# Veterinary Fluid Therapy: Crystalloids vs Colloids ## Key Takeaways - Balanced crystalloids are generally preferred over 0.9% saline for large-volume resuscitation in dogs and cats due to their ability to avoid hyperchloremic metabolic acidosis and renal vasoconstriction, which can be induced by the supraphysiological chloride concentration of saline. - Synthetic colloids, particularly hydroxyethyl starch (HES), have faced significant restrictions in human medicine due to adverse effects including acute kidney injury and coagulopathy, prompting a reassessment and limitation of their use in veterinary practice, with a 2016 survey indicating 70% of veterinarians had changed their colloid practices. - Crystalloids distribute throughout the extracellular space, requiring 2-3 times the estimated deficit for resuscitation, whereas colloids remain primarily intravascular, necessitating approximately equal volume to the deficit, but this volume-sparing benefit must be weighed against colloid-specific risks. - The choice of fluid therapy is guided by a structured assessment of patient volume status, suspected pathophysiology (e.g., hemorrhagic vs. septic shock), and patient-specific risks such as renal disease or coagulopathy, with monitoring of perfusion parameters, lactate, urine output, and body weight being critical for reassessment. - While synthetic colloids may have a role in severe hypovolemia refractory to crystalloids or in cases of low colloid osmotic pressure, their use should be judicious, documented, and reserved for specific indications, avoiding them in patients with known coagulopathy, active bleeding, renal disease, or sepsis. - Albumin, a natural colloid, is theoretically safer but lacks proven survival benefits in human medicine, is expensive and not widely available in species-specific forms, and carries risks of xenotransfusion reactions in veterinary patients. --- The choice between crystalloid and colloid solutions for volume resuscitation remains one of the most consequential decisions in veterinary emergency and critical care. This article compares these fluid classes for the practicing veterinarian, examining their physicochemical properties, hemodynamic effects, safety profiles, and the evidence base that informs their use across species. It addresses the clinical question of which fluid to select for a given patient presentation, with particular attention to shock resuscitation, colloid osmotic pressure support, and the controversies surrounding synthetic colloids. The topic has shifted substantially over the past two decades. Synthetic colloids, particularly hydroxyethyl starch (HES), were once preferred resuscitation fluids in many countries, but accumulating evidence of adverse effects has prompted major restrictions in human medicine and a corresponding reassessment in veterinary practice. A 2016 internet-based survey of 1,134 veterinarians from 42 countries found that 70% of respondents had changed their colloid practices in recent years, mostly by limiting use, largely due to emerging safety concerns [survey of small animal fluid practices](https://pubmed.ncbi.nlm.nih.gov/28929101/). This article synthesizes the physiological principles and clinical evidence that should guide fluid selection in dogs, cats, and other species. ## At a Glance | Parameter | Crystalloids | Colloids | |---|---|---| | Intravascular persistence | Short, approximately 20-25% of infused volume remains intravascular | Longer, 80-100% remains intravascular depending on molecular weight and membrane integrity | | Volume required for resuscitation | 2-3 times estimated deficit | Approximately equal to estimated deficit | | Cost | Low | High, particularly for albumin | | Primary risk | Tissue edema, dilutional coagulopathy with large volumes | Renal injury, coagulopathy, anaphylaxis, volume overload | | Balanced vs saline | Balanced solutions avoid hyperchloremic acidosis | No equivalent distinction within class | | Evidence in human medicine | Balanced crystalloids favored over saline | Synthetic colloids restricted, albumin benefit unproven | | Veterinary evidence base | Extensive clinical experience, limited controlled trials | Mostly retrospective or small experimental studies | ## Physiological Basis of Fluid Distribution Fluid movement between the intravascular and interstitial compartments follows the Starling forces: hydrostatic pressure drives fluid out of capillaries, while colloid osmotic pressure from plasma proteins draws fluid in. The endothelial glycocalyx, a layer of glycosaminoglycans lining the capillary lumen, modifies this classic model by restricting protein extravasation and contributing to the oncotic