**Core Concept**
The cardiac index is a critical parameter in assessing cardiac function, especially in postoperative patients. It represents the cardiac output (CO) adjusted for the patient's body surface area. In this scenario, the patient's cardiac index has decreased, indicating a potential decrease in cardiac output.

**Why the Correct Answer is Right**
The patient's decreased cardiac index, despite a normal left ventricle, suggests a possible issue with the coronary perfusion or a decrease in venous return. The most likely cause is a decrease in venous return due to a high central venous pressure (CVP) or a decrease in systemic vascular resistance (SVR). Administering a vasodilator, such as nitroglycerin, would decrease SVR, increasing venous return and subsequently cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Increasing the heart rate would not directly address the issue of decreased cardiac index in this scenario. A heart rate of 55 beats per minute is within the normal range, and increasing it would not necessarily improve cardiac output.

**Option B:** Administering a positive inotrope, such as dobutamine, would increase contractility, but the patient's left ventricle is normal, and the issue lies with decreased venous return, not contractility.

**Option C:** Administering a diuretic would decrease fluid volume and potentially worsen venous return, making this option incorrect.

**Option D:** Administering a vasoconstrictor would increase systemic vascular resistance, potentially worsening venous return and decreasing cardiac output.

**Clinical Pearl / High-Yield Fact**
In postoperative patients, a decrease in cardiac index can be due to various factors, including decreased venous return, decreased contractility, or increased systemic vascular resistance. Identifying the underlying cause is crucial in selecting the appropriate treatment.

**Correct Answer:** B.