Anesthesia for Rodent ERG and VEP

There are two common anesthetics used for ERG testing in animals. The most common method is an injection of ketamine-xylazine for a combination of dissociative and paralytic effects. The second method is gas anesthetic, commonly Isoflurane Gas. In choosing the right anesthetic for testing, weigh the pros and cons of each method.

Breath Rate

Under optimal anesthesia, the breath rate will be approximately 55-65 breaths per minute without any muscle movement or corneal reflex. Under insufficient anesthesia the breath rate will be greater than 70 breaths per minute and the corneal reflex will be intact. Too much anesthesia will result in <50 breaths per minute and heavy breathing (gasping) which can be seen as noise artifact in the recording.

Ketamine-Xylazine

Pros and Cons of Ketamine/Xylazine

PRO: Ketamine/Xylazine is the most common anesthetic used for ERG testing.

CON: Xylazine can cause transient cataracts. These cataracts often dissipate once the drug wears off, however, the opacity may obscure the amount of light received by the retina. Anesthesia may negatively affect blood flow to the retina, repeated testing over multiple days can affect lower ERG amplitudes. Allow for a long recovery period for the ketamine to fully wash out fully before conducing repeat testing.

CON: Ketamine/Xylazine can certainly be a difficult drug combination for animals to endure. It runs a higher risk for overdose and death and is much more likely to kill geriatric mice.

Suggested Dosage of Ketamine/Xylazine

Please check with your animal use committee or onsite vet prior to dosing the animals.

When using ketamine-xylazine, it is important to weigh the animal first to calculate the correct dose. Improper dosing can lead to the death, or failure to become fully anesthetized.

Ketamine @ 80 mg/kg [75 – 150] + Xylazine @ 20 mg/kg [16-20]. Some animals, including particularly young wild types, may require additional ketamine. Suggested dosage range is indicated in square brackets following the suggested dosage.

Ketamine @ 50mg/kg [50-75] + Medetomidine @ 1.0 mg/kg [0.5 to 1]. Some animals, including particularly young wild types, may require additional ketamine. Suggested dosage range is indicated in square brackets following the suggested dosage.

Isoflurane Gas

Pros and Cons of Isoflurane Gas

PRO: One of the primary advantages of isoflurane gas is that the mouse is only anesthetized during direct gas inhalation. Thus once the test ends, the animal experiences a smooth and swift recovery.

CON: When using gas anesthesia, air is blown directly into the animal's snout, often causing dryness of the eyes. Closely monitor eye hydration and reapply eye drops or gel as needed. Keep in mind that dry eyes are less conductive, which could potentially decrease ERG amplitudes.

CON: Gas anesthetics like isoflurane suppress VEP responses.

CON: Isoflurane can also cause eye movements (or the eyes to roll back in the head) which can introduce substantial noise and variability into the trace.

CON: When animals inhale gas anesthesia, they tend to take deeper breaths, causing significant baseline deviations in the trace that increase signal variability. To counteract this, it may be necessary to take more measurements for better averaging of results.

Isoflurane Nose Cone

There are a number of gas anesthetic delivery systems for which Diagnosys can provide flexible tubing and fitted nose cones. For more, see our Isoflurane Nose Cone Kit.

References

Bell BA, Bonilha VL, Hagstrom SA, Anand-Apte B, Hollyfield JG, Samuels IS. Prolonged Ocular Exposure Leads to Formation of Retinal Lesions in Mice. Experimental Eye Research. (2019)

Calderone L, Grimes P, Shalev M. Acute Reversible Cataract Induced by Xylazine and by Ketamine-Xylazine Anesthes in Rats and Mice. Experimental Eye Research. (1986) 42: 331-337.

Ewald AJ, Werb Z, Egeblad M. Monitoring of Vital Signs for Long-Term Survival of Mice under Anesthesia. Cold Spring Harb Protoc (2011)

Koehn D, Meyer KJ, Syed NA, Anderson MG. Ketamine/Xylazine-Induced Corneal Damage in Mice. PLOS ONE. (2015) 10, 1371.

Nair G, Kim M, Nagaoka T, Olson DE, Thule PM, Pardue MT, Duong TQ. Effects of Common Anesthetics on Eye Movement and Electroretinogram. Doc Ophthalmol (2011) 122:163-176.

Ridder WH, et al. Causes of Cataract Development in Anesthetized Mice. Experimental Eye Research. (2002) 75: 365-370.

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