The nuts and bolts of blood pressure measurement (Proceedings)
Why Measure Blood Pressure?
Blood pressure measurement is often not a routine part of small animal practice. This partially has to do with the equipment available to measure blood pressure as well as our patients (they tend not to hold still like we have to). This does not however change the fact that measuring blood pressure is
a) good medicineb) a diagnostic test with considerable owner acceptance
c) a way to generate income with good medicine
Blood pressure can be determined through direct or indirect means. The direct method requires catheterization of a peripheral artery, generally a procedure reserved for monitoring of critical patients. Indirect means of blood pressure determination are more practical for day to day use. The indirect methods are non-invasive and cause little discomfort. The increasing availability of simple and relatively inexpensive means to indirectly measure blood pressure has lead to increased clinical application of this diagnostic tool.
Doppler ultrasonic blood pressure machines have a transmitting and receiving transducer. The ultrasound waves generated by the transducer are used to detect motion of the arterial wall or the blood cells themselves. If an object is moving a frequency change occurs (Doppler effect) with the ultrasound waves, making the reflected sound beam go from ultrasonic range to audible range. This reflected signal is amplified by the Doppler machine. An inflatable cuff with an aneroid pressure gauge is usually also needed. The cuff applies pressure to a peripheral artery and the pressure values are displayed by the gauge. The cuff is inflated to levels greater than systolic pressure. This occludes the artery and wall motion stops so that a signal is not received. The cuff is slowly deflated to a level below systolic pressure, which then means that the pulses are heard again. This method can be used in practice. There are a variety of downsides to this method. The method is operator dependent and widely varying numbers can be obtained by different people. The noise of the unit can contribute to stress in the animal as does shaving to improve probe contact. The set up time is relatively long making this method poorly adaptable to routine blood pressure measurement. Generally two skilled people are needed, one to restrain the pet, the other to obtain the reading. One of the most significant downsides to this technology is the inability to routinely and accurately determine diastolic blood pressure.
Oscillometric blood pressure devices measure oscillations within a cuff bladder. The pulse wave traveling through an artery causes these oscillations. Several commercial devices are available. The machines automatically inflate the cuff and deflate it slowly. Pressures above systolic pressure are initially used to occlude the artery and stop wall motion. Once deflated to systolic pressure, oscillations begin. Though relatively reliable in dogs, to date this technique has been very difficult to use in cats. Since there is no hunt for an artery as with Doppler, in many cases getting a reading is more rapid with oscillometric. Each individual reading takes longer in that the devices automatically inflate and then deflate slowly. This is actually an advantage, because this means that blood pressure is sampled over a longer period of time. As a result, a more accurate reflection of blood pressure can be determined.
High Definition Oscillometry (HDO)
HDO is also based on oscillometry, however that is similar to saying that a model T car is the same as a Lexus. HDO represents a generational leap in oscillometry. The processor is considerably more powerful allowing real time analysis of the oscillometric curve and exact control of the valves that determine inflation and deflation. This makes HDO accurate over a wider range of pressures (from 300 to 25 mmHg). It is also possible to visualize the oscillations as they are occurring using a computer. Seeing the curve allows the veterinarian to make important decisions as to whether the readings are accurate. With HDO it is possible to obtain blood pressure readings off the tail, even in awake ferrets. It is even possible to send the HDO unit home with the owner and most owners can obtain readings that are free from in hospital stress. This unit allows blood pressure measurement to truly become a routine and relatively easy diagnostic test in small animal patients.