Introduction

• The automated oscillometric technique (oscillometry) can be used as a method for non-invasive measurement of arterial blood pressure (ABP).
• The accuracy of oscillometry is questionable, particularly in certain groups of patients (discussed below); however, it may provide a useful trend of ABP.
• Oscillometry is often incorporated into multi-parameter patient monitors where it may be labeled as NIBP (non-invasive blood pressure). It is also available in a variety of stand-alone units.
• The DINAMAP ( device for indirect noninvasive automatic mean arterial pressure) Veterinary Blood Pressure Monitors (Critikon, Tampa, Florida, USA) did much to establish the veterinary commercial market and that trade name is sometimes erroneously used to describe all oscillometric pressure monitors.
• Recently a refinement on the traditional oscillometric devices has been developed. This is the 'HDO' or high definition oscillometry.
• The rationale for measuring ABP and normal values are detailed elsewhere Blood pressure: direct measurement.

Technical details

• All non-invasive techniques for measuring ABP require an inflatable cuff (occlusion or tourniquet cuff) connected to a manometer to measure the pressure within the cuff. The cuff is placed around an extremity. The system also requires a means to detect blood flow in an artery distal to the cuff.
• The pressure in the occlusion cuff is inflated until blood flow ceases. The pressure is then slowly released; the pressure in the cuff at this point that blood flow resumes is equal to the systolic ABP.
• In human practice, stethoscopy is frequently used as the detector system for blood flow in the distal artery; however, this technique does not perform well in small animal practice.
• Doppler ultrasound techniques (Dopplers) Blood pressure: Doppler ultrasound and oscillometry are relatively widely used in small animal practice for non-invasive measurement of ABP.
• In oscillometry the occlusion cuff and the means of detecting blood flow are combined. If the pressure in the cuff is less than the systolic ABP and greater than the diastolic pressure, during systole the artery under the cuff will snap open and closed. This causes a spike in the pressure within the cuff. These spikes can be detected by the patient monitor and will vary in size according to the pressure within the cuff.

How automated oscillometry works

• The occlusion cuff  is placed around the patients limb or tail (site selection is discussed later) and secured. The single-bladder cuff will have one or two tubular connections (2), made of non-distensible plastic, to the machine.
• The NIBP monitor includes a microprocessor that controls inflation and deflation of the cuff using an air pump.
• A pressure transducer (3) measures the pressure signals, which in turn are interpreted by the microprocessor.
• The oscillometric technique is based on detection of pressure oscillations within the cuff bladder produced by changes in diameter of the underlying artery .
• Measurements may be initiated by the machine, at preset intervals, or manually by the operator.
• The data is analysed electronically and values for systolic, mean and diastolic pressures will be displayed with the heart rate (or more correctly pulse rate).
• If the patient moves, it may give an erroneous reading; however, the software may be sophisticated enough to detect this and repeat the measurement.
• The pressure within the occlusion cuff is shown in gray. The cuff is inflated to exceed systolic ABP and then it is slowly deflated in a stepwise fashion. Some units require a number of inflation/deflation cycles to determine values; others vary the speed at which deflation occurs according to the pulse rate so that emptying the cuff bladder will take longer when the pulse rate is low.
• The actual arterial pressure waveform is shown in red; it is not possible to visualize this waveform with oscillometery.
• When the cuff pressure is just below the systolic ABP a sharp spike or fluctuation (shown in blue) in the pressure of the cuff will be detected. This corresponds to the walls of the artery opening and closing. The amplitude of the spikes reaches a peak at the mean ABP and cease when the cuff pressure is below diastolic ABP.
• HDO works slightly differently to conventional oscillometry in that it performs real time analysis of the pulse wave forms detected under the inflation cuff. This proportedly gives faster and more accurate readings of systolic, mean and diastolic arterial blood pressures. Deflation of the occlusive cuff is more closely controlled and happens in a linear rather than stepwise fashion. This increases the sensitivity of the device and also allows pressures to be detected over a wider range of heart rates up to 500. This sensitivity makes it suitable for animals in which conventional oscillometric machines would not be suitable, eg rats. However, in one study the HDO device was not found to correlate well with the Dinamap nor with direct blood pressure measurements and the publication could not recommend its use in anesthetized dogs. A study comparing Doppler and HDO in cats found similar lack of consistency in readings with the HDO with it apparently over reading low pressure and under reading high pressures.

Technique

Advantages and disadvantages

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Petric A D, Petra Z, Jerneja S et al (2010) Comparison of high definition oscillometric and Doppler ultrasonic devices for measuring blood pressure in anaesthetised cats. J Feline Med Surg 12 (10) 731-737 PubMed.
• Wernick M, Doherr M, Howard J et al (2010) Evaluation of high-definition and conventional oscillometric blood pressure measurement in anaethetised dogs using ACVIM guidelines. JSAP 51 (6), 318-324 PubMed.
• Love L & Harvey R (2006) Arterial blood pressure measurement: physiology, tools, and techniques. Compend Contin Educ Pract Vet 28 (6), 450-461 VetMedResource.
• Wagner A E & Brodbelt D C (1997) Arterial blood pressure monitoring in anesthetized animals. J Am Vet Med Assoc 210 (9), 1279-1285 PubMed.

Other sources of information

• Trim C M (1994) Monitoring the anesthetized cat. In: Anesthesia of the Cat. Eds L W Hall & P M Taylor. London: Bailliere Tindall.