Fluid therapy challenges (Proceedings)

In order to appropriately administer fluid therapy, you must answer the following five questions:

1. Is the need emergent (ie. Are we treating shock or dehydration?)

2. What kind of fluid should be given?

3. What route should the fluid be given?

4. How much fluid does the animal require?

5. What is the frequency/rate of administration?

Is the need emergent?

Hypovolemia and dehydration are not the same. Hypovolemia refers to a decrease in effective circulating blood volume resulting in circulatory shock. Decreased vascular volume leads to decreased venous return of blood to the heart. Consequently, stroke volume is decreased, cardiac output is decreased, and tissue delivery of oxygen suffers. This represents an emergency situation, and rapid replacement of extracellular fluid volume through intravenous fluid bolusing is required. Situations that lead to an acute or rapid fluid loss include acute blood loss, severe vomiting and diarrhea, heatstroke, and gastric dilatation-volvulus ("bloat").

In contrast, dehydration is defined as a decrease in total body water. Dehydration usually develops over a period of time, and the body is consequently able to preserve vascular volume by shifting fluid from the intracellular space to the vascular space. Thus blood pressure is typically preserved, and vital signs are relatively normal. In contrast to hypovolemia, significant dehydration cannot be corrected rapidly. If vascular volume is normal and a rapid bolus of intravenous fluids is administered, mechanisms are activated to increase urine production resulting in a loss of some of the administered fluids. Therefore, dehydration is typically corrected over several hours time.

The first goal of fluid therapy then is to assess whether hypovolemia or dehydration is present. Heart rate is one of the easiest tools used for identification of hypovolemia. Because cardiac output is a product of heart rate and stroke volume, when circulating blood volume is decreased, the body compensates by increasing heart rate to maintain cardiac output. Normal heart rate in the dog is approximately 60-120 beats per minute. Elevation in resting heart rate above this range should be considered a possible warning of shock. Other parameters that may be useful in assessment of hypovolemia include pale mucous membrane color, prolongation of capillary refill time, decreased level of consciousness, weak or "bounding" pulse quality, and decreased body temperature. If more advanced diagnostic modalities are available, measurement of arterial and central venous blood pressures, urine output, and venous lactate levels may also be helpful.

The degree of dehydration is best assessed from the physical exam. Skin turgor (pinching a fold of skin and assessing how long it takes to return to its normal position), and mucous membrane assessment are the clinical standards. The clinical range of chronic fluid deficit (dehydration) is 5-15% of body weight. Dehydration <5% is not detected on physical exam. When dehydration is 12–15%, shock is marked and death is imminent. You will detect dehydration as mild (6-7%, loss of skin pliability, dryness of mouth), moderate (8-9%, also: depression of eyes, mental depression), or severe (10-12%, very dry mm, complete loss of skin turgor, eyes sunken and dull, shock, altered mentation/loss of consciousness). Exactly estimating percent dehydration from the physical exam is an inexact science, and re-evaluation and re-assessment of therapy is indicated.

There are other methods to evaluate the degree of dehydration. Theoretically, body weight is the most exact/correct. Acute loss of 1 kg body weight indicates a fluid loss of 1 liter. Unfortunately, pre–dehydration body weight is seldom known. In the hospital setting, daily weighing of patients may be helpful in monitoring fluid therapy. In the absence of renal disease, urine output is also an excellent indicator of fluid volume, dehydration, and adequacy of fluid resuscitation. Dehydration results in increased specific gravity (>1.040) and decreased urine output if renal function is normal. Rehydration will result in lowering of the specific gravity towards 1.010 and increasing urine output towards normal (1-2 ml/kg/hr). Excessive fluid load will result in excessive urine output.

What kind of fluids should be given?

The choice of fluid is based on three factors: a) knowledge of the disease process b) laboratory data (e.g. electrolyte or acid-base imbalances, hypoproteinemia), and c) purpose of fluids (i.e. replacement or maintenance). For most purposes, a crystalloid is sufficient, and 0.9% sodium chloride, lactated Ringer's, Normosol-R, and others may all be used interchangeably in most situations. Severe hyperkalemia, hypercalcemia, hyponatremia, or hypochloremia may dictate a need for sodium chloride specifically.

Other fluid types used in special circumstances include hypertonic saline, hypotonic fluids, dextrose-containing fluids, and colloids (hetastarch, albumin). Hypertonic saline is used most commonly for small volume fluid resuscitation. The elevated sodium concentration draws water out of the cell to rapidly expand the vascular volume. Its main advantage is that the patient can be resuscitated with less volume, quicker. This is useful in shock states where vascular access is limited, or where excessive shifting of fluids into the interstitial and intracellular spaces could be deleterious (eg head trauma). This may also be of importance in field use (don't have to carry as much volume).

Hypotonic fluids such as 0.45% saline, normosol-M, or plasmalyte 56 are typically used to replace free water deficits (for example, a severely hypernatremic patient) or to meet maintenance needs in chronically hospitalized animals. They may also be used in patients not likely to tolerate high sodium loads, including patients with severe heart or liver disease. These fluids should not be administered subcutaneously. They are also not meant for rapid IV administration, as red cell lysis may result from the hypotonicity.

