Abstract

Domoic acid (DA) exposure has been associated with neurological sequelae as well as cardiomyopathy in humans, California sea lions (Zalophus californianus), and sea otters (Enhydra lutris).^1-4 The degenerative cardiomyopathy seen in DA affected sea lions follows along the conduction pathway of the heart, and thus has been hypothesized to disrupt the normal electrocardiogram (ECG) wave pattern.⁴ Protocols for the collection of ECGs from California sea lions, including electrode placement, were needed and the effect of chemical immobilization and waveform orientation evaluated. A standardized protocol for lead placement was developed and ECG wave character evaluated using a hexaxial ECG reference system in rehabilitating California sea lions. Electrocardiograms were recorded from California sea lions (n = 31), that were sedated or anesthetized for routine procedures at The Marine Mammal Center. Sea lions exhibiting clinical signs consistent with acute or chronic domoic acid poisoning were excluded. Three placement protocols were examined in yearling sea lions under isoflorane anesthesia (n = 15). Amplitude varied between individuals and decreased as electrodes were moved more distally. There was no significant difference between heart rate, PR interval, QRS duration, QT interval, or mean electrical axis in any of the three electrode positions, evaluated in yearlings anesthetized with isoflorane. A standard protocol was developed with the cranial electrodes (RA, LA) placed at the skin at the trailing edge of the thoracic appendage at the axillary insertion, and the caudal electrodes (RL, LL) placed at the skin at the lateral aspect of the pelvic appendage at the insertion. This position was chosen as consistent placement of electrodes across age classes and animal size was possible, the fewest inaccurate recordings (unrecorded P waves or immeasurable waveforms) were obtained, and the least baseline interference was seen on recordings. Using the standardized protocol, ECG recordings were compared between yearlings (n = 22) and non-yearlings (subadult and adult, n = 9) immobilized with isoflorane (n = 23) or midazolam (0.2 mg/kg) and butorphanol (0.2 mg/kg, n = 8). Overall, ECG tracings had variable P wave amplitudes, characteristically stunted Q and R wave morphology, deep S waves, and tall narrow T waves. The mean PR interval was significantly longer in non-yearlings when compared to yearlings (p = 0.03) but no significant differences in mean heart rate, QRS complex duration, QT duration, or mean electrical axis. There was no significant difference between any of the duration parameters measured in animals anesthetized with isoflorane compared to those sedated with the midazolam and butorphanol. The technique and protocol developed in this study may be used to detect and compare deviations of ECG waveform and duration, hypothesized to occur in sea lions with cardiac changes associated with previous domoic acid intoxication.

Acknowledgements

The authors wish to thank all the staff and volunteers of The Marine Mammal Center, specifically Marjorie Boor for her donation of ECG equipment, and Drs. Kittleson, Griffiths, Jones, and Hsu from UC Davis for their input into the study design and assistance with ECG interpretation and analysis. Funding support for this project was provided by UC Davis School of Veterinary Medicine and made possible through a grant from the Summer Training in Advance Research (STAR) program.

References

1.  Gill S, Pulido O. 2005. Glutamate receptors in peripheral tissue: distribution and implications for toxicology. In: Gill S, Pulido O, editors. Glutamate Receptors in Peripheral Tissue: Excitatory Transmission outside the CNS. New York: Kluwer Academic/Plenum Publishers; p 3-21.

2.  Gulland F, Haulena M, Fauquier D, Langlois G, Lander M, Zabka T, Duerr R. 2002 Domoic acid toxicity in California sea lions (Zalophus californianus): clinical signs, treatment and survival. Vet Rec 150: 475-480.

3.  Kreuder C, Miller M, Lowenstine L, Conrad P, Carpenter T, Jessup D, Mazet J. 2005. Evaluation of cardiac lesions and risk factors associated with myocarditis and dilated cardiomyopathy in southern sea otters (Enhydra lutris nereis). Am J Vet Res 66: 289-299.

4.  Zabka T, Goldstein T, Cross C, Mueller RW, Kreuder-Johnson C, Gill S, Gulland F. 2009. Characterization of a degenerative cardiomyopathy associated with domoic acid toxicity in California sea lions (Zalophus californianus). Vet Pathol 46: 105-119.