How to measure blood pressure: direct and indirect techniques
Abstract
Monitoring blood pressure has become more widely recognized as a vital part of patient care. Veterinary nurses often carry out this procedure in conscious and anaesthetized patients. As most veterinary practices will use indirect methods of measurement, standardization of the measurement process is vital to ensure accurate results are achieved. Direct monitoring is still considered to be the gold standard form of measurement particularly in critically ill patients where accuracy is vital. It is widely held that direct monitoring is less commonly performed due to the increased costs and skills required to place and maintain an arterial line. This article examines the monitoring equipment available and how to standardize the measurement process in order to achieve accurate results.
The importance of detecting alterations in arterial blood pressure (BP) has become more widely recognized as a vital part of patient care, due to hypotension and hypertension having significant effects on patient health. Direct monitoring is proven to be more accurate than indirect methods and enables continuous measurements to be taken. This would be a distinct advantage in critically ill patients or for the detection of rapid changes in BP. As a result direct monitoring is considered to be the gold standard for BP monitoring. Most veterinary practices use indirect methods to estimate BP and make treatment decisions based on the values achieved from those methods. This highlights the importance of using a standard measurement protocol to improve accuracy and achieve repeatability of results.
The veterinary nurse (VN) should be knowledgeable with regards to the normal ranges of blood pressure (BP) in dogs and cats, the physiological effects of anaesthesia on BP and consequences of prolonged periods of hypotension or hypertension. During anaesthesia a mean arterial pressure (MAP) of >60 mmHg is required to ensure adequate perfusion of vital organs, if this is not maintained then signs of shock and organ dysfunction may be seen (Egger, 2007). MAP can be assessed with direct monitoring and with the oscillometric method of indirect measurement, but the Doppler method only measures systolic pressure. Systolic pressure should be >80 mmHg to prevent hypotension in anaesthetized patients (Macintire, 2010). Stepien (2010) defined hypotension in conscious patients as a systolic pressure of <100 mmHg. Hypertension results in increased workload on the heart and increases myocardial oxygen demand. Long-term hypertension can cause heart, renal disease and ocular changes such as retinal haemorrhage or retinal detachment (Love and Harvey, 2006). Hammond and Walters (1999) defined normal BP ranges as 112–192 mmHg (systolic) and 56–110 mmHg (diastolic) in conscious dogs, and 120–170 mmHg (systolic) and 70–120 mmHg (diastolic) in conscious cats.
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