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.
Other tips for accurate measurement include:
Use headphones to ensure that the sound does not stress the patient. This also allows you to hear the sound of the machine more effectively, increasing your accuracy.
Shave the area for the transducer very short, and apply adequate ultrasound gel so that good contact is ensured. If an assistant can hold the transducer in place, this will likely help because the artery tends to roll around under the skin, and minor adjustments in position often need to be made. Using tape is the alternative, but apply it so that it can be quickly removed and adjusted as needed.
Just the right amount of pressure on the transducer is essential. Too much and the flow will be reduced, too little and it will slip out of position. Make sure that if a limb is being used, that its position does not compromise blood flow to the extremity where the transducer is in place. Use a correctly sized cuff for the patient. Its width should be about 1.5-2 times the diameter of the antebrachium. Inflate the cuff slowly if the patient is awake to ensure they are not startled.

    The Doppler Technique

    1. Assemble the Doppler unit before addressing the cat. Plug in the headphones and place the ear pieces over your ears.
    2. Position the cuff proximal to the hock, proximal to the elbow, or at the base of the tail, depending on which artery is to be used. Note: cuff size in width should be 40% of the circumference of the limb at the site of placement.  When the median tibial artery is used, a 2-to 4cm cuff is placed above the hock. The width of the cuff is critical when oscillometric technology is used, but it is not critical with Doppler technology. View cuff placement on a cat or a dog
    3. Turn the unit on and set the volume in the high range. If the volume is too low, arterial sound may not be detected.
    4. Apply a thin coat to coupling gel to the skin.
    5. Apply coupling gel to the transducer.
    6. Position the transducer over the artery. The median tibial artery is usually slightly medial to the midline. It is necessary to move the transducer until the “swishing” of the blood is heard in the artery. Hold it in place with your thumb. It may be necessary to apply firm pressure to assure proper contact between the transducer and the skin (too firm a pressure may occlude the artery).
    7. Squeeze the bulb on the manometer until the pressure in the cuff exceeds that of the artery. At that point, the swishing of the blood in the artery will no longer be heard. Notice that filling the cuff too rapidly may frighten your patient.
    8 Slowly reduce the pressure in the cuff by squeezing the manometer’s trigger. When the pressure in the cuff falls below the arterial pressure, the arterial swishing sound can again be heard. This should be noted and recorded.
    9 Seven measurements should be taken. Discard the high and low readings and average the remaining five. This is considered the systolic blood pressure and is measured in mmHg.

    Hypertension is defined as the following:Cats systolic> 170-1 80 mm Hg; diastolic> 120 mm Hg
    Dogs systolic> 180 mm Hg; diastolic> 100 mm Hg
    Note the diastolic pressure can be detected in some cats by closely watching the dial of the manometer. It oscillates at the diastolic pressure level. However, a reproducible reading is not always possible, especially in cats that are stressed due to catecholamine influence   (which makes the vessels more stiff than normal), cats that are cold, and very small patients. However, the important measurement in cats is the systolic blood pressure.
 

    Technique tips for using the Doppler:
    1. Do not tape the transducer in place, as is recommended by some. The slightest movement of the cats foot or limb often moves the transducer enough that the arterial sounds are lost. Instead, hold the transducer in place with your thumb. If the artery is lost, slight         movement of your thumb can relocate it.
    2. Use the maximal volume that is comfortable to your ears. Use the headphones. Although they are somewhat cumbersome, they minimize the stress of the procedure. The sounds of the external speaker frighten many cats, elevating their blood pressure. The headphones bypass the external speaker so the cat no longer hears the harsh sounds.
    3 The transducer is fragile (and expensive to replace). It may be damaged by dropping it or by striking it against something solid. Treat it with care.  4. Use of ultrasound gel is essential. Do not use ECG paste or baby oil or alcohol. These products may cause deterioration of the resin surface of the transducer. Ultrasound gel is water soluble. Remove it from the transducer after each use.
    5 Comfortably extend the cat’s limb. If it is flexed, the blood flow through the artery may be abnormally restricted giving no readings or falsely low readings.

Treatment of Hypertension

The treatment for hypertension should focus on the underlying cause, if possible. Hyperthyroid cats that are hypertensive should be treated simultaneously for the hyperthyroidism and systemic hypertension. In cats with renal disease and systemic hypertension, medication is usually needed indefinitely to control the hypertension. The treatment of hypertension includes sodium restriction and specific medications. In the majority of cases, a low-sodium diet with a calcium channel blocker (amlodipine, for cats: 0.625 mg per cat every 24 hours). Angiotensin - converting enzyme inhibitors are sometimes also indicated if no response to a calcium channel blocker or, on some occasions, the two have to be given together. The blood pressure does need to be monitored on a periodic basis and to evaluate for any possible complications (especially retinopathy and glomerular disease). In the majority of patients, hypertension can be controlled with appropriate therapy.
 

Update on Hypertension in Cats

The domestic cat is a species in which the prevalence of hypertension and its effects, especially in geriatric or chronically ill cats have only been recently elucidated. Each practitioner should be vigilant, and indications for checking blood pressure ideally should be extended to all cats as a preventive health care screen, but be especially targeted for those patients older than l0 years of age, those with ocular, neurological, or renal dysfunction signs, or with hormonal disturbances such as hyperthyroidism.
Causes and Signs
Hypertension is often sub-clinical 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 veni-puncture, 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 over-stimulation 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.
 

Treatment

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 KID® is prescribed, but like any new food in cats, should be gradually introduced to ensure that the patient adjusts to the new taste. Amlodipine (NorvascTM), a calcium channel blocker, is most commonly selected to counteract hypertension. The usual initial dosage is 0 625 rng/cat/24h (1/4 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 dose is approximately 2.5 mg/day (for a 2.5-5 kg cat). Sometimes amlodipine is compounded into a suspension for easy dministration. 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 (LasixTM) is dosed at 1 mg/kg! 1 2 hr P0. Close monitoring of blood pressure is important because hypotension may potentially occur, especially with combination therapy.
 

References

Tilley. L.P. and Goodwin, J.: Manual of Canine Feline Cardiology. 3rd Edition. New York, WE. Saunders, 1999 (in press).
Stiles, J. Et al. The prevalence of retinopathy in cats with systemic hypertension and chronic renal failure or hyperthyroidism. J Am An Hosp Assoc 1994:30:564.

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