Indirect Blood Pressure Measurement

Maintenance of appropriate systemic arterial blood pressure is vital for survival. Because many common situations and conditions (eg, general anesthesia, renal or cardiovascular disease) place animals at risk for dangerously high or low blood pressure, blood pressure monitoring is a fundamental necessity.

In clinical practice, blood pressure is usually measured indirectly. The two most common methods of indirect blood pressure measurement are Doppler ultrasound using a sphygmomanometer and oscillometry. Both techniques use an inflatable cuff placed around an extremity. High-definition oscillometry, a newer indirect technique, is not used often in clinical practice.

Doppler Ultrasound Measurement
Doppler ultrasound measurement is likely the most common blood pressure measurement technique used in small animal practice. The procedure is straightforward, and the equipment is relatively inexpensive and reliable.

Doppler ultrasound reliably measures only systolic blood pressure in dogs and cats; it cannot be used to measure mean or diastolic blood pressure.

Standard Oscillometry Measurement
In clinical practice, oscillometers are often built into multiparameter monitors and usually have a stat feature that allows continual, repeated measurements. Thus oscillometers are commonly used for critically ill or anesthetized patients that may require more intensive monitoring.

Oscillometers report systolic, diastolic, and mean blood pressures; however, they are unreliable in patients with cardiac dysrhythmias because measurement relies on rhythmic arterial pulsations.

General Techniques
While actual blood pressure measurement varies between the techniques described here, some important points apply to both.

Patient Acclimation & Restraint
Ideally, blood pressure should be measured in a quiet, comfortable environment after the patient has become acclimated (ie, without disturbance) for at least a few minutes, but acclimation is rarely possible for acutely or critically ill patients.

The patient should remain still during measurements to optimize accuracy and, if necessary, can be physically restrained gently. However, some patients cannot be restrained in lateral recumbency because of agitation, respiratory distress, or other reasons. Chemical restraint should be avoided, as it often alters results.

Cuff Height
To acquire the most accurate blood pressure values, cuff height should be as close to the level of the right atrium as possible (Step 1A). The cuff can be placed on the antebrachium, distal pelvic limb, or tail.

If the vertical distance between the cuff site and the right atrium exceeds 10 cm, a correction factor must be integrated to interpret blood pressure measurement results (see Correcting for Cuff & Right Atrium Level Disparity). When the cuff is much lower than the right atrium, pressure values are falsely high because of the weight of the blood column between the atrium and cuff site.

Multiple Measurements
The first blood pressure measurement should be discarded. At least 3 additional measurements should be obtained for a minimum of 3 to 7 consecutive, consistent measurements with less than 20% variability. Once these measurements have been obtained, they should be averaged to yield the blood pressure measurement.

What You Will Need

• Doppler ultrasound with attached Doppler probe
• Headphones, if desired
• Sphygmomanometer
• Tape measure with metric scale
• Cuff of appropriate size
• Clippers with clean blade
• Ultrasound transmission gel

• Oscillometer with pressure tubing attached
• Tape measure with metric scale
• Cuff of appropriate size

Step-by-Step Doppler Ultrasound Blood Pressure Measurement

Here, the midpoint of the cuff sits approximately 27 cm below the right atrium. If the measured systolic pressure at this level were 142 mm Hg, correct the measurement as follows to yield a more accurate reading:

27 cm × 0.8 mm Hg/cm = 21.6 mm Hg
142 mm Hg – 21.6 mm Hg = 120.4 mm Hg

Author Insight: Most commercial cuffs have Velcro closures that can loosen during inflation. Medical tape can be placed to secure the cuff’s closure, but be certain not to further tighten the cuff with the tape, which would invalidate the measurement. This is true for both techniques.

Author Insight: Consider using headphones to minimize ambient noise and to avoid frightening the patient.

Author Insight: Deflate the cuff only a few mm Hg/sec in order to hear the first pulse wave passing under the cuff, heralding systolic pressure. Opening the valve too quickly results in falsely low readings because the true systolic pressure is missed.

Step-by-Step Standard Oscillometry Blood Pressure Measurement

Author Insight: Support the dog’s limb from underneath while restraining the dependent limb. Many dogs resent having their paws handled, and this technique minimizes patient movement and tension in the limb, which can adversely affect results.

JAMIE M. BURKITT CREEDON, DVM, DACVECC, is head of emergency and critical care at Red Bank Veterinary Hospital in Cherry Hill, New Jersey. Her clinical interests include monitoring techniques and endocrinopathy in critical illness. Dr. Burkitt Creedon recently coedited the textbook Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care. She has also instructed at numerous NAVC Conferences. She received her DVM from University of California, Davis, and completed a rotating small animal internship at University of Pennsylvania and a residency in small animal emergency and critical care at UC Davis.