Colloids (hetastarch, dextrans, or albumin) are solutions containing relatively large molecules that do not pass readily across capillary membranes. The presence of these molecules has the effect of retaining water within the vascular space for long periods of time, unlike crystalloid fluids that equilibrate quickly to the interstitial and intracellular spaces. Colloids are therefore useful in low volume fluid resuscitation as well as in patients with hypooncotic states resulting from decreased blood protein levels.

What route of administration should be used?

If an animal is assessed as having urgent need of fluids due to hypovolemia, intravenous fluid administration is required as this is the only way that rapid replacement of vascular volume can be effected. Intravenous fluids should also be considered in cases of moderate to severe dehydration where the need for ongoing fluid therapy is expected, or when poor tolerance of oral fluids is likely due to vomiting. If dehydration is only mild to moderate, or if fluid therapy is being provided as a means for pre-hydration or prevention of dehydration in an at-risk animal, then oral or subcutaneous routes may be considered.

How much fluid?

If the animal is assessed as hypovolemic, a shock rate of fluid should be administered. The shock dose of fluids may be calculated as 90 ml/kg/hr in dogs and 66 ml/kg/hr in cats. Not all animals in shock will require this amount of fluids so frequent reassessment is needed. Typically, one-quarter to one-third the calculated volume is bolused over 10-15 minutes and vital signs (HR, CRT, pulse quality, level of consciousness) are then reassessed. Fluid administration should continue until normalization of vital signs is achieved.

For treatment of dehydration, the total amount of fluid to give is equal to the deficit (estimated percent dehydration x body weight in kg), plus maintenance (48-60 ml/kg/day) plus any ongoing losses (estimate). For example, a 40 kg German Shepherd assessed as being moderately dehydrated (ie 8%) would get its deficit (0.08 x 40 kg= 3.2 liters) plus maintenance needs (60 ml/kg x 40 kg= 2.4 L) plus any ongoing losses assessed, for a total of approximately 6 L over the next 24 hours. The calculated total quantity of fluids should be administered, monitoring variables (physical exam, urine output, weight) serially to assess progress in resolving dehydration.

What is the frequency/rate of administration?

The rate or frequency of administration will be dictated by the need of the patient. If there is an urgent need, as in hypovolemic shock, then fluids should be given rapidly (20-90 ml/kg within the first hour as described above) to expand the vascular volume to normal and restore hemodynamic stability. Once vitals have returned to normal, remaining fluid deficits may be replaced more gradually over 24-36 hours.

In contrast, if fluid therapy is being administered because of dehydration, the patient will benefit from a more gradual fluid administration. Mild dehydration may be treated with subcutaneous fluids administered 1-2 times per day. Moderate to severe dehydration should be addressed by calculating the fluid requirements as described above and replacing this volume intravenously over 24-36 hours. Using our previous example, 6L ÷ 24 hours= 250 ml/hr. Alternatively, some clinicians will give half the calculated deficit over the first 6 hours and the remainder over the following 18 hours.

Challenging scenarios

Anuric renal failure

Any time a patient is suspected of having acute renal failure, getting an accurate assessment of urine production is crucial to case management. For this reason, placement of a urinary catheter to allow quantitation of urine output is strongly recommended. Normal urine output in the dog or cats is generally 1-2 ml/kg/hour. Oliguria is defined as the production of a smaller than normal volume of urine (i.e. less than 1 ml/kg/hour) and anuria is defined as the absence of urine production. The development of oliguria should always be considered to be of pre–renal cause until proven otherwise. For this reason, adequate volume resuscitation should be ensured before diuretics are started as described below.

A plan for management of the oliguric or anuric patient should include

1. Identify treatable diseases or underlying cause. Discontinue nephrotoxins (eg. Aminoglycosides).

2. Assess volume status

     • electrolytes, acid-base status, PCV/TS, CVP, urine output, blood pressure.

3. 3. Treat severe hyperkalemia

4. 4. Replace volume deficits (4 - 6 hrs): body weight (kg) X estimated % dehydration

     • or give fluid boluses of 20 ml/kg IV until urine production is noted or evidence of slight volume overload is present

5. Re-assess urine output.

     • Output < 1-2ml/kg/hr – Give more fluids if not overhydrated. Overhydration may be confirmed through a combination of physical exam findings, evidence of excessive weight gain, and/or CVP >8 cm H2O. If overhydrated, see #6 below.

     • Output > 1-2ml/kg/hr adequate. Resume fluid therapy at a rate that will account for maintenance needs, estimated deficit, and any ongoing losses.

6. If CVP >8 and/or excessive increase in body weight but urine output is still inadequate, diuretics may be added.

     • Lasix: 2-4mg/kg IV bolus q4-6 hours, or continuous rate of infusion (CRI) 0.1-1mg/kg/hr

     • Can also use mannitol 0.25-0.5 g/kg IV bolus followed by 1 mg/kg/min CRI

7. Once urine output is restored in a previously anuric patient, maintain the animal in a slight state of overhydration. This may be accomplished early on by matching the rate of fluid administration to the rate of fluid loss (urine output).

