Which cardiac medication acts to decrease cardiac contractility and increase vasodilation resulting in a reduction of the heart's oxygen demand?

Calcium channel blockers are indeed effective medications for decreasing cardiac contractility and promoting vasodilation. They work by inhibiting the influx of calcium ions into cardiac and smooth muscle cells. This reduction in calcium levels leads to decreased contractility of the heart muscle (myocardium), which results in a lower demand for oxygen as the heart works less vigorously.

In addition, calcium channel blockers cause dilation of blood vessels, which helps to reduce systemic vascular resistance and lowers blood pressure. The combination of decreased contractility and increased vasodilation collectively lowers cardiac workload and oxygen demand, making these medications beneficial for conditions such as hypertension and angina.

In contrast, the other medication classes listed do not primarily focus on these effects. Antiarrhythmics mainly work to stabilize heart rhythms rather than directly impacting contractility or causing vasodilation. Angiotensin-converting enzyme inhibitors primarily affect the renin-angiotensin-aldosterone system to lower blood pressure, which can involve vasodilation but does not significantly decrease contractility. Antithrombotics are used to prevent blood clot formation and do not directly influence cardiac contractility or vascular tone.