Pulse Oximetry in Exotic Animal Species

Key Points

  • 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 pulse oximeter is fast, easy to use, and relatively inexpensive. This non-invasive monitor utilizes the pulsatile nature of arterial blood to provides four pieces of real-time information: 3,8,12

  • 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.
This mammalian oxygen-hemoglobin dissociation curve illustrates the nonlinear relationship between the partial pressure of oxygen and the percentage of total hemoglobin saturated with oxygen at normal body temperature and pH.

Figure 1. This mammalian oxygen-hemoglobin dissociation curve illustrates the nonlinear relationship between the partial pressure of oxygen and the percentage of total hemoglobin saturated with oxygen at normal body temperature and pH. Photo credit: Diberri via Wikimedia Commons

Equipment

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.
Pulse-oximeter probes. From left to right: flat reflectance probe, flat wrap probe, small clip probe, large clip probe, flat reflectance (rectal) probe

Figure 2. Pulse oximeter probes. From left to right: flat reflectance probe, flat wrap probe, small clip probe, large clip probe, flat reflectance (rectal) probe. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

rabbit pulse ox

Figure 3.  Rabbit (Oryctolagus cuniculus) with a large clip pulse oximeter probe on the tongue (left) and the pinna (right). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge

pulse ox foot pads

Figure 4. Placement of pulse oximeter probes on the footpad of a ferret (Mustela putorius furo) (left) and a guinea pig (Cavia porcellus) (right). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Ferret in which a clip-style pulse oximeter probe was left on for an extended period

Figure 5. Ferret (Mustela putorius furo) in which a clip-style pulse oximeter probe was left on for an extended period. The tip of the tongue sloughed and required debridement. Photo credit: Katrina Lafferty, CVT, VTS.

Table 1. Pulse oximetry probe placement options in select exotic animals
RabbitsFerretsRodentsBirdsLizards, CheloniansSnakes
TailC

(some species)
C

(some species)
Toes/legsCCCCC
WingsC
Cloaca/RectumRRRRRR
Scrotum/VulvaCCC
PinnaC
Ear canalR
TongueCCCCC

(some species)
Oral cavityRRRRRR
Esophagus/cropRRRRRR
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

Bearded dragon with the pulse oximetry probe clipped to the thigh. The monitor gives an accurate heart rate of 91 bpm, and a saturation reading of 98%.

Figure 6. Bearded dragon (Pogona vitticeps) with the pulse oximetry probe clipped to the thigh. Pulse oximeter accuracy and reliability can be affected by a variety of factors so always compare the heart rate displayed with the rate determined by some other means, such as ECG or Doppler. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

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
  • Poor probe positioning*

  • Venous pulsations**

  • Excessive movement

  • IV pigmented dyes (e.g. methylene blue)

  • Severe anemia (with concomitant hypoxemia)

  • Methemoglobinemia

  • Sepsis, septic shock

  • Bright light

  • Hemoglobin variants

  • Carbon monoxide poisoning

  • Methemoglobinemia

  • Poor probe positioning*

  • Sepsis, septic shock

SpO2: hemoglobin oxygen saturation in arterial blood
IV: intravenous

* 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.

With most species, hemoglobin oxygen saturation in arterial blood (SpO2) should exceed 95%.7,12

A sugar glider with a small clip pulse oximeter probe and a Doppler ultrasound probe attached

Figure 7. A sugar glider (Petaurus breviceps) with a small clip pulse oximeter probe and a Doppler ultrasound probe attached. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Birds

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

Duck with a pulse oximetry probe placed on a digit. In some situations, readings can also be achieved when a probe is placed on the webbing between digits

Figure 8. It is often difficult to find a suitable site for the pulse oximeter probe in the avian patient. Shown here, a small clip probe on the interdigital webbing of a duck. Pigment can sometimes make this site unsuitable for a transmittance probe. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Reptiles

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

Painted turtle with a pulse oximeter probe placed on the forelimb

Figure 9. Painted turtle (Chrysemys picta) with a pulse oximeter probe placed on the forelimb. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Snake with a small, flat reflectance probe placed within the cloaca. Maintaining a constant, proper position can be a challenge with “rectal probes” and frequent adjustments to assure adequate tissue contact are often necessary.

Figure 10. Snake with a small, flat reflectance probe placed within the cloaca. Maintaining a constant, proper position can be a challenge with “rectal” probes and frequent adjustments to assure adequate tissue contact are often necessary. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

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

Conclusion

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.

References