Queensland News - Australian Veterinary Association

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AVA Queensland News - August 2009Continued from pg. 13Given that diastolic blood pressure is more difficult to measure in our patients, it is fortunate that isolateddiastolic hypertension is not thought to be common in veterinary patients.AetiologySystemic Arterial Hypertension may be divided into:• essential (primary) hypertension (common in humans)• secondary hypertension (common in veterinary patients).The most common causes of SAH are:In dogs: renal disease (↑ BP in 60%);In cats: renal disease (↑ BP in 61-73%) and hyperthyroidism (↑ BP in up to 87%).Other causes include:hyperadrenocorticism (↑ BP in 60% dogs); diabetes mellitus (↑ BP in 50-70% dogs); pheochromocytoma(episodic ↑ BP in 50% dogs); acromegaly; hypothyroidism (atherosclerosis); hyperaldosteronism; obesity;chronic anaemia; polycythemia; hypergammaglobulinaemia (ie increased viscosity).DiagnosisThe clinical diagnosis of hypertension should never be established on the basis of asingle blood pressure measurement in the absence of overt clinical disease consistentwith the diagnosis (Cowgill LD, ACVIM 2001)This statement supports the balanced clinical approach to the diagnosis of hypertension, rather than onlytreating a BP measurement, taken on only one occasion.What is normal BP?Dog:Cat:By consensus, a systolic BP between 160-165 mmHg and a diastolic BP between 90-95 mmHgare considered to be the upper thresholds for normotension in the dog.The Veterinary Blood Pressure Society (VBPS) proposed a mild elevation (systolic/diastolicmmHg) at > 150/95, moderate elevation > 160/100 and severe elevation at 180/120.Sighthounds tend towards higher normal pressures at approx 145-150 mmHg (systolic).Similar to the dog, but perhaps 5 mmHg less.For adequate perfusion of the central organs BP must be above:Direct: systolic >70 mmHg, mean >50 mmHg;Indirect: systolic > 90 mmHg, mean > 70 mmHg.Moderate hypotension is: Indirect: systolic 80-90 mmHg, mean 60-70 mmHgSevere hypotension is: Indirect: systolic < 75 mmHg, mean < 60 mmHgMeasurement TechniquesDirectDirect arterial puncture – with measurement via either an aneroid manometer or a pressure transducer.IndirectDoppler – (e.g. Parks Medical)Oscillometric – (e.g. Dinamap)Plethysmography – (e.g. Finapress).14
AVA Queensland News - August 2009Continued from pg. 14This discussion will mostly look at the Doppler technique as this is the most popular ‘in practice’ method ofBP measurement.DopplerUses a piezoelectric crystal to send and receive an ultrasound signal that detects blood flow in an arterydistal (usually superficial palmar or plantar arterial arch or the coccygeal artery e.g. tail in cats) to an inflatablecuff.Accurate assessment requires:• good, repeatable operator technique• correct cuff size (cuff width to be 40% of circumference of limb [dog], or 30-40% of circumference [cat])• calm patient.Averaging of approx 5 measurements is ideal.Only reliably measures systolic BP (not diastolic BP).Oscillometric – (e.g. Dinamap)Measures the magnitude of arterial pulsations produced in an air-filled cuff: these are inaccurate withpatients that are moving, have arrhythmias, hypotension or bradycardia.3. Central venous pressure (CVP) assessmentCVP is a simple way to evaluate right ventricular function.CVP is a measure of the right atrial pressure, which is the chamber of the heart into which venous returnfrom the all of the body (except the lungs) occurs.This is measured via the placement of a long catheter into the entrance of the right atrium, via the jugularvein.CVP is particularly useful for regulating the volume and rate of IV fluids that are administered to a patient. Ifthe CVP is low despite fluid therapy, the rate of fluid therapy can be increased, provided there is no left heartfailure. If the CVP is elevated, fluid therapy can be discontinued and possibly re-commenced at a slowerrate when the CVP is at an acceptable range.Procedure: extension tubing is attached to the CVP/jugular catheter, then a water manometer is attachedto the extension tubing via a 3 way stopcock. An administration drip set and bag of fluids is attached to theother port of the three way tap. The manometer can be attached to the cage with the zero marking at the leftof the right atrium (i.e. at approximately the point of the shoulder in sternal recumbency, or mid-sternum inlateral recumbency).Normal range is 3-7 cm of water.Some reasons for a high CVP: occlusion of the jugular vein; over-administration of fluids; right ventricularheart failure.If the CVP is low, patient has reduced blood volume.4. End-tidal gas analysis – capnographyA capnograph measures infra-red absorption to determine the concentration of CO2 (carbon dioxide) inexpired gases. It gives a digital report of the inspired and expired CO2 concentrations during each ventilatorycycle.It is therefore only of use in the anaesthetized patient.It can assist in rapidly diagnosing airway and breathing failure.In patients breathing supplemental oxygen the capnograph can detect ventilation problems earlier than apulse oximeter. It measures the CO2 directly at the endo-tracgheal tube through an adaptor.Normal peak CO2 expiratory is 30-50 mmHg.15
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