gradient across a much narrower space than previously appreciated. Damage to the glycocalyx, which occurs in hemorrhagic shock, sepsis, and ischemia-reperfusion injury, increases capillary permeability and diminishes the theoretical advantage of colloids [review of fluid effects on microcirculation](https://pubmed.ncbi.nlm.nih.gov/20502873/). Crystalloids are electrolyte solutions that distribute according to the body's water compartments. Isotonic crystalloids equilibrate across the entire extracellular space, meaning only a fraction of the infused volume remains in the vasculature. Normal saline (NaCl 0.9%) has a supraphysiological chloride concentration of 154 mmol/L, which can produce hyperchloremic metabolic acidosis and renal vasoconstriction in animal and human models. Balanced crystalloids, formulated with organic anions such as lactate, acetate, or gluconate, avoid this chloride load and are not associated with these changes [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). Colloids contain macromolecules that exert oncotic pressure and are classified as natural (albumin) or synthetic (HES, dextrans, gelatins). Their intravascular persistence depends on molecular weight, the integrity of the capillary barrier, and in the case of synthetic colloids, the rate of enzymatic degradation and renal clearance. The clinical relevance of these differences depends heavily on the underlying disease process. ## The Resuscitation Fluid Controversy The fundamental trade-off in fluid selection is volume efficacy versus safety. Colloids achieve intravascular volume expansion with smaller infused volumes, which is attractive in patients at risk of edema, including those with pulmonary or cerebral compromise. However, the evidence that this translates into improved outcomes is limited. A systematic review of preclinical studies comparing resuscitation fluids after hemorrhagic shock found substantial heterogeneity in microcirculatory endpoints and no clear superiority of any single fluid class [systematic review of preclinical resuscitation studies](https://pubmed.ncbi.nlm.nih.gov/27465755/). The authors noted that translatability to clinical practice was often poor. In human medicine, the safety profile of synthetic colloids has become the dominant consideration. Semisynthetic colloids are associated with increased incidence of renal failure and mortality, leading to substantial restrictions on HES administration by medicines authorities beginning in 2013 [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). These restrictions have influenced veterinary practice, though the evidence base in animals is far less robust. The veterinary literature consists largely of retrospective evaluations and experimental studies with small numbers of patients, predominantly dogs [colloid therapy review](https://pubmed.ncbi.nlm.nih.gov/34277747/). ## Crystalloid Solutions ### Balanced Crystalloids Versus Saline The chloride content of resuscitation fluids has emerged as a clinically relevant variable. Normal saline infusion produces higher serum chloride concentrations and metabolic acidosis compared with balanced solutions, and renal vasoconstriction has been demonstrated in experimental models [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). For this reason, balanced crystalloids are generally preferred for large-volume resuscitation in dogs and cats, a position reflected in recent consensus guidance [AAHA/AAFP fluid therapy guidelines](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/). ### Limitations of Crystalloid Resuscitation Large-volume crystalloid resuscitation carries specific risks. Interstitial edema is the most predictable consequence, particularly when capillary permeability is increased. In the brain, the relationship between oncotic pressure and water content is complex. Experimental work in rabbits with cryogenic brain injury found that hemodilution with saline required approximately twice the fluid volume of hetastarch or albumin to maintain hemodynamic targets, yet the saline group did not show increased brain water content compared with colloid groups [cerebral effects of crystalloid and colloid solutions](https://pubmed.ncbi.nlm.nih.gov/2457341/). This suggests that in the setting of an intact blood-brain barrier, crystalloid-induced reductions in oncotic pressure may have less cerebral impact than traditionally assumed. ## Colloid Solutions ### Synthetic Colloids Hydroxyethyl starch solutions were the most commonly used synthetic colloids in veterinary practice, selected by 75% of survey respondents for colloid osmotic pressure support [survey of small animal fluid practices](https://pubmed.ncbi.nlm.nih.gov/28929101/). Their