25 Jan 2016
Sarah Caney
Job Title
Figure 1. Ocular evidence of systemic hypertension is evident in this cat. Bilateral mydriasis, bilateral retinal detachment and some retinal haemorrhage (left eye) can be seen when assessing the eye with a pen torch.
Systemic hypertension is recognised as a common problem in elderly cats. It is often referred to as a “silent killer”, so proactive blood pressure assessment is important – especially in older cats and those at increased risk of the condition.
In most cases, systemic hypertension is thought to occur secondary to another illness. Chronic kidney disease is the most important of these. Other diseases associated with the problem include hyperthyroidism, primary hyperaldosteronism, hyperadrenocorticism, erythropoietin therapy and chronic anaemia.
Diagnosis can be challenging, since cats are vulnerable to stress-associated (or “white coat”) hypertension. For this reason, a single high blood pressure reading is not diagnostic for this condition. If ocular changes consistent with systemic hypertension are documented, this confirms the diagnosis. The author prefers the Doppler methodology for measuring systolic blood pressure.
Treatment is often successful and oral amlodipine is generally highly effective when used alone, with an angiotensin-converting enzyme inhibitor, or with an angiotensin receptor blocker.
Systemic hypertension (SH) is a common diagnosis in elderly cats. As with people, SH is often said to be a “silent killer”, since clinical signs of this condition are not always readily visible.
Unfortunately, for a significant number of cats the diagnosis is only made when severe consequences of SH develop, such as sudden onset blindness (Figure 1).
Proactive screening of cats at risk is important to facilitate diagnosis and treatment at an early stage, to prevent catastrophic consequences.
SH is commonly secondary to another disease. Chronic kidney disease (CKD), hyperthyroidism and primary hyperaldosteronism are among the diseases associated with SH.
At least 20% of cats with CKD are thought to suffer from SH as a complication of their renal disease, and there does not appear to be an association between severity of azotaemia and likelihood of SH (Syme et al, 2002).
SH has also been reported in cats suffering from chronic anaemia and in up to half of cats receiving treatment with erythrocyte-stimulating agents, such as erythropoietin and darbepoetin.
Primary SH has been documented in cats, but is thought to affect no more than 20% of cats suffering from SH.
The author follows recommendations developed by charity International Cat Care (ICC; www.icatcare.org), as part of its WellCat guidelines, for life stage-appropriate health checks of cats.
ICC recommends blood pressure is measured:
Blood pressure tests should be included in the assessment of all patients with diseases known, or suspected, to be associated with SH, such as CKD, hyperthyroidism and others mentioned earlier. Blood pressure should be assessed every three to six months in these patients.
Blood pressure assessment is also important in patients with potential clinical evidence of SH. Persistent elevation of blood pressure causes damage to sensitive “target organs” (target organ damage; TOD). The four target organs are the kidneys, heart, brain and eyes, hence clinical signs of TOD include:
Most specialists, including the author, favour the Doppler methodology as being the most reliable technique to assess systolic blood pressure in conscious cats.
Blood pressure measurement should be performed in a quiet room, away from barking dogs and telephones. Ideally, allow the cat 10 minutes to acclimatise to the surroundings before taking the measurements. This “acclimatisation” period helps to reduce the incidence of “white coat hypertension” – stress and anxiety stimulate the sympathetic nervous system, leading to falsely high blood pressure readings. For some cats, having the owner present limits the effect of stress on blood pressure readings.
After the acclimatisation period, the cat is restrained as gently as possible for the procedure – usually all that is required is gentle steadying of the cat while the cuff is placed and readings are taken (Figure 2).
The forelimb is the most popular location, but take care to not overextend the elbow, as this is a common site for osteoarthritis in older cats. Some cats do not like their paws to be held; when this is the case it can be simplest to use the base of the tail (coccygeal artery) instead.
An inflatable cuff (2.5cm wide for most cats, with the ideal width 30% to 40% of the limb circumference) is placed just below the elbow. Surgical spirit can be used to wet the area over the common digital artery, located on the palmar surface of the forelimb between the carpal and metacarpal pads. A liberal amount of ultrasound coupling gel is then applied over this area to ensure a good signal. Ultrasound coupling gel is also applied to the Doppler probe.
The Doppler probe is placed over the common digital artery, maintaining the Doppler crystals perpendicular to the limb axis and, therefore, the blood vessel. To avoid excessive noise, it is preferable not to switch the Doppler unit on until after the probe has been placed on the skin. Where possible, headphones should be worn so the cat does not hear any of the noise associated with measuring blood pressure.
If pulsatile blood flow cannot immediately be heard, it may be necessary to gently move the probe over the skin, between the carpal and metacarpal pads, until a signal is detected. Additional ultrasound coupling gel is often useful if blood flow still cannot be detected.