8. Once the animal has become polyuric (urine output >2 ml/kg/hour) discontinue diuretics. Fluid rates may then be gradually decreased. Fluid therapy is ideally continued until renal values normalize or plateau.

Cardiac disease

When dealing with heart failure patients, fluid therapy is generally best avoided unless clearly needed. It is not uncommon for patients to develop mild azotemia following initiation of diuretic and vasodilator therapy. Often the best answer in these situations is to decrease the dose of diuretic and provide a bowl of water. Trying to juggle simultaneous parenteral fluid and diuretic administration is usually more complicated than the situation requires. For animals that are reluctant to drink due to their illness, a nasoesophageal tube is a useful way to meet free water needs.

When a patient with significant cardiac disease does require fluid therapy due to decreased intake and ongoing losses, caution must be taken to avoid excessive sodium load. Patients with heart disease (failure) are prone to sodium and water retention, so fluids with lower sodium concentration (0.45% NaCl, plasmalyte 56) are frequently selected unless hyponatremia or hypovolemia are present. Colloids should be avoided in heart failure cases. Fluid therapy should be titrated to effect. Once it is determined that a true need for fluid therapy exists, arbitrarily picking a "1/2 maintenance rate" is not likely to provide for the animal's deficit and ongoing losses. Careful monitoring of body weight, urine output, CVP, PCV/TS, Azo, and electrolytes during fluid therapy can help with decisions related to ongoing fluid and cardiac therapy.

Pulmonary contusion

Following pulmonary contusion, excessive administration of fluids has been shown to result in increases in lung water in dogs, potentially worsening oxygen exchange. However, concurrent traumatic injury and hypovolemia frequently necessitate aggressive fluid resuscitation. Fluid therapy should not be withheld in these patients. Results of studies comparing colloids, crystalloids, and hypertonic saline do not show a clear benefit to any particular fluid type. Current recommendations for fluid therapy in patients with pulmonary contusion are to provide crystalloids or colloids as needed to maintain adequate tissue perfusion. Monitoring of vital signs, blood gases, lactate, and central venous pressure may be helpful in assessing adequacy of resuscitation. Once adequately resuscitated, excessive fluid therapy should be avoided.

Traumatic brain injury

Periods of hypoxemia or hypotension have been implicated as predictors of poor neurologic outcome in human patients that have suffered head trauma. Fluid therapy should therefore never be restricted in the head trauma patient. Options for fluid resuscitation of the head trauma patient include:

Isotonic Crystalloid Fluids: Isotonic crystalloid solutions (Normosol-R, 0.9% Saline) are reasonable resuscitation fluids for the patient that has sustained TBI. Only that volume necessary to restore euvolemia, provide maintenance, and balance out ongoing losses should be administered. Under-resuscitation should be avoided. Similarly, over-resuscitation may predispose to worsening cellular edema and increased intracranial pressure and should also be avoided.

Hypertonic Saline: Hypertonic saline resuscitation has the advantages of smaller volume resuscitation, rapid restoration of intravascular volume, improved contractility, and its osmotic effect at the level of the brain decreasing intracranial pressure. There is some evidence that hypertonic saline may disrupt the blood brain barrier and thus nullify some of its beneficial effects. Hypertonic saline may be administered at a dose of 4ml/kg of a 7.5% solution by slow IV infusion. Diligent monitoring of serum sodium concentration is critical after administration especially with concurrent administration of other hypertonics such as mannitol. The effects of hypertonic saline may be short lived, but may be extended by concurrent administration of a colloidal solution (Hetastarch or Dextran). Concurrent isotonic crystalloid administration for maintenance purposes and provision for ongoing losses will be required.

Colloidal Solutions: Colloid solutions are considered by some to be the resuscitation fluid of choice in the patient that has sustained TBI. Benefits include a smaller volume necessary for resuscitation (when compared to crystalloid solutions), persistence in the vascular system (long half-life) and thus support of CPP, potential for minimizing vascular leakage, and prolonging the intravascular effects of hypertonic solutions such as mannitol and hypertonic saline. Hetastarch (6%) (5-10ml/kg IV) is probably the most readily available colloid for resuscitation of the canine patient with TBI. In cats, the bolus (2.5-5ml/Kg) must be given over 20-30minutes.

Blood Products: Blood products are a very desirable resuscitation fluid in the patient with concurrent injuries resulting in hemorrhage and hypovolemia. Packed red blood cells, fresh whole blood, and fresh frozen plasma are all acceptable options. The use of hemoglobin based oxygen carriers (HBOCs) in head trauma requires further investigation.

Conclusion

Numerous clinical scenarios pose unique challenges when administering fluid therapy. It should be remembered that in all cases, fluid therapy is a dynamic process that must be tailored to the needs of the individual. Adjustment of fluid rates and volumes based upon frequent reassessment of vital signs and hydration status is the key to providing maximum benefit with a minimum of adverse effects.