Page 40 - Basic Monitoring in Canine and Feline Emergency Patients

Table 2.1. Normal blood pressure values in dogs and using either Doppler flow detection or oscillometry
cats. (see below for more details).
VetBooks.ir Systolic blood Mean arterial Diastolic Indirect blood pressure measurement
pressure
pressure
pressure
Dogs 90–140 mmHg 60–100 mmHg 50–80 mmHg Doppler
Cats 80–140 mmHg 60–100 mmHg 55–75 mmHg
The Doppler flow detection unit (ultrasonic
method) employs a probe containing two piezoe-
lectric quartz crystals (Fig. 2.6). One crystal serves
many additional variables may interfere with precise as the transmitter and emits a constant ultrasound
non-invasive blood pressure monitoring. From this wave at a set frequency (2–10 MHz). The second
table, one may deduce that a normal or strong pulse crystal functions as a continuous receiver. The
pressure is 40–60 mmHg (i.e. the difference between Doppler unit probe has a concave surface which is
the systolic and diastolic pressures). placed against the skin surface (any fur must be
shaved prior to placing the probe) and positioned
over a peripheral artery parallel to the direction of
arterial blood flow.
The ultrasound signal is emitted from the trans-
2.2 How the Blood Pressure
Monitor Works mitting crystal and waves reflect back from moving
red blood cells within the artery and are detected
Direct arterial blood pressure measurement by the receiving crystal. These reflected waves have
a different wavelength dependent on the velocities
The gold standard method of measuring arterial of the moving red blood cells. If the red blood cells
blood pressure is via an arterial catheter, typically are moving toward the probe, the sound wave fre-
placed in a hindlimb dorsopedal artery. Although quency is increased and if the red blood cells are
less commonly placed, femoral artery, auricular moving away from the probe, the sound wave fre-
artery, or coccygeal artery catheterization may also quency is decreased. The change in wavelength is
be performed. The arterial catheter is in turn con-
nected to a transducer via a continuous column of
saline (i.e. direct arterial blood pressure measure-
ment). The transducer converts the pressure to an
electrical signal which is displayed on a monitor
that amplifies the signal and filters the noise.
There are several important advantages to direct
arterial blood pressure monitoring: (i) providing
beat-to-beat monitoring; (ii) production of accu-
rate readings at low pressures; and (iii) avoiding
repeated cuff inflations. However, there are several
disadvantages including discomfort and skill level
required for placing the arterial catheter, risk of
thrombosis in the arterial system due to catheter
placement, and the challenge in maintaining a func-
tional arterial catheter in an ambulatory cat or dog.
Although direct blood pressure monitoring may be
used with some frequency in anesthetized patients,
anecdotal impressions suggest that maintaining
function and patency of these catheters for >24
hours post-anesthesia is difficult. Fig. 2.6. A close-up photograph of the piezoelectric
For these reasons, in a clinical setting, indirect quartz crystals in the Doppler ultrasonic unit. One of
blood pressure monitoring is performed more fre- these crystals serves as a transmitter and emits an
quently, especially in non-anesthetized patients. ultrasound wave at a frequency of 2–10 MHz. The
Indirect blood pressure monitoring is performed second crystal serves as a receiver.

32 D.S. Foy