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Lidocaine is primarily used in cardiac care as an antiarrhythmic agent. Its main purpose is to stabilize cardiac membrane excitability, specifically in cases of ventricular arrhythmias. When administered, lidocaine works by blocking sodium channels during the cardiac action potential, which helps to reduce the occurrence and severity of abnormal electrical activity in the heart. This action can indirectly lead to a decrease in heart rate, particularly in conditions where there is rapid ventricular activation.

In terms of vasodilation, while lidocaine does not have a primary effect as a vasodilator, its ability to help restore normal heart rhythm can contribute to improved cardiac output and perfusion, indirectly affecting vascular resistance. This function supports overall cardiovascular health, particularly in emergency care settings, such as during or after a myocardial infarction, where rhythm stabilization is critical.

The other choices do not accurately represent the primary action and purpose of lidocaine in cardiac care. It is not used to increase heart rate, serve as an antidiuretic, or enhance blood coagulation. Therefore, the focus on both heart rate regulation and the indirect effects on vascular response aligns with the correct choice.