Page 84 - Basic Monitoring in Canine and Feline Emergency Patients

an enzyme called methemoglobin reductase, which 4.2 How Does Pulse Oximetry Work?
reduces the iron back to the ferrous state. Normally, Pulse oximetry (aka ‘pulse-ox’) is a noninvasive,
VetBooks.ir this mechanism maintains very low levels of MetHb continuous monitoring modality that uses two
in the blood. However, if this system is over-
wavelengths of light to estimate the SaO in pulsatile
whelmed, as seen with various conditions including
2
toxicities (see Table 4.1), high levels of MetHb can arterial blood. Red light (wavelength 660 nm) and
impair oxygen delivery by essentially creating a near infrared light (940 nm) are projected into tis-
2+
functional anemia. Because the change from Fe to sues via a light-emitting diode (LED) on the pulse-ox
Fe impacts the electrochemical structure of the probe. This light penetrates into tissues easily.
3+
porphyrin ring, it changes the wavelengths of light Oxygenated Hb absorbs greater amounts of infrared
absorbed by the molecule, giving blood with a high light and less red light compared with deoxygenated
content of MetHb a brown appearance. Hb. How much light of each wavelength is absorbed
vs how much was transmitted is detected by the
photodiode sensor on the probe. The Lambert–Beer
Carboxyhemoglobin law states that the concentration of a substance is
proportional to the absorption of light of a specific
The most common abnormal form of Hb encoun-
tered in veterinary medicine is carboxyhemoglobin wavelength by the substance and the distance
(COHb). This results from inhalation of carbon through which the light travels. Therefore, the dif-
monoxide (CO), a compound that binds to Hb with ferential light absorption between OxyHB and
an affinity approximately 200–250 times greater DeOxyHb can be used to calculate the amount of
than oxygen. This high-affinity binding means the OxyHb versus DeOxyHb that is present.
Hb molecules that have CO bound at one site will As the light travels through the tissue bed, it will
remain in the relaxed state, increasing the affinity for be absorbed by Hb in the arterial, capillary, and
binding O at the other open sites. This also causes venous blood. To differentiate the Hb in arterial
2
the OxyHb dissociation curve to shift to the left (Hb blood from these other areas, the pulse-ox probe
binding oxygen more readily). must be placed in an area where arterial pulsation
OHb is detrimental in two ways. First, CO binds can be detected by the sensor. The sensor detects
to a place that should be occupied by O , resulting the absorption of light in these non-pulsatile areas
2
in less O overall in the blood. Secondly, by shifting as a ‘static’ signal, and the absorption of light in the
2
the oxygen–hemoglobin dissociation curve to the arterial blood as an alternating signal (Fig. 4.5). It
left, COHb reduces offloading of any O present on then uses a ratio of these signals and compares this
2
the Hb to the tissues. Since the iron-binding site on ratio to proprietary formulas stored within the
Hb is occupied by CO and the porphyrin ring is not machine to calculate the SpO . It is crucial that the
2
changed versus normal, COHb absorbs very simi- pulse-ox be able to detect an accurate pulsatile flow
lar wavelengths of light as OxyHb. Therefore, the to be able to distinguish arterial Hb saturation
blood color with COHb is red, despite the fact that from the Hb within other tissues.
patients may have only a small amount of actual
O bound to Hb and decreased delivery of O to
2
2
tissues. Probe types
Two types of pulse oximetry probes are commonly
used in veterinary medicine. Transmission probes
Table 4.1. Common causes of methemoglobinemia in (Fig. 4.6) are the type of probe commonly placed on
veterinary medicine. a human finger or earlobe. The tissue is sandwiched
between the LED and sensor portions of the probe.
Medications Toxins
This type of probe is commonly placed on the
Acetominophen Skunk musk tongue, lip, vulva, prepuce, toe webbing, flank, or ear
Benzocaine Onions of small animal patients. The tissue must be thin
Prilocaine Garlic enough to allow light transmission (i.e. no thicker
Lidocaine Naphthalene than can comfortably fit within the ‘clip’ of the trans-
Metoclopramide (mothballs) mission probe), but well-perfused enough to give a
Sodium nitroprusside/nitroglycerin good pulsatile signal. Therefore, mucous membranes
Hydroxyurea
(tongue, lip, prepuce, vulva) tend to work best.

76 K.A. Marshall and A.C. Brooks