Why does a patient in renal failure have decreased cardiac output?

Not always related. Renal failure in and of itself is not realted to cardiac output, there are many things that can cause both renal failure and decreased cardiac output, but they are not always related. However if someone has really low cardiac output, one could develop renal failure b/c not enough blood is getting to the kidneys.

Related Questions

For what reasons might a patient in renal failure have decreased cardiac output?

Direct vs. indirect. Kidney failure can weaken the heart muscle (uremic cardiomyopathy), but this is pretty rare. A more common problem is fluid accumulation around the heart (pericardial efffusion), which can literally choke the heart (tamponade) but improves with extra dialysis. Or, more likely, the conditions that caused the kidney disease (commonly hypertension and/or diabetes) also are causing heart problems. Read more...

Why would a patient in renal failure get decreased cardiac output?

They wouldn't. But the reverse is true. Someone with low cardiac output underperfuses the kidneys and commonly gets "pre-renal azotemia" which is a form of poor kidney function. People with advanced renal failure are commonly volume overloaded which is often mistakenly called "heart failure" (really it's circulation failure). Is that what you mean? Read more...
Complex. Even though the mechanisms are complex, one way to understand is that if you lose the ability to filter your blood and eliminate extra fluid from your body (one of the kidney's main functions) your heart will be burdened with pumping a lot more blood than it was intended to do and will therefore fatigue and not pump as well (same as any other muscle would if forced to work "overtime"). Read more...

Why? Would someone in renal failure have decreased cardiac output?

Many reasons. Decreased cardiac output can contribute to reduced kidney function. Kidney disease is often associated with heart problems that reduce cardiac output. High blood pressure and volume overload with kidney failure can also reduce heart output. It's often difficult to sort out cause and effect in these situations. Read more...

Why would a patient in renal failure get decreased cardiac function?

Other way around. Usually it is the decreased cardiac function that leads to kidney failure due to lack of blood flow. However, the two illnesses in general share many contributing factors like high blood pressure and diabetes. Read more...