“Cannon” – a wave is seen in-
**Core Concept**
The "Cannon" wave is a phenomenon observed in electrocardiography (ECG) that represents the initial upward deflection in the QRS complex, caused by the rapid depolarization of the ventricles during a heart attack or myocardial infarction (MI).
**Why the Correct Answer is Right**
The Cannon wave is a result of the abnormal activation of the ventricles, which occurs when there is a blockage in one of the coronary arteries, leading to ischemia and eventual necrosis of the myocardial tissue. This abnormal activation is mediated by the activation of the Purkinje fibers, which are a network of specialized fibers that conduct electrical impulses from the AV node to the ventricles. The rapid depolarization of the ventricles results in an initial upward deflection in the QRS complex, known as the Cannon wave.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Cannon wave is not typically associated with atrial depolarization, which is represented by the P wave in the ECG.
* **Option B:** This option is incorrect because the R wave is a normal component of the QRS complex and is not a characteristic of the Cannon wave.
* **Option C:** This option is incorrect because the S wave is also a normal component of the QRS complex and is not associated with the Cannon wave.
**Clinical Pearl / High-Yield Fact**
The Cannon wave is a classic ECG finding in acute myocardial infarction and is often used as a diagnostic criterion. However, it is essential to note that the presence of a Cannon wave does not necessarily confirm a diagnosis of MI, as it can also be seen in other conditions such as ventricular tachycardia or pre-excitation syndromes.
**Correct Answer: D.**