• Many anesthetic drugs cause hypotension.
• Hypotension can contribute to some postanesthetic complications, eg postanesthetic myoneuropathy Muscle: myopathy - postanesthetic and anesthetic deaths.
• The aim of blood pressure monitoring Anesthesia: monitoring - cardiac output and blood pressure is to detect early signs of hypotension Anesthesia: peri-operative complications - hypotension in order to correct the problem before serious complications result. Treatment can then be initiated, eg reducing the depth of anesthesia, fluid therapy and drug therapy using sympathomimetics such as dobutamine Dobutamine.

Uses

• Indicates the depth of anesthesia in conjunction with other parameters.
• Warns of a deterioration in the status of the cardiovascular system.
• Warns of shock or exaggerated response to the anesthetic drugs.

Advantages

• Both direct and indirect methods are relatively easy to use.
• Direct method allows access to arterial blood for blood gas analysis Blood: gas analysis.
• Hypotension may be detected and dealt with early enough to prevent more serious sequelae.

Disadvantages

• Direct method requires arterial catheterization Cardiovascular: arterial catheterization.
• Manual or mechanical ventilation will affect the values; therefore readings should be taken between respiratory excursions.
• Cost of equipment for both direct and indirect methods can be high.

Technical problems

• Thrombus formation may result if the arterial catheter is not flushed before each reading is taken and this can be both dangerous to the patient and can affect the results.
• Arterial catheterization can lead to hematoma formation after the catheter is withdrawn if sufficient pressure is not immediately put on the site and maintained for at least 2 min.
• Indirect measurement relies on the technician's ability to detect changes in pitch or frequency of sound emitted by the monitor to distinguish between systolic and diastolic values.
• Indirect oscillometric method requires accurate placement of the cuff around the base of the tail .

Alternative techniques

• Pulse pressure gives an indication of blood pressure, especially when combined with capillary refill time and color of the mucous membranes .
• Ocular reflexes Anesthesia: monitoring - overview give a good indication of anesthetic depth in most horses.

Time required

Preparation

• Placing of an arterial catheter can be time consuming for the less experienced operator when the direct method is being used. 5-15 min should be allowed for preparation, placement and securing of the catheter .
• Placing a cuff, eg around the horse's tail, and connecting to the measuring equipment takes <1 min.

Procedure

• Flushing the catheter with heparinized saline before each reading is taken by the direct method involves a total time of only a few seconds.
• The cuff must be inflated and then deflated (the latter slowly), either manually or automatically in order to obtain readings by the indirect method. This can take up to 1 min for each reading. Multiple readings (3-6) should be taken and averaged.

Decision taking

Criteria for choosing test

• Any anesthetic procedure expected to last for more than 45 min should involve some form of blood pressure monitoring.
• Heavy, well-muscled or very fit horses are more prone to complications related to hypotension than most other patients and so blood pressure monitoring is especially important in these groups.

Risk assessment

• Horses with cardiovascular compromise already present before anesthetic administration, eg for emergency gastrointestinal surgery Gastrointestinal: neoplasia, should always be monitored continuously and any fluid deficits corrected prior to induction.
• Certain anesthetics, eg most inhalation agents, can cause hypotension Anesthesia: peri-operative complications - hypotension.