adverse effects in human patients include acute kidney injury, coagulopathy, and tissue accumulation. Whether these effects occur to the same degree in dogs and cats remains uncertain, but the precautionary principle has led many veterinary clinicians to restrict their use. Dextrans and gelatins are used by some European practitioners but have their own safety concerns, including anaphylactoid reactions [colloid therapy review](https://pubmed.ncbi.nlm.nih.gov/34277747/). ### Albumin Natural colloids such as albumin are theoretically safer than synthetic colloids, but evidence of a positive effect on survival is lacking in human medicine [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). In veterinary practice, species-specific canine albumin is expensive and not widely available, while human serum albumin carries known risks of xenotransfusion reactions. A substantial proportion of surveyed veterinarians, particularly those in Australia and New Zealand, used no albumin product at all [survey of small animal fluid practices](https://pubmed.ncbi.nlm.nih.gov/28929101/). ## Clinical Decision Framework for Fluid Selection The choice between crystalloid and colloid therapy begins with a structured assessment of the patient's volume status, the suspected cause of hypovolemia, and the presence of comorbid conditions that alter the risk-benefit balance of each fluid class. The [AAHA/AAFP fluid therapy guidelines for dogs and cats](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/) recommend that fluid selection follow a stepwise evaluation of perfusion parameters, hydration status, and ongoing losses instead of a reflexive preference for one fluid class. The first decision point is whether the patient requires resuscitation, rehydration, or maintenance. Resuscitation targets intravascular volume deficits and is time-critical. Rehydration addresses interstitial and intracellular deficits. Maintenance replaces ongoing losses. Crystalloids address all compartments, whereas colloids exert their primary effect within the intravascular space. This distinction drives the initial choice. The second decision point is the suspected pathophysiology. Hemorrhagic shock, septic shock, and cardiogenic shock impose different constraints on fluid type and rate. The [RECOVER Initiative veterinary CPR guidelines](https://recoverinitiative.org/) address fluid selection specifically in the peri-arrest and post-arrest period, where the margin for error narrows considerably. The third decision point is patient-specific risk. Renal disease, coagulopathy, cerebral injury, and hypoalbuminemia each modify the safety profile of synthetic colloids and albumin. A patient with acute kidney injury or a high bleeding risk is a poor candidate for hydroxyethyl starch, regardless of the theoretical volume-sparing benefit. ### Shock States and Fluid Prioritization | Shock category | First-line fluid | Second-line or adjunct | Primary constraint | Monitoring emphasis | |---|---|---|---|---| | Hemorrhagic shock | Isotonic crystalloid, balanced preferred | Blood products when available, hypertonic saline as temporising adjunct | Coagulation status, ongoing blood loss | Perfusion parameters, lactate, hemoglobin | | Septic shock | Isotonic crystalloid, balanced preferred | Albumin in hypoalbuminemic patients, synthetic colloids generally avoided | Renal function, endothelial integrity | Blood pressure, lactate, urine output, perfusion | | Hypovolemic shock, non-hemorrhagic | Isotonic crystalloid | Colloid only if crystalloid volume is excessive or oncotic support is needed | Volume tolerance, underlying cause | Perfusion, body weight, fluid balance | | Cardiogenic shock | Minimal or no crystalloid | Inotropic support preferred | Volume overload, pulmonary edema | Central venous pressure, lung auscultation, oxygenation | | Traumatic brain injury with hypovolemia | Isotonic crystalloid, balanced preferred | Hypertonic saline for cerebral edema | Osmolality, cerebral perfusion pressure | Neurologic status, osmolality, blood pressure | The table reflects a synthesis of current consensus guidance. In hemorrhagic shock, crystalloids remain the initial resuscitation fluid in most veterinary settings because blood products are not immediately available. The [RECOVER guidelines](https://recoverinitiative.org/) emphasize that crystalloid boluses should be given in aliquots with frequent reassessment, and that blood products should be introduced early when hemorrhage is severe. In septic shock, the balance shifts further toward crystalloids. The [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/) notes that semisynthetic colloids carry an adverse