• Open Access

A Comparison of Hypertonic (7.2%) and Isotonic (0.9%) Saline for Fluid Resuscitation in Horses: A Randomized, Double-blinded, Clinical Trial

Authors


  • This study was performed at the 2009 Western States 100-mile endurance ride in Auburn, CA. A portion of this study was presented at the 2011 ACVIM Forum.

Corresponding author: C. Langdon Fielding, Loomis Basin Equine Medical Center, P.O. Box 2059, Loomis, CA 95650; e-mail: langdonfielding@yahoo.com.

Abstract

Background:

Hypertonic saline solution (7.2%) (HSS) can quickly replace intravascular volume deficits. HSS more recently has been advocated in the treatment of traumatic brain injury, but its use in dehydrated patients remains controversial.

Hypothesis:

Hypertonic saline solution will show a significant improvement in both clinical and laboratory hydration parameters as compared to isotonic (0.9%) saline solution (ISS).

Animals:

Endurance horses eliminated from the 2009 Western States 100-mile (220-km) endurance ride and requiring IV fluid therapy were eligible for enrollment in the study.

Methods:

Twenty-two horses were randomly assigned to receive 4 mL/kg of either HSS or ISS along with 5 L lactated Ringer's solution (LRS). After this bolus, horses were treated with additional LRS in varying amounts. Blood and urine samples were collected before, during, and after treatment. Data were compared using 2-way ANOVA with repeated measures.

Results:

As compared to ISS, HSS horses showed greater decreases in PCV (P = .04), total protein (P = .01), albumin (P = .01), and globulin (P = .02) concentrations. HSS horses showed greater increases in sodium and chloride (P < .001) as compared to ISS horses. Horses receiving HSS had a shorter time to urination (P = .03) and lower specific gravity (P < .001) than those receiving ISS.

Conclusions:

Results of this study indicate that HSS may provide faster restoration of intravascular volume deficits than ISS in endurance horses receiving emergency medical treatment. More marked electrolyte changes should be expected with HSS, however, and additional fluids after HSS administration likely are needed.

Abbreviations
ECFV

extracellular fluid volume

HSS

hypertonic saline solution

ICFV

intracellular fluid volume

ISS

isotonic saline solution

The use of hypertonic saline solution (HSS) has been described for the treatment of intravascular volume deficits in a variety of species secondary to controlled hemorrhage, sepsis, and other emergency conditions associated with hypovolemia.[1-6] The initial proposed mechanism responsible for its beneficial effects was its osmotic attraction of fluid from the intracellular space.[1, 7, 8] More recent research indicates that other benefits may include the indirect release of vasopressin and activation of aquaporin channels, immunomodulatory effects, and alterations of the vascular endothelium.[9-11] In the clinical setting, HSS is now recommended for the treatment of traumatic brain injury.[12]

There have been primarily experimental studies in horses evaluating the effects of HSS in a variety of clinical models including endotoxemia and controlled hemorrhage.[13-17] A recent clinical trial examined the effects of HSS before anesthesia for abdominal surgery.[18] The results of this study showed greater cardiovascular benefit with a colloid solution than with HSS. Additional clinical trials are needed to determine if there is a place for HSS in the emergency treatment of horses.

Controversy exists about the use of HSS in horses with dehydration.[19, 20] The concern is that these animals may have clinically relevant intracellular fluid deficits and therefore should not be treated with a hyperosmotic agent.[19] Endurance horses requiring emergency treatment represent a group of dehydrated patients requiring fluid resuscitation.[21] HSS could be an ideal fluid for this patient population if it is able to rapidly expand the vascular volume as it has done in other equine studies.

The purpose of this study was to compare the effects of a 7.2% HSS solution with those of a 0.9% saline solution (ISS) in a randomized, double-blinded clinical trial. The study groups consisted of horses eliminated from endurance competition because of signs of dehydration and hypovolemia, as well as metabolic failure requiring emergency medical treatment. We tested the hypothesis that HSS would more rapidly improve clinical and biochemical variables associated with intravascular volume expansion than would ISS.

