Page 85 - Basic Monitoring in Canine and Feline Emergency Patients

Pulse
VetBooks.ir Arterial blood

Absorption Venous blood

Tissue

Time
Fig. 4.5. Light passing through tissues will be absorbed by hemoglobin (Hb) in multiple compartments. By analyzing
the ratio of absorption between the signal that varies with pulsatile flow versus the signal that is static, the pulse
oximeter differentiates between the saturation of Hb in arterial blood and other tissues and reports only the saturation
of the Hb in arterial blood. Therefore, detection of pulsatile flow is critical to the accuracy of the reading.

LED

Fig. 4.6. Transmission pulse oximetry
Sensor probe (Massimo, Irvine, California, USA).
LED, light emitting diode.

Transmission probes are generally poorly tolerated LED
for long-term use unless the patient is anesthetized or
moribund since they fall off easily with movement.
Also, transmission probes will gradually compress
the tissue bed and can compress the vessels, reducing
the all-important arterial blood flow and causing the
probe to not give a reading. When using a transmis- Sensor
sion probe, the LED and sensor portions of the probe
must be in correct alignment or the signal can be
distorted, resulting in inaccurate measurements. Fig. 4.7. Reflectance pulse oximetry probe (Massimo,
.
Reflectance probes (Fig. 4.7) have the LED and Irvine, California, USA). LED, light emitting diode.
sensor on the same side of the probe. The emitted
light is reflected off the bone and is then received
by the sensor. To allow for proper reflection back probe in order to allow the emitted light to reflect
to the sensor, the reflectance probe must either be back to the sensor.
placed over a flat bone (e.g. the forehead in humans In anesthetized animals, reflectance probes can
or the ventral aspect of the first coccygeal bone in be used on the hard palate or within the esophagus.
animals) or held at just the right angle to a more In awake animals, these probes can be used on the
curved bone to allow proper reflection of the emit- underside of the base of the tail, over the femoral
ted light back to the sensor. The distance from the artery/proximal femoral bone, on the dorsal sur-
LED to the bone must not be any farther than the face of the metatarsus, or the plantar side of the
distance between the two sides of the transmission carpus. Versions of reflectance probes made to be

Pulse Oximetry 77