Blood Pressure Measurement

Introduction

Blood pressure (BP) is a useful measurement to help assess the cardiovascular status of a patient during an anaesthetic, or as a diagnostic test for an underlying cause of ocular changes in cats as a result of high blood pressure. Many agents used during anaesthesia have an affect on the cardiovascular system, and blood pressure in particular so it is important to monitor and correct this if necessary. Some surgical procedures such as portosystemic shunt surgery, for example may have a direct effect on blood pressure and BP must be closely monitored throughout the procedure. It tends to be difficult to obtain an accurate blood pressure reading in the conscious animal due to the effects of the situation. Some cats, for example, may suffer from the "white coat effect". That is their stress levels have already increased due to being in an unusual environment, and when the vet attempts to take the blood pressure, this will stress them out even more. This has the effect of increasing the blood pressure. That said however, there are still ranges of normal blood pressure that take this into account. It is also difficult to obtain an accurate reading in a sedated or anaesthetised animal because most drugs have an effect on blood pressure. However, measurements can still be taken during surgery, for example, to compare throughout the procedure.

Normal Arterial Blood Pressures

• Dogs – Systolic 140, Mean 100, Diastolic 75 mm Hg
• Cats - Systolic 180, Mean 135, Diastolic 100 mm Hg
• Horses - Systolic 110, Mean 90, Diastolic 70 mm Hg

Dogs are subject to breed specific variations in blood pressure reference ranges.

Non Invasive Methods

Doppler Flow Detection

Method:

1. Usually measured using the metacarpal and metatarsal artery, although the base of the tail can also be used.
2. Once a pulse has been detected, the doppler probe is placed in that position on the limb to check that it is audible and the probe can then be taped to the limb.
3. A cuff is selected and placed proximal to the doppler probe and attached to a manometer.
4. The cuff is inflated until the pulse is no longer audible on the doppler.
5. The cuff is gently released and a reading is taken when the pulse becomes audible again.

• This measures systolic pressure only.
• Cuff width should ideally be 40% of the circumference of the limb chosen for this method.

Advantages

• Cheap and available in most practices.
• Useful in hypotensive patients.

Disadvantages

• Not a reliable method in horses.

Oscillometric Method

Method:

1. The specialised oscillometric cuff is placed on a distal limb or tail.
2. The cuff automatically inflates.
3. As it deflates, oscillation in the cuff is detected electronically.

• Measures systolic, diastolic and mean arterial blood pressure as well as pulse rate. The first oscillation of blood flow detected is the systolic pressure, the largest oscillation is equal to the mean pressure, and the oscillations subside at diastolic pressure.

Advantages

• Good in dogs.
• Automatic.

Disadvantages

• Less reliable in cats and hypotensive patients.
• Expensive.

Invasive Methods

Arterial Catheterisation

Method:

1. A catheter is placed in a peripheral artery and attached to either an anaeroid manometer or electronic transducer.
2. An anaeroid manometer gives a mean arterial blood pressure reading; an electronic transducer gives systolic, mean and diastolic arterial blood pressure readings.
3. In dogs, the dorsal pedal artery is commonly used.
4. In horses, the facial, transverse facial or metatarsal arteries are usually used.

Advantages

• Continuous measurement.
• Catheterisation allows for additional sampling for blood gas analysis.

Disadvantages

• Invasive
• Requires the correct equipment, which can be expensive.

Central Venous Pressure

Central venous pressure (CVP) is an indicator of blood return to the heart. It indicates adequacy of fluid therapy, heart performance and volume loading of the heart. It is measured by the placement of a central line, usually via the jugular vein. The method then used is that described above for invasive arterial blood pressure measurements.

• Low or falling CVP suggests hypovolemia.
• High or rising CVP suggests volume overload or a failing heart.

Literature Search

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Blood Pressure Measurement publications