safety profile with increased renal failure and mortality in human patients, and the same concerns have driven changes in veterinary practice. A survey of small animal practitioners found that 70% of respondents had changed their colloid use in recent years, mostly by limiting it, largely due to safety concerns [survey of current trends in volume replacement therapy](https://pubmed.ncbi.nlm.nih.gov/28929101/). ### Volume Required and the 1:3 Ratio A crystalloid bolus distributes across the extracellular space, so only roughly one quarter to one third remains in the intravascular compartment after equilibration. A colloid remains largely within the vasculature, so the volume required to achieve the same intravascular expansion is smaller. The traditional teaching is a 1:3 colloid-to-crystalloid ratio, meaning 10 mL/kg of colloid approximates 30 mL/kg of crystalloid. This ratio is a clinical approximation, not a precise equivalence. The [preclinical systematic review of fluid resuscitation after hemorrhagic shock](https://pubmed.ncbi.nlm.nih.gov/27465755/) found that colloids restore microcirculatory flow with smaller volumes, but the clinical significance of this difference is uncertain. The practical implication is that a patient who requires repeated large crystalloid boluses may benefit from a colloid to reduce cumulative volume, particularly when there is concern about tissue edema or when ongoing losses are substantial. The ratio does not justify colloid use as a default. The [review of colloid therapy in dogs and cats](https://pubmed.ncbi.nlm.nih.gov/34277747/) concludes that the evidence for superior outcomes with colloids is limited, and that the safety concerns with synthetic colloids argue against routine use. The volume-sparing effect must be weighed against the specific risks of the colloid chosen. ### Monitoring Parameters That Change the Decision Fluid therapy is a dynamic intervention. The correct choice at the start of resuscitation may be wrong thirty minutes later. Monitoring parameters must be selected to detect the transition points where the fluid plan should change. Perfusion parameters, including mucous membrane color, capillary refill time, heart rate, pulse quality, and extremity temperature, are the first-line reassessment tools. They change rapidly with volume expansion and are available in every setting. A patient whose perfusion normalizes after one crystalloid bolus may not need further resuscitation fluid at all. Blood pressure, measured directly or indirectly, adds objective data. Hypotension that persists after an adequate crystalloid challenge suggests either ongoing losses, vasodilation, or myocardial dysfunction. The response to a fluid bolus, measured as a change in blood pressure or perfusion, is more informative than a single absolute value. Lactate concentration is the most useful biochemical marker of tissue hypoperfusion. A falling lactate after fluid resuscitation indicates that the microcirculation is being recruited and oxygen delivery is improving. A rising or static lactate despite fluid therapy suggests inadequate resuscitation, ongoing ischemia, or a non-perfusion cause of hyperlactataemia. The [review of fluid effects on microcirculation and tissue oxygenation](https://pubmed.ncbi.nlm.nih.gov/20502873/) notes that improving microcirculatory flow is the ultimate goal of volume replacement, and lactate is the most practical clinical surrogate. Urine output is a downstream indicator of renal perfusion. Oliguria after resuscitation should prompt reassessment of volume status, blood pressure, and the possibility of acute kidney injury. This is particularly relevant when synthetic colloids have been used, given the association with renal injury [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). Body weight is the most sensitive indicator of net fluid balance. Serial weights, measured on the same scale at the same time daily, detect fluid accumulation before it becomes clinically apparent as peripheral edema or pulmonary crackles. A patient who has gained 8% of body weight during resuscitation has received substantial fluid, and further volume expansion should be justified by persistent perfusion deficits. ### When Colloids Are Indicated Synthetic colloids have a narrow but genuine role in veterinary practice. The [review of colloid therapy](https://pubmed.ncbi.nlm.nih.gov/34277747/) identifies the primary indications as patients with severe hypovolemia that cannot be corrected with crystalloids alone, and patients with low colloid osmotic pressure contributing to tissue edema. The survey of practitioners found that 75% of respondents used hydroxyethyl starch for colloid