Materials and Methods

The study took place at the 2009 Western States 100-mile (220-km) endurance ride (Tevis Cup). The study was approved by the Clinical Trials Review Board of the William R. Pritchard, Veterinary Medical Teaching Hospital University of California, Davis and client consent was obtained.

Horses were treated at the 40, 58, 111, or 160-km mark on the racecourse depending on the location where they were eliminated. All horses designated as disqualified for “metabolic” reasons by a ride veterinarian were eligible for entry into the study. These metabolic conditions included acute abdominal disease (colic), exertional myopathy, and poor cardiovascular recovery, and have been previously described in endurance horses.[21] Horses were excluded from the study if they had been treated with IV fluids before admission to the treatment center. The owner of 1 eligible horse declined enrollment in the study.

Study Design

All personnel were blinded to the treatment group until after the study was completed. After admission to the treatment center, each horse had a routine physical examination performed. Rectal temperature, heart rate, respiratory rate, mucous membrane color, and capillary refill time were recorded. Blood samples were collected from the jugular vein and were placed into evacuated tubes containing potassium EDTA and sodium heparin. One of the tubes containing sodium heparin was centrifuged and the plasma collected and transferred to storage at −70°C within 12 hours.

A 14 gauge IV catheter1 was placed in a jugular vein and fluids were administered by gravity flow through an IV infusion set.2 Each horse was assigned a treatment code with a corresponding IV fluid that was blinded to the personnel administering the fluid and to the treatment veterinarians. A treatment letter was assigned to 1 of the 2 fluid types before the start of the study according to a code produced by a random number generator. Numbers were assigned according to a block randomization design. Horses in the HSS group received an initial bolus of 2 L of 7.2% saline3and 5 L of lactated Ringer's solution (LRS)4 simultaneously followed by additional LRS as deemed necessary by the treating veterinarian. Horses in the ISS group received an initial bolus of 2 L of 0.9% saline5 and 5 L of LRS followed by additional LRS as deemed necessary by the treating veterinarian. The initial 5 L of LRS given to all horses also contained 250 mL of 23% calcium gluconate solution.6

After the initial 7 L bolus of fluids (HSS or ISS + 5 L LRS), physical examinations were repeated and blood samples were collected. The study was terminated when the treating veterinarians determined that the horses no longer required IV fluid administration. This determination was based on the clinical examination findings and serum biochemical results available. Urine samples were collected by voiding when horses were observed to urinate. Urine volume and specific gravity were recorded upon collection. Specific gravity was estimated using refractometry. A sample of the urine was centrifuged and transferred to storage at −70°C within 12 hours. Horses were allowed free access to water during the study, but the amount of oral fluids consumed could not be recorded. Some horses with colic initially were restricted from feed but all other horses had free access to grass hay and alfalfa hay.

Sample Analysis

Biochemistry profiles were analyzed on plasma within 12 hours using a commercial chemistry analyzer.7 PCV was determined using the micro-hematocrit method. Plasma total protein (TP) concentration was measured using refractometery. Whole blood lactate was measured using a commercial lactate analyzer immediately upon collection.8 Frozen plasma and urine samples were analyzed for electrolytes using a commercial chemistry analyzer (Na+, K+, Cl, and ionized Ca++).9 Simplified strong ion difference (SSID) was calculated as: [sodium] – [chloride].

Statistical Analysis

Data are reported as mean (± SD). Data were assessed for normality using the Kolmogorov and Smirnov method. Admission and posttreatment data were analyzed using an unpaired t-test or Mann–Whitney test depending on whether they were normally distributed or not, respectively. Fisher's exact test with calculation of odds ratios was used to evaluate for differences in categorical variables in baseline data between the 2 groups. Clinical and clinicopathological data between treatment groups were compared over time using 2-way ANOVA with repeated measures. A significance level of P < .05 was used.

Results

The baseline data from the 2 groups, including clinical diagnoses, are shown in Table 1 and there were no statistically significant differences. Group sizes were equal with 11 horses in each. The horses in the ISS group received 21.5 ± 5.2 L of fluids during the study whereas the HSS group received 17.9 ± 3.0 L (P = .14). All horses enrolled in the study recovered completely with treatment and none of the horses required referral to an equine hospital or euthanasia. Comparisons after treatment are shown in Table 2 and are separated after receiving 7 L of fluids (2 L of study fluid + 5 L LRS) and then a total of 17 L of fluids (ie, additional 10 L of LRS). There were no significant differences in mucous membrane color or capillary refill time between the 2 groups.

