Cardiac Output: Preload and Afterload Part 2

Before going over the main topic – preload and afterload, we will first touch on cardiac output and stroke volume to get a clearer background as to how the mechanisms work.

Cardiac Output

Cardiac output is the amount of blood that is being ejected from the heart’s left ventricle to the aorta, then to the rest of the body in a minute. Normally, an average person has about five liters of blood circulating throughout the body’s systems. Therefore, if the pumping organ is compromised and there is no or insufficient cardiac output, blood that is responsible for transporting oxygen that sustains life, will decrease; thus, resulting in tissue death.

For instance, if a tourniquet is placed around the finger, blood supply is cut off. What follows is a series of manifestations for tissue death – turning pale, cold, and cyanotic. Cardiac output is the sum of a person’s heart rate times the stroke volume. But what is stroke volume?

Stroke Volume

Stroke volume is the amount of blood pumped in every beat – how much the heart is pumping in one clean push from the left ventricle. A typical example of how stroke volume is affected in cases of heart ailments is when left ventricular hypertrophy happens.

Left Ventricular Hypertrophy

For instance, if a client has a cardiac failure or congestive heart failure, there is increased pressure being back up from the rest of the body due to high blood pressure. The left ventricle is doing its best to keep up with the significant amount of pressure to pump out. When this happens, stroke volume is lessened because the left ventricle cannot pump all at once throughout the body. Swelling of the left ventricle occurs due to its effort of pushing against the resistance. Thus, the client is diagnosed with left ventricular hypertrophy. Remember, when stroke volume is compromised, cardiac output is compromised as well.

Brain Natriuretic Peptides

To measure the left ventricle’s hyperinflation, the client’s brain natriuretic peptides (BNP) are measured. Brain natriuretic peptide is compensatory mechanism released from the brain, to allow communication from the brainstem to the left ventricle, asking the heart to pump more blood to the rest of the body due to lack of oxygen. Inside the brainstem, there’s a vasomotor center that regulates and controls blood pressure.

The normal BNP is around 100; anything that goes beyond this number indicates a heart problem. Clients with congestive heart failure have a BNP of more than 300. Severe congestive heart failure clients can even reach over 25,000 of BNP.

Now that we’ve explained what cardiac output and stroke volume is, let’s go to preload and afterload.

Preload

Preload is the stretch in the ventricle, whether right or left. Ventricles stretch and squeeze to distribute blood adequately. However, if there’s too much pressure being backed up due to cardiac issues, the ventricles tend to stretch extensively, taking it longer to squeeze, resulting in an abnormal contraction.

Afterload

Afterload is the degree of pressure inside the aorta to overcome the push of blood. Afterload is just a fancy term for how much pressure the ventricles need to exert during systole.

Going back to clients with congestive heart failure or even hypertension, the backing up of pressure will cause a wider stretch, increasing preload and afterload.

Decreasing Preload and Afterload

Nitroglycerin and Morphine are often given to bring down the preload and afterload; these two drugs, having a direct effect on the preload and afterload, are commonly given to clients with acute myocardial infarction.

Nitroglycerin dilates the pathways of the heart, relaxing the left ventricle. Morphine, a central nervous system opioid analgesic, also widens and relaxes the blood vessels by acting on the BNP of the brainstem.

Hopefully, we’ve shed some light on what is cardiac output, stroke volume, preload, and afterload, and how they are intertwined whenever there are heart issues like congestive heart failure.