is
a very complex subject which tends to make my eyes
glaze over. However, it is
thought to occur in 65-75% of cats with kidney failure, so it is important
to check for it and treat it if it is present.
Fortunately, you don't really need to know too much about it,
you simply need to know the basics, i.e. what it is, what symptoms a cat might exhibit, how
it is diagnosed, and most importantly, how it is treated.
Essentially, metabolic acidosis means that levels of acid in the cat's
body are too high. Acid is produced in the body as a result of diet. In
healthy cats, the kidneys help to balance acid levels in the body in two
ways:
Bicarbonate ions (which are alkaline) in the kidneys help protect against
acid build-up in the body;
Any excess acids
that do arise are flushed from the body by the
kidneys.
Unfortunately the excessive urine flow of CKD washes the protective bicarbonate ions out of
the kidneys. The damaged kidneys may also not flush the acids from the
body properly. As a result of these damaged
mechanisms, acidity levels in the
blood rise (i.e. the body’s pH becomes too low). This is known as
acidosis, and can cause muscle wasting and weight loss, particularly lean
muscle loss, partly because acidosis prevents
the cat's body from using protein properly.
"Metabolic" means that the acidosis is caused by kidney failure. This is
to differentiate it from another type of acidosis known as respiratory
acidosis, which is caused by the lungs not expelling carbon dioxide
properly.
I know a lot of people get confused by the word "acidosis" and think it is
the same thing as
excess stomach acid,
but that is not the case. Excess stomach acid is a
separate problem with different causes and treatments, so please be sure you
are dealing with the correct condition. It is possible for a CKD cat to
have either excess stomach acid or metabolic acidosis, or both at the same time.
Metabolic
acidosis can also cause potassium imbalances, bone loss and if severe,
heart disturbances. In
Renal disease (2006), Dr D
Polzin mentions that metabolic acidosis may increase the risk of pulmonary
oedema in cats on fluid therapy. In
Chronic Renal Failure (2001), he states that "potassium depletion and metabolic acidosis
may promote potentially fatal reductions in plasma
taurine
Despite the many risks and side effects of metabolic acidosis, unfortunately few vets bother to diagnose or treat
it - in
Current concepts for the management of chronic renal
failure in the dog and cat - early diagnosis and supportive care
(2005), a presentation to the World Small Animal
Veterinary Association 30th World Congress, Dr S Sanderson states that
"at least in veterinary medicine, it appears that metabolic
acidosis tends to be undertreated in patients with CKD."
This may be because metabolic acidosis can be difficult to
measure accurately. Below are the usual tests used to diagnose
metabolic acidosis.
If a cat has metabolic acidosis, the cat's
potassium
levels may appear high or normal in blood tests, but may subsequently fall
after the metabolic acidosis is treated. You should ensure that your cat's
potassium levels are monitored, because Dr D Polzin mentions in
Chronic Renal Failure (2001) that "potassium depletion and metabolic acidosis
may promote potentially fatal reductions in plasma
This is the most accurate way of testing for metabolic acidosis, but it can be difficult to find a laboratory where this test can be done,
and it requires sedation. It is therefore usually only done at specialist vet
schools.
Global RPh explains how to interpret arterial blood gas tests.
TCO2
TCO2 or total carbon dioxide in the blood is a
way of measuring levels of bicarbonate in the body. If TCO2 is low, it can
be a sign that the blood's protective bicarbonate levels are depleted from
too much acid.
TCO2 has to be
measured on a special blood gas machine, which not many vets and not all
laboratories have. It is also easy to make mistakes in taking the
measurement because if the sample is exposed to air, the dissolved gas
escapes and makes the reading look lower than it really is. Since it is a
tricky test to run, some laboratories, such as Antech in the USA, may only
measure it on request
(with Antech, you need to ask for Test No. T115).
The usual range
for TCO2 is about 17 to 23. In
Renal disease (2006) Dr D Polzin
recommends treating for metabolic acidosis when tests show the level
is below 15 mmol/l on more than one occasion.
The Merck Veterinary Manual states that
treatment "may be indicated if the animal is severely
acidotic (plasma bicarbonate <15 mEq/L) or remains acidotic 2-3 wk after
diet change."
If your vet or
his/her laboratory cannot measure TCO2 at all, ask your vet if it is possible to test for carbon dioxide (CO2) levels instead -
if they are low, they may also indicate metabolic acidosis.
Anion Gap
This is the
difference between measured concentrations of cations and anions in the
blood. It is calculated as follows:
AG = [Na+ + K+] - [Cl-
+ HCO3]
which in
English means:
anion gap = [sodium + potassium]
– [chloride + bicarbonate].
