Fluid Therapy What, when and how much? Michael Ethier DVM, DVSc, DACVECC Toronto Veterinary Emergency Hospital

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1 Fluid Therapy What, when and how much? Michael Ethier DVM, DVSc, DACVECC Toronto Veterinary Emergency Hospital Intravenous fluids are one of the most common therapies administered to hospitalized patients, yet remain one of the most misused. One of the first decisions a veterinarian must make when faced with an ill patient is whether or not IV fluid therapy is required. Once the decision is made to hospitalize for IV support, a carefully formulated fluid plan is essential. The goals of the following paragraphs are to review the basic understanding of fluid physiology and treatment options in order to facilitate proper fluid administration for common clinical scenarios. Fluid Physiology Approximately 60% of a patient's body weight is composed of water. Approximately, 40% of the volume of total body water is within the intracellular compartment (ICS), with the remaining 20% found within the extracellular space (ECS). The ECS can be further divided into the interstitial space (ITS-15%) and the intravascular space (IVS-5%). Although the IVS is the smallest of the fluid compartments, it plays a vital role in organ perfusion and oxygen delivery to tissues. Water shifts across the three compartments (e.g. ICS, ITS and IVS) through osmosis. Osmosis is the movement of water across a semipermeable membrane due to differences in solute concentration. Until equilibrium is established, water will shift from a low solute concentration to a high solute concentration. Sodium and glucose are the two most important effective osmoles within the body. Sodium is found in abundance within the ECS and is considered the primary osmole contributing to water shifts. It is only during periods of hyperglycemia that glucose contributes clinically to water movement between compartments. The osmolality of a solution is a measure of the number of osmotically active particles within a particular space. Effective Osmolality = 2 (Na+K+) + glucose A second force contributes to fluid movement between the IVS and ITS, called Starling's Forces. Net filtration = Kf [(Pc Pi) (π c -π i )] The principles of Starling's Forces dictate that the volume of the IVS is due to the balance between the forces favoring filtration of fluid through the vascular endothelium (capillary hydrostatic pressure and tissue oncotic pressure) and forces favoring reabsorption of fluid within the IVS (plasma oncotic pressure and tissue hydrostatic pressure). Oncotic pressure is generated by the concentration of plasma proteins, mainly albumin. Although hydrostatic pressure is generated by the heart, autoregulation at the levels of the capillaries (e.g. precapillary sphincter) limits the systemic effects of blood pressure on fluid translocation. Fluid losses On a daily basis, the body's homeostatic mechanisms attempt to maintain a zero fluid balance by ensuring total fluid intake equals total fluid output. The volume of water required daily to maintain this zero fluid balance is referred to as an animal s maintenance fluids. Water loss occurs through both sensible losses (e.g. urine, feces, saliva) and insensible losses (e.g. evaporation from skin and lung epithelium). Sensible water losses are generally regarded as losses that can be easily quantified and therefore, in clinical veterinary medicine it refers to urine volume only. Maintenance Fluid Needs = Sensible + Insensible Losses

2 Daily maintenance fluid needs for healthy, adult animals are defined as 40-60mL/kg/day ( ml/kg/hr), with larger animals requiring less than smaller animals. Unfortunately, a thorough understanding of a patient s daily renal solute load (affected by diet), degree of respiratory effort, ambient temperature and humidity, activity level and underlying disease would be required daily to truly maintain zero balance. In fact, an inactive sick animal with decreased to no food intake may require less water than defined maintenance needs. Although, water balance is clearly more complex with no single maintenance water rate applicable to all patients, it remains reasonable to use standard rates as a starting point. In addition to daily maintenance losses, veterinary patients often have increased losses due to underlying illnesses. Increased fluid loss usually occurs due to renal disease (e.g. polyuria), GI disease (e.g. diarrhea