- Heart rate and oxygenation should ideally be monitored during every anesthetic event.
- Hemoglobin oxygen saturation in arterial blood should exceed 95% in most species.
- Even when small patient size (exotic companion mammals) or lack of validation (birds and reptiles) limits the accuracy of pulse oximetry readings, trends can be monitored during the course of anesthesia that can provide useful clues to patient clinical status.
- This article is part of a series on anesthetic monitoring in exotic animal patients. Additional topics available include: blood pressure, capnometry, electrocardiography, and monitoring vital signs.
How does pulse oximetry work?
- The percentage oxygen saturation of arterial blood (SpO2) is measured by detecting changes in the absorption of light across tissues. The device is calibrated using the mammalian oxygen hemoglobin dissociation curve (Fig 1).
- The pulse oximeter also measures variations in this signal with each heartbeat and uses this information to calculate heart rate. Some monitors can be adjusted to count pulses up to 400 beats per minute.
- The strength of the pulsatile signal, usually displayed as a bar graph or waveform, provides some indication of blood flow through the tissues.
- The pulsatile waveform corresponds with heart rhythm.
Pulse oximeter probes come in many shapes and sizes and can be adapted and utilized in a variety of situations (Fig 2) (Table 1). Suitable sites for probe placement include the tongue, ear, tail, nail bed, and across the footpad.
- A small, flat reflectance probe can be tucked into the pinna. The probe can also be taped over a superficial artery, often after fur has been clipped away.
- Flat probe wraps can be placed around a digit, limb, tail, or laid flat against a mucosal membrane.
- A clip probe can be placed on the tongue, pinna, digit, or tail (Fig 3, Fig 4). Use caution as the clip can act like a tourniquet and occlude blood flow (Fig 5).
- A large, flat reflectance probe can be placed within the rectum/cloaca, pinna, esophagus, or against the palate. This probe can also be taped over an artery, where fur has been clipped away.
|Table 1. Pulse oximetry probe placement options in select exotic animals|
|R = Flat reflectance or rectal probe C = Clip style probe|
Troubleshooting & limitations
There are many factors that affect pulse oximetry (Fig 6) (Table 2).12 If a poor signal is obtained, try shaving fur or repositioning the pulse oximetry probe. When a probe on the tongue provides a poor signal, try moistening the tongue and then replacing the probe. Some sensors also function poorly in bright light, so try shielding the probe with a drape or sponge.8
Pulse oximeters are reasonably accurate and reliable at normal oxygen saturation levels but become increasingly inaccurate and unreliable as levels falls (Table 2).8 Pulse oximeters are also sensitive to movement artifact, which can cause difficulty during the later stages of recovery.8 A fall or loss of the oxygen saturation signal can be caused by a decrease in tissue perfusion due to cardiovascular failure, a fall in blood pressure, hypothermia, vasoconstriction caused by anesthetic agents, particularly alpha-2 agonists, as well as shock.8,12
|Table 2. Causes of false SpO2 readings 7|
|Falsely low||Falsely normal||Falsely high|
|SpO2: hemoglobin oxygen saturation in arterial blood|
* Resulting in decreased absorption of red and/or infrared light. Always check and reposition probe position when preparing to take action to correct hypoxia 7
** Venous pulsations can occur when a probe is placed too tightly
Exotic companion mammals
Although the absorption spectra of hemoglobin vary, the values are similar enough to allow oximeters designed for use in people to be used successfully in most mammals.8 Pulse oximeters are particularly useful in larger species. In small patients, the oximeter will not work as well due to the low volume of tissue available.8 Despite this technical limitation, pulse oximetry is considerably more reliable than clinical assessment alone in small mammals (Fig 7).8 There are some instruments designed for small patients. I have had good success with Nonin Medical (Plymouth, MN), Surgivet (Smiths Medical, Norwell, MA), and Sentier Vetcorder (Brookfield, WI) pulse oximeters.
Pulse oximetry has not been validated in birds. 10 The absorption characteristics of oxygenated and deoxygenated avian and human hemoglobin differ leading to an underestimation of hemoglobin saturation in avian patients.10,13 Nevertheless, oximeters are often used to measure trends in oxygen saturation and heart rate (Fig 8). 12
The use of pulse oximetry remains controversial in reptiles due to technical limitations and high susceptibility for measurement errors and artifacts. 2,6,11,12. Transmittance probes do not provide consistent readings when applied to thick, pigmented reptile skin (Fig 9). A reflectance probe placed in the esophagus or cloaca can maximize oximetry readings (Fig 10). 2,5
In green iguanas (Iguana iguana), the absorbencies for oxyhemoglobin (660 nm) and deoxyhemoglobin (990 nm) were similar to mammalian values, 2,6 however the reptilian oxygen hemoglobin disassociation curve is often markedly different when compared to mammals. 2, 4,9 As in birds, pulse oximetry is generally used only to monitor trends in reptile patients rather than absolute saturation values. 2
Although pulse oximetry can be challenging in exotic animal patients, it is possible to maximize the functionality of this monitoring technique with practice, patience, and creativity. Each taxonomic group has anatomic variances and it is important to understand expected monitoring parameters.