Med
Calc will calculate it for you if you input the
appropriate values from your cat’s bloodwork.
The normal range for cats
is around 10-27, though it does vary greatly from lab to lab. If the anion
gap is increased in a CKD cat, it may indicate metabolic acidosis, so if
your vet is unable to test TCO2 or CO2, but has checked the anion gap,
this may be an indicator (although you need to be aware that there are
other causes of metabolic acidosis too which may not be reflected in the
anion gap).
Some vets
diagnose metabolic acidosis from the pH levels of the urine but in
Renal disease (2006) Dr D Polzin states: "Because
urine pH is often insensitive as a means of assessing the need for or
response to treatment, it is not recommended for this purpose."
The first step for mild metabolic acidosis is to try to feed a
prescription diet. Most renal diets are pH neutral or even slightly
alkalinising. If your cat will not eat prescription food, at least try to
avoid
foods for urinary tract health, which are acidified and therefore not
suitable for CKD cats generally and cats with metabolic acidosis in
particular.
Fluid Therapy
Subcutaneous fluids
are used to help cats who are unable to remain hydrated without them. It
is therefore not appropriate to give sub-Qs purely in order to control
metabolic acidosis. However, if you are already giving subcutaneous fluids, using
Lactated Ringers Solution
(LRS) (which is the most commonly used sub-Q fluid for CKD cats) may be sufficient to correct mild acidosis, because the lactate is metabolised by the liver where it is converted to bicarbonate, which helps
correct the acidosis.
Potassium Citrate
Cats with both low potassium and metabolic acidosis can be given
potassium citrate. In fact, Dr D Polzin says in
Renal disease (2006) that other treatments for metabolic
acidosis in cats who also have low potassium may only be of limited use.
Potassium citrate is an effective treatment for both problems but you may
need to watch for crystals forming in your cat's urine.
If you are using potassium citrate, you should give it at least two hours
apart from any
phosphorus binders containing aluminium, because citrate may increase
the absorption of aluminium.
Potassium citrate should also not be used or its usage should be stopped
for cats with high potassium levels.
Bicarbonate of Soda
Bicarbonate of soda or baking
soda (sodium bicarbonate) may be used to treat metabolic acidosis. It
works by replenishing the
bicarbonate lost from the body. It is available in powder form in the
baking section of most supermarkets. In the USA, you can also buy it in
pill form, in which case you would probably have to crush it before use.
Dosage
Dr Katherine
James of the Veterinary Information Network
suggests an initial dose of 5-10mg per kg of body weight every twelve
hours. A kg is 2.2 lbs, so a 10 lb cat weighs 4.55 kg. A cat of this
weight would be given 22.75 - 45mg of bicarbonate of soda every twelve
hours.
In Renal disease (2006) Dr D Polzin
suggests a similar but slightly higher initial
dose of 8 to 12 mg/kg body weight every 8 to 12 hours. A 10lb (4.55kg) cat
would thus receive 36 - 55 mg of bicarbonate of soda every 8 to 12 hours.
You can
either dissolve the bicarbonate of soda in a small amount of water
and syringe it into your cat's mouth several times a day, or you can
give it in a gelcap.
When Thomas was first diagnosed, he had metabolic acidosis and my vet
recommended giving him the following to drink:
10 fluid ounces (0.3l) of water
2 tbsps powdered glucose
(available from pharmacies)
1 pinch of salt
1 pinch of bicarbonate of soda
Not every cat will drink this but it may be worth trying so ask your vet
about it. The cat does not
have to drink the entire amount each day. You should not allow cats
without metabolic acidosis to drink this, so if you have more than one
cat, it might be better to give it to your cat separately (see above). If
your cat's metabolic acidosis is severe, it would probably be safer to
give more accurate dosages as outlined above. Be guided by your vet.
Bicarbonate supplementation slows progression of CKD and improves
nutritional status (2009) de
Brito-Ashurst I, Varagunam M, Raftery MJ & Yaqoob MM Journal of the
American Society of Nephrology20(9) pp2075-2084 found that " bicarbonate
supplementation slows the rate of progression of renal failure to ESRD and
improves nutritional status among patients with CKD." This was a study
with human patients.
It is not clear whether treating the metabolic acidosis generally or the
use of bicarbonate of soda specifically helped in these studies.
Never give your cat bicarbonate of soda without your vet's knowledge and approval, since too
much can cause the opposite problem of excess alkalinity, which can be very
dangerous.
Drugs has some information about
bicarbonate of soda.
Acid-base, electrolytes and renal failure (1999) Polzin
DJ, Osbourne CA, James K Supplement toCompendium
on Continuing Education for the Practising Veterinarian21
11(K)) has information on metabolic acidosis and CKD.