Table 1. Baseline characteristics of horses receiving hypertonic (7.2%) and isotonic (0.9%) saline solution (HSS and ISS) (mean ± SD).
VariableISSHSSP-values
Sample size1111N/A
Age (years)11.3 ± 2.513 ± 3.9.23
Breed7 Arabians
4 Half-Arabians
8 Arabians
1 Half-Arabian
1 Rocky Mountain
1 Unknown
.48
Gender8 Geldings
2 Mares
1 Stallion
5 Geldings
6 Mares
.18
Diagnosis3 Synchronous diaphragmatic flutter
3 Failure to recover
2 Colic
3 Rhabdomyolysis
3 Synchronous diaphragmatic flutter
5 Failure to recover
3 Colic
 
Heart rate (bpm)63 ± 1064 ± 18.93
Temperature (°F)99.7 ± 1.3100.4 ± 1.1.25
PCV (%)49 ± 746 ± 7.31
TPP (g/dL)8.4 ± 1.18.5 ± 0.6.68
Lactate (mmol/L)2.3 ± 0.82.4 ± 0.9.79
Na (mEq/L)135 ± 6137 ± 3.99
K (mEq/L)3.3 ± 0.43.4 ± 0.7.565
Cl (mEq/L)97 ± 799 ± 4.965
Ca++ (mmol/L)1.2 ± 0.31.3 ± 0.1.532
SSID (mEq/L)39 ± 338 ± 3.629
TCO2 (mEq/L)27.1 ± 1.827.5 ± 2.5.64
CK (IU/L)5716 ± 62713528 ± 4103.72
AST (IU/L)791 ± 622590 ± 378.49
BUN (mg/dL)35 ± 1631 ± 5.45
Creatinine (mg/dL)1.6 ± 0.41.6 ± 0.4.92
T Bil (mg/dL)3.2 ± 1.23.1 ± 0.8.89
GGT (IU/L)20.6 ± 10.222.7 ± 5.6.09
Albumin (g/dL)3.4 ± 0.43.5 ± 0.4.67
Globulin (g/dL)4.5 ± 0.74.6 ± 0.5.81
Table 2. Posttreatment characteristics of the horses receiving hypertonic (7.2%) and isotonic (0.9%) saline (HSS and ISS).
 Post-7 L of FluidsPost-17 L of Fluids
VariableISSHSSP-valueISSHSSP-value
  1. a

    Denotes values that are statistically different (P-value listed in table).

  2. Comparisons between fluid types are made at two time points during treatment: (1) after 7 L of administered IV fluids in the first set of three columns and (2) after 17 L of administered IV fluids in the second set of three columns.

Heart rate (bpm)46 ± 645 ± 9.8848 ± 1147 ± 5.32
Temperature (°F)98.9 ± 1.7100.1 ± 0.5.1098.5 ± 1.699.8 ± 0.7.09
PCV (%)40 ± 833 ± 4.01537 ± 634 ± 7.118
TP (g/dL)7.2 ± 0.66.2 ± 0.3.00076.4 ± 0.46.3 ± 0.5.507
Lactate (mmol/L)2.0 ± 0.81.8 ± 0.8.6462.1 ± 0.71.6 ± 0.7.089
Na (mEq/L)136 ± 4143 ± 3.0012135 ± 3139 ± 3.0086
K (mEq/L)2.8 ± 0.63.1 ± 0.4.2853.1 ± 0.43.3 ± 0.2.274
Cl (mEq/L)99 ± 4108 ± 6.0027102 ± 3106 ± 4.0208
SSID (mEq/L)38 ± 535 ± 5.27033 ± 233 ± 2.426
Ca++ (mmol/L)1.6 ± 0.21.6 ± 0.2.6921.3 ± 0.11.3 ± 0.1.944
TCO2 (mEq/L)26.8 ± 2.524.9 ± 2.0.0725.5 ± 2.224.5 ± 4.0.99
CK (IU/L)5487 ± 67112789 ± 4391.7625233 ± 67762588 ± 4411.762
AST (IU/L)859 ± 743422 ± 263.115713 ± 604432 ± 282.181
BUN (mg/dL)34 ± 1328 ± 4.19434 ± 1226 ± 4.067
Creatinine (mg/dL)1.4 ± 0.31.3 ± 0.3.4621.4 ± 0.41.2 ± 0.3.133
T Bil (mg/dL)2.7 ± 1.12.3 ± 0.6.2932.7 ± 1.02.2 ± 0.6.268
GGT (IU/L)16 ± 716 ± 5.75116 ± 717 ± 8.916
Albumin (g/dL)2.9 ± 0.32.5 ± 0.2.00652.8 ± 0.52.6 ± 0.2.777
Globulin (g/dL)3.9 ± 0.53.3 ± 0.2.023.7 ± 0.53.4 ± 0.3.14

