be of great benefit for resource planning in the ICU.