Fluid therapy challenges (Proceedings)
In order to appropriately administer fluid therapy, you must answer the following five questions:
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.