osmotic support, indicating that this remains a common clinical application despite safety concerns [survey of current trends in volume replacement therapy](https://pubmed.ncbi.nlm.nih.gov/28929101/). The decision to use a synthetic colloid should be explicit and documented. The clinician should record the indication, the expected benefit, the volume administered, and the monitoring plan. The [AAHA/AAFP guidelines](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/) recommend that synthetic colloids be avoided in patients with known coagulopathy, active bleeding, renal disease, or sepsis, and that they be used at the lowest effective dose for the shortest duration. Albumin is a separate consideration. Species-specific albumin is expensive and not always available. Human serum albumin carries a risk of immunologic reactions in dogs. The [review of colloid therapy](https://pubmed.ncbi.nlm.nih.gov/34277747/) notes that evidence for a survival benefit with albumin is lacking, and that xenotransfusion has known side effects. Albumin may be considered in patients with severe hypoalbuminemia and oncotic pressure-dependent edema, but the evidence base is thin. ### Species and Setting Modifications The crystalloid-versus-colloid decision differs across species. Dogs and cats have been the subjects of most veterinary colloid research, but the findings do not transfer automatically to other species. Ruminants and horses have different fluid distribution dynamics, and their response to colloids is less well characterized. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) provides species-specific guidance on fluid therapy that should be consulted before extrapolating small animal protocols. Production animals present additional constraints. Cost, availability, and withdrawal considerations influence fluid choice in food animals. The [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) address the regulatory framework for veterinary products in food animals, and the [AVMA practice resources](https://www.avma.org/resources-tools) provide professional guidance on responsible antimicrobial and fluid therapy in production settings. A synthetic colloid that is cost-effective in a 30 kg dog may be prohibitively expensive in a 600 kg cow. Equipment availability also changes the decision. A practice without blood pressure monitoring or point-of-care lactate measurement must rely on physical examination parameters alone, which favours a more conservative approach to colloid use. A referral hospital with continuous monitoring can titrate colloid therapy more precisely and detect adverse effects earlier. The cerebral patient deserves specific mention. The [experimental study of crystalloid and colloid effects after cryogenic brain injury in rabbits](https://pubmed.ncbi.nlm.nih.gov/2457341/) found that saline required approximately twice the volume of hetastarch or albumin to maintain hemodynamic stability, and that the saline group experienced a greater reduction in oncotic pressure. However, the study did not demonstrate a significant difference in brain water content between groups. This suggests that the theoretical concern about crystalloid-induced cerebral edema may be less clinically important than the volume required to maintain perfusion. In a brain-injured patient, the priority is maintaining cerebral perfusion pressure without causing hypervolaemia, and the fluid choice should be guided by the patient's overall volume status instead of by oncotic pressure alone. ## Recognized Complications and Early Detection Fluid therapy failure often presents as persistent hypotension, worsening tachycardia, or declining mentation despite adequate volume administration. The most common complication is volume overload, particularly in patients with reduced cardiac reserve or oliguric kidney injury. Early detection relies on serial body weight measurement, which remains the most sensitive bedside tool, combined with thoracic auscultation for crackles, assessment of jugular venous distension, and monitoring of respiratory effort. A weight gain exceeding 10% of admission weight during resuscitation should trigger immediate reassessment of fluid rate and choice. Hyperchloraemic metabolic acidosis complicates high-volume saline resuscitation. The [physiological basis and clinical evidence review by Severs and colleagues](https://pubmed.ncbi.nlm.nih.gov/24463187/) describes how supraphysiological chloride content produces metabolic acidosis and renal vasoconstriction in animal and human models. Detection requires serum electrolyte and acid-base assessment, not clinical examination alone. Balanced crystalloids avoid this pattern, which supports their