If John's electrocardiogram (ECG) shows an extra P wave, it indicates the presence of an additional electrical signal originating from a different location than the normal sinus node. This condition is known as atrial premature complex (APC) or premature atrial contraction (PAC).
In the normal conduction pathway of the heart, the electrical signal originates in the sinus node (located in the right atrium) and travels through the atria, causing them to contract and generating the P wave on the ECG. Then, the signal reaches the atrioventricular (AV) node, which acts as a gateway between the atria and the ventricles. After a slight delay at the AV node, the signal is conducted to the ventricles, causing their contraction and generating the QRS complex on the ECG.
However, in the case of an extra P wave, there is an additional electrical signal originating from a different location within the atria. This could be due to an ectopic focus, an area of abnormal electrical activity that fires prematurely. The exact location of the ectopic focus determines the appearance and characteristics of the extra P wave on the ECG.
It's important to note that an extra P wave doesn't necessarily indicate a malfunction or blockage in the conduction pathway itself. Instead, it suggests that there is an additional electrical signal originating from an ectopic focus within the atria, resulting in an abnormal ECG finding.