Two-way ANOVA with repeated measures revealed statistically significant time-treatment interactions with decreases in PCV (P = .04), TP (P = .01), globulin (P = .02), and albumin (P = .02) concentrations in the HSS group as compared to the ISS group after the initial bolus. The plasma sodium and chloride concentrations showed statistically significant time-treatment interactions and main treatment effect, with statistically significant increases in sodium and chloride in the HSS group (P < .001 after the initial bolus, and P < .01 after the 17 L fluid administration (2 L HS or ISS + 10 L LRS).

Electrolyte results from urine samples are shown in Table 3. No significant abnormalities were detected between the groups in urine electrolyte concentrations. The time to first urination was shorter for horses in the HSS group (69 ± 36 minutes) than for horses in the ISS group (128 ± 66 minutes) and was statistically significant (P = .03). The specific gravity from the first urination sample in the ISS group was 1.046 ± .007, which was significantly higher than the specific gravity of 1.026 ± .012 observed in the first urination of the HSS group (P = .0007).

Table 3. Urine electrolyte values comparing horses receiving hypertonic saline (9.2%) solution and isotonic (0.9%) saline solution (HSS and ISS).
Urine electrolyteHSSISSP-value
  1. Urine samples are from the first urination after the start of treatment.

Na (mEq/L)99 ± 3762 ± 54.130
K (mEq/L)50 ± 5260 ± 44.678
Cl (mEq/L)148 ± 57210 ± 122.227
Ca++ (mmol/L)3.5 ± 2.52.9 ± 1.6.581
SSID (mEq/L)−49 ± 56−137 ± 173.198

Discussion

The greater decrease in both PCV and TP observed in the HSS group as compared to the ISS group in this clinical trial is consistent with previous experimental studies.[17] This observation supports a greater expansion of plasma volume by HSS than ISS. Changes in hemoglobin concentration have been used to estimate changes in plasma volume and appear to be inversely related, although this relationship may be less reliable in horses.[22, 23] The change in TPP has been used as an assessment of the change in the size of the plasma volume assuming no change in the overall amount of protein.[24-26] Based on this assumption, the plasma volume may have expanded as much as 29.1 ± 4.0% in the HSS group. A similar calculation for the ISS group revealed a change of only 12.0 ± 14.6% which was significantly smaller (P < .0001). Treatment with HSS may increase the plasma volume more than twice as much as ISS.

There are 2 likely mechanisms for this larger expansion. First, the HSS administration may have osmotically drawn fluid from the interstitial space, intracellular fluid space or both into the vascular space.[1, 7] This would result in dilution of the PCV and TP as observed. The other possibility is that the increase in plasma sodium concentration created by the HSS induced a greater thirst response.[27] These horses may have consumed more water during the initial period which would have allowed a more rapid expansion of the vascular volume. From a clinical standpoint, both mechanisms likely contributed to the observed results and either would result in a larger expansion of the plasma volume which is likely to be beneficial in dehydrated and hypovolemic endurance horses.

