We offer several arguments about why age and DMV are plausible determinants of outcomes. First, age is unquestionably an independent variable affecting physiologic function. For example, lung and kidney function diminish with age irrespective of disease. Superimposition of acute disease on reduced functional physiologic reserve increases the likelihood that acute disease will result in life-threatening critical illness. Advancing age also provides the opportunity for accumulation of environmentally and genetically determined illnesses and, therefore, greater risk of acute or chronic end-stage organ failure. Second, duration of MV often is a marker of not only respiratory system insufficiency, but also multisystem insufficiency. Patients may initially require MV for a primary lung process, but prolonged ventilation often is caused by a constellation of contributors, including acute or chronic disease, infections, malnutrition, complications of medications, and processes of care that unnecessarily prolong DMV. Moreover, positive-pressure ventilation,2–4 and interventions (eg, endotracheal tube placement, sedation, pain relief, and tube feeding) applied to facilitate MV3 may increase the risk for complications. Accordingly, it is physiologically plausible that DMV is strongly associated with outcomes. In many cases, DMV is a composite, surrogate marker for overall failure (where it is both cause and effect) because it marks both the severity of diseases that prolong MV and the erosive effects of the intervention itself. Thus, our findings may inform day-to-day care discussions. For example, if an 80-year-old patient is in the 5th day of receiving MV and is not rapidly improving, our study findings suggest that the likelihood of hospital discharge to home is low. It is worth emphasizing that such estimates are mere likelihoods and not certainties.16 Ultimately, decisions about the level of subsequent care should be informed by empirical data. Finally, some will argue that chronic and acute illnesses are the independent variables that predict outcome and that DMV is a dependent covariable. Unfortunately, daily acuity-of-illness scores (eg, acute physiology and chronic health evaluation, simplified acute physiology score) were not available. Hospital discharge diagnoses were available, which allowed us to adjust for illness severity based on Charlson score. The association of DMV and age with hospital outcomes persisted after adjustment for Charlson scores. We acknowledge that this methodology may not ensure complete adjustment for severity of illness.