Final answer:
A partial block in the cardiac conduction system will show up on an ECG as some P waves not being followed by the QRST waves, indicating that not all electrical signals are causing the heart to contract. This occurs in a second-degree or partial heart block, where there are occasional 'dropped beats.' A full or third-degree block, by contrast, causes a complete dissociation between the P waves and the QRS complexes.
Step-by-step explanation:
A partial block in the cardiac conduction system would be reflected on the ECG as some P-waves without corresponding QRST waves because this indicates that not all the electrical signals are successfully causing the heart to contract. Specifically, a second-degree or partial block, which is a type of heart block, can be identified on the ECG when some P waves are not followed by the QRS complex and T waves, while other P waves are followed normally. This is typically seen as a pattern where the PR interval remains consistent, but occasionally there is a P wave that is not followed by a QRS complex, known as 'dropped beats.'
Additionally, we can understand how the ECG correlates with the mechanical action of the heart. The P wave on the ECG represents the atrial depolarization, signaling the atria to contract and push blood into the ventricles. This should be followed by the QRS complex, which signifies ventricular depolarization and the subsequent contraction of the ventricles to pump blood out of the heart. A partial block can occur at various levels of the conduction system but commonly occurs at the atrioventricular (AV) node. A delay or block here can mean that the synchronization between atrial and ventricular contraction is lost, which is reflected as a disconnection between P waves and subsequent QRS complexes on an ECG.
In a complete or third-degree block, the atrial activity (shown by the P wave) is completely dissociated from the ventricular activity (shown by the QRS complex) resulting in 'AV dissociation.' This is a more serious but a different type of block from the partial block described above and represents a failure of the electrical signals to propagate from the atria to the ventricles.