preferential use when large volumes are anticipated. Synthetic colloids carry specific risks that crystalloids do not. The [review by Adamik and Yozova](https://pubmed.ncbi.nlm.nih.gov/34277747/) summarizes the renal injury and mortality concerns that led to substantial restrictions on hydroxyethyl starch administration in human medicine. In veterinary patients, acute kidney injury, coagulopathy, and tissue accumulation are recognized concerns. Early detection of nephrotoxicity requires serial creatinine and urine output monitoring in any patient receiving synthetic colloids, particularly those with pre-existing renal disease or sepsis. Coagulation abnormalities may present as prolonged bleeding from venepuncture sites, petechiae, or unexplained hematoma formation. ## Common Errors and Corrective Actions Less experienced clinicians frequently confuse hypovolemia with dehydration. Hypovolemia is an intravascular deficit requiring rapid volume expansion, whereas dehydration is a total body water deficit that can be corrected more gradually. Administering a large crystalloid bolus to a dehydrated but normovolemic patient risks interstitial and pulmonary edema without improving perfusion. Another recurring error is failure to reassess after each bolus. Fluid therapy is a dynamic intervention, and the response to a bolus determines whether further volume is needed or whether vasopressor support should begin. The [AAHA and AAFP fluid therapy guidelines](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/) emphasize repeated clinical reassessment instead of adherence to a predetermined total volume. A third error is using colloids when crystalloids would suffice. The perception that colloids are inherently superior persists despite evidence that their volume-sparing effect is modest and their adverse effect profile is significant. The [internet-based survey of small animal practitioners](https://pubmed.ncbi.nlm.nih.gov/28929101/) found that 70% of respondents had limited their synthetic colloid use in recent years, largely in response to safety concerns. The corrective action is to reserve colloids for patients with documented hypoalbuminaemia and ongoing losses, or for those who fail to respond to adequate crystalloid resuscitation. ## Troubleshooting Table | Observation | Likely Cause | Discriminating Check | |---|---|---| | Persistent tachycardia after 2 boluses | Ongoing hemorrhage or inadequate volume | Repeat blood pressure, lactate, hematocrit, surgical site assessment | | Weight gain with worsening respiratory effort | Volume overload | Thoracic ultrasound or radiography, central venous pressure if available | | Progressive azotaemia after colloid administration | Synthetic colloid nephrotoxicity | Urine output, urine sediment, fractional excretion of sodium | | Metabolic acidosis after large saline volume | Hyperchloraemic acidosis | Serum chloride, anion gap, blood gas analysis | | Hypotension despite adequate volume | Vasodilatory shock, cardiac failure | Echocardiography, lactate trend, vasopressor trial | ## Limitations of Current Evidence Veterinary fluid therapy evidence is largely extrapolated from human medicine, and the [critical appraisal by Severs and colleagues](https://pubmed.ncbi.nlm.nih.gov/24463187/) notes that data on clinical outcomes associated with crystalloid infusion are heterogeneous. Large prospective randomised veterinary trials comparing crystalloids and colloids are lacking. The [review by Adamik and Yozova](https://pubmed.ncbi.nlm.nih.gov/34277747/) states that veterinary data on synthetic and natural colloid safety and efficacy are limited to retrospective evaluations and experimental studies with small numbers of patients, mainly dogs. Expert opinion still differs on the role of albumin. Species-specific albumin is not widely available, and xenotransfusion with human serum albumin carries known side effects. Some clinicians advocate colloid administration for oncotic support in patients with severe hypoalbuminaemia, while others argue that crystalloid therapy and treatment of the underlying disease are sufficient. The [preclinical systematic review of hemorrhagic shock resuscitation](https://pubmed.ncbi.nlm.nih.gov/27465755/) found that optimal fluid choice for microcirculatory restoration remains unknown, with studies reporting divergent results across rodent, canine, and porcine models. ## Referral and Escalation Criteria Patients that fail to stabilize after two or three fluid boluses warrant escalation. Referral to a specialist or emergency facility is appropriate when vasopressor support is required, when invasive monitoring such as arterial catheterization or central venous pressure