Blood pressure: direct measurement

Introduction

• Arterial blood pressure is the product of stroke volume, heart rate and total peripheral resistance.
• There is some variation between published values for 'normal' arterial blood pressure in different sources. Suggested values: systolic pressure 105-145 mmHg; diastolic pressure 55-85 mmHg; mean arterial pressure 90-110 mmHg.
  Mean pressure is the hydrostatic intravascular pressure in the vessel during one complete cardiac cycle, NOT the arithmetic mean of the systolic and diastolic pressures
• Almost all general anesthetic drugs and some premedicant drugs cause hypotension by affecting one or more of the following factors: total peripheral resistance, stroke volume, heart rate. Their effects are often dose related.
• Adequate arterial blood pressure is necessary to maintain sufficient tissue perfusion to avoid cellular damage and/or death.
• In anesthetic monitoring, mean arterial pressure should be maintained above 70 mmHg.

• Accurate monitoring of arterial blood pressure enables appropriate corrective measures to be taken to ensure adequate tissue perfusion if hypotension develops.
• Particularly beneficial in animals which are hypovolemic prior to surgery, have pre-existing cardiovascular system disease, or are undergoing procedures where significant blood loss or large fluctuations in blood pressure are anticipated.
• Mean arterial pressure and cardiac output are maintained at normal levels until 10% of circulating blood volume has been lost.
• Mean arterial blood pressure is maintained by constriction of arterioles at deficits of circulating blood volume between 10 and 20%, but cardiac output is reduced.
• At deficits of circulating blood volume in excess of 20%, mean arterial blood pressure is no longer maintained and hypotension develops.

• Most accurate estimation of arterial blood pressure.
• Equipment required for determination of mean arterial pressure alone is inexpensive compared with that for oscillometric indirect technique.
• Arterial cannulae can also be used to collect samples for blood gas analysis.

• Requires arterial cannulation with attendant risks, eg hemorrhage, sepsis, thromboembolism.
• Arterial cannulation technique requires practice.
  Gain practice on a peripheral artery in medium to large breeds after anesthesia has been induced, eg dorsal pedal (metatarsal) artery. Pressure dressing can be used to control any hemorrhage/hematoma formation more easily than in larger, more proximal vessels.
• Equipment required to display and record systolic, diastolic and mean arterial blood pressure is expensive.

• Auscultatory method (as used in man), is not sufficiently accurate in the dog.
• Doppler technique .
• Oscillometric technique .

• Collect all necessary equipment.
• Prepare selected site aseptically.

Sources

Publications

Refereed papers

• Bodey A R & Rampling M W (1999) Comparison of hemorrheological parameters and blood pressure in various breeds of dog. JSAP 40 (1), 3.
• Bodey A R, Young L E, Bartram D H, Diamond M J & Michell A R (1994) A comparison of direct and indirect (oscillometric) measurements of arterial blood pressure in anesthetised dogs, using tail and limb cuffs. Res in Vet Sci 57 , 265-269.
• Kittleson M D & Olivier N B (1983) Measurement of systemic arterial blood pressure 13. Vet Clin of N Am - Small Anim Pract, 321-336.