Final answer:
Two P-waves in a cardiac transplant are caused by the existence of two SA nodes: the recipient's own and the one from the donor heart. These may operate independently and initiate separate atrial contractions, visible as P-waves on an ECG.
Step-by-step explanation:
The reason for having 2 P-waves in a cardiac transplant is due to the presence of two sinoatrial (SA) nodes in the transplanted heart. When a heart is transplanted, the recipient's original SA node is often left in place, and the transplanted heart comes with its own intrinsic SA node. This can result in two separate sources of electric pulses, each initiating a P-wave on an electrocardiogram (ECG). The conduction pathway of the transplanted heart may not be fully integrated with the recipient's natural cardiac conduction system, leading to the possibility of each SA node activating the atria at different times, thus producing two distinct P-waves.
The normal cardiac conduction system consists of the SA node, which initiates an electrical charge causing the atria to contract, followed by the impulse reaching the atrioventricular (AV) node, which pauses before spreading to the ventricular myocardium via the bundle of His, and then through the left and right bundle branches and Purkinje fibers. However, if there is a complete AV block after the transplant, each SA node may act independently, leading to dual P-waves, although one node's impulses might not result in a ventricular response if it cannot pass through the AV node.