Diagnosing and treating hypertension (Proceedings)
Causes and Signs
Hypertension is often subclinical initially, and as the blood pressure increases, clinical signs may occur both due to the hypertension itself, or be linked to frequently associated medical conditions. Associated signs may reflect the cat's geriatric life stage, cardiac disease, or be associated with the two most commonly linked conditions— hyperthyroidism and renal failure.
One serious clinical finding associated with hypertension is sudden blindness. Retinal detachment and hemorrhage are typical findings. The sooner the cat is treated, the better the prognosis. Ideally, therapy should start immediately, or at least within 24 hours of hemorrhage or detachment in order to optimize prognosis for sight. When the retina is not yet detached, mild hemorrhage in the eye may lead to anisocoria or just a reduced PLR if in the early stages, rather than blindness. Ophthalmoscopic examination will allow visualization of retinal changes, though severe retinal detachment can sometimes be seen with the naked eye. Dilation and engorgement of the optic disc-associated blood vessels may be noted especially in early cases, pre-hemorrhage.Other findings commonly noted in cats presenting in a hypertensive state include PU/PD, gallop rhythm, tachycardia and increased PMI, and bounding pulses. An echocardiogram often reveals thickened LV free wall and IV septum, with a decreased LV volume and internal diameter. If hemorrhage occurs in other organs or nervous system, variable presentations including stroke signs may occur. An interesting note aside is that during venipuncture, it may be observed that a syringe may fill more quickly than normal, and post-procedure bleeding at the entry point may be more pronounced than usual.
When associated with hyperthyrodism, the hypertension is due to thyrotoxic cardiomyopathy leading to an increase in cardiac output. A large percent of cats with hyperthyroidism have associated hypertension.
In chronic renal disease, the hypertension is thought to be due to fluid and sodium retention related to overstimulation of the rennin-angiotensin-aldosterone axis, resulting in increased vascular compartment volume. Reduced vessel capacitance, especially in the venous side occurs along with increased adrenergic and renopressor activation; all factors contributing to an increased cardiac load. Approximately 65% of cats with chronic renal failure are thought to be hypertensive. A portion of Persian cats with polycystic kidney disease may develop increased mean arterial pressure.
Measuring Blood Pressure
Though there is some variation in reported parameters in the literature, it is commonly accepted that a systolic pressure above 170 mmHg indicates clinically significant high blood pressure in cats. Because cats are prone to the so-called "white coat syndrome", which is a stress response due to cortisol release as a result of handling, restraint, or just being in a clinic rather than in a familiar home environment, results of a single blood pressure check should not be considered accurate. Due to this variability associated with performing blood pressure measurements, it is recommended that seven measurements be taken, and that the highest and lowest values be discarded, with the remaining five averaged.
Treatment of Hypertension
In cats, the standard approach is to complete a thorough diagnostic evaluation and ideally, establish an underlying cause and treat it. Concurrently, the use of an anti-hypertensive agent will assist with normalization of blood pressure. Hyperthyroid cats may not need lifelong medication to control blood pressure once the hormonal disturbance is controlled. A low sodium diet such as Hills K/D® is prescribed, but like any new food in cats, should be gradually introduced to ensure that the patient adjusts to the new taste. Amlodipine (Norvasc™), a calcium channel blocker, is most commonly selected to counteract hypertension. The usual initial dosage is 0.625 mg/cat/24h (¼ of a 2.5 mg tab), and if not effective after 48 hours, the dose can be doubled. If still not providing good improvement, an ACE inhibitor (e.g., benazepril) is sometimes added to the regimen. Benazepril has been approved in Europe for treating kidney failure in the feline. The suggested European label dose is 0.5 mg/kg SID. Sometimes amlodipine is compounded into a suspension for easy administration. It is very important to shake the suspension well before each dosing to make sure that it is properly distributed in the liquid. In moderate to severe cases, diuretics may be used in the first few days to enhance blood pressure reduction. Furosemide (Lasix™) is dosed at 1 mg/kg/12 hr PO. Close monitoring of blood pressure is important because hypotension may potentially occur, especially with combination therapy.
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