Positive inotropic medications

Positive inotropic medications, as their name implies, are a diverse group of medications that increase the strength of heart muscle contraction.

As a result, they increase the stroke volume and thus, the cardiac output.

Positive inotropic medications include cardiac glycosides, like digoxin; beta agonists, like dobutamine; and phosphodiesterase inhibitors, like milrinone.

They are used in conditions where the heart can’t pump enough blood to the body’s tissues, like in systolic heart failure.

Alright, the heart needs to squeeze out a certain volume of blood each minute, called the cardiac output, which can be rephrased as the heart rate multiplied by the stroke volume, which is the volume of blood squeezed out with each heartbeat.

Okay, now the stroke volume depends on the preload, or the amount of blood that returns to the heart; the afterload, or peripheral resistance; and the strength of the contraction, or contractility, of the cardiac muscle.

Now, muscle contraction is initiated with an action potential which modifies receptors allowing calcium ions to flow from the sarcoplasmic reticulum into the sarcoplasm.

This allows myosin heads to bind to the actin.

These two proteins are ultimately responsible for cell contraction.

In order for a muscle to relax, calcium ions must be pumped back into the sarcoplasmic reticulum.

When the frequency of stimulation is increased, more calcium ions accumulate in the sarcoplasm, and the strength of contraction increases.

Alright, now there are conditions in which the strength of the heart’s contraction is impaired, and the heart can’t pump out enough blood to meet the body’s demands, this is called heart failure.