or vomiting), and/or increased evaporative losses (e.g. pyrexia, respiratory disease, exudative skin lesions). These increased losses must either be quantified or estimated and an appropriate fluid volume added to the patients maintenance needs to ensure correction. Aside from anorexia and inactivity, oliguric or anuric patients usually truly have lower than maintenance needs due to reduced obligate water loss via urine. Continued delivery of maintenance rates will likely result in fluid overload if close monitoring is not provided. Increased fluid loss can also occur secondary to blood loss, resulting in a decrease in intravascular volume. Recognition and correction of hypovolemia is essential for establishing adequate tissue perfusion. Fluid Plan Any patient with a history of increase fluid loss, whether from water and/or electrolytes loss or blood loss, should have an appropriate fluid plan in place. A fluid plan consists of a thorough assessment of the patient s intravascular volume and hydration status with the intent of calculating an estimated total fluid deficit. The following is a the minimum database recommended to properly determine a patient's fluid plan Physical examination Body weight Blood pressure PCV/TP Electrolytes Urine specific gravity BUN/Creatinine +/- blood gas & lactate The calculation of the patient's total fluid deficit is crucial to avoid the tendency to administer IV fluids in multiples of maintenance. A comment statement is to place the patient on twice maintenance when in fact calculation of the actual fluid deficit is almost always significantly greater. Prior to calculation of any deficit, the 1 st step is to correct hypotension. Hypotension can arise from either 1) severe dehydration leading to intravacular volume depletion, 2) 3 rd spacing for total body water resulting in ineffective circulating volume and 3) blood loss. Regardless of the cause, intravenous bolus administration should be provided until a consistent minimum mean arterial pressure of 60 mmhg is obtained. Bolus doses vary depending on the clinical status and underlying disease; however, the following are common bolus ranges. Isotonic crystalloids Dogs: 15-30mL/kg over 10-15min Hypertonic crystalloids Dogs: 2-5 ml/kg over 5-10min Colloids (synthetic) Dogs: 5-10 ml/kg over 10-15min Cats: 10-15mL/kg over 10-15min Cats: 1-3mL/kg over 5-10 min Cats: 2.5-5mL/kg over 10-15min Once the patient is hemodynamically stable, one must determine if fluid losses are due to simple dehydration or blood loss. Dehydration is characterized by the loss of water and solutes from

3 the body, and if severe enough can lead to intravascular volume depletion as a result of fluid shifts. In these incidents, only the plasma volume component of blood is affected while the blood cell volume remains unchanged. Estimates of dehydration are not exact. However through careful evaluation of the patient's history, physical examination findings and simple laboratory tests, a reasonable range can be determined. The following table is a guideline to assist in assessing the hydration status of the patient: Clinical Sign Percent Dehydrated 5-7 % 7-9% 9-11% 11%+ Mentation Likely normal Quiet Quiet to dull Dull to obtunded Resp rate Normal Normal Tachypneic Tachypneic to shallow MM/CRT Tachy/<2sec Tachy/<2 sec Tachy//< or > 2sec Tachy/ > 2 sec Heart rate Normal Normal to Increased Increased/decreased Increased/decreased Pulse pressure Strong Good to weak Weak to absent Absent Blood pressure Normal Normal to decreased Decreased Decreased to absent Skin tent Normal to mild Mild Moderate Marked Azotemia None to mild Mild Moderate Marked Hemoconcentration None to mild Mild Moderate Marked Lactate Normal Normal-mild increase Mild to mod increase Mod to marked increase ** note: < 5% dehydration is considered subclinical but would apply to any patient with increased losses (e.g. vomiting, diarrhea, PU/PD) where appropriate increases in oral intake are not noted Once an estimate of the degree of dehydration is obtained, the following formula should be used to calculate the total volume deficit. Dehydrated patient: Volume to be infused = % dehydration + maintenance + ongoing losses In contrast to dehydration, both the blood cell volume (e.g. RBCs and WBCs) and plasma volume are decreased during blood loss. Similar to estimates of dehydration, estimates of blood loss are not exact. The patient's cardiac and renal function in addition to the