Conventional oscillometric unit
Doppler Ultrasound
The first noninvasive blood pressure measurement technology on the market was Doppler ultrasound. It is based on the Doppler shift effect—the change of frequency due to moving blood cells and movement of the arterial wall. The return of an audible pulse wave is supposed to equal systolic arterial pressure (SAP). Diastolic arterial pressure (DAP) generally cannot be determined reliably.
There are concerns regarding Doppler accuracy in detecting SAP. In cats under anesthesia, it has been shown that SAP determined via Doppler ultrasound correlated more closely with mean arterial pressure than SAP.1
Conventional Oscillometry
Both conventional and high-definition oscillometry are based on arterial wall motion (oscillations). Initially oscillometry was rather inaccurate and not very sensitive, complicating its use in veterinary medicine. However, as it is microprocessor based, it improved as this technology improved over time.
Conventional oscillometry measures mean arterial pressure (MAP) and pulse rate, and calculates SAP and DAP. Most of these devices are relatively accurate within a range of 70 to 160 mm Hg (linearity range) but tend to underestimate higher and overestimate lower pressures. In low-pressure situations, these units may not provide a reading.
High-Definition Oscillometry
With high-definition oscillometry (HDO), SAP, DAP, MAP, and pulse rate are measured, but other information beyond blood pressure is also gathered. Its 32-bit processor and PC software allow visible real-time measurements and analysis. In addition, features include:
-Assessment of initial blood pressure reading; then individual adjustment of all measurement parameters to maximize subsequent readings
-Gain function to increase sensitivity, allowing detection of weak pulses
-Pulse-dependent deflation rate (faster with higher heart rates) to cope with high and low heart rates.
In animals with high heart rates, such as cats or ferrets, readings can be obtained in as little as 8 seconds. Additionally, the real-time projection of oscillations (pulse wave analysis) allows the practitioner to determine if the rhythm is regular and may also provide information on cardiac output and arterial elasticity.
Accuracy has been proven in different animals against direct line/telemetry. Due to its entire linearity (not limited to 70–160 mm Hg) even the lowest pressure situations can be measured. Although the units can be used without a computer, it is highly recommended that one be used so the examiner can visualize the reading versus receiving only a numerical result.
Technology Learning Curves
Handling of the different devices has become easier as the technology has become more advanced.
The most experience and intensive training is needed for Doppler devices because blood pressure values obtained can be significantly influenced by the examiner. Cuff selection and placement, maximum inflation pressure, and deflation rate are determined by the operator. These factors all influence the accuracy of the readings obtained. However, its accuracy as a flow meter is less operator-dependent.
With conventional oscillometry, the examiner’s predominant task is to keep the animal calm for up to 2 minutes (depending on the unit). HDO is as easy to use as conventional oscillometry and due to the pulse-adapted deflation rate; the examiner only has to keep the animal calm for a short time. In addition, readings can be taken at the base of the tail of a standing or moving animal. Since every single heart beat can be visualized on a computer screen, artifacts can be noted while the reading is being taken.
