Page 43 - Basic Monitoring in Canine and Feline Emergency Patients

documented. The initial Doppler measurement the cuff and no wall oscillations are present. The
should be discarded as the patient may be acclimat- unit then gradually deflates the cuff at a rate of
VetBooks.ir ing to the environment or the sensation of the 3–4 mmHg per second until arterial pulsations are
detected. Once arterial blood flow returns, a slight
inflated cuff. As long as the measurements are rea-
sonably consistent (i.e. within 10% of each other)
the movement of blood past the cuff and through a
and appear to have plateaued, between five and increase in the blood volume of the limb occurs and
seven measurements should be recorded and aver- partially compressed artery creates a detectable
aged. If the measurements continue to drop or arterial wall vibration. The oscillometric unit
increase with subsequent readings, measurements detects these small arterial wall vibrations and the
should continue until a plateau is reached. In some variation in the vibrations allows for assessment of
situations (e.g. patient temperament), obtaining five the MAP, systolic, and diastolic blood pressures.
to seven readings may not be feasible. As long as the The units commonly used in veterinary medicine
obtained readings are relatively consistent, fewer measure heart rate and MAP and calculate the sys-
than five to seven measurements may be acceptable. tolic and diastolic pressures.
The first pressure at which small oscillations are
detected corresponds to the systolic arterial pres-
Oscillometry
sure. As the cuff is slowly deflated and the pressure
Oscillometry is the second method used for indirect decreases, greater pulsatile flow occurs, which
blood pressure monitoring. The oscillometric unit leads to greater oscillations in the arterial wall. The
(Fig. 2.10) is connected to a blood pressure cuff; peak amplitude of the oscillations is considered the
the size of the cuff is selected in the same way as MAP. Finally, as the cuff pressure is reduced suffi-
outlined above in the Doppler ultrasonic method ciently, blood flows normally through the artery,
(see Fig. 2.8). The oscillometric unit inflates the cuff the arterial wall vibrations will no longer be detect-
to a pressure greater than the arterial blood flow; able and this reading is considered the diastolic
at that point, no arterial blood flow passes distal to pressure. Although some units used in people do
measure all three pressures, the units currently
available in the veterinary marketplace use the data
collected at different pressures to determine the
MAP. Proprietary algorithms are then used to ana-
lyze the data and calculate the systolic and diastolic
pressures. Based on the pulsatile flow and changes
in cuff pressure, the oscillometric unit will also
measure a heart rate. Specifically, the machine
interprets each vibration or oscillation of the vessel
wall as originating from the heart, and therefore,
can determine a heart rate.
If the detected signal of arterial wall vibration is
insufficient or inconsistent or the algorithm cannot
convert the collected data into a calculated systolic
or diastolic pressure, the unit will re-cycle and
repeat the attempt to measure the patient’s blood
pressure. Dependent on the pulse palpation and
numbers calculated for the systolic and diastolic
Fig. 2.10. An example of an oscillometric blood pressures, the operator may determine if the read-
pressure unit. This unit is attached to a cuff and inflates ings are reliable. For example, if the palpable pulse
the cuff to a pressure necessary to occlude arterial pressure in a patient (e.g. physical palpation of a
flow. The unit will gradually deflate the cuff (at a rate of patient’s femoral pulse) is subtle but a large differ-
3–4 mmHg per second). As the cuff deflates, arterial ence is noted between the systolic and diastolic
wall vibrations are detected and the intensity and pressures reported by the oscillometric unit (i.e.
change in arterial wall vibrations allows the unit to
measure the mean arterial pressure. Once the MAP is reported systolic minus diastolic value is a large
calculated, a proprietary algorithm is used to calculate number), the operator may interpret the findings
the systolic and diastolic pressure. of the oscillometric unit as poor or potentially

Blood Pressure Monitoring 35