Page 454 - WSAVA2018
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 25-28 September, 2018 | Singapore
pressure during anaesthesia does not always correlate with good perfusion. That is, increased blood pressure > vasoconstriction > decreased blood flow.
Direct measurement of blood pressure involves catheterisation of a peripheral artery – dorsal metatarsal, femoral, lingual and carotid. Indirect measurement is commonly done using a Doppler ultrasound unit to detect arterial blood flow and a pressure cuff.
Using the Doppler
· Cuff should be placed above hock for dorsal metatarsal or above carpus for dorsal pedal artery
· Follow the cuff manufacturer's instruction on choosing a size that is appropriate for the limb. In general, the width of the cuff should measure 30 – 50% the circumference of the limb
· Ensure snug fit
· Measurement should be performed several times to
· Clip area over artery to be used (dorsal metatarsal, dorsal pedal)
· Apply liberal amount of gel to area probe is to be placed
· Position probe perpendicular to the artery
· Tape in place
· When pulse audible – increase pressure until no longer audible
· Slowly decrease pressure until pulse just becomes audible once more. This is the systolic blood pressure reading
· This should be repeated at least 3 times to ensure correct reading
Pressure readings are expressed in mmHg (millimetres of mercury) e.g. 120mmHg.
The pressure (or tension) of the blood within the arteries is maintained by the contraction of the left ventricle, the resistance of arterioles & capillaries, the elasticity of arterial walls and viscosity and volume of blood. Changes to any one of these factors will affect the blood pressure.
Systolic blood pressure results from systolic contraction of the cardiac chamber. The highest arterial blood pressure reached during any given ventricular cycle
Diastolic blood pressure results from diastolic relaxation of the cardiac chamber. The lowest arterial blood pressure reached during any given ventricular cycle.
*Blood pressure ranges can vary depending on referring text
A mean arterial blood pressure of greater than 60mm
of mercury (mmHg) is required to ensure adequate perfusion of vital organs such as the brain and kidney.
To be cautious, it is best that the anaesthetised patient maintain a mean arterial blood pressure of greater than 70mmHg. Because the Doppler ultrasound measures only systolic pressure and because the systolic pressure is about 20 to 30mmHg higher than mean pressure, Doppler readings of greater than 90mmHg are desirable.
In the dog it is time for action if the Doppler readings fall below 80mmHg. In the cat this time for action occurs
at 65mmHg (there is an overestimation of mean arterial pressure in cats with this device).
Monitoring of the Respiratory System
It is not just good enough to have blood being delivered at an adequate rate and pressure to the body, there must be oxygen in that blood for any good to be done.
Pulse Oximetry
Use a pulse oximeter to monitor that the patient is oxygenating sufficiently. The pulse oximeter measures the pulse rate and the degree of oxygen saturation of arterial blood, by measuring the absorbency of particular wavelengths of red and infrared light.
Most instruments require a probe placed on non- pigmented skin or mucous membrane (most
frequently the tongue). They are susceptible to movement, and interference to blood flow by the
clamp resulting in erroneous measurements. Also, saturation measurements may be inaccurate in cases of hypovolaemia.
Pulse oximeter readings of Sp02:
ensure correct.
· 98 - 99% · <95%
· <90%
· <75%
Normal Hypoxemia
Severe Hypoxemia Lethal Hypoxemia
Areas where a probe may be placed include: · Tongue
· Lip
· Ear
· Between toes · Prepuce / vulva
If placing the probe on the tongue, place a damp/wet swab over the tongue to keep it moist, then place probe on over this.
 Normal Systolic Blood Pressure in animals
 100 – 160 mmHg
 Normal Diastolic Blood Pressure in animals
  60 – 100 mmHg
  Normal Mean blood pressure in animals
   80 – 120 mmHg

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