Blood Pressure Measurement

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Introduction

Blood pressure is an useful measurement to help assess cardiovascular status of a patient during an anaesthetic. As many agents used for anaesthesia have an affect on the cardiovascular system, and blood pressure in particular it is important to detect this early so that it can be corrected if necessary. It is also essential for some surgical procedures as the surgery itself may have a direct effect on the blood pressure of that animal and so requires to be closely monitored, for example portosystemic shunt surgery. There are various methods of measuring blood pressure both invasive and non invasive.

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

Non Invasive Methods

Doppler Flow Detection

• Usually measured using the metacarpal and metatarsal artery, although the tail can also be used.
• Method

1. Once a pulse has been detected, the doppler probe is placed over to check that it is audible and the probe is taped to the limb.
2. A cuff is selected and placed proximal to the doppler probe and attached to a manometer.
3. The cuff is inflated until the pulse is no longer audible on the doppler.
4. The cuff is the gently released and a reading is taken when the pulse becomes audible again.

• The measurement is systolic pressure.
• Cuff width should measure as 40% of the circumference of the limb being used.
• Advantages
  • Cheap and available in most practices.
  • Useful in hypotensive patients.
• Disadvantages
  • Not very reliable in horses.

Oscillometric Method

• Method

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

• Measures systolic, diastolic and mean arterial blood pressure as well as pulse rate.
• Advantages
  • Good in dogs.
  • Automatic.
• Disadvantages
  • Less reliable in cats and hypotensive patients.
  • Expensive.

Invasive Methods

Arterial Catheterisation

• Catheter is placed in a peripheral artery and attached to either a anaeroid manometer or electronic transducer.
• An anaeroid manometer only gives a mean arterial blood pressure reading.
• An electronic transducer gives systolic, mean and diastolic arterial blood pressure readings.
• In dogs, the dorsal pedal artery is commonly used.
• In horses, the facial, transverse facial or metatarsal arteries used.
• Advantages
  • Continuous measurement.
  • Catheterisation allows for blood gas analysis sampling.
• 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 same method is then used for invasive arterial blood pressure measurement.

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