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 systemic high blood pressure. Bear in mind that in a conscious animal, a stress free environment is useful in preventing a rise in blood pressure which is induced by anxiety, particularly in cats. Many agents used during anaesthesia have an affect on the cardiovascular system, and blood pressure in particular so it is important to monitor sequential readings and correct BP changes 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.

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 to be accurate. The cuff should be applied so that only a small finger can be inserted between the cuff and the leg, to ensure that it is neither too tight nor too loose.

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.

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