Fluid therapy: Choosing the best solution for each patient - Veterinary Medicine
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Fluid therapy: Choosing the best solution for each patient
Which intravenous solution is best in a patient with metabolic acidosis and a low sodium concentration? In a patient experiencing hypercoagulability and thrombosis? This emergency clinician helps you select the right fluid for each patient and discusses an efficient way to help hypoalbuminemic patients.



Table 1 Estimating the Percentage of Dehydration
The percentage of dehydration can be subjectively estimated based on the presence and degree of loss of body weight, mucous membrane dryness, decreased skin turgor, sunken eyes, and altered mentation (Table 1).1 These parameters are largely subjective because they can also be affected by decreased body fat and increased age.

The more severe stages of dehydration are also accompanied by signs of hypovolemic shock. Other factors, including hemorrhage and third spacing of body fluids, can also result in a decrease in intravascular circulating volume, resulting in signs of hypovolemia. Severe hypovolemia resulting in more than a 15% depletion of effective circulating volume leads to a transcompartmental fluid shift from the interstitial to the intravascular compartments, which occurs within one hour of fluid loss.6 When fluid loss is so severe that intravascular fluid volume is affected, hypovolemia can result in tachycardia, prolonged capillary refill time, decreased urine output, and hypotension.

The vascular space is sensitive to changes in the amount of circulating volume. During states of normovolemia, baroreceptors in the carotid body and aortic arch sense vascular wall tension and send pulsatile continuous feedback via vagal afferent stimuli to decrease heart rate. In the early stages of hypovolemic shock, the baroreceptors sense a decrease in vascular wall stretch or tension and blunt the tonic vagal stimulation. This allows sympathetic tone to increase heart rate and contractility in an attempt to normalize cardiac output. Later, decreased blood flow and sodium delivery to receptors in the juxtaglomerular apparatus activate the renin-angiotensin-aldosterone axis, stimulating sodium and fluid retention to replenish intravascular volume.1-3


When clinical signs of hypovolemic shock are present, intravascular fluids must be replaced immediately. Calculated fluid volumes for patients in shock are 90 ml/kg for dogs and 44 ml/kg for cats.1 A simple guideline to follow is to replace one-fourth of the calculated fluid volume as rapidly as possible and then reassess perfusion parameters including heart rate, blood pressure, capillary refill time, and urine output. In dogs, a simple method to calculate one-fourth of the fluid volume for treating shock is to take an animal's weight in pounds and add a zero, giving you the amount of fluid in milliliters to administer as a bolus.

About 80% of the volume of crystalloid fluid infused will re-equilibrate and leave the intravascular space within one hour of administration. A constant-rate infusion of a crystalloid fluid is recommended to provide continuous fluid support in patients that are dehydrated and have ongoing losses. In some cases, the fluid required to restore intravascular and interstitial volume can cause hemodilution and dilution of oncotically active plasma proteins, resulting in interstitial edema formation. In such cases, a combination of a crystalloid fluid along with a colloid-containing fluid can help restore oncotic pressure and prevent interstitial edema.6

Once immediate life-threatening fluid deficits are replaced, provide additional fluid based on the estimated percentage of dehydration and maintenance needs. Basic dehydration estimates can be calculated based on the fact that 1 ml water weighs about 1 g and by using the following formula:

Body weightkg estimated percent dehydration 1,000 ml/L

This formula helps you determine the amount of fluid deficit in liters. A frequent mistake when replenishing fluid deficits is to arbitrarily multiply a patient's daily water requirement by a factor of two or three to replenish intravascular and interstitial deficits. This practice frequently underestimates a patient's fluid needs and does little to treat volume depletion and interstitial dehydration. Instead, it is better to use the formula above and add the result to daily maintenance fluid requirements and ongoing losses.


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