Blood pressure: A critical factor (Proceedings)
It has been stated that: "Systemic hypertension associated with kidney disease is a very real problem, and has been diagnosed in over 60% of cats with chronic renal disease. Hypertension can have multi-systemic effects if left untreated, including left ventricular hypertrophy and cardiac failure, retinal detachment and blindness, cerebrovascular hemorrhage, and progression of renal dysfunction." (Rosemary Henik, DVM, MS, DACVIM) While this is true, let's look at the applicability of measuring blood pressure, methods of assessment and the interpretation of results in clinical practice.
1. Systemic arterial pressure is the pressure within the arteries and arterioles2. Systolic pressure refers to the pressure when the aortic valve is open and the heart is ejecting blood (120 mmHg)
3. Diastolic pressure refers to the pressure when the aortic valve is closed and the heart is resting (80 mmHg)
4. Mean arterial pressure is closer to diastolic as the heart spends most of its time resting in diastole (90 mmHg)
5. Excitation, stress and pain can *transiently* raise the values, but as there are mechanisms in place to limit the elevations, consistent systolic values exceeding 170 mm Hg (range 168-180) are accepted as reflecting hypertension.1-9
6. To minimize the effects of "white coat syndrome", allow the patient to acclimate to the environment for ten minutes before measuring BP. Measure BP before performing any other evaluations (TPR, examination, etc.) Take measurements over several minutes until a series of five values are obtained that vary by no more than 10 mm Hg.
7. A mean arterial pressure of >60 mmHg are necessary to maintain perfusion to the brain, heart and kidneys.
8. Doppler measurement of systolic pressure underestimates values obtained by direct invasive measurement of arterial pressure. This may be corrected by the equation: Doppler + 14 mm Hg = direct systolic pressure.
Whose blood pressure should we measure?
Non-invasive, indirect arterial measurements of blood pressure should be made in all anesthetized, critical or high-risk patients to detect and monitor management of hypotension. This technique should be used widely as a screening method for the pre-clinical detection of hypertension in patients with renal disease, hyperthyroidism, ocular changes consistent with hypertension, a cardiac murmur, or left ventricular hypertrophy, neurological dysfunction and all cats over eight years of age.
How to measure
In an anesthetized patient, Doppler or oscillometric methods are reliable. In conscious cats, oscillometric measurements using devices reported in published papers do not correlate with radiotelemetrically obtained values; Doppler, PetMap or Memo methodology should be used. To minimize the effects of "white coat syndrome", allow the patient to acclimate to the environment for ten minutes before measuring BP. Measure BP before performing any other evaluations (TPR, examination, etc.). Use of forelimb or hindlimb is equally valid. It is very important to use the appropriate cuff size. The cuff chosen must measure 40% of the circumference of the limb at the cuff placement site (see attached table). Shaving helps achieve good probe contact but is not essential. In fact, I do not recommend shaving a conscious patient as this raises their fear level. Wetting the fur in the metatarsal or metacarpal area with alcohol is adequate. It is important to avoid alcohol touching the probe; use gel generously. Gentle inflation and deflation of the cuff will reduce strangeness of the experience for the patient. Use of a stereo headset will help reduce noise for the cat as well as make it easier for the operator to hear the signal. Take measurements over several minutes until a series of five values are obtained that vary by no more than 10 mm Hg. Record the limb and cuff size used, for future comparisons, in the medical record.
Causes of artificially high values: fear, noise and the sensation of the cuff inflating and deflating.... be quiet, use headset and inflate gently; take the readings on the client's lap whenever possible. Using a cuff that is too small will also cause artificially high BP readings.
Other methods of measurement
* Oscillometric measurement (Critikon, Dinamap, Datascope Passport) is not reliable in conscious cats and small dogs (<25 lbs). These are appropriate for monitoring anesthetized patients.
* Central Venous Pressure (CVP) measurement:
CVP an easy, cheap and is an under-utilized technique and is the most accurate, but requires invasive procedure and is not an out patient procedure. While it reflects right atrial pressure associated with volume changes, arterial BP assesses adequacy of perfusion of vital tissues.