Final answer:
A. fib, SVT, and w/ HR refer to atrial fibrillation, supraventricular tachycardia, and conditions involving heart rate respectively, all of which are types of heart arrhythmias. Atrial fibrillation is not immediately life-threatening if ventricles pump adequately, while ventricular fibrillation is an emergency needing urgent defibrillation. A third-degree block can result in a decreased heart rate (pulse).
Step-by-step explanation:
The possible options mentioned, A. fib (Atrial fibrillation), SVT (Supraventricular Tachycardia), and w/ HR (with Heart Rate), refer to different types of heart arrhythmias. Atrial fibrillation is characterized by an abnormal electrical pattern before the QRS complex and increased frequency between QRS complexes. SVT, which includes conditions like atrial flutter and paroxysmal supraventricular tachycardia, is a type of arrhythmia stemming from above the heart's ventricles and also involves a rapid heart rate. Ventricular arrhythmias, like ventricular fibrillation and ventricular tachycardia, are more serious and may require emergency treatment with a cardiac defibrillator.
Heart arrhythmias such as these are generally due to problems with the heart's electrical conduction system. For example, atrial fibrillation can be serious but is not immediately life-threatening as long as the ventricles continue pumping blood. Conversely, ventricular fibrillation is a medical emergency where the heart cannot pump blood effectively, potentially leading to brain death without prompt treatment. Cardiac defibrillators are devices that can deliver an electrical shock to the heart to 'reboot' it and restore a normal rhythm, especially useful in cases of ventricular fibrillation.
When considering the scenario presented in a third-degree block, where some impulses initiated by the SA node do not reach the AV node, and the P waves are not followed by the QRS complex, we would expect to see a decrease in heart rate (pulse) since the ventricles may not be receiving regular signals to contract.