Transthoracic impedance in adult Thoroughbred horses is similar to other species.

Overview

• This study measured transthoracic impedance (TTI) in adult Thoroughbred horses to understand factors influencing it and compared the values with those reported in other species.

Background

• Transthoracic impedance (TTI) is an important measure affecting the success of transthoracic cardioversion or defibrillation, as it represents the resistance the electrical shock encounters passing through the thorax.
• Understanding TTI in horses is essential because the efficacy of defibrillation or cardioversion may be impacted by species-specific differences in thoracic anatomy and electrical properties.
• This study examined TTI in Thoroughbred horses, which are large animals, to assess if TTI values differ from other species, and what factors may influence TTI values during procedures.

Study Design and Methods

• The study was divided into two phases using a randomized crossover design involving 19 adult Thoroughbred horses:
• Phase 1: Measured TTI in 11 deceased horses immediately after euthanasia to avoid ethical concerns with repeated shocks in live animals.
• Measurements were taken with horses in dorsal and lateral recumbency positions.
• TTI was measured with and without manual pressure applied on the electrodes.
• Multiple shocks were delivered (shock 1, shock 2, shock 4) to see if repeated shocks affected TTI.
• Phase 2: Measured TTI in 8 anesthetized live horses during inspiration and expiration phases of respiration, all in dorsal recumbency.
• A biphasic exponential defibrillator was used for TTI measurements.
• Statistical models (mixed-effects linear regression) were used to analyze the influence of patient factors (e.g., weight, respiratory phase) and technique factors (e.g., position, manual pressure, number of shocks) on TTI.

Key Findings

• The median TTI across all horses and conditions was 56 ohms, with a range from 44 to 133 ohms.
• Phase 1 (deceased horses):
• TTI decreased with subsequent shocks: shock 4 had significantly lower TTI (54.2 Ω) than shock 1 (67.2 Ω) and shock 2 (71.1 Ω).
• This suggests repeated shocks may reduce impedance, possibly related to changes in electrode contact or local tissue factors.
• Phase 2 (live anesthetized horses):
• TTI increased slightly by approximately 0.2% per kilogram increase in body weight.
• The specific terminal study to which the horses belonged also influenced TTI, indicating factors related to study conditions or horse populations may matter.
• Patient position (dorsal vs lateral), manual pressure on electrodes, and respiratory phase (inspiration vs expiration) did not significantly affect TTI.

Interpretations

• The typical TTI values in adult Thoroughbred horses were comparable to those reported in other species, meaning that the electrical resistance through their thoraxes is not uniquely high.
• This indicates that TTI itself may not be the primary limiting factor for successful transthoracic cardioversion or defibrillation in horses.
• Factors like patient weight and the number of electrical shocks administered can influence TTI, and thus should be considered during treatment.

Limitations

• Use of a convenience sample of horses may limit generalizability.
• Phase 1 data came from euthanized horses, which could differ physiologically from live animals.
• Phase 2 live horse subjects were from terminal studies, potentially introducing variability or bias based on their health conditions or treatments.

Conclusion

• This research demonstrates that transthoracic impedance in Thoroughbred horses aligns closely with reported values in other species, suggesting that challenges with transthoracic cardioversion in horses are unlikely due to unusually high impedance.
• Understanding patient weight and repeated shock effects on TTI can help improve defibrillation strategies in equine medicine.