Noninvasive Blood Pressure Monitoring in the Canine and Feline Patient

Noninvasive blood pressure (NIBP) monitoring provides valuable information about hemodynamic stability of canine and feline patients. It is less stressful for the patient than catheterization of peripheral arteries. Results can be used to guide the veterinary team’s determination of treatment and prognostic factors. Accuracy of the results can be altered by the type of equipment used to obtain the reading, blood pressure cuff size and placement, and psychosocial and physiological factors of the patient. Reference ranges for blood pressure are shown in TABLE 1.

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Maximizing the Setting

Before beginning the measurement process, consider the location of the patient, if the patient can be transported, who should be present, and what type of NIBP will be performed. A recent study that used oscillometric units showed a notable difference in NIBP results obtained at home versus in the hospital, confirming “white-coat hypertension” in canine and feline patients.²

For hospitalized patients, their location within the hospital can determine the level of noise and traffic, which can affect their psychosocial stress. Being in the hospital, whether for an examination or illness, increases stress for any patient, and increased stress can lead to inaccurately elevated readings. If the patient is stable, plan NIBP monitoring during quiet times.

For patients visiting for an appointment, NIBP monitoring can be performed in a quiet examination room or treatment area, with or without the client present (depending on hospital protocol). Often the client’s presence reduces the stress associated with inaccurately elevated readings. NIBP measurement of cats is more accurate if performed while the cat is still in the carrier, as opposed to on the examination table.³

Methods

Doppler Method

Doppler ultrasonography devices (FIGURE 1) generate ultrasound waves that audibly indicate velocity of arterial blood flow. A cuff is placed over the artery and inflated until it impedes blood flow, at which time the sound is no longer audible. As the cuff is deflated and arterial flow resumes, the first returning audible sound corresponds to the systolic arterial pressure.

Figure 1A. Doppler ultrasonography unit, all components

The sound is amplified by a probe containing 2 piezoelectric crystals, which replaces use of a stethoscope. One of the crystals transmits energy and the other receives the echo from the artery; as sound waves travel through the ultrasonography gel, the echo of the pulsing artery is transmitted. Use of headphones will reduce external noise, which will help calm the patient for an accurate reading. To ensure optimal contact between the probe and the skin (as sound waves passing through air become distorted), the probe and skin should be clean and the hair over the artery may be shaved.⁴

Figure 1B. Probe, showing crystals.

Locating small arteries can be challenging; making small, slow, lateral movements with light pressure of the probe can be helpful. Too much pressure can occlude the artery, making it challenging to locate. After the artery is located, the patient’s heart rate can be assessed, if indicated. Doppler ultrasonography has historically been proven to be unreliable for measuring diastolic blood pressure as accurately recording the flow sounds is audibly challenging; however, the quality of newer Doppler ultrasonography units is increasing, enabling accurate diastolic pressure measurement.

Oscillometric Method

Oscillometric units (FIGURE 2) measure air pressure fluctuations inside the cuff, generated by pulsations of the underlying artery. Oscillometric units work automatically and can be set to record blood pressure in increments from immediately to hourly. The digital display shows systolic pressure, diastolic pressure, and mean arterial pressure. Oscillometric blood pressure measurement of small patients (<7 kg [15.4 lb]) or excessively moving patients can be inaccurate.⁵ For patients of any size, the reading may be inaccurate if the patient is squirming/struggling, shaking, or panting. To determine if the reading is inaccurate, compare the oscillometric pulse rate to the patient’s pulse rate; if they do not match, the reading can be considered inaccurate.

Figure 2. Oscillometric unit.

Differences and Similarities

Both methods are readily available and easy to use (TABLE 2). Regardless of which method is used, when serial blood pressure measurements from a given patient are needed, the same method (unit, cuff size, cuff location, patient position, patient environment) should be used each time. A recent study of NIBP measurement in conscious cats showed that results from oscillometric and Doppler ultrasonography units differ, and thus they cannot be used interchangeably.⁶

With either method, proper cuff size selection and placement is essential for obtaining an accurate result. Arteries accessible for NIBP are shown in FIGURE 3. Both methods are best used on arteries proximal to the carpus or hock or under the tail.

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With use of a Doppler ultrasonography device, the cuff should be located above the joint proximal to the probe; with use of an oscillometric unit, the cuff should be placed proximal to the carpus or hock joints or at the tail base (FIGURE 4). No matter which method is used to obtain the readings, 5 to 7 readings should be obtained to ensure accuracy, and all readings along with the type of unit, cuff size, cuff location, and patient position should be properly recorded in the patient’s record.

Figure 4. Proper noninvasive blood pressure cuff size.

Steps for NIBP Monitoring

For both types of units, the initial steps are the same.

1. Collect the supplies required, depending on the type of unit to be used.
2. Choose a quiet, low-traffic area for the procedure.
3. Gently restrain the patient into ventral or lateral recumbency; if necessary, ask for assistance with restraint.⁷
4. Determine the proper size cuff for circumference of the location on the patient (proportion = 30% for cats, 40% for dogs⁸) (FIGURE 5).
5. Place the cuff securely around the limb, proximal to the carpus or hock joint, or the base of the tail.

Figure 5. Proper noninvasive blood pressure cuff size.

Doppler

Follow steps 1 through 5 as above.

6. Prepare the location for the Doppler ultrasonography probe placement as mentioned above.

7. Apply ultrasonography gel to the probe.

8. Attach the gauge to the cuff.

9. Place the probe distal to the cuff.

10. Inflate the cuff until arterial sound is no longer heard or until a standard is reached (200 mm Hg in humans) unless the sound is still present at 200.

11. Slowly release pressure on the cuff.

12. Record systolic blood pressure at which arterial sound returns.

13. Record each reading in the patient’s record.

14. Repeat 5 to 7 times. Between each reading, completely release air from the cuff and wait 3 to 5 seconds for the integrity of the vessel wall to return.

Oscillometric

Follow steps 1 through 5 as above.

6. Attach monitor tubing to cuff.

7. Start the blood pressure unit.

8. Record each reading in the patient’s record.

9. Repeat 5 to 7 times.

Summary

Obtaining and recording accurate NIBP measurements is a skill that veterinary nurses can use to proactively care for their patients. Ensuring accurate results requires assessing all risk factors associated with inaccurate readings, avoiding as many psychosocial and physiological stressors for the patient as possible, precisely measuring cuff size, properly placing the cuff, and properly positioning the patient.