Chapter 1
Body water physiology

C. Langdon Fielding

Loomis Basin Equine Medical Center, Penryn, CA, USA

Introduction

The topic of fluid therapy usually focuses on the ideal fluid type and rate that should be administered to equine patients for specific clinical conditions. While the remainder of this textbook addresses these important questions, a brief introduction to the distribution of administered fluids is needed as a basis from which to interpret subsequent chapters. Specifically, concepts including the physiologic fluid spaces, effective osmolality, and oncotic pressure are important foundations to understand prior to important foundations to understand prior to formulating a fluid therapy plan.

Physiologic fluid spaces

The physiologic fluid spaces are typically divided into total body water (TBW), extracellular fluid volume (ECFV), and intracellular fluid volume (ICFV) as shown in Figure 1.1. It is important to remember that these fluid spaces are both physiologic (not anatomic) and dynamic. They represent a volume estimate at a point in time and therefore are constantly changing based on a number of physiological principles. Much of the attention in clinical medicine is focused on the ECFV; blood sampling for laboratory testing comes from this fluid space.

Figure 1.1 Relationship between total body water (TBW), extracellular fluid volume (ECFV), and intracellular fluid volume (ICFV) in a normal horse. Diagram represents a simplified "single cell" model.

Total body water (TBW)

Total body water represents the total volume of water within the animal. Values in adult horses have ranged from 0.55 to 0.77 L/kg depending on the measurement technique used (Dugdale et al., 2011; Fielding et al., 2004; Latman et al., 2011). A consensus from most of the research would suggest that a typical horse has a volume of TBW between 60 and 70% of its weight. A value of 2/3 is often used by many textbooks and is easy to remember. The majority of studies determining TBW have used deuterium oxide dilution, but this is not practical in a clinical setting. Acute changes in body weight are likely the best determination of changes in TBW in sick horses and foals. Monitoring of weight change should be done frequently (1-2 times per day if possible), in order to recognize acute loss or gain of body water.

Extracellular fluid volume (ECFV)

Extracellular fluid volume represents the volume of TBW that is not contained within the cells. This includes the plasma volume (PV), interstitial volume (IV), and transcellular compartments (gastrointestinal tract, joint fluid, etc.). The ECFV has also been measured using a number of different dilution techniques and reported values in adult horses have ranged from 0.21 to 0.29 L/kg (Dugdale et al., 2011; Fielding et al., 2004; Forro et al., 2000). A good approximation of the ECFV is about 1/3 of TBW.

In addition to evaluating fluid balance, monitoring the size of the ECFV is clinically useful in determining the dosage of some medications. In disease states, the ECFV is the space from which fluid losses often occur (e.g. sodium-rich fluid loss in diarrhea); it is also the space where fluids are administered and often remain (i.e. intravenous isotonic crystalloids). Three techniques that have been used clinically to monitor changes in the ECFV are bioelectrical impedance analysis (BIA), sodium dilution, and volume kinetics (Fielding et al., 2008; Forro et al., 2000; Zdolsek et al., 2012).

Plasma volume is easier to estimate as compared to the other fluid spaces and has been reported as 0.052 to 0.063 L/kg in healthy adult horses (Marcilese et al., 1964). Clinical monitoring of the PV is essential as excessive expansion or contraction can lead to clinical derangements such as edema and shock, respectively. Changes in packed cell volume (PCV) over time may give an indication of PV alterations, but the role of splenic contraction makes the use of this measurement somewhat complicated in horses. Total plasma protein concentration may be a more useful tool for monitoring PV; however, abnormal protein losses can make interpretation problematic.

Intracellular fluid volume (ICFV)

Intracellular fluid volume is the volume of fluid contained within the cells. It is usually estimated as the difference between TBW and the ECFV. Bioimpedance technology has been used to make estimates of this fluid space in horses, but dilution techniques cannot be easily applied to the ICFV. Reported values for ICFV are between 0.356 and 0.458 L/kg in horses (Dugdale et al., 2011; Fielding et al., 2004; Forro et al., 2000). Monitoring of the ICFV is typically not performed in clinical practice, but BIA may offer the best assessment available at this time.