rapidity of the blood loss and concurrent comorbidities (e.g. pain, pulmonary disease etc) must be taken into account. Nonetheless, the following table is a guide to determining blood loss. Clinical Sign 15% (10-12mL/kg) 15-30% (10-12mL/kg) Percentage of Total Blood Volume Lost 30-40% (25-32mL/kg) >40% (>32mL/kg) Mentation Normal Normal to quiet Quiet to dull Dull to obtunded Resp rate Normal Increased Increased +/- effort Dyspneic to shallow MM/CRT Pink/<2sec Pale pink/2 sec Pale to white/> 2sec White/ > 2 sec Heart rate Normal to increased Increased Increased/decreased Increased/decreased Pulse pressure Strong Good to weak Weak to absent Absent Blood pressure Normal Normal to decreased Decreased to absent Decreased to absent Temp Normal Normal to low Low Low

4 Again, once an estimate of the amount of blood loss is obtained, the following formula should be used to calculate the total volume deficit. Hemorrhaging patient (ongoing therapy): Volume to be infused = % blood loss + maintenance fluids + ongoing losses Once a calculated volume of fluid is determined, the next step is choosing the appropriate fluid to administer. In general, dehydration is corrected using crystalloids with the occasional need for colloid supplementation in hypoproteinemic patients. Whereas blood loss often corrected using a combination of crystalloids, colloids and blood components depending on the degree of blood loss. The particular crystalloid administered is often determined following evaluation of the patient s electrolyte and acid/base status. Electrolyte abnormalities, specifically potassium, sodium and glucose, should be monitored and appropriate measure taken to ensure correction. Although the details of electrolyte abnormalities and acid/base are beyond the scope of this lecture, generalizations will the discussed in the subsequent section entitled Fluid Choices as well as with the case discussions. The rate over which the calculated deficit volume is administered is dictated by the magnitude and rapidity of the fluid loss. Recall, replenishment of intravascular volume must occur quickly to limit further hypoperfusion. Following which acute deficits should be replaced within 6-12 hours, while more chronic losses over 24hrs. Special considerations should be given to patients scheduled for general anesthesia. Whenever possible, fluid deficits should be replaced before anesthesia and surgery to avoid hypotension and impaired organ perfusion. Fluid choices The focus of this lecture is on IV fluid resuscitation. However, in patients with mild dehydration both oral rehydration, if gastrointestinal dysfunction (e.g. vomiting, diarrhea or gastric stasis) is absent, and subcutaneous fluids, remain viable, and less costly, options. Intravenous fluid choices can be broken into 3 simple categories 1) crystalloids, 2) synthetic colloids and 3) blood products. The individual choice will depend on the patients physical and laboratory findings. Often more than one fluid type will be administered during the course of a patient s treatment plan. Crystalloids Crystalloids are sterile water based solutions containing electrolyte and buffer solutes that easily enter all 3 body fluid compartments. The electrolyte concentration varies and determines whether a particular crystalloid is classified as a replacement, maintenance, hypertonic or dextrose in water solution. The composition of replacement solutions approximates that of plasma water (e.g. high sodium and low/no K+), allowing rapid IV fluid administration for the treatment of shock secondary to hemorrhage or severe volume depletion secondary to marked dehydration. Unfortunately, only 20-25% of the infused volume remains within the IVS 1 hour after infusion due to extravasation into the ITS and ICS. As such, a limited duration of intravascular volume expansion occurs with a risk of interstitial and cellular edema. The choice of a particular replacement solution is usually based on serum electrolyte and acid/base status. Maintenance solutions are hypotonic relative to plasma water resulting in less than 10% of the infused volume remains within the IVS 1 hour after infusion. The electrolyte composition mimics that of the extracellular fluid, making it most suitable for delivery of a patient's daily maintenance fluid volume. Dextrose in water (D5W) is reserved for the replacement of pure water loss (e.g. stable diabetic) or to facilitate the delivery of drugs. Despite the administration of 5% dextrose, it is not a suitable replacement for enteral nutrition as it only supplies 200kcal/L, which does not approach even the smallest patient s daily caloric needs. Hypertonic crystalloids, as the name implies, contain solute concentrations that are in excess of plasma water. Administration results in

