What is the primary effect of a positive inotropic drug on cardiac performance when preload is normal?

What is the primary effect of a positive inotropic drug on cardiac performance when preload is normal?

• A. Increase LVSW and CO
• B. Decrease PCWP
• C. Increase SVR
• D. Decrease MAP

Answer: (A)

Boosting contractility in a heart with normal filling (preload) mainly makes the ventricle contract more forcefully, so it ejects a greater portion of its filled volume with each beat. This increases the left ventricular stroke work, which is the energy or work the heart performs to move blood, and, if heart rate stays the same, raises cardiac output. Preload isn’t the main driver here—the filling pressure is already adequate—so the primary change is in how hard the heart squeezes and how much blood it pushes out, not in how much it fills. The other options reflect changes in pressures or resistance that aren’t the core effect of increasing contractility with normal preload. For example, preload-related pressures or systemic vascular resistance and mean arterial pressure depend on many factors beyond basic inotropy and are not the direct primary outcome of a positive inotropic effect when preload is already normal.

Boosting contractility in a heart with normal filling (preload) mainly makes the ventricle contract more forcefully, so it ejects a greater portion of its filled volume with each beat. This increases the left ventricular stroke work, which is the energy or work the heart performs to move blood, and, if heart rate stays the same, raises cardiac output. Preload isn’t the main driver here—the filling pressure is already adequate—so the primary change is in how hard the heart squeezes and how much blood it pushes out, not in how much it fills.

The other options reflect changes in pressures or resistance that aren’t the core effect of increasing contractility with normal preload. For example, preload-related pressures or systemic vascular resistance and mean arterial pressure depend on many factors beyond basic inotropy and are not the direct primary outcome of a positive inotropic effect when preload is already normal.