**Core Concept**
Improving oxygenation using a ventilator involves optimizing the ventilator settings to enhance gas exchange and increase oxygen delivery to the patient's tissues. This can be achieved through various techniques that manipulate the ventilator's mode, rate, tidal volume, and inspiratory pressure.

**Why the Correct Answer is Right**
The correct answer is the method that does not effectively improve oxygenation using a ventilator. The other options (A, B, and C) are methods that can enhance oxygenation by increasing the patient's inspired oxygen fraction, using a higher tidal volume or respiratory rate, or applying positive end-expiratory pressure (PEEP). PEEP helps to keep the alveoli open, increasing the area available for gas exchange and thus improving oxygenation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because increasing the **inspiratory time** can actually decrease oxygenation by allowing more time for nitrogen to diffuse into the alveoli, reducing the effective partial pressure of oxygen.
**Option B:** This option is incorrect because increasing the **peak inspiratory pressure** without adequate PEEP can lead to overdistension of alveoli, causing damage to lung tissue and potentially decreasing oxygenation.
**Option C:** This option is incorrect because using a **higher respiratory rate** without adequate PEEP can lead to alveolar collapse and decreased gas exchange, thus worsening oxygenation.

**Clinical Pearl / High-Yield Fact**
In mechanically ventilated patients, PEEP is essential to prevent alveolar collapse and maintain adequate oxygenation. The optimal PEEP level should be determined by the patient's lung mechanics and gas exchange parameters, rather than relying on a fixed value.

**Correct Answer: D. Increasing inspiratory time. Increasing inspiratory time can actually decrease oxygenation by allowing more time for nitrogen to diffuse into the alveoli, reducing the effective partial pressure of oxygen.**