5 rapid, but temporary (e.g. 30 minutes), volume expansion due to the osmotic draw of water into the IVS from the ITS. Classification Example Important points Replacement PlasmaLyte-A, LRS Normosol-R Alkalinizing solution due to buffers (e.g. lactate, gluconate). Potassium containing solution 0.9% NaCl Acidifying solution due to high chloride content. No potassium Maintenance P56 Hypotonic solution suitable for delivery at maintenance fluid rates only Hypertonic (HTS) 3-21% NaCl Rapid but short volume expansion using doses of ~5mL/kg over 5-10min of 3-7% HST solution Combine with synthetic colloid 1:1 for longer duration of effect (2.5mL/kg colloids + 2.5mL/kg HTS) Dextrose + water D5W (5% dextrose) Not a significant source of calories Synthetic colloids Synthetic colloids are high-molecular weight compounds that resist extravasation. As a result they are effective in expanding and maintaining intravascular volume. Their main indications are for rapid IV volume replacement in patients with clinical blood loss and low volume resuscitation protocols to reduce the risk of fluid leakage. They are also effective in providing colloid osmotic support in hypoalbuminemic patients. They have a limited role in dehydrated patients unless accompanied by concurrent hypoproteinemia and refractory hypotension. Historical concerns with colloid administration are volume overload, coagulopathies, renal failure and anaphylactic reactions. The use of newer colloids, proper patient selection and careful monitoring during fluid resuscitation can result in safe and effective use of synthetic colloids in even the most critical patient. Blood products A range of blood products from fresh whole blood to stored component therapy are available for veterinary practitioners. Details of each product are beyond the scope of this lecture. However, a few important details warrant mention. Initial fluid resuscitation efforts do not require the administration of whole blood or blood products in most scenarios. Acute life threatening concerns are associated with hypovolemia and poor tissue perfusion, both of which can quickly be restored by synthetic colloid and crystalloid administration. Concerns over diluting the PCV should not limit initial fluid administration, as adequate blood volume and perfusion are critical. Recent human evidence support more conservative prbcs transfusion triggers, in critically ill patients. A target PCV of 21-27% has been shown to be equally effective in the majority of patients. Exceptions necessitating higher targets (e.g > 29%) are patients with head trauma or primary myocardial disease. Also, as a generalization, patients undergoing general anesthesia and invasive surgical interventions should ideally have a PCV of >25%. Furthermore, both fresh frozen or stored plasma are inefficient, expensive sources for albumin replacement as approximately 22.5 ml/kg of plasma is required to raise a patient s albumin by 4-5g/L. A more suitable choice for plasma oncotic support in most patients is the administration of a synthetic colloid. Monitoring As with any therapeutic intervention, continuous monitoring is vital to ensure target goals are attained as well as patient tolerance. The first priority when administering fluids is to restore intravascular volume. Reversing shock and achieving cardiovascular stability usually coincides with improved mentation, correction of tachycardia and confirmed normotension (systolic > 90mmHg and

