Abstract
It is important for every veterinary nurses to possess blood pressure measurement skills. This skill is vital for the purpose of diagnosing and monitoring the blood pressure of patients.
Measuring patient’s blood pressure accurately is key to ensuring they receive the needed care and treatment as at when due. Failure to follow the right procedure in a careful manner can lead to error, and can have an adverse effect on patient management. Though most veterinary practices will settle for indirect measurement methods, using the standard procedure is essential for recording accurate results. Adequate training should be organized for nurses who are charged with the duty of performing measurement of blood pressure. They should also be made up to date in the right use of equipment. The focus of this article is the pronciples of monitoring blood pressure. It also points out various error sources in blood pressure monitoring.
Introduction
Measuring arterial blood pressure accurately is vital for diagnostic and monitoring tools in many cases of clinical conditions. Attention has been brought to this as an important aspect of patient care. This is as a result of the significant effects hypotension and hypertension have on patient health. Study has shown that direct monitoring is more accurate than the indirect methods and also facilitates continuous measurements (Figure 1 and 2). This proves advantageous in the case of a critically ill patients or when the aim is to detect rapid changes in BP. For this reason, direct monitoring is recognized as the gold standard for BP monitoring. Becausecof the high cost and the skill requirements for placing and maintenance of an arterial line, direct monitoring is less commonly performed. In veterinary practice, what is most commonly obtainable is the use of indirect monitoring for the estimation of BP and making of treatment based on the values gotten using this method. This lends credence to the sanctity of using a standard measurement method to improve result accuracy and make repeatability of results possible.
Figure 1. Arterial catheter placement in the dorsal pedal artery
Figure 2. Direct arterial blood pressure monitoring
Intravascular Pressures
Intravascular pressures are commonly measured in acute and critical illness. Blood exerts pressure on the vessel wall. This pressure is what is known as intravascular or “blood” pressure. This pressure is vital since the change in it, recorded at any two points in the vascular network determines the force for the circulation of blood.
The amount of pressure at the base of the aorta outweighs the pressure in the vena cava, this allows blood flow from the arterial to the venous side, releasing oxygen and other important nutrients to cells along its path. Intravascular pressures most commonly measured in small animals via peripheral arterial blood pressure (ABP). The measurement of ABP is most measured indirectly using surface applied cuff pressure and flow-detection methods such as Doppler ultrasound or oscillometry.
General principles of measurement
Doppler ultrasonic blood pressure monitoring is one of the most common NIBP measurement methods. This technique involves placing a Doppler probe with two piezoelectric crystal against the skin atop a peripheral artery, distal to a pressure cuff applied circumferentially to the animal’s limb or tail (Figure 3). The probe produces an audible sound of blood flow through an artery, causing Inflation until the sound becomes inaudible and then followed by gradual deflation of the cuff until the sound again becomes audible, allowing the determination of systolic pressure.
Figure 3.
The Doppler technique is not used in dogs and cats for measuring mean arterial pressure or diastolic arterial pressure, however, the technique remains popular in veterinary practice. This is because it is not expensive and it is easy to perform.
The Doppler technique is a relatively sensitive non-invasive blood pressure (NIBP) measurement method in hypotensive and feline patients (Burkitt and Davis, 2012).
The oscillometric technique, compared to the Doppler ultrasonic technique, is much less time consuming and does not require much technical skills. In Oscillometry, pressure cuff are connected to a device which detects arterial wall oscillations while blood flow as the cuff deflates.
Oscillometric machines display MAP, SAP, DAP, and pulse rate (Figure 4). This method is not advisable for use with patients with cardiac arrhythmias, significant tachycardia or bradycardia, vasoconstriction, or where there is patient movement, because it may be diffcult to to obtain accurate results (Thomas and Boller, 2018; Sierra and Savino 2015).
Figure 4. Non-invasive blood pressure can be measured using the oscillometric technique
determining systolic, diastolic, mean arterial pressure and pulse rate.
Limb selection and patient position
To avoid incorrect readings, limb selection and patient positioning must be done carefully. The animal must be in a position in which it is most comfortable. It is preferable for the the animal to lie immobile, in lateral recumbency and in a relaxed position, with the cuff positioned on the limb at the level of the right atrium (Rondeau et al 2013).
The cuff may be placed on the forelimbs, tail, and hindlimbs (Durham, 2019) (Figure 5) with the areas just proximal to the carpus or tarsus working well. The most suitable location in patients such as dachshunds and basset hounds is the tail. Cats often tolerate a tail application of the cuff better than a limb.
Figure 5. The cuff may be placed on the forelimbs, tail, and hindlimbs
Cuff Size and placement
Using the appropriate cuff size is important when measuring BP. If a cuff is too wide, it can lead to erratic readings with the possibility of underestimating BP, also, if a cuff is too narrow, can return incorrectly high readings narrow can lead to falsely high readings (Schauvliege, 2016). The circumference of the limb on which the BP is to be measured is related to the correct cuff size. The width should be 40% of the limb circumference for dogs, while for cats, 30% will be more appropriate. In a situation where measurement determines a cuff size between two available cuffs, the wider of the two should be selected. Cuff selection is the most common error in measuring BP (Durham, 2016).
