Page 89 - Basic Monitoring in Canine and Feline Emergency Patients

Table 4.2. Possible causes of inaccurate pulse oximetry readings.

VetBooks.ir Abnormality Example Effect on SpO reading
2
Patient in recent house fire
Carboxyhemoglobin
Spuriously high readings
Methemoglobin Exposure to toxins or drugs Spurious readings in the 80s
(see Table 4.1)
Vasoconstriction Hypothermia, shock, Reduced pulsatile signal, increased risk of motion
vasoconstrictive drugs artifacts, variable effects on readings
Vasodilation Hyperthermia, SIRS/sepsis Venous congestion may lead to venous pulsation
and artifactually low readings as pulsing venous
blood is misread as arterial
Excessive motion Non-compliant patient If interpreted as pulsation, may cause erroneous
readings (both falsely high and low)
Pigmented tissues Black pigment on gums/lips Low signal quality as less light will reach the
sensor (absorbed by pigment)
Other light sources/electrical Fluorescent lights, electrocautery Variable effects if light is absorbed erroneously by
energy detected by sensor; better compensated for in newer units
photodiode
Lack of pulsatile blood flow Cardiac arrest, thrombosis Other artifacts (e.g. motion) may be interpreted
of tissue bed as pulsatile flow and give erroneous readings.
Pulse oximetry should not be considered
accurate in low flow/no flow states
Anemia Blood loss, hemolysis Generally accurate except at extremely low
(<75%) saturation or anemic shock resulting in
vasoconstriction
Intravenous dyes Methylene blue Spuriously low readings, pigment affects amount
of light reaching the sensor
Hyperbilirubinemia/icterus Hemolysis, liver disease Generally accurate assuming minimal presence of
other dyshemoglobinemias (e.g. COHb)
Shifted OxyHb dissociation Changes in body temperature, The correlation between SpO and PaO will be
2
2
curve acid–base status, hyper/ shifted (see Fig. 4.4); the practitioner must take
hypoventilation this into account when interpreting SpO values
2
and using them to predict PaO 2
Arrhythmias Atrial fibrillation, VPCs Correlate with ECG, but if signal quality is good
and SpO reading is stable, generally do not
2
affect accuracy of readings
COHb, carboxyhemoglobin; ECG, electrocardiogram; OxyHb, oxyhemoglobin; PaO , partial pressure of oxygen in plasma; SIRS,
2
systemic inflammatory response syndrome; SpO , pulse oximetry; VPC, ventricular premature contraction.
2
hypoxemic. In this setting, despite all measured val- COHb, the pulse oximeter would read 95%, which
ues being normal in the blood and mucous mem- is falsely inflated by 15%. In this setting, a PaO
2
branes appearing bright pink/red, the patient will MUST be measured to determine the amount of
still have a significant tissue oxygen derangement. oxygen in the plasma available to tissues.
Dyshemoglobinemias, such as MetHb and MeHb absorbs infrared and red light equally well,
COHb, are also particularly confounding to pulse which is more infrared light than either OxyHb or
oximetry. The two wavelengths of light emitted in DeOxyHb absorb. It also absorbs a similar amount
traditional pulse oximetry cannot differentiate of red light to DeOxyHb. Therefore, when there are
OxyHb from COHb. Pulse oximeters therefore significant MetHb levels in the blood (typically
measure COHb as OxyHb and will give spuriously >30%), this can lead to a pulse oximetry machine
high readings that do not reflect the true SaO . For reading in the 80–85% range, regardless of the
2
example, if a patient has 80% OxyHb and 15% actual amount of OxyHb. This might over or

Pulse Oximetry 81