Which symptom is a sign of impending respiratory failure?

The symptom of drooling due to an inability to swallow can indicate impending respiratory failure because it often suggests significant dysfunction of the neuromuscular system or an obstruction in the airway. When a patient cannot swallow, it may be a result of impaired muscle control or nerve function, which can compromise the ability to maintain an open airway. This symptom is particularly concerning in cases of conditions like myasthenia gravis, stroke, or severe respiratory distress, where the patient may also struggle to manage secretions effectively, increasing the risk of aspiration and further respiratory compromise.

In contrast, the other symptoms, while potentially serious, are not direct indicators of impending respiratory failure. Coughing with purulent sputum typically suggests a respiratory infection but does not indicate that respiratory failure is imminent. Fever with petechiae may suggest an infection or systemic condition but again does not directly imply respiratory failure. Surgical site pain without drainage is more related to post-operative complications rather than a signal of respiratory decline. Therefore, drooling due to an inability to swallow is the most indicative sign of potential respiratory failure among the options provided.