What is hypertension?
Hypertension is an increase in the blood pressure outside the normal range. The body is designed to keep the pressures within the blood vessels between 60 and 160 mmHg. If the pressure drops below this, there is not enough delivery of oxygen and nutrients to the tissues. If it goes above this it causes damage to tissue. It is a common condition in older cats and also occasionally occurs in dogs.
Why does it happen?
Certain medical conditions can lead to hypertension. The most common of these are kidney disease, hyperthyroidism, heart disease and Cushings, but about 20% of cases have no known underlying cause.
What does it do?
The main organs that are affected are the eyes, kidneys, heart and brain.
In the kidneys, increased pressure within the small blood vessels damages their walls. This affects the delivery of blood to the kidneys which results in a reduced capacity of the kidneys to filter the blood and remove impurities. It also often causes a leakage of protein from the kidneys into the urine. The damage to the kidneys also start off a chain of events within the body resulting in the release of chemicals that elevate the blood pressure even more. This then causes further damage to the kidneys.
In the eyes, the blood vessels at the back of the eye (retina) become damaged, which affects the ability of the eye to process images. The damaged vessels also leak blood into the eye which can lead to cataracts, retinal detachment and glaucoma.
In the heart, having to pump blood that is at a higher pressure is hard work, and so the heart muscle has to work more than usual. The consequence of this is that the heart muscle becomes thicker and this results in a reduction in size of the chambers of the heart. Therefore, to continue to pump the same amount of blood round the body, the heart has to pump more times and so we see an increased heart rate. The thicker muscle is supplied by the same number of blood vessels as the normal heart but needs more oxygen, so areas of the muscle may not get enough oxygen. This can result in abnormal heart beats, which will affect the heart’s ability to pump blood.
In the brain, the blood vessels are fragile and easily damaged. Where this damage affects blood supply, areas of the brain will be starved of nutrients and die off. This can result in changes often assigned to getting older (e.g. dementia). In areas where the vessels become leaky, it will result in increased pressures within the brain. Not only will this also kill off brain cells but will also cause pain.
What are the consequences for my pet?
- Neurological signs (behaviour changes, increased vocalisation)
- Heart disease
- Kidney disease
How is it diagnosed?
The first step is to check what your pet’s blood pressure is. As hypertension affects about 20% of all elderly cats, it is recommended that all cats over 10 years of age have their blood pressure checked at least once a year, and more often if they are showing any clinical signs or are diagnosed with any of the underlying conditions.
The pressure is checked in a similar way to humans. A small cuff is placed around the top of your pet’s front leg (or base of tail) and a pump is used to blow it up which stops the blood flow to the foot. Then as we listen for a pulse in the foot, we gradually reduce the pressure in the cuff. Once the pulse returns, the pressure on the cuff at this point is the same as the pressure within the arteries.
If the pressure is above 160 mmHg, this equates to hypertension. Sometimes small elevations can be caused by stress, and so if the pressure is around 160-170 mmHg we may allow your pet to settle for a bit and repeat again. If repeated checks are elevated then a diagnosis of hypertension is reached. The next step is to check for any underlying conditions.
The other tests that are required are blood and urine samples to rule out kidney, thyroid or other hormonal conditions. A heart scan is a quick test that can be done without any sedation or anaesthetic to rule out the heart conditions.
What are the treatment options?
The first priority is to treat any underlying condition (kidney disease, hyperthyroidism, heart disease, Cushings).
If there is no underlying condition, treatment is with a tablet containing a chemical called amlodipine. It is usually a once daily treatment and will control the blood pressure in almost all cases. It is essential to recheck pressures regularly to ensure that treatment is being effective.
A lot of the clinical problems caused by hypertension can be reversed once the blood pressure is under control (even blindness if caught early enough). Hence it is crucial to monitor blood pressure as your pet gets older