measurement is needed, or when the patient has concurrent cardiac, renal, or pulmonary disease that complicates fluid management. Laboratory involvement is indicated for serial blood gas, electrolyte, lactate, and coagulation assessment in any patient receiving large-volume resuscitation or synthetic colloids. Regulatory reporting obligations vary by jurisdiction. The [AVMA practice resources](https://www.avma.org/resources-tools) and [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) provide guidance on reportable adverse events and notifiable diseases, but clinicians must consult their local regulatory authority for specific requirements. Suspected adverse drug reactions to synthetic colloids should be reported through the appropriate pharmacovigilance pathway in the relevant country. ## Frequently Asked Questions ### How do I choose a fluid when cost limits my options? Isotonic crystalloids remain the most affordable and universally available resuscitation fluid. Balanced crystalloids are preferred over saline when available because saline carries risks of hyperchloremic acidosis and renal vasoconstriction, as described in the [critical appraisal of intravenous fluids](https://pubmed.ncbi.nlm.nih.gov/24463187/). When synthetic colloids are unaffordable, larger crystalloid volumes can achieve similar hemodynamic targets, though tissue edema risk rises. Hypertonic saline offers a low-volume alternative for large animals where cost or logistics limit crystalloid volume. Avoid substituting dextrose-containing solutions for resuscitation, they provide negligible volume expansion. Document the resource limitation in the medical record and adjust monitoring frequency, since crystalloid-only resuscitation requires closer reassessment of perfusion parameters. ### What should I do when blood products are unavailable for hemorrhagic shock? Crystalloids remain the first-line replacement when blood products are unavailable, but the clinician must accept that oxygen-carrying capacity cannot be restored. Use balanced crystalloids in aliquots of 10 to 20 mL/kg in dogs and 5 to 10 mL/kg in cats, reassessing perfusion after each bolus. Synthetic colloids do not replace the oxygen-carrying function of blood and carry renal and coagulation risks that argue against their routine use in hemorrhage, per the [review on synthetic colloid use in small animals](https://pubmed.ncbi.nlm.nih.gov/28929101/). Transfusion should be arranged as soon as feasible. Monitor lactate, base deficit, and hematocrit trends to detect ongoing blood loss. If packed red cells are unavailable, consider whole blood from a screened donor where permitted by regional regulations. ### Does the crystalloid versus colloid decision differ in cats compared with dogs? Yes. Cats tolerate volume overload poorly and require smaller, slower boluses with more frequent reassessment. The [AAHA and AAFP fluid therapy guidelines](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/) emphasize lower resuscitation volumes in cats and caution against synthetic colloids given limited safety data in this species. Cats also have a higher incidence of adverse reactions to human serum albumin, making xenotransfusion particularly risky. In dogs, synthetic colloid use has declined following extrapolated human safety concerns, but some practitioners still use them for colloid osmotic pressure support. For both species, balanced crystalloids are the default resuscitation fluid. Colloid use in cats should be reserved for specific indications such as severe hypoalbuminemia with refractory hypovolemia, and only after crystalloid optimization fails. ### How should I document fluid selection and response in the medical record? Record the indication for fluid therapy, the specific solution chosen, the rationale for that choice, and the total volume administered. Include baseline and serial measurements of heart rate, blood pressure, mucous membrane color, capillary refill time, lactate, and urine output. Note any complications such as prolonged capillary refill, worsening respiratory effort, or peripheral edema, and document the corrective action taken. The [RECOVER initiative CPR guidelines](https://recoverinitiative.org/) illustrate the value of structured documentation during resuscitation, the same principle applies to fluid therapy. If a colloid was declined due to cost or availability, record that decision and the discussion with the owner. Serial body weight measurements provide an objective record of fluid balance and should be logged at each reassessment. ### How do I explain the crystalloid versus colloid choice to a concerned owner? Explain that the primary goal is restoring blood