Estimation of Acute Fluid Shifts Using Bioelectrical Impedance Analysis in Horses

Current Concepts in Fluid Therapy in Horses

Despite the frequent inclusion of fluid therapy in the treatment of many conditions in horses, there are limited studies available to provide evidenced-based, species-specific recommendations. Thus, equine fluid therapy is based on the application of physiology and extrapolation from evidence in other veterinary species and human medicine. The physiologic principles that underly the use of fluids in medicine are, at first glance, straightforward and simple to understand. However, in the past 20 years, multiple studies in human medicine have shown that creating recommendations based on theory in combination with experimental and/or small clinical studies does not consistently result in best practice. As a result, there are ongoing controversies in human medicine over fluid types, volumes, and routes of administration. For example, the use of 0.9% NaCl as the replacement fluid of choice is being questioned, and the theoretical benefits of colloids have not translated to clinical cases and negative effects are greater than predicted. In this review, the current body of equine research in fluid therapy will be reviewed, connections to the controversies in human medicine and other veterinary species will be explored and, where appropriate, recommendations for fluid therapy in the adult horse will be made based on the available evidence. This review is focused on the decisions surrounding developing a fluid plan involving crystalloids, synthetic colloids, and plasma.

Transcranial bioimpedance measurement in horses: a pilot study

OBJECTIVE

This pilot study aimed to evaluate the feasibility of transcranial bioimpedance (TCBI) measurement and variability of TCBI values in healthy conscious horses and to study effects of body position and time on TCBI in anaesthetized horses.

STUDY DESIGN

Prospective, observational study.

ANIMALS

A total of four research horses and 16 client-owned horses presented for surgery.

METHODS

After establishing optimal electrode position using computed tomography scans of cadaver heads, TCBI [described using impedance at zero frequency, R_0, (Ω)] was measured in four conscious, resting horses to investigate the feasibility and changes in TCBI over time (80 minutes). Data were compared using a paired t test. TCBI was then measured throughout anaesthesia (duration 92 ± 28 minutes) in 16 horses in dorsal and lateral recumbency. Data were analysed using a general linear model; gamma regression was chosen as a model of characteristic impedance [Z_c; (Ω)] against time. Data are presented as mean ± standard deviation.

RESULTS

No change in R₀ was seen in conscious horses (age = 15.3 ± 7.3 years, body mass = 512 ± 38 kg) over 80 minutes. The technique was well tolerated and caused no apparent adverse effects. In 16 horses (age = 7.4 ± 4.7 years; body mass = 479 ± 134 kg) anaesthetized for 92 ± 28 minutes, Z_c fell during anaesthesia, decreasing more in horses in lateral recumbency than in horses in dorsal recumbency (p = 0.008). There was no relationship between Z_c and body mass or age.

CONCLUSIONS AND CLINICAL RELEVANCE

TCBI is readily measured in horses. TCBI did not change with time in conscious horses, but decreased with time in anaesthetized horses; this change was greater in horses in lateral recumbency, indicating that TCBI changes in anaesthetized horses may be related to the effects of recumbency, general anaesthesia, surgery or a combination of these factors.

Determining dehydration and its compartmentation in horses at rest and with exercise: a concise review and focus on multi-frequency bioelectrical impedance analysis

Multi-frequency bioelectrical impedance analysis (MFBIA) has been, and likely will increasingly be, used to rapidly and non-invasively assess the time course of volume losses and recovery in horses. Dehydration in performance horses is frequently the cause of health and performance problems, and presently used techniques for objectively quantifying optimum hydration are time consuming and challenging to perform accurately. Dehydration can take a number of different forms, with a balanced loss of water and electrolytes from both extra- and intracellular fluid compartments, or a primarily extracellular or intracellular dehydration. This review summarises the current state of knowledge regarding the quantification of dehydration, losses of water and electrolytes from extra- and intracellular fluid compartments. The effects of dehydration on exercise performance, muscle function, cardiovascular function, thermoregulation and feeding are briefly summarised. The review provides a quantitative description of the magnitude and time course of compartmental fluid losses and recovery in horses in response to feeding and due to exercise at different intensities and durations representing the endurance horse to the track race horse. Effective rehydration requires knowledge of the losses from the main body fluid compartments, which is now possible using MFBIA technology. The present review outlines the key approaches that have been used to assess dehydration in horses, including the new technique of MFBIA.

Application of the sodium dilution principle to calculate extracellular fluid volume changes in horses during dehydration and rehydration

OBJECTIVE

To apply the principle of sodium dilution to calculate the changes in the extracellular fluid (ECF) volume (ECFV) and intracellular fluid volume (ICFV) that occur during dehydration and rehydration in horses.

ANIMALS

8 healthy horses of various breeds.

PROCEDURES

Horses were dehydrated over 4 hours by withholding water and administering furosemide. Saline (0.9% NaCl) solution was administered IV during the next 2 hours (20 mL/kg/h; total 40 mL/kg). Horses were monitored for an additional hour following IV fluid administration. Initial ECFV was determined by use of multifrequency bioelectrical impedance analysis, and serum sodium concentration was used to calculate total ECF sodium content. Sodium and fluid volume losses were monitored and calculated throughout the study and used to estimate changes in ECFV and ICFV during fluid balance alterations.

RESULTS

Changes during dehydration and rehydration primarily occurred in the ECFV. The sodium dilution principle estimated an overexpansion of the ECFV beyond the volume of fluid administered, indicating a small contraction of the ICFV in response to fluid administration. Serum and urinary electrolyte changes were recorded and were consistent with those of previous reports.

CONCLUSIONS AND CLINICAL RELEVANCE

The sodium dilution principle provided a simple method that can be used to estimate the changes in ECFV and ICFV that occur during fluid administration. Results suggested an overexpansion of the ECFV in response to IV saline solution administration. The sodium dilution principle requires further validation in healthy and clinically ill horses, which could provide clinical applications similar to those in other species.