The bladder of the cuff should be placed directly over the artery, lining up the tubing of the cuff with the artery. Most cuffs have an artery arrow imprinted on them (Figure 6), which does not need to be pointed in the direction of blood flow but rather the arrow should overlie the anatomic site of the vessel in the extremity. The tubing should be directed away from the patient and towards the monitoring device.
When using medical type to secure the cuff, the cuff should be placed closely and carefully around the the extremity but should not be too tight, as it can lower blood flow, leading to underestimation of the pressure
Figure 6. The artery mark indicates proper cuff positioning. Place the cuff over the limb with the artery mark positioned directly over artery
Tip #1: Small file folder or coupon organiser to separate cuffs and keep them neatly organized in an easily accessible place.
Box 1. Definitions of terms
- Blood pressure – the pressure of blood against the walls of the main arteries
- Systolic blood pressure – peak blood pressure in the artery following ventricular systole (contraction)
- Diastolic blood pressure – level to which the arterial blood pressure falls during ventricular diastole (relaxation)
- Hypertension – high blood pressure: consistently >140/90mmHg
- Hypotension – low blood pressure: typically a systolic reading of <90mmHg
Protocol 10.1 Doppler ultrasonic blood pressure
measurement
Items Required
• Clippers with clean blade
• Ultrasonic conductance gel
• Correct size pressure cuff
• Sphygmomanometer
• Charged (or plugged in) Doppler ultrasound machine, with
headphones if desired
• Assistant, if required
Procedure
1. Collect necessary supplies.
2. Position patient in lateral or sternal recumbency while
holding the planned cuff site at approximately the level
of the right atrium (RA). (If this is not possible, see Note
following the list of procedures.)
3. Clip hair from area over artery.
4. Secure a deflated cuff proximal to the artery and attach
the sphygmomanometer. Cuff should be of a size that
is approximately 40% of limb circumference in dogs
or 30%–40% of limb circumference in cats. When
measuring Doppler on the forelimbs, secure cuff on the
radius. When measuring on the hind limbs, secure the
cuff to proximal to the hock. Medical adhesive tape
may be required to secure the cuff well.
5. Apply ultrasonic gel to concave surface of the Doppler
probe.
6. Place gelled probe on the clipped skin overlying
the artery, keeping the probe’s cord parallel to the
limb. Adjust with fine movements until the rhythmic
“whooshing” arterial sound is audible.
7. Inflate sphygmomanometer to 30–40 mm Hg past
the point at which the arterial sounds are no longer
detectable.
8. Slowly deflate the cuff until the first sounds are
detected, marking the systolic blood pressure.
9. Allow cuff to completely deflate, allowing blood flow to
return to the limb.
10. The first measurement should be discarded and the
average of three to seven consistent, consecutive
readings recorded.
11. Record results, cuff size, and cuff location in patient
record.
Protocol 10.2 Standard oscillometric sphygmomanometry
blood pressure measurement
Items Required
• Correct size pressure cuff
• Oscillometer, with power cord
• Assistant, if required
Procedure
1. Collect necessary supplies.
2. Position patient in lateral or sternal recumbency while
holding the planned cuff site at approximately
the level of the right atrium (RA). (If this is
not possible, see Note following the list of procedures.)
3. Secure a deflated cuff of a size that is approximately
40% of limb circumference in dogs or 30%–40% of limb
circumference in cats.
4. The bladder of the cuff (the portion that fills with air)
should be centered over the palpable pulse if possible.
a. Cuff should be measured for and placed above the
carpus in small dogs and in cats.
b. Cuff should be measured for and placed over the
metatarsal in larger dogs.
5. Push the button on the oscillometer that begins pressure
readings. Discard the first reading and record the
average of at least three to seven consecutive, consistent
readings.
6. Record the average SAP, DAP, and MAP, and the cuff size
and location in the patient record.
Key Points
The gold standard for blood pressure measurement is the direct monitoring. For increasing the accuracy of results when using indirect measurement, standardization of the measurement process is essential. The results obtained using indirect method should be carefully interpreted alongside other parameters. If any inaccuracies are observed, they should be discussed.
Conclusion
The nurses who perform blood pressure measurement should be adequately trained. Also, they should be updated on the procedure necessary for handling the equipment correctly.
Repeatability of results is desirable and this can be achieved when the measurement equipment are standardized. Results should be interpreted accurately and trends in observation should be recorded carefully. This will provide veterinary practices with a reliable diagnostic tool. The gold standard for BP measurement is direct monitoring and it is recommended for achieving accurate results, especially when it involves critical patients.