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 RACE-approved Anesthetic Monitoring teaching module. Visit the articles on monitoring the degree of central nervous system depression (anesthetic depth), blood pressure, capnometry, electrocardiography, and vital signs for additional information in exotic animal patients.
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.
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.
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.
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.
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.
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 | ||||||
|---|---|---|---|---|---|---|
Rabbits | Ferrets | Rodents | Birds | Lizards, Chelonians | Snakes |
|
| Tail | C | C (some species) | ||||
| Toes/legs | C | C | C | C | C | |
| Wings | C | |||||
| Cloaca/Rectum | R | R | R | R | R | R |
| Scrotum/Vulva | C | C | C | |||
| Pinna | C | |||||
| Ear canal | R | |||||
| Tongue | C | C | C | C | C | |
| Oral cavity | R | R | R | R | R | R |
| Esophagus/crop | R | R | R | R | R | R |
| 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
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 |
|
|
|
| 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
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
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
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.
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
References
1. Bailey JE, Pablo LS. Anesthetic monitoring and monitoring equipment: Application in small exotic pet practice. Semin Avian Exot Pet Med 7(1):53-60, 1998.
2. Bertelsen MF. Squamates (snakes and lizards). In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015:657-659.
3. Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med 107(6):789-799, 2013.
4. Damsgaard C, Storz JF, Hoffman FG, Fago A. Hemoglobin isoform differentiation and allosteric regulation of oxygen binding in the turtle, Trachemys scripta. Am J Physiol Regul Integr Comp Physiol 305(8):R961-R967, 2013.
5. DeVoe RS. Reptilian cardiovascular anatomy and physiology. Proc Annu Conf Am Board Vet Pract 2011.
6. Diethelm G, Mader DR, Grosenbaugh DA, et al. Evaluating pulse oximetry in the green iguana (Iguana iguana). Proc Annu Conf Assoc Reptilian Amphibian Vet 1998: 11-12.
7. Eatwell K, Mancinelli E, Hedley J, et al. Use of arterial blood gas analysis as a superior method for evaluating respiratory function in pet rabbits (Oryctolagus cuniculus). Vet Rec 173(7):166, 2013.
8. Flecknell PA, Thomas AA. Comparative anesthesia and analgesia of laboratory animals. In: Grimm KA, Lamont LA, Tranquilli WJ et al (eds). Veterinary Anesthesia and Analgesia: The Fifth Edition of Lumb and Jones. Ames, Iowa: Wiley Blackwell; 2015: 758.
9. Frische S, Bruno S, Fago A, et al. Oxygen binding by single red blood cells from the red-eared turtle Trachemys scripta. J Appl Physiol 90(5):1679-1684, 2001.
10. Hawkins MG, Zehnder AM, Pascoe PJ. Cagebirds. In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015:770-772.
11. Hernandez-Divers SM, Schumacher J, Stahl S, et al. Comparison of isoflurane and sevoflurane anesthesia after premedication with butorphanol in the green iguana (Iguana iguana). J Zoo Wildl Med 36(2):169-175, 2005.
12. Ozeki L, Caulkett N. Monitoring. In: West G, Heard D, Caulkett N (eds). Zoo Animal and Wildlife Immobilization and Anesthesia, 2nd ed. Ames, IA: Wiley Blackwell; 2015: 149-165.
13. Schmitt PM, Göbel T, Trautvetter E. Evaluation of pulse oximetry as a monitoring method in avian anesthesia. J Avian Med Surg 12(2):91-99, 1998.
FURTHER READING
Nevarez JG. Monitoring during avian and exotic pet anesthesia, Semin Avian Exot Pet Med 14(4):277-283, 2005.
Storz JF, Natarajan C, Moriyama H, et al. Oxygenation properties and isoform diversity of snake hemoglobins. Am J Physiol Regul Integr Comp Physiol 309(9):R1178-R1191, 2015.
Thawley V, Waddel LS. Pulse oximetry and capnometry. Topics in Companion Animal Medicine 28(3):124-128, 2013.
Torsoni MA, Stoppa GR, Turra A, Ogo SH. Functional behavior of tortoise hemoglobin Geochelone denticulata. Braz J Biol 62(4A): 725-733, 2002.
Lafferty K, Pollock CG. Pulse oximetry in exotic animal species. May 17, 2018. Available at https://lafeber.com/vet/pulse-oximetry/