**Question:** What is pin index of O2-CO2 (CO2 > 7.5%):-

A. 6
B. 7
C. 8
D. 9

**Core Concept:** The pin index is a measure used to calculate the partial pressure of oxygen (O2) and carbon dioxide (CO2) in arterial blood samples. This index helps in assessing the patient's respiratory status, oxygenation, and ventilation. In this context, the pin index represents the ratio of partial pressure of CO2 to O2.

**Why the Correct Answer is Right:** The pin index is calculated as follows: Pin = (Pco2 / Po2) × 100. When CO2 partial pressure (Pco2) is greater than 7.5%, the pin index will be less than 100 (%pin < 100). In this case, the correct answer is D (90), as it represents a pin index of 90, which indicates a situation where CO2 is above 7.5%. **Why Each Wrong Option is Incorrect:** A. 60 (Option A): This value (60) is too low, suggesting hypocapnia (low CO2 levels), rather than CO2 levels above 7.5%. B. 100 (Option B): This value (100) is too high, indicating a situation where CO2 levels are within normal range (around 40-50 mmHg), not above 7.5%. C. 80 (Option C): This value (80) is still higher than the normal range (40-50 mmHg), not above 7.5% as required by the question. **Clinical Pearl:** An elevated pin index (CO2 partial pressure greater than 7.5%) can be seen in conditions such as hyperventilation, respiratory alkalosis, or hypercapnia. On the other hand, a low pin index is indicative of hypocapnia, respiratory acidosis, or hypoventilation. Understanding the pin index helps in assessing respiratory status and guiding appropriate interventions.