Physiologic fluid spaces in foals

Physiologic fluid spaces in newborn foals have been described (Table 1.1). In general, there is an increased size of the ECFV and TBW as compared to adults (Fielding et al., 2011; Spensley et al., 1987). Values of TBW in newborn foals appear to be larger (0.74 L/kg) as compared to adults, which is consistent with other species (Fielding et al., 2011). Estimations of ECFV in foals are also significantly larger than in adults and have been reported to be between 0.36 and 0.40 L/kg in newborn foals; this decreases to 0.290 L/kg in foals at 24 weeks of age (Fielding et al., 2011; Spensley et al., 1987). The PV was estimated to be 0.090 L/kg (Fielding et al., 2011; Spensley et al., 1987), which represents an increase as compared to adults. Interestingly, the ICFV of foals is approximately 0.38 L/kg, which is similar to that in adult horses (Fielding et al., 2011). The ratio of ICFV to ECFV is approximately 1:1 in newborn foals as compared to adults with a ratio of approximately 2:1 (Figure 1.2).

Table 1.1 Physiologic fluid spaces in horses and foals.

Figure 1.2 The relative sizes of the intracellular fluid volume, extracellular fluid volume and total body water in adults and foals.

These differences in physiologic fluid spaces in foals alter the volume of distribution for common medications that have a high degree of water solubility (i.e. aminoglycoside antibiotics). This is one reason why the dosing of some medications differs in neonates as compared to adult horses. Fluid therapy plans must also take into consideration the different fluid physiology of the neonate.

Concepts in fluid balance

Perhaps the two most important physiologic concepts in water balance and fluid therapy are:

1. Effective osmolality (tonicity) - this guides the intracellular to extracellular fluid balance.
2. Starling's law of net filtration - this guides the intravascular to interstitial fluid balance.

Osmolality

Osmolality refers to the number of osmoles per kilogram of solvent (or water). The osmotic effect exerted by solutes is based on the total number of particles regardless of the size or weight of those particles. Osmolality is measured in serum by the method of freezing-point depression. Serum osmolality in adult horses was reported to range from 271 ± 8 to 281 ± 9 mOsm/kg H2O (Carlson et al., 1979; Carlson & Rumbaugh, 1983; Pritchard et al., 2006). Serum osmolality in foals has been reported as 245 ± 19 to 267 mOsm/kg H2O (Brewer et al., 1991; Buchanan et al., 2005).

Osmolality in serum can also be estimated with a calculation that is based on the use of the primary osmotically active substances in serum. One of the available equations for calculation is:

The values for sodium and potassium are doubled to estimate the contributions of the anions (given that positive and negative charges are always balanced). Glucose and body urea nitrogen (BUN) are divided by their molecular weights in order to convert milligrams per deciliter to millimoles per liter. While not extensively reported, normal values for calculated osmolality in horses would likely range from 295 to 300 mOsm/kg H2O based on reported ranges for these ion concentrations in horses.

The osmolal gap is the difference between measured and calculated osmolality. Reference ranges for the osmolal gap in horses have not been reported; based on available information, it is anticipated that the calculated osmolality may be greater than the measured osmolality. This same observation has been reported in cats and has been attributed to laboratory error (Wellman et al., 2012). A wide osmolal gap represents the presence of unidentified osmoles. The clinical usefulness of this test in horses may be more limited than in small animal medicine, as ethylene glycol toxicity is not commonly reported in horses. Other unidentified osmoles could be suspected using this calculation, however.

Effective osmolality (tonicity)

Effective osmoles are those that do not freely move across a membrane, and therefore exert tonicity. When considering horses (or other animals), the cellular membrane dividing the ECF from the ICF...