Dehydration - Correcting fluid imbalnces

DEHYDRATION : CORRECTING FLUID IMBALANCES

CORRECTING FLUID IMBALANCES

  • When there are clinical signs of hypovolemic shock, 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. A simple guideline to follow in day to day practice 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.
  • 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.
  • 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 weight in kg × estimated percent dehydration × 1,000 ml/L
  • This formula helps to 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.
  • Eighty percent of the calculated fluid deficit can be replaced in the first 24 hours. More rapid administration of an animal's estimated fluid deficit can result in diuresis and loss of the fluid administered. After successfully treating hypovolemic shock and replacing fluid deficits estimated based on the percentage of dehydration, we need to administer only maintenance fluids until the animal can maintain hydration on its own, provided no signs of dehydration or ongoing excessive fluid losses are present. An objective way to assess whether the fluid volume is adequate is to evaluate body weight regularly throughout the day. Acute weight loss is commonly associated with fluid loss and can be used to determine whether the patient is at risk of becoming dehydrated again.
  • Vomiting results in loss of H2O, H+, Cl-, Na+, K+, and HCO3-. If vomit is primarily stomach contents, 1o loss is HCl, H2O.
  • Most vomit includes proximal duodenal contents, therefore HCO3- also lost.
  • Conclusion: H2O is consistently lost in vomiting, other electrolytes/acid base are best assayed.
  • Diarrhea results in loss of H2O and electrolytes, resulting in dehydration, electrolyte depletion/imbalance, acid-base imbalance, and shock.
  • Intestinal contents are basically ECF; also can lose large amounts of K+.
  • Fluid losses from diarrhea can be particularly severe in the cow and horse (salmonellosis, neonatal calf diarrhea).
  • The primary acid-base disturbance is metabolic acidosis.

Plasma osmolality

  • Ratio of body solute to body water.

Effective circulating volume

  • Part ECF in the vascular space Depends on SNS, angiotensin II, and renal sodium excretion.
  • Regulates by increasing vasoconstriction, and renal sodium resorption (RATS).
  • Hypovolemia causes activation of RATS. If < 5%, PE is normal. If 5%, dry mm but no panting. If 7%, decreased skin turgor, dry mm, mild tachycardia.
  • If 10%, dec skin turgor, tachycardia, dry mm, dec pulse pressure.
  • If > 12%, marked loss of skin turgor, dry mm, shock.
  • In mild dehydration, s/c route (isotonic fluids, max. 5 to 10 ml/lb at each injection site).
  • Need multiple sites. I/p route is quick, easy but can cause dyspnea. IV route indicated with dehydration < 7%.

Amount of fluid

  • The deficit volume - only 75% to 80% of the deficit should be replaced during the first 24 hours, as it can worsen dehydration.
  • Total Deficit Replacement Volume (24 hrs) = Deficit Volume(% dehydration x body weight (lb/kg) x 454/1000 x 0.80) + Maint. Volume

Maintenance volumes

  • 2/3 sensible (urine and feces) and 1/3 insensible (panting or sweating). (30 X BWKg) + 70.
  • A 22-lb (10 kg) dog, 7% dehydrated will need - Volume (ml) required = deficit volume + maintenance volume= [0.07 x 22 lb x 454 x 0.80] + [(10 x 30) + 70]= [560] + [370] = 930 ml

Continuing losses during the replacement

  • Estimate the volume of fluid loss and then double this estimate.
  • How to know if animal is receiving an inadequate fluid volume.
  • If the animal is losing body weight while being given crystalloid fluids, the animal is likely receiving inadequate volumes of fluid.
  • One group of patients where body weight may fool you is in animals that are third-spacing fluids (peritonitis, pyometritis, pleural effusions).
  • In these animals the animal may still be dehydrated but the body weight may not have changed.
  • Additionally, if renal function is adequate, an animal which is dehydrated will have a urine specific gravity above 1.025.

Clinical signs of overhydration

  • Increased serous nasal discharge, followed by chemosis, and finally pulmonary congestion will be ausculated before edema ensues.
  • Clinically, pulmonary edema is the terminal event of overhydration!
    • non-respiratory acidosis (HCO3-). may result from: excess ingestion of H+, decreased elimination of H+ (renal), increased production of H+ (anaerobic metabolism), or increased elimination of HCO3- . It is the most common acid-base disturbance in dogs, cats, and horses.
    • non-respiratory alkalosis ( HCO3-) excess ingestion of HCO3-, excess admin. of HCO3-, excess loss of H+ (vomiting), or sequestration of H+ (functional 3rd space loss). This acid-base disturbance is common in the cow (displaced abomasum).
    • respiratory acidosis (CO2) hypoventilation. This is common in the anesthetized horse.
    • respiratory alkalosis (CO2). hyperventilation, pain, excitement, and artificial ventilation that is excessive.
  • Shock therapy with crystalloid fluid: (no head trauma or pulmonary edema) - Dog – 90 mL/kg/hour. Cat – 60 mL/kg/hour
  • Blood transfusion (PCV < 20%): 20 ml/kg fresh whole blood. 15-30 ml/kg Oxyglobin.
  • Shock therapy for head trauma or pulmonary contusions: Hypertonic saline + Hetastarch or dextran. Total dose = 5 ml/kg. Draw up 1/3 volume as 23% saline, 2/3 as colloid.
  • Small volume resuscitation: 5ml/kg IV hetastarch or dextran. Repeat every 5-10 minutes until HR, pulses and color improves.
  • Crystalloids: Run very fast. Doesn’t stay in vascular space, so need to give 3-4 times what they have lost.
  • Avoid in animals w/ interstitial edema (head trauma, pulmonary contusions, hypoproteinemia).
  • RL - Buffered pH of about 7.4 which is good for acidosis, The lactate is converted to bicarb for acidosis, Lactate is metbolized in the liver and has calcium.
  • Avoid in cows (alkalosis). Normalsol R - Buffered pH of about 7.4 which is good for acidosis, The acetate in normalsol R is converted to bicarb for acidosis.
  • Colloids: Help in retention of fluid in the vascular space. Increases oncotic pressure b/c are not filtered in the glomerulus. Give smaller volume to restore circulation.

Indications

  • Hypoproteinemia, 3rd space loss, Head trauma, pulmonary edema, leaky capillaries, SIRS.
  • Eg. Hetastarch, Dextran 70, Whole blood – if PCV drops below 20% and TP < 3.5 (1 mL of blood per pound will raise hematocrit 1%) then give 20 mL/kg,

Oxyglobin

  • Same as above at a rate of 15-30 ml/kg, Whole plasma
  • If there is evidence of ongoing blood loss into the abdominal cavity, a snug compressive bandage should be applied, being careful not to impair respiration.
Last modified: Monday, 28 May 2012, 6:33 AM