A 74 year old woman, in otherwise good health, tripped and injured her right leg 2 days previously and has been bedridden since the accident. Two hours ago, she became delirious. On physical examination, her temperature is 99 F, blood pressure is 120/70 mmHg, hea rate is 110, and respiratory rate is 32. Pulse oximetry shows an oxygen saturation of 80%, and a chest x-ray film is normal. Which of the following is the most likely diagnosis?

**Core Concept:** Delirium is a neuropsychiatric syndrome characterized by acute onset of confusion, inattention, and altered level of consciousness. It is usually multifactorial, involving both systemic and neurological causes. Hypoxia can lead to cerebral hypoperfusion, which is a risk factor for delirium.

**Why the Correct Answer is Right:** The patient's oxygen saturation (SpO2) of 80% on pulse oximetry is significantly low. This indicates severe hypoxia, which is the primary cause of her recent onset delirium. Furthermore, her respiratory rate is elevated (32 breaths/minute), suggesting that she is working harder to compensate for her hypoxia.

**Why Each Wrong Option is Incorrect:**

A. Hypotension (120/70 mmHg) is not a sufficient cause for her delirium, especially considering her elevated respiratory rate.
B. A chest X-ray is normal, ruling out pulmonary causes like pneumonia or congestive heart failure as potential etiologies for her hypoxia.
C. The patient's age is mentioned, but age alone is not the cause of her hypoxia or delirium.
D. The patient's blood pressure (120/70 mmHg) is within normal limits, making it less likely as an explanation for her hypoxia and delirium.

**Clinical Pearl:** Though the patient has a history of trauma and immobility, it is not the primary cause of her severe hypoxia. The patient has developed delirium secondary to her hypoxia, and treatment should focus on addressing the oxygenation issue first.

**Correct Answer:** D. Delirium can be a manifestation of hypoxia, regardless of age.

✓ Correct Answer: D. Pulmonary thromboembolism