Guide to Blood Pressure Monitoring

PhysiologyThe blood pressure waveform is like a sinewave, having an upstroke (systole or SAP =ejection phase) and a downstroke (diastole orDAP = filling phase). Mean arterial BP (MAP) isdefined as the area under the BP curve.Because more time is spent in diastole, it hasfar greater than systole on MAP:MAP = DAP + 1/3 (SAP - DAP) orMAP = (SAP + 2xDAP) / 3

Always assess mucous membranecolour when evaluating BP.

BP = Blood Flow x ResistancePink = good flow, BP may be low

if vasodilated Pale = lower flow, BP may be high

if vasoconstriced

Clinical BP monitors:1. Invasive, direct monitors that use an

arterial catheter connected via atransducer to measure systolic, diastolic, +mean BP

2. Non-invasive, indirect blood pressure(NIBP) monitors that use a pneumaticcuff placed around a limb or the tail toestimate pulse rate, systolic and in somecases, diastolic and mean BP. In dogsNIBP monitors most accurately estimatesystolic BP.

Common NIBP monitorsDoppler ultrasonic devices that use a bloodflow transducer placed on a distal limb arteryto indicate return of blood flow when apneumatic cuff is deflated manually. Dopplersare the most consistent at detecting systolicBP in conscious cats.Oscillometric devices (eg. CAS 740) thatdetect pulsations in the pneumatic cuff whenblood flow returns to the limb as the cuff is

automatically deflated. In general,oscillometric devices must accurately detectthe pulse rate in order to accuratelydetermine BP.Using Pneumatic Cuffs Most errors of measurement are due toproblems with cuffs.1, 2

1. Cuff bladder width should usually be 40%of the circumference of the limb at thelevel of application

2. Positiona) Thoracic limb - above the carpusapproximately 1/3 distance towardsthe elbowb) Pelvic limb - just above the hock (smalldogs) or just below the hock (large dogs)c) Tail - close to the base of the tail

3. The occlusive bladder should be directlyover the artery to be occluded, usually onthe underside of the appendage.

4. Clipping off somehair will help inanimals with longor thick coats.

5. Short, bent forelegs are achallenge (egDachshunds) -consider using apelvic limb or tail

6. Pneumatic cuffs and flowprobes should be positioned at the level ofthe heart to avoid errors caused bygravity.

7. Motion at the measuring site will generallyaffect any NIBP results.

8. Deflate the cuff betweenreadings or it will restrictblood flow to the limb causingcatecholamine release whichgenerally causes BP to rise.

9. In anesthetized cats NIBPdevices tend to under-estimatesystolic BP (may more closely

estimate mean BP); thereforeadd 15 mm Hg to SBP readings.1, 2

10. Doppler flow detectors are‘microphones’ and are affected byexternal noise (eg clippers), electricalinterference (eg electrocautery) andultrasonic feedback noise.

11. Occasionally polishing the connectors onthe Doppler box will reduce static noise.

12. Probe damage is the most commoncause of Doppler problems.

Hypertension & Hypotension1. Cat NIBP measurement

variations arein part due to smallpatient limbs.

2. Treat hypotension duringanesthesia by reducingAnestheticdepth,administering IV fluids orinotropes.Increasing HR with atropine may raise BP

3. BP is dynamic and single measurementsmay not be representative. ConsistentNIBP readings should be obtained overseveral hours or days to diagnosehypertension (average 3 sets ofmeasurements).3, 4

If readings are questionable considercomparing BP readings from different sitesin the same animal.Vasodilator therapy cancause complications in normal, healthyanimals!

4. Treatment for hypertension should beconsidered in animals with clincal signs(eg retinal haemorrhage) even if BP valuesare not in the ‘hypertensive’ range.3, 4

5. Stress (‘white coat’ disease) causesBP to rise in conscious animals so allowanimals to rest, either with the owner orin a cage, prior to making BPmeasurements.

References:1. Grandy JL, Dunlop CI, Hodgson DS et al. Doppler

blood pressure measurement in cats.Am J Vet Res53: 1166-1169;1992

2. Pederson KM, Butler MA, Ersboll AK etal. Evaluationof an oscillometric blood presure monitor inanaesthetized cats. JAVMA 221:646-650; 2002

3. Elliott J, Barber PJ, Syme HM et al. FelineHypertension: clinical findings and response toantihypertensive treatment in 30 cases.JSAP 42:122-129; 2001

4. Snyder PS & Cook KL. Systemic hypertension.In Consultations in Feline Internal MedicineIV.WB Saunders pp 277-282; 2001

Steps to Better Understanding & Measurements

Interpretation of blood pressure valuesSystolic AP mm Hg Dog Cat

Normal conscious 135 +/- 10 155 +/- 10

Hypertension 180 +/- 10 190 +/- 10

Hypotension 85 +/- 5 70 +/- 5

Severe Hypotension < 75 < 60

Systemic arterial blood pressure (BP) monitoring is widely used during anesthesia in part because depth is difficultto establish simply using physical signs. Changes in BP and CO2 level occur with Anestheticdepth, painful stimulation, cardiovascular

dysfunction or changes in vasomotor tone such as occur with hypothermia.A mean BP > 60 mm Hg is required for adequateperfusion of the brain and to ensure production of glomerular filtrate by the kidneys. For these reasons frequent monitoring of BPis used in critical care. Routine clinical BP assessment has also led to better understanding and treatment of hypertension in dogs

and cats.