Reliable measurement of blood pressure is required for the appropriate management of patients with hypertension, both for the identification of patients in need of anti-hypertensive therapy and monitoring of response to treatment. While invasive measurement, obtained by the direct insertion of a catheter into an artery, is the most accurate method for determining blood pressure this is rarely practical outside of a critical-care facility. Non-invasive measurement of blood pressure has inherent limitations in terms of accuracy of the measurements that are obtained but is more practical than direct measurement in most clinical settings.

It has been demonstrated that both oscillometric and Doppler methods correlate reasonably with direct measurement of blood pressure in anaesthetised patients although they may underestimate the true values especially when it is low. However, the oscillometric technique correlates poorly with direct measurements in awake cats, and traditional oscillometric units do not reliably predict which cats will have retinal lesions.¹ For this reason the Doppler method has generally been preferred for diagnosing hypertension in this species. High-definition oscillometry (HDO) devices have been shown to correlate with direct blood pressure measurements in recent studies² and been shown to detect changes in blood pressure associated with amlodipine treatment³ so its use may become more widespread in the future. This discussion will, however, focus on the Doppler method.

Wherever possible the blood pressure is measured with the owners present and before admitting the patient to the hospital. Ideally measurements should be made having allowed the patient to acclimatise to its environment for 5–10 minutes and before any stressful procedures are performed. The fore-limb, hind-limb or tail can be used for measurement but it is important to be consistent since different measurements may be obtained from the various sites. The width of the cuff should be approximately 40% of the limb circumference.

To maximise acoustic coupling between the Doppler probe and the cat's skin various approaches can be taken. The fur can be clipped and this may be essential in cats with very dense hair coats. However, the sound of the clippers can be stressful and in the majority of cats clipping is not necessary. Instead an alcohol (spirit) soaked swab can be used to wet the hair in the area that the probe is to be applied. A small amount of ultrasound coupling gel can also be gently rubbed into the fur.

Before the probe is applied to the limb a liberal amount of ultrasound coupling gel should be placed onto the Doppler probe. It should be ensured that either the amplifier is turned off, or the volume turned to zero, before the probe is applied to the limb otherwise very loud 'static' noise will result which is likely to upset the cat. Once the probe is in direct contact with the skin the volume setting is increased and the probe gently moved around without losing contact with the skin until arterial pulsations are detected. The pulse can usually be found close to the carpal pad and slightly to the medial aspect of the limb. With practice it is not necessary to look at where the probe is being placed so the pulse can be found with the minimum of limb manipulation.

The pressure in the cuff is increased to about 20 mm Hg above the point above which the pulse can no longer be heard and then is slowly deflated while listening for the point at which the pulsations resume and viewing the dial of the sphygmomanometer so that this pressure can be recorded. This is the systolic blood pressure. The cuff is then fully deflated so that blood flow to and from the limb is unimpeded and then the process is repeated several times. The first measurement is disregarded and then between 3 and 5 consecutive measurements of blood pressure are made in the same manner. Ideally they should all be within 10 mm Hg of each other. If there is a downward progression in the series of measurements then the process should be repeated until the measurements become consistent.

The decision to treat a cat that has definitive evidence of end-organ damage (most often ocular changes) as a result of systemic hypertension is usually quite straightforward. The decision to treat an asymptomatic patient for hypertension on the basis of blood pressure measurements is essentially a compromise between the probabilities of unnecessarily treating for 'white-coat' hypertension (a transient blood pressure increase that occurs due to the stress of the hospital environment) and of not treating a patient that subsequently develops end-organ damage. The balance of these possibilities is, in part, dependent on the severity of the blood pressure elevation which is reflected in the ACVIM consensus statement guidelines for the risk of end-organ damage as outlined in the table below.⁴ It must be appreciated however, that the increase in blood pressure due to the 'white-coat effect' can be profound, up to 80 mm Hg, and it is not easily predicted by the external demeanour of the cat.⁵

IRIS (International Renal Interest Society) blood pressure sub-stages

When interpreting the results of blood pressure measurement it is important to also consider the pre-test probability that the patient is hypertensive. If the patient is old and has CKD the pre-test probability may be 20–30%, whereas if the cat is young and healthy then the likelihood of the patient being truly hypertensive is really very low. Measuring blood pressure (for the purpose of detecting hypertension) is not recommended in young, healthy cats because even if the measurement is high the likelihood of the patient being truly hypertensive is low.

For this reason it is not possible to give definitive cut-off points at which patients are designated as hypertensive; the decision to start therapy will depend on the clinical status of the individual patient. However, in the author's practice anti-hypertensive therapy is instituted in any cat with systolic blood pressure measurements over 160 mm Hg and compatible ocular lesions. If no ocular lesions are present then the cat is not started on therapy on the basis of measurements made at a single visit (irrespective of how high these are) but the owner is asked to return the cat to the clinic in 1–2 weeks for blood pressure measurement to be repeated. It is only if blood pressure is also high (>170 mm Hg) on this second visit that therapy is instituted. Screening for hypertension in asymptomatic cats is only performed if they are over 10 years of age unless they are also azotaemic, or have some other identifiable risk factor for development of hypertension.

References

1.  Jepson RE, Hartley V, Mendl M, Caney SM, Gould DJ. A comparison of CAT Doppler and oscillometric Memoprint machines for non-invasive blood pressure measurement in conscious cats. J Feline Med Surg. 2005;7(3):147–152.

2.  Martel E, Egner B, Brown SA, et al. Comparison of high-definition oscillometry - a non-invasive technology for arterial blood pressure measurement - with a direct invasive method using radio-telemetry in awake healthy cats. Journal of Feline Medicine and Surgery. 2013;15(12):1104–1113.

3.  Huhtinen M, Derre G, Renoldi HJ, et al. Randomized placebo-controlled clinical trial of a chewable formulation of amlodipine for the treatment of hypertension in client-owned cats. J Vet Intern Med. 2015;29(3):786–793.

4.  Brown S, Atkins C, Bagley R, et al. Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats. J Vet Intern Med. 2007;21(3):542–558.

5.  Belew AM, Barlett T, Brown SA. Evaluation of the white-coat effect in cats. J Vet Intern Med. 1999;13(2):134–142.