Measuring blood pressure
The most accurate way to measure blood pressure is by catheterizing a suitable artery (e.g., femoral) and using a transducer to measure the pressure directly.
This method is technically challenging, uncomfortable for patients, and not practical in the clinical setting. As a result, veterinarians rely on indirect methods of estimating blood pressure. Doppler flow detection, oscillometry, and photoplethysmography are the indirect measurement methods that have been most closely studied in veterinary medicine.
Regardless of the technique, it is important to remember that these methods are not absolutely accurate and not every method is suitable for every patient. Operator variations can greatly influence blood pressure measurements, so a consistent operator and technique are essential for accurate results. It is also important to remember the significance and possibility of the "white coat" effect: artificially increased blood pressure resulting from stress. A single elevated blood pressure value in an otherwise asymptomatic animal must be interpreted with caution, especially if that patient does not have an underlying disease compatible with hypertension.
Blood pressure should always be evaluated in a quiet area away from other animals and distractions, after the patient has been allowed to acclimate. When possible, the owner should be present because this is likely to have a calming effect. Several sequential blood pressure measurements should be taken. The first attempt can be used to allow the animal to become accustomed to the procedure, and the following pressure measurements should be compared with each other to verify accuracy. If there are great variances in the individual readings, the series should be repeated. Although it may be ideal to measure the blood pressure of all patients, this is not practical in most clinical settings. Therefore, it is recommended that efforts focus on patients with known predisposing clinical conditions or patients receiving drugs known to increase blood pressure.
Doppler Flow Detector
Doppler flow detection is a common method of measuring blood pressure in the clinical setting. Doppler ultrasound machines (Figure 3) work by emitting ultrasound waves and then "listening" for the waves to be reflected back. If there is movement, such as blood cells through an artery, the frequency of the reflected ultrasound wave is shifted (the Doppler effect), and this change in frequency is amplified and made audible. The change in frequency is heard as the characteristic "swoosh" sound when the transducer is placed over an artery. The only materials needed other than the Doppler flow detector is a sphygmomanometer, selection of cuffs of different sizes, and ultrasound gel. Although measuring systolic blood pressure is easy, measuring diastolic (and therefore mean arterial) pressure is technically more challenging.
Any leg or distal artery can be used, but it is best to always measure blood pressure on the same leg to develop a consistent technique.3 If a hind leg is used, an area over the dorsal surface of the tarsal region should be clipped and the Doppler probe placed over the dorsal pedal branch of the cranial tibial artery. Ultrasound gel should be placed on the probe to ensure proper conduction. A cuff with a diameter that is 30% to 40% of the limb's circumference should be placed halfway between the stifle and hock and fitted snugly. Selecting an appropriately sized cuff is extremely important because cuffs that are too small generate artificially elevated pressure measurements, whereas cuffs that are too large result in measurements that are lower than they should be. Using a tape measure to measure the circumference of the limb can help in selecting the correct cuff.
Once the clear and consistent sound of pulsating blood can be heard via the Doppler detector, the pressure can be measured. The cuff should be inflated using the sphygmomanometer until blood flow can no longer be heard.26 It is best to inflate the cuff 20 to 30 mm Hg past the point at which blood flow was last detected. The point at which flow sounds return while the pressure in the cuff is slowly released is the systolic blood pressure. A second sound can often be heard as the cuff pressure continues to be slowly released; this sound represents the diastolic blood pressure. This process should be repeated three to five times to ensure an accurate reading.
An oscillometric blood pressure device works by inflating a cuff around an extremity until arterial blood flow stops; then, while slowly reducing the inflation, the unit monitors pulse waves generated by arterial pulsations. These devices are conveniently automated, measure systolic and diastolic pressure as well as mean arterial pressure (MAP), and are reasonably accurate for medium-sized and large dogs. However, these devices should not be used in small dogs or cats because they consistently underestimate blood pressure in these animals.
Photoplethysmography is a technique used in human medicine that relies on infrared light transmission to measure arterial volume on a digit. Although its use is limited to cats and small dogs that weigh approximately 22 lb (10 kg) or less, this technique can provide continuous blood pressure measurements. These devices are not widely available for veterinary use except in some teaching hospitals and research settings.
- Binns SH, Sisson DD, Buoscio DA, Schaeffer DJ (1995) Doppler ultrasonographic, oscillometric sphygmomanometric, and photoplethysmographic techniques for noninvasive blood pressure measurement in anesthetized cats. J Vet Intern Med 9:405-414
- Branson KR, Wagner-Mann CC, Mann FA (1997) Evaluation of an oscillometric blood pressure monitor on anesthetized cats and the effect of cuff placement and fur on accuracy. Vet Surg 26(4):347-353
- Stepien RL, Rapoport GS, Henik RA, et al (2003) Comparative diagnostic test characteristics of oscillometric and Doppler ultrasound methods in the detection of systolic hypertension in dogs. J Vet Intern Med 17(1):65-72
- Belew AM, Barlett T, Brown SA (1999) Evaluation of the white-coat effect in cats. J Vet Intern Med 13(2):134-142