What typically occurs first when monitoring the earliest physiologic response to breathing 100% O2?

When breathing 100% oxygen, the body's immediate physiological response is typically an improvement in oxygenation. This occurs because high concentrations of oxygen can significantly increase the amount of dissolved oxygen in the blood. This leads to more efficient transport of oxygen to tissues, which is especially beneficial in situations where the patient may be experiencing hypoxemia or other forms of respiratory distress.

Increased respiratory rate, substernal chest pain, and hyperventilation may occur as subsequent physiological responses or as a reaction to other underlying conditions but are not the primary or earliest responses to breathing pure oxygen. For instance, while some individuals may experience increased respiratory rate and hyperventilation due to hyperoxic conditions, these are generally compensatory reactions rather than immediate responses.

The improvement in oxygenation is crucial because it signifies that the body is responding positively to the treatment, thereby enhancing tissue oxygen delivery and supporting metabolic functions. This answer underscores the practical importance of recognizing how the body reacts to medical gas therapies and allows for effective monitoring of patient responses during oxygen therapy.