It is important to hold the probe gently over the skin and to not apply excessive pressure that could impede blood flow.
Once regular pulsatile blood flow is heard, the cuff should be inflated using the hand pump sphygmomanometer, to a pressure of about 20mmHg above that required to occlude blood flow. Air is then allowed to slowly bleed through the valve at the back of the sphygmomanometer.
The point at which blood flow can first be detected clearly and consistently again in the artery is taken as the systolic blood pressure (SBP). The procedure should be repeated five times over two to three minutes and the SBP taken as an average of these readings.
Some cats show a sharp drop (20mmHg to 30mmHg) in SBP over the first two to three readings due to stress. Where this occurs, the initial readings are discarded.
The diastolic blood pressure (DBP) is the pressure at which the pulsatile flow becomes a more continuous sound. Unfortunately, it is not always possible to determine this using the Doppler technique.
A number of “reference ranges” have been published for normal cats citing normal SBP readings from 107mmHg to 181mmHg. When it is possible to measure it, the DBP of normal cats should be lower than 95mmHg.
White coat hypertension, or stress-induced increases in the SBP, are a significant issue when interpreting results. On average, the white coat effect increases SBP by 15mmHg to 20mmHg; however, the effect is highly variable and can be as much as 75mmHg.
The International Renal Interest Society has published a classification system according to risk of target organ damage (Table 1).
Cats with SBP above 180mmHg are generally hypertensive and therapy is justified. However, some healthy cats may transiently have SBP above 180mmHg. Hypertension should therefore never be solely treated on the basis of a single abnormal blood pressure reading.
If TOD is evident, hypertension diagnosis is confirmed and treatment can be instituted. Patients with pre-existing CKD are considered to have TOD.
In the absence of TOD, it is prudent to recheck the SBP on another occasion before pursuing treatment. The author recommends the following steps be taken in cats with SBP readings above 180mmHg:
SBP readings persistently between 160mmHg and 180mmHg are believed to pose a moderate risk. Persistence is defined as being present on several occasions over a two-month period.
If there is evidence of TOD (for example, hypertensive retinopathy), or if the cat is known to have CKD or any other condition known to be associated with hypertension then antihypertensive therapy is justified.
In the absence of either of these, it might not be possible to rule out white coat hypertension. Further monitoring might be more appropriate.
Cats in this group may have mild hypertension, but many normal cats will also give blood pressure readings in this range due to the white coat effect. Treatment is not normally recommended, unless there is evidence of TOD.
For cats with conditions known to predispose to hypertension, monitoring blood pressure and evaluating evidence of TOD is recommended every one to three months once readings above 150mmHg are obtained.
Most normal cats have SBP readings of 120mmHg to 150mmHg. This should be viewed as the “target range” following treatment.
A detailed ophthalmic examination is essential both in the diagnosis and assessment of the extent of ocular disease.
Abnormalities consistent with SH include retinal oedema and detachment, intraocular haemorrhage, arterial tortuosity, variable diameter of retinal arterioles, papilloedema and glaucoma. Foci of retinal degeneration (hyper-reflectivity) may develop at the sites of any previous damage.
Distant indirect ophthalmoscopy is the easiest way to carry out a thorough ocular examination. This requires the following:
Direct ophthalmoscopy can be used to have a closer look at any lesions identified.
The management goals are to:
Amlodipine is a licensed treatment for feline hypertension. Most cats can be stabilised on 0.625mg or 1.25mg daily (Table 2). Response to therapy should ideally be monitored after 7 to 10 days of treatment by measuring SBP and monitoring TOD.
In successfully treated cases, the blood pressure should drop to between 120mmHg and 150mmHg within 7 to 10 days of starting therapy.
Post-treatment blood pressure readings between 150mmHg and 160mmHg are acceptable, as long as there is no evidence of continued TOD. In some cases, it may be necessary to use amlodipine with benazepril or telmisartan to achieve an adequate response.
Once blood pressure is stable, patients should be assessed every one to two months, reducing the frequency to a minimum of once every three months in very stable patients.
Further investigations are indicated to determine the cause of the SH, where possible. Investigations should include serum thyroxine (T4), blood urea and creatinine and urinalysis including specific gravity, assessment of urine protein to creatinine ratio and bacterial culture.
Where possible, cardiac and renal ultrasounds are helpful. A degree of left ventricular hypertrophy is a common echocardiographic finding in hypertensive cats and generally does not require specific treatment.
Additional diagnostic tests that may be considered include:
The long-term prognosis depends on the presence, nature and extent of any underlying disease. In primary hypertensive cases, it is usually possible to manage the hypertension and prevent future complications.