flow to vital organs, and that balanced crystalloid solutions accomplish this safely in most patients. Describe colloids as volume expanders that stay in the bloodstream longer but carry increased risks of kidney injury and bleeding complications, as summarized in the [review on colloid therapy](https://pubmed.ncbi.nlm.nih.gov/34277747/). Use plain language: crystalloids are like adding water to a sponge, while colloids are like adding gel that holds water in the vessels. Reassure owners that the choice is based on current evidence and their pet's specific condition. If a colloid is recommended, explain the specific indication and the monitoring plan. If cost is a factor, discuss the option of larger crystalloid volumes and the associated trade-offs without judgment. ### When is it reasonable to use a synthetic colloid despite the safety concerns? Synthetic colloids may be considered when a patient has severe hypovolemia refractory to adequate crystalloid resuscitation and blood products are unavailable or contraindicated. Examples include severe hypoalbuminemia with nonhemorrhagic volume loss where albumin products are unavailable. The [survey of small animal fluid practices](https://pubmed.ncbi.nlm.nih.gov/28929101/) found that most veterinarians have limited their synthetic colloid use, reflecting growing safety concerns. Before administering a synthetic colloid, document that crystalloid therapy was optimized, identify the specific hemodynamic target not achieved, and rule out ongoing hemorrhage. Use the lowest effective dose and discontinue as soon as perfusion parameters stabilize. Monitor renal function, coagulation status, and fluid balance closely for 24 to 48 hours after administration. ## Related Clinical & Scientific Guides * [Toxicology in Emergency Practice: Common Poisons and Diagnostic Approach](/knowledge/veterinary-medicine/emergency-critical-care/toxicology-emergency-practice-common-poisons-diagnostic-approach) * [Veterinary Cardiopulmonary Resuscitation: Post-Cardiac Arrest Care](/knowledge/veterinary-medicine/emergency-critical-care/veterinary-cardiopulmonary-resuscitation-post-cardiac-arrest-care) * [Fluid Therapy Guidelines for Dogs and Cats: A Practical Update](/knowledge/veterinary-medicine/emergency-critical-care/fluid-therapy-guidelines-dogs-cats-practical-update) ## References and Further Reading - [A critical appraisal of intravenous fluids: from the physiological basis to clinical evidence.](https://pubmed.ncbi.nlm.nih.gov/24463187/). 2015. - [Current Trends in Volume Replacement Therapy and the Use of Synthetic Colloids in Small Animals-An Internet-Based Survey (2016).](https://pubmed.ncbi.nlm.nih.gov/28929101/). 2017. - [Searching For the Optimal Fluid to Restore Microcirculatory Flow Dynamics After Hemorrhagic Shock: A Systematic Review of Preclinical Studies.](https://pubmed.ncbi.nlm.nih.gov/27465755/). 2016. - [Colloids Yes or No? - a "Gretchen Question" Answered.](https://pubmed.ncbi.nlm.nih.gov/34277747/). 2021. - [The impact of fluid therapy on microcirculation and tissue oxygenation in hypovolemic patients: a review.](https://pubmed.ncbi.nlm.nih.gov/20502873/). 2010. - [Acute cerebral effects of isotonic crystalloid and colloid solutions following cryogenic brain injury in the rabbit.](https://pubmed.ncbi.nlm.nih.gov/2457341/). 1988. - [RECOVER Initiative Veterinary CPR Guidelines](https://recoverinitiative.org/). Veterinary Emergency and Critical Care Society. - [AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats](https://www.aaha.org/resources/2024-aaha-fluid-therapy-guidelines-for-dogs-and-cats/). AAHA. - [MSD Veterinary Manual, Professional Edition](https://www.msdvetmanual.com/). MSD Veterinary Manual. ## Related Articles - [Subcutaneous Fluid Therapy in Dogs: Indications, Volumes, and Monitoring](/knowledge/veterinary-medicine/emergency-critical-care/subcutaneous-fluid-therapy-dogs-indications-volumes-monitoring) - [Fluid Therapy Guidelines for Dogs and Cats: A Practical Update](/knowledge/veterinary-medicine/emergency-critical-care/fluid-therapy-guidelines-dogs-cats-practical-update) - [Goal-Directed Fluid Therapy in Sepsis: Resuscitation, Optimization, and Maintenance](/knowledge/veterinary-medicine/emergency-critical-care/goal-directed-fluid-therapy-sepsis-resuscitation-optimization-maintenance) - [Veterinary Shock: Fluid Resuscitation Strategies](/knowledge/veterinary-medicine/emergency-critical-care/veterinary-shock-fluid-resuscitation-strategies) - [Complications of Oxygen Therapy in Veterinary Patients](/knowledge/veterinary-medicine/emergency-critical-care/complications-oxygen-therapy-veterinary-patients) > This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.