Estimations of changes in the extracellular fluid volume (ECFV) in the horses of this study suggest a greater increase with the administration of HSS. The previously described sodium dilution principle can be used to give an estimated range of the change in ECFV that may have occurred in these horses.[28] Assuming an ECFV of between 80 and 120 L before treatment, horses in the HSS group would have experienced an estimated expansion of between 15.9 and 17.9 L using previous studies for approximate weights of endurance horses and approximate size of the equine ECFV.[29, 30] Conversely, horses in the ISS group would have an estimated expansion of only 7.5–7.7 L which is approximately the amount of fluid administered. A comparison anywhere in the physiologic range of ECFV would yield a highly significant difference between expansion with HSS and ISS (P < .0001). These data support an estimated increase in ECFV of approximately 8–10 L beyond the volume of fluid administered IV. These are similar to the results observed when TP is used to estimate changes in plasma volume. Calculations using the sodium dilution principle are subject to a number of limitations and assumptions as have been previously discussed.[28] A more rigorous study using an exogenously administered dilution indicator would be needed to confirm these estimated results. However, if the expansion of the ECFV was a result of contraction of the ICFV, replacement of this loss is likely warranted with additional IV or PO fluids.

The HSS group had a shorter time to urination; increases in urinary frequency have been previously observed with HSS in horses.[17] The present study also indicated significantly more dilute urine after HSS administration. These observations also are consistent with a more rapid increase in the plasma volume. The increased sodium load likely also contributed to diuresis.[7] Faster onset to urine production could indicate improved renal perfusion. This could be of potential benefit in this group of horses, as it might provide some degree of protection from renal failure associated with dehydration, rhabdomyolysis, or both.[31]

The present study did not specifically evaluate the need for additional fluids after HSS administration. However, the clinical experience of the authors suggests that it is extremely important that HSS administration is followed by either isotonic or hypotonic IV or PO fluids. This same recommendation has been made by other authors.[32, 33]

The groups appear to have been randomized well based on the analysis of the admission variables studied. However, one important limitation was the uneven distribution of horses with exertional rhabdomyolysis. All 3 of the horses with this condition were randomized to the ISS group. In a previous study, these horses did not have significantly different clinical signs and electrolyte changes as compared to other endurance horses with different conditions.[21] Some authors have recommended alkalinization of urine in human patients with rhabdomyolysis and associated renal failure.[31] In a previous study in horses, the administration of sodium chloride-rich fluids produced a metabolic acidosis but did not evaluate urinary changes in electrolytes attributable to HSS administration.[14] The present study did not evaluate the effects of HSS on acid-base status or the appropriateness of fluids in horses with exertional rhabdomyolysis specifically.

One of the major limitations of this study was sample size. However, we were limited by the number of endurance horses requiring emergency treatment. A study that was performed at multiple endurance competitions (ie, larger study population) would have improved the statistical power and allowed these results to be applied to a larger population.

An additional limitation of the study was our inability to quantify the amount of water consumed and urine produced by each horse. Horses entered into the study were treated in a race setting where multiple water sources were available. Many of the horses would only drink out of larger water troughs and we did not believe that it was ethically appropriate to restrict water intake for the purposes of the study. In addition, a urinary collection system could not be placed on these client-owned horses and therefore total urine output could not be easily quantified.

Based on the results of this study, the administration of approximately 4 mL/kg of 7.2% HSS appears to be safe in the treatment of horses disqualified for dehydration, metabolic problems or both encountered in endurance horses. When the fluid is followed by the administration of LRS, it appears to expand plasma volume, and ECFV to a greater degree than 0.9% saline solution.

Footnotes

  1. 1

    BD Angiocath, BD, Sandy, UT

  2. 2

    IV 1000 STAT, International WIN, Kennett Square, PA

  3. 3

    Hypertonic saline 7.2%; VETone, Meridian, ID

  4. 4

    Lactated Ringer's; Baxter, Deerfield, IL

  5. 5

    0.9% sodium chloride, Baxter

  6. 6

    23% Calcium gluconate; VEDCO, St. Joseph, MO

  7. 7

    Vet Scan VS2; Abaxis, Union City, CA

  8. 8

    Accutrend Lactate Analyzer; Roche Diagnostics, Indianapolis, IN

  9. 9

    ABL 805, Radiometer Medical ApS, Bronshoj, Denmark

Acknowledgments

This study was supported in part by the Western States Trail Foundation and by Abaxis.

Ancillary