True statement about ‘instantaneous mean vector’ a) Equal & same as mean QRS vectorb) It is drawn through the centre of vector in a direction from base toward apexc) Summated vector of generated potential at particular instant caused by inflowing septal depolarizationd) When a vector is exactly horizontal and directed toward the person’s left side, the vector is said to extend in the direction of 0 degrees

## **Core Concept**
The instantaneous mean vector refers to the net direction and magnitude of electrical activity in the heart at a specific instant during the cardiac cycle, often discussed in the context of the QRS complex in electrocardiography (ECG). This concept is crucial for understanding how electrical depolarization waves spread through the heart and are represented on an ECG.

## **Why the Correct Answer is Right**
The correct statement about the 'instantaneous mean vector' is related to its definition and representation. The instantaneous mean vector indeed represents the **summated vector of generated potential at a particular instant** caused by specific depolarization events, such as septal depolarization. This vector is a momentary representation of the heart's electrical activity and can be visualized as an arrow that points in the direction of the net electrical force at that instant.

## **Why Each Wrong Option is Incorrect**
- **Option A:** "Equal & same as mean QRS vector" - This statement is not entirely accurate because while related, the mean QRS vector is a specific type of instantaneous mean vector that represents the overall direction of ventricular depolarization during the QRS complex, not all instantaneous mean vectors.
- **Option B:** "It is drawn through the centre of vector in a direction from base toward apex" - This describes a general orientation for cardiac vectors but does not specifically define an instantaneous mean vector.
- **Option D:** "When a vector is exactly horizontal and directed toward the person's left side, the vector is said to extend in the direction of 0 degrees" - This statement refers to the orientation of vectors in the hexaxial reference system but does not directly define or describe an instantaneous mean vector.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **instantaneous mean vector** changes continuously throughout the cardiac cycle, reflecting the changing pattern of electrical activation. Understanding this concept is essential for interpreting ECGs and recognizing abnormal patterns of electrical activity.

## **Correct Answer:** C. Summated vector of generated potential at particular instant caused by inflowing septal depolarization

✓ Correct Answer: C. bcd