Practical pulse oximetry for the Veterinary Nurse

Reliability and Limitations

When validating an SpO₂ reading, the plethysmograph should have a strong waveform which mimics an arterial blood pressure trace and it should also match the heart rate. The upward wave represents systole (the heart contracting) and the downwards wave represents diastole (when the heart is filling before the next beat). A dicrotic notch can be seen on the downwards wave when the aortic valve closes and the distended aorta contracts, creating a brief change in pressure into the arterial circulation. Both the waveform and dicrotic notch can be interpreted alongside other parameters to understand the patient's cardiovascular status:

Pulse oximetry provides a reading based on the red blood cells present as they pass the probe sensors. A limitation in SpO₂ readings occurs in anaemic animals where there are fewer red blood cells but they are fully saturated giving a normal SpO₂ , but there is still a reduced oxygen carrying capacity overall because there is less haemoglobin. Haemoglobin is estimated as ⅓ of the packed red blood cell volume (haematocrit) and is normally 12.1-20.3g/dL in dogs, and 9.3-15.9g/dL in cats (Klaassen, 1999).

Even if both the SpO₂ and PaO₂ are normal, it does not always indicate that the Oxygen Content of Arterial Blood (CaO₂) is normal. To fully assess the oxygen carrying ability of the patient, the haemoglobin levels must be measured. A normal CaO₂ in dogs has been reported between 14.2-17.8mL/dL (Haskins et al., 2005) and can be calculated as follows:

CaO₂ (mL/dL) = (1.36 × [Hb] × % saturation) + (PaO₂ × 0.003)

To estimate the PaO₂ , the FiO₂ is multiplied by five; e.g. room air is 21% oxygen and when multiplied by five, the expected PaO₂ is 80-100mmHg. In a patient under general anaesthesia breathing 100% oxygen, the PaO₂ should be 400-500mmHg. A limitation of pulse oximetry when the patient is breathing an oxygen rich concentration (high levels of PaO₂ are achieved), is that the SpO₂ will read 99-100%. Therefore, the PaO₂ must fall to a dangerously low level for there to be noticeable changes in the SpO₂ reading in this instance. A small change in the SpO₂ percentage actually reflects a huge change in PaO₂. Desaturation (SpO₂ <90%) can occur within 60 seconds in dogs breathing room air, compared to 300 seconds in those that were preoxygenated for 3 minutes prior to anaesthesia induction (McNally, Robertson and Pablo, 2009) despite a PaO₂ that would have been rapidly decreasing.

Two other forms of haemoglobin exist but they do not carry oxygen;

• Carboxyhemoglobin - occurs when a patient is exposed to carbon monoxide (e.g. house fire) which provides falsely high readings.
• Methemoglobin - occurs in paracetamol toxicity which lowers the reading towards 80-85%.

As mentioned previously, pulse oximetry provides information on oxygen transport, not oxygen delivery to the tissue. Oxygen delivery is determined by cardiac output and haematocrit (available haemoglobin). Additionally, it does not provide information on ventilation, and a hypoventilating patient under anaesthesia breathing 100% oxygen can have a normal SpO₂ but a dangerously high level of CO₂.

Troubleshooting abnormal readings

Although there are few limitations with pulse oximetry use, inaccurate readings can be given. Readings can be erroneously provided, and they are usually artificially low rather than artificially high (Haskins, 2015).

• Alpha-2 agonists (e.g. medetomidine) cause peripheral vasoconstriction resulting in limited pulsatile blood flow available to obtain a reading. It usually occurs in the early stages of premedication and will eventually wane. 
• Vasoconstriction can also occur in hypothermic patients, and actively warming the patient can improve blood flow to the periphery.
• Hypotension can reduce blood flow to the capillary beds and readings may be difficult to obtain. Measures to correct the blood pressure should be taken. 
• Patient movement makes it difficult for the pulse oximetry probe to sense arterial blood flow, and a reading may not be provided at all.
• Ambient room light, especially fluorescent can reduce the ability of the two probes to differentiate oxyhaemoglobin from deoxyhemoglobin. Covering the probe with a drape or blanket helps to block out the room light.
• The pulse oximetry probe must make contact with the tissue, and the use of a wet swab placed between the probe and the tissue keeps it moist and encourages contact. Some probes may also have a tight clip and compress the blood from the capillary bed, providing an underestimation of true oxygen saturation - these may need to be wedged open (a clear syringe cap helps).

Using Pulse Oximetry in the Pre and Post Anaesthetic Period

Pulse oximetry is widely used in the peri-anaesthetic period, but it can also be used in the pre- and post-anaesthesia period.

A pre-anaesthesia reading can help guide the expected SpO₂ reading in the recovery period. For example, brachycephalic breeds are often chronically hypoxic and a “normal” reading prior to anaesthesia can manage expectations post operatively.

In the anaesthesia recovery period, hypoventilation still occurs due to residual sedative effects and when coupled with breathing room air (21% oxygen), can cause hypoxemia. If the patient is experiencing a delayed recovery, consider taking an SpO₂ reading and providing supplemental oxygen.

Conclusion

While pulse oximetry monitoring is very prevalent and useful in practice, interpretation against its limitations is important.

Author: Courtney Scales

If you are interested in pulse oximeter, pls visit the links beloe:

//www.chinautech.com/handheld-pulse-oximeter-with-charger-base.html

//www.chinautech.com/upm50-handheld-pulse-oximeter-5015696300888753.html

//www.chinautech.com/product-4987383350735856.html