Page 49 - Basic Monitoring in Canine and Feline Emergency Patients

Changing conditions or personnel may also cre- cmH O by multiplying it by a conversion factor of
2
ate inconsistency in blood pressure monitoring. It is 1.36. However, this approach to blood pressure
VetBooks.ir important to be as consistent as possible when measurement is not advisable as it is difficult to
measuring serial blood pressures. The patient’s
accurately measure the distance between the cuff
position (e.g. left lateral recumbency), limb utilized
tioning will likely be difficult to recreate for subse-
for the measurement (e.g. right forelimb), and cuff and the right atrium. Furthermore, precise posi-
size should be accurately recorded. In addition, the quent monitoring leading to poor repeatability of
environment, patient’s disposition or attitude, and the measurement between time points.
indirect monitoring device should all be docu- All oscillometric measurements must be inter-
mented. The staff obtaining the measurements is preted carefully. The unit provides systolic and
also important as positioning of the patient, cuff diastolic pressures as well as MAP. When evaluating
location, and cuff tightness may vary between per- the results, the clinician must consider the quality of
sonnel. Subsequent checks should take pains to the pulse pressure on physical examination. The
recreate a similar environment and condition each palpated pulse may be interpreted as strong, fair,
time a blood pressure is measured to reduce poor, or absent, dependent on the situation. When
variability. the oscillometric unit displays the systolic and dias-
Frequent mistakes include the choice of cuff size, tolic pressures, a calculated pulse pressure can be
patient positioning, and the type of indirect moni- determined from the measurements (see Equation
toring equipment used. Cuff size is important as 2.2). On occasion the machine may provide either
using too large a cuff will generally result in falsely very wide/large or very narrow/small pulse pres-
low blood pressure readings. Conversely, using a sures, and this data should be compared carefully
cuff that is too small will result in falsely increased with the clinician’s impression of the pulse pressure
blood pressure measurements. Cuffs that are too noted during examination. As mentioned previ-
small transmit pressure inefficiently to the underly- ously, a normal or strong pulse pressure is approxi-
ing tissues, meaning a greater pressure than appro- mately 40-60 mmHg. If the subjective assessment of
priate is necessary to occlude the arterial blood the pulse strength is inconsistent with the calculated
flow. The opposite is the case when too large a cuff pulse pressure, the result displayed on the oscillo-
is selected for the patient. metric unit should be interpreted cautiously.
Blood pressure measurement (i.e. the location of Likely attributable to difficulty sensing and assess-
the cuff) should occur at the level of the right atrium ing the arterial wall oscillations, cats and small dogs
which is generally estimated by the location of the (<10 kg) should not have their blood pressure meas-
thoracic inlet in lateral recumbency or the point of ured with oscillometric equipment as this tool tends
the shoulder in sternal recumbency. Patients having to underestimate blood pressure. There is also some
a blood pressure measured while in a sternal or controversy as to the interpretation of Doppler
standing position often have significant vertical dis- blood pressure readings in small dogs and cats.
tance between the cuff site (usually on a limb) and Some sources state that Doppler blood pressure
the right atrium. This leads to false elevation of the readings in these animals (especially in hypotensive
blood pressure due to the weight of the blood col- patients) may be more representative of MAP than
umn between the right atrium and the cuff site. If a systolic blood pressure. However, there is no consist-
blood column exists between the cuff site and the ent research data to support this claim.
right atrium, the measured blood pressure includes
both the actual blood pressure (generated by left
ventricular contraction) and the weight of the verti- Patient factors
cal blood column which exerts force on the cuff. Patients with peripheral limb disease affecting the
An adjustment for the cuff distance (in centime- vasculature or interstitium may have unreliable
ters; cm) below the right atrium may be incorpo- readings by either indirect method. Any state of
rated to adjust the blood pressure and appropriately shock may create pulses with a lower pulse pres-
decrease the measured blood pressure. The cuff sure, leading to a greater degree of difficulty in
distance below the right atrium (measured in cm) pulse detection and measurement. Furthermore,
can be subtracted from the measured blood pres- states of obstructive shock (such as gastric dilata-
sure (in centimeters of water; cmH O). The meas- tion and volvulus) can compromise blood flow to
2
ured blood pressure (in mmHg) can be converted to the caudal aspect of the body and make blood

Blood Pressure Monitoring 41