6 means > 60 mmhg). Urine output should also return to normal (defined as > 1.0 ml/k/hr). Unfortunately, correction of physical examination parameters does not always correlate to adequate global tissue perfusion. When available, fluid resuscitation end points should also include normalization of serum lactate to less than 2.5 mmol/l ideally within 12 hrs. Once hypoperfusion has resolved, fluid rates and patient status should be evaluated every 4-8hrs to ensure current fluid rate remains appropriate. Depending on the electrolyte abnormality and severity, concurrent electrolyte reassessment (q1-4hrs) is often required during the early stages of fluid therapy to ensure correction or at least improvement. Special attention should be placed on serum sodium levels in chronically ill patients, if presenting levels are either greater than 168 mmol/l or less than 128 mmol/l as rapid correction (greater than 12 mmol/l within a 24hr period) may result in severe neurological abnormalities. Once deficits have been corrected, further fluid administration should be based on regular physical and laboratory findings, trending body weight as well as quantification of urine output (e.g. see minimum database above ). Overhydration can be as detrimental as dehydration, especially in patients with significant underlying cardiopulmonary disease or renal disease. If overhydration is noted, consider possible underlying renal disease (e.g oliguria or anuria) or excessive rate of fluid administration. Potential signs of fluid overload in patients receiving IV fluids include: Patient parameter Body weight PCV/TP Urinary Skin Respiratory rate Behaviour Cardiovascular Description Increase in weight above estimate fluid loss (e.g. 100mL = 0.1kg) Evidence of hemodilution beyond expected deficit BUN below normal suggesting medulary wash out Prominent polyuria with low USG (isosthenuria to hyposthenuria) Peripheral edema-most noticeable over hocks or intermandibular Chemosis or serous nasal discharge Tachypnea with or without crackles (late stage sign), Coughing Restlessness or shivering Tachycardia (also noted if rate too fast) Failure to achieve rehydration or normovolemia (in the case of blood loss) within the desired period should prompt the veterinarian to reevaluate their initial fluid plan. Reasons for ongoing fluid deficits may include one or more of the following: Calculation errors Ongoing sensible losses larger than expected Third spacing of fluids (e.g. ascites) Excessive fluid rate infusion resulting in obligatory urinary diuresis Uncontrolled hemorrhage resulting in ongoing blood loss Underestimation of initial fluid deficit Ongoing insensible losses larger than expected Technical error (e.g. blown IV, pump error) Summary For any therapy to be effective, it must be appropriate for the underlying condition. The choice of fluid is determined based on the type and cause of fluid loss, duration of loss, concurrent electrolyte and

7 acid/base imbalance and overall patient status. Prior to any fluid administration, a fluid plan should be calculated to avoid the tendency to treat in multiples of maintenance. Once an appropriate fluid plan is chosen, the patient must be closely monitored. Ongoing assessments should tailor future fluid support to ensure the desired response is achieved in an appropriate period. Side effects, as with any therapy, can occur, further emphasizing the importance of ongoing patient monitoring. **Dosage and rates listed above are only suggestions, and patient variables must be considered prior to the administration** of any drug or therapy. Case examples 1. 3 yr old MN Labx. HBC 30 min prior to presentation. PE findings: Alert. Tachypneic (66 bpm). Pale pink MM. CRT<2sec. Pupils equal and responsive. Sinus tachycardia (165 bpm) with good, synchronous dorsal pedal pulses. Harsh lung sounds unilaterally. Temp 38.9C. Small lacerations/abrasions around caudal abdomen/prepuce, face/head. Tense on abdominal palpation. Ambulatory Quats: PCV 58%, TP 65g/L. BG 7.1 mmol/l. Na+ 147, K+ 3.3, Cl 115. Lactate 1.9 mmol/l ph 7.288, pc , HCO 20.7, ABE -1.9, Azostix 5-15 Fluid Plan: 2. 4 yr old DSH with 48hrs history of profuse vomiting and anorexia. History of dietary indiscretion. PE findings: Quiet. Tachypneic at 36bpm. MM pink but tachy. CRT<2sec. Mild skin tent. Sinus tachycardia at 230bpm with weak dorsal pedal pulses. Chest clear. Temp 39.5C. Marked abdominal pain on palpation. Palpable foreign body mid abdomen. Quats: PCV 40%, TP 75g/L. BG 9.9 mmol/l. Na+ 155, K+ 3.5, Cl 112. Lactate 1.6 mmol/l ph 7.283, pc , HCO 21.7, ABE -5.1, BUN 16.3, USG Fluid Plan: 3. 5 yr old MN Papillon. Acute onset profuse hematochezia and hematemesis. No known dietary indiscretion but did have guests over for a BBQ. PE findings: Obtunded. Shallow RR (18 bpm). MM injected and tachy. CRT <1 sec. Prolonged skin tent. Tachycardic (180 bpm) with absent dorsal pedal pulses. Chest clear. Temp 37.3C. Tense on abdominal palpation. Quats: PCV 78%, TP 45g/L. BG 1.9 mmol/l. Na+ 145, K+ 2.7, Cl 109. Lactate 4.6 mmol/l ph 7.183, pc , HCO 16.7, ABE -10, BUN 13.3, Creatinine 143, USG Fluid Plan: