Heart Failure and How the Heart and Kidney Work Together

When I first heard my vet tell me, “your cat has congestive heart failure,” I thought Rudy would be on life support and immediately ready to be put down. The image in my mind of “congestive heart failure” was an unconscious person hooked up to oxygen on a hospital bed– they had suffered a major heart attack and family members waited quietly nearby to say goodbye. As it turns out, that’s not at all what “heart failure” means. This post describes what heart failure means and how the kidney plays a vital role in managing your cat’s heart disease.

Veterinary medicine is really complicated business and caring for a pet with heart disease takes years of specialized education and tons of experience. I don’t have either of those qualifications. But as a pet owner, I do have experience from the patient’s perspective– and the frustrating feeling of not having a clue about what heart disease meant when our journey started. This post is intended as a beginner’s guide at a high-level for the non-specialist. Think of it as just giving a rough lay of the land to help you understand what your veterinarian tells you and not as an avenue for second-guessing your pet’s care.

What is “Heart Failure?”

Heart failure doesn’t mean that the heart has stopped or collapsed. It simply means that the current output from the heart isn’t meeting the body’s demands. This doesn’t mean that death is around the corner– instead, it just means that we need to either improve the heart’s output or lessen the body’s demands in order for the heart to keep up at the right pace. A cat or dog can live perfectly happily for years with “controlled” heart failure– and many do. Add the word, “congestive” and all it means is that the heart’s weakness is causing things to back up– usually, the blood supply from the lungs is too strong for the left ventricle to keep up.

I think my dread over the term, “heart failure” is that the word, “failure” has such negative connotations in modern English usage. I tend to use the word “fail” in the same context as words like “lost” or “defeated.” For me, “failure” also has a finality about it that’s hard to get past– when in many cases, medication can enable a heart that is in “heart failure” to keep up just fine. I think a more accurate phrase like “cardiac insufficiency” would be much easier for me. I’m fine with the word “congestive” (I use that word all the time when I just have a cold). So, if you have the same sensitivity to language that I do and your cat has been diagnosed with “congestive heart failure,” just tell yourself she has (hopefully temporary) “congestive heart insufficiency” instead.

Why Did My Vet Tell Me to Consider a Kidney Diet?

A lot of veterinarians advise their patients who have CHF to consider a low-protein diet that is more appropriate to kidney issues. They do this for a few reasons– the least of which is to sell you expensive prescription kidney diet food. The real reason has to do with the fact that the medication used to treat your cat for heart disease is going to put a burden on the kidneys– and so starting them on a diet specific to kidney issues helps lessen the load.

Given Enough Time, the Kidneys Will Fail

The kidneys serve a vital role in every mammal by filtering out metabolic waste products in the blood. Without the protection of modern civilization, the average life span for a human being is about 30-35 year. For a cat in the wild, it’s only about 4-5 years. The normal kidney is easily capable of filtering the blood for that long but it’s not designed for longevity– none of our organs are. After all, Mother Nature’s rule of “survival of the fittest” doesn’t mean “survival for the longest.” Instead, evolution favors the species that could most successfully live to the age where they reproduced– after that, any genes favoring longevity were unnecessary and potentially even counterproductive. Organs like the kidneys and heart don’t typically regenerate like skeletal muscles (e.g. your bicep or leg muscles). After all, it is highly likely that our ancestors may have injured their feet or arms while growing up or hunting game– and so being able to regenerate these structures was favored by evolution. All internal organs (including our kidneys and heart) have a limited amount of capability, eventually they will fail in a human, a dog, and any other mammal. The kidneys don’t necessarily fail first but often they do. The kidneys in all mammals are under stress, but the kidneys in cats are particularly stressed because, after millions of years of evolution in desert environments, Nature favored cats that had a strong ability to concentrate their urine to conserve water.

Your vet may have recommended in the past that you include some wet food in your cat’s diet in place of dry food. The reason is mostly because of exactly this added pressure that cats naturally place on their kidneys. Cats rarely lap water at home and aren’t really designed to. Instead, they should obtain almost all of their water needs from the juicy prey that they ate. Giving your cat wet canned food simulates this natural way for cats to take in water.

A Pure Kidney Diet May Not be Appropriate

As beneficial as a prescription kidney diet may be for your cat’s kidneys, I’ve also read that a purely kidney-focused diet may not be the best thing for a cat with congestive heart failure.

Cats with CHF tend to lose weight. No one knows exactly why but a likely reason is that, with less cardiac output, every system in the body becomes less efficient. This includes the digestive system. Thus, your cat may have a harder time extracting the same nutrition and so it tends to lose weight. As obligate carnivores (i.e. animals that naturally eat almost exclusively meat), a cat has an extremely high need for meat-based proteins– and some of the amino acids found in high proportions in meat. With a slower digestion system caused by CHF, however, less protein is being absorbed by the body– and so a kidney diet may act as a “double whammy” to intensify this effect.

The problem is that the heart has pretty high protein needs. For instance, a cat needs protein in order to give the heart plenty of L-carnitine and L-taurine, two amino acids vital to the health of your cat’s heart. Among other things, L-carnitine helps with the heart muscle’s energy needs– and when a cat has CHF, her heart needs all the energy it can get. Unlike humans, which can synthesize L-taurine from other amino acids, cats need to get taurine from their diet. This makes sense because taurine is abundant in meat so there was no reason for cats to evolve the ability to synthesize their own. Our human ancestors probably ate a lot more vegetable protein sources so we had to evolve the ability to synthesize taurine.

Just in terms of diet, you can see that the heart and kidneys have different and competing needs. What’s great for one isn’t so great for the other. Unfortunately, diet is just the beginning of where that tension exists. On the flip side, I had great success using supplements like azodyl and renadyl (granted, in reasonably high doses) to achieve a good balance through diet– enabling Rudy to keep a high protein diet (good for Rudy’s heart) while reducing toxins through his gut instead of the bloodstream (good for Rudy’s kidneys). While your mileage may vary in terms of azodyl’s effectiveness, I personally have did notice a correlation between increases in Rudy’s azodyl with decreases in his BUN and creatinine.

How the Kidneys Make it Harder for the Heart

If it were just a matter of taking care of the heart, managing heart failure would be so much easier. Just sprinkle in an ever increasing amount of diuretics and fluids stop accumulating in the chest even as heart function decreases. Then, maybe add a vasodilator to help the blood going everywhere it should. Lastly, add in the latest veterinary miracle drug for heart failure (pimobendan) to improve cardiac output and your cat is as good as new. Easy peasy.

The problem is that the other bodily systems don’t like getting the lowered blood flow that’s part of heart failure and the lowered blood volume associated with diuretics– and probably the biggest complainers in the body are the kidneys. Through a complex set of hormonal signals called the “Renin-Angiotensin-Aldosterone System” (just remember “RAAS”), the kidneys react to the lower blood flow by increasing extracellular volume (extra fluid)– and this shows up as the “congestive” symptoms of CHF and fluids accumulate more quickly. But it’s that fluid accumulation that caused you to start with the diuretics in the first place! In addition to increasing extracellular volume, the RAAS also tends to trigger vasoconstriction, which lowers peripheral blood flow.

As much as it’s tempting to consider the kidneys as the “bad guys” in congestive heart failure, the RAAS system does serve an important purpose. In a normally functioning organism, the RAAS system can be important– even potentially lifesaving (e.g. blood loss from a significant wound) in some instances. The problem is that the body of a cat with congestive heart failure isn’t functioning normally— and the kidney’s reaction to low perfusion (oxygenated blood flow) through the RAAS system is actually counterproductive.

So it shouldn’t come as a surprise that interfering with the RAAS system is a vital component of managing heart insufficiency in current medicine, both for people and for animals. There are actually a number of smaller steps along the RAAS (the three major steps of renin, angiotensin, and aldosterone is an oversimplification) and interfering with one of those smaller steps sometimes does a great job– and other times, it’s important to block a different step or maybe even more than one step– in what cardiologists call the “RAAS blockade.” In different research articles, some scientists aptly describe the problem as a small amount of one step (e.g. angiotensin) “leaking” or “escaping” the effects of one drug and so they try to interfere with another step.  And modern medicine seems to have an ever-increasing number of different drugs to do exactly that. For instance,

  • ACE Inhibitors. Your vet may prescribe an Angiotensin Converting Enzyme (ACE) inhibitor, like enalapril or benezepril. These break the first part of the chain by interfering with the angiotensin part. Together with a diuretic, this is probably the first step that most veterinarians go to with heart failure.
  • Spironolactone. Originally, it was thought that spironolactone helped heart failure by serving as a mild diuretic. When used with humans, adding spironolactone had a pretty big effect in helping people with heart failure– more than just what a mild diuretic could achieve. It turned out that spironolactone also hit the other end of the RAAS cycle by blocking the aldosterone end. Consequently, spironolactone is another medication your vet may suggest at some point.
  • ARBs. Another class of medications work by blocking another segment of the RAAS chain by blocking a point in between where angiotensin is picked up. These angiotensin receptor blockers (ARBs) are a relatively recent innovation in animal health care and drugs like telmisartan have been used successfully with dogs where ACE inhibitors alone have failed and more recently been marketed to veterinarians treating cats for kidney issues like proteinuria. While most of this work with ARBs has focused on proteinuria, I have a hunch that there may be a lot of potential with ARBs for heart failure as well. Drugs like entresto (which combines an ARB with a a medication that improves blood flow by inhibiting a vasocontricting hormone called neprilysin) have reportedly had great results in human subjects.

Often the trick to managing CHF is simply finding the right “cocktail” of medications– between diuretics, RAAS blockade medications, and other drugs (e.g. inotropics)– and I get the sense that finding that cocktail is often more art than science. Hit the right combination and even some animals who are at death’s door can stabilize and live happily for years. Unfortunately, biological systems are subject to constant change– and even the most perfect cocktail can be out of balance with the body’s needs at any minute. And sometimes, a cat’s heart disease just can’t be adequately adjusted and the heart disease will just continue to get worse. This isn’t a journey for the faint-hearted and a pet owner needs to be learn to accept a great deal of uncertainty.

Kidney Disease and Quality of Life with CHF and CKD

Photo of CaryUnfortunately, I’m all too familiar with chronic kidney disease (CKD). About 18 years ago, I had a darling sheltie (Cary) who died from kidney failure at only six years old. Like Rudy, Cary was the most perfect boy and bravely fought his disease for months and months before his short life ended. I could ask him to do a “stand stay” (like in the photo on the right) and he would proudly stay there. In fact, the two photos here were taken as I walked around Cary– and he kept his head pointed steadily in the same direction like a statue as I walked around him.

As awful as CHF is, I personally think that CKD is worse– or at least it was 18 years ago. With CHF, there is a decrease in blood flow that contributes to tiredness. I liken it to the fatigue that accompanies old age. My 11 year cat often acted (and probably felt) more like an 18 year old cat. Every veterinarian I talked to has told me that congestive heart failure doesn’t cause pain. It’s true that there is some discomfort from have fluids accumulate in the chest cavity, which limits the size of each breath, and there is the discomfort from visits to the vet for

Cary in profile

thoracocentesis (draining chest cavity fluids)– but these are only a few hours out of several days, weeks, or months of otherwise pretty normal life. Granted, Rudy didn’t have quite the same spring in his step after his CHF diagnosis but he still had some of his best moments ahead of him. By contrast, kidney disease causes pretty awful and constant feelings of nausea. I can still remember how hard it was to encourage Cary to eat, which for a dog like Cary was pretty unusual. And this battle with food went on for months. I can honestly say that Rudy rarely had the same awful days that Cary had.

Prerenal Kidney “Failure”

During the course of your kitty’s months or years with CHF, at some point you may hear your vet mention the words “renal failure” or more specifically “prerenal kidney failure.” Given my past experience with Cary, a small part of me died inside when I heard those words used to describe Rudy’s condition. If you hear them used with your kitty, take a deep breath as it’s not as bad as it sounds.

First, there’s that word “failure” again. “Kidney failure” doesn’t mean that your kitty’s kidneys have shut down. Instead, it means that the kidneys just aren’t keeping up with the body’s demands. More specifically, the BUN and creatinine levels (a rough indicator of waste products) have reached a sufficiently high threshold. My vet roughly described the BUN as the “how you feel” measure (although some cats– like Rudy– can feel fine with extremely high BUN values so it’s a highly individual number) and the creatinine as the “how functional is the kidney” measure.

Second, there are really two kinds of kidney failure that are relevant here. One kind of kidney failure represents true tissue damage to the kidneys. This is what got Cary and represents true CKD. The second kind of kidney failure can spring up when the kidney are perfectly healthy but can’t function for some other reason that happens “before” the kidneys (usually insufficient blood flow in cases such as CHF). This second kind of renal failure is called “prerenal kidney failure.” This distinction isn’t perfect, however, because, as noted above, kidney function naturally declines just through the process of living. So if your cat is older, an already compromised (but otherwise healthy) kidney can function as if it is a lot worse due to CHF. It also gets more complicated because the drugs used to treat CHF can strain the kidneys even more and accelerate the degeneration that naturally comes with aging (to add to the confusion, there is also a third kind of kidney failure– postrenal kidney failure– that can occur when an obstruction in the urinary tract below the kidneys causes waste to build up in the kidneys but that usually isn’t relevant in cases of CHF).

While this may all seem terribly confusing, the important point is that prerenal kidney failure isn’t as awful as it sounds. It doesn’t mean that your kitty’s kidneys have shut down. And it also means that, with a little prompting, it may be possible to win back a bit more kidney function through tricks like increasing blood perfusion. And I think some of those tricks may be capable of ameliorating the prerenal kidney part more than would be possible for the non-prerenal part of a compromised kidney.


In a more perfect world, we would have implantable devices to replace a failing heart or kidney and life could continue just fine. When it comes to kidney function, the second best option commonly available is dialysis– and for decades plenty of people with kidney failure have relied on dialysis to stay alive. This option has been available to cats and dogs for some time for those of us who are lucky enough to live near one of the relatively few veterinary dialysis centers. One of those centers happens to be in Seattle (where we live) and I had a long conversation with one of the vets in the dialysis center. In general, dogs and cats can do quite well with dialysis and tend not to feel as awful afterwards as humans. On the other hand, a small amount of blood gets lost in the machinery of the dialysis machines– and, with the tiny size of cats, it’s usually enough blood loss to require a transfusion. Thus, dialysis isn’t appropriate for kitties with chronic conditions that will require frequent bouts of dialysis.

Biological Systems are Imperfect and Finicky

I am an extremely analytical person by nature. My father was an engineer and my mother was an architect. I studied mathematics and became a lawyer. I’m used to very precise answers and a clear relationship between cause and effect.

Biological systems, however, are highly imperfect. With a lot of CHF medications, some cats will react immediately, some more slowly, some won’t react at all or will react badly. And, even if it takes a long time to react, it doesn’t necessarily mean that that reaction will be gradual. Or sometimes it just appears that way because the body approaches a tipping point and appears to make sudden changes while staying on the same course of treatment. Worse yet, research studies that you find on the internet seem so black-and-white and appear to give clear, predictable answers when the reality is that there is always a good deal of gray.

For me, the imperfect nature of biological systems was a very frustrating lesson to learn. For instance, Rudy was having trouble with accumulating fluid in his chest cavity and it seemed as if diuretics simply weren’t working. We slowly increased his dosage and still he accumulated fluids. Then, after several weeks on the same dosage, he suddenly stopped accumulating fluids. Looking for a clear reason, I attributed his sudden improvement to a recent relocation of his litter box (so that Rudy didn’t have to climb two sets of stairs each time he used the litter box). While I still believe that moving the litter box helped (certainly it didn’t hurt), looking back I think that his body was simply “catching up” to his diuretics– and that should have been a warning to me that I should have looked out for dehydration and increased kidney values through weekly blood checks to make sure that he didn’t go over the tipping point.

Also, early in Rudy’s course of treatment, our veterinarian suggested using an ACE inhibitor– either benazepril and enalepril. Because benazepril is metabolized in the liver and enalepril in the kidneys, we opted to use benazepril to prevent kidney issues down the road. Within a day after starting benazepril, Rudy looked simply awful and we reluctantly took him off the medication. Months later, we tried enalepril and he took it in stride without the slightest problem. Our vets seemed a bit perplexed but not surprised– sometimes different cats react unexpectedly to different medications that should have pretty identical side effects.

The Bottom Line

In summary, the heart and kidney have a delicate balance in even the healthiest cats. Add a condition that compromises heart function and kidney function can go haywire (the reverse can happen as well). As you and your kitty’s veterinarian struggle with heart failure, it often becomes a delicate balancing act between the heart and the kidneys. And managing that balance requires constant vigilance.

Rudy’s condition was exceptionally grave from the start– and hopefully your kitty is in a much more stable and controllable condition than he was. But, even in rough cases like Rudy’s, it’s still possible to find and maintain the balance, if you and your vet have a good amount of luck and experience. But, knowing the nature of the disease, what to look for, what questions to ask, and how to cope with the frequent ups and downs will certainly stack the deck in your favor– and help you keep a level head and make life awesome for you and your kitty!

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