Montana Anesthesia Services

Crystalloids and Colloids in Anesthesia 

Many of the fluids used in medicine and anesthesia can be classified as either crystalloids or colloids, two different types of water-based mixtures with varying properties. A crystalloid is an aqueous solution of small soluble molecules, like salts, glucose, and electrolytes, that does not contain insoluble molecules.1 Crystalloid solutions can be classified by tonicity, which is a measure of a solution’s ability to modify the volume of cells by altering their water content.2 Colloid solutions contains larger, insoluble molecules, like proteins and certain kinds of starches, that are suspended in a crystalloid solution. Blood, milk, and synthetic compounds like gelatins and complex polysaccharides are examples of colloids.1 

Many surgeries involve the administration of intravenous fluids in order to maintain a balance of fluids and electrolytes and to ensure sufficient blood circulation.3 Crystalloids typically pass easily into tissues, while colloids are usually better at expanding the volume of circulating blood because their larger molecules make it easier for the fluid to remain within the vasculature and increase osmotic pressure.4 Whether crystalloids or colloids are preferred depends partially on how a certain procedure and the administered anesthesia affect a patient’s fluid levels and other medical criteria.  

A major side effect of spinal anesthesia, which is commonly used for C-sections, among other surgeries, is hypotension. In a 2018 study published in Anesthesiology and Pain Medicine, researchers sought to determine whether crystalloids or colloids administered after anesthesia would more effectively reduce hypotension.4 They found that the colloid solution was more effective at controlling both systolic and diastolic blood pressure, as well as in controlling heart rate. The type of intravenous fluid had no effect on Apgar score, but patients in the colloid solution group experienced a lower incidence of nausea and vomiting after spinal anesthesia. 

Colloids were also found to be more effective in a study involving patients undergoing major elective abdominal surgery. Use of the colloid hydroxyethyl starch was associated with a significantly lower score on the postoperative morbidity survey (POMS) and lower overall complications.5 The authors suggested that the beneficial effect of the colloid solution may be due to the lower volumes of fluid required for colloids compared to crystalloids. 

However, it is unclear whether colloids should be the preferred choice broadly. The American Society of Anesthesiologists acknowledges that the “crystalloid vs. colloid” debate remains unresolved, but they recommend isotonic crystalloid fluid as the primary maintenance fluid for all major surgery.6 Crystalloids, due to their ability to freely permeate vascular membranes, provide immediate fluid resuscitation,7 which is critical for patients who are severely dehydrated or are experiencing a severe degree of fluid loss. On the other hand, some studies show no significant difference between the two types of fluids: in a nearly decade-long trial comparing colloids and crystalloids in ICU patients with hypovolemic shock, intravenous fluid type did not result in a significant difference in 28-day mortality.8  

The anesthesiologist and surgeon must consider each patient’s medical needs and expected impacts of the particular procedure to be performed to decide whether to administer crystalloids or colloids during anesthesia and surgery. Healthcare providers should be prepared to quickly adapt to sudden changes in patient status. 


1. Rudloff, E. & Hopper, K. Crystalloid and Colloid Compositions and Their Impact. Front. Vet. Sci. 8, (2021), DOI: 10.3389/fvets.2021.639848 

2. Maldonado, K. A. & Mohiuddin, S. S. Biochemistry, Hypertonicity. in StatPearls (StatPearls Publishing, Treasure Island (FL), 2024), 

3. Voldby, A. W. & Brandstrup, B. Fluid therapy in the perioperative setting—a clinical review. J. Intensive Care 4, 27 (2016), DOI: 10.1186/s40560-016-0154-3 

4. Gousheh, M. R., Akhondzade, R., Asl Aghahoseini, H., Olapour, A. & Rashidi, M. The Effects of Pre-Spinal Anesthesia Administration of Crystalloid and Colloid Solutions on Hypotension in Elective Cesarean Section. Anesthesiol. Pain Med. 8, e69446 (2018), DOI: 10.5812/aapm.69446 

5. Joosten, A. et al. Crystalloid versus Colloid for Intraoperative Goal-directed Fluid Therapy Using a Closed-loop System: A Randomized, Double-blinded, Controlled Trial in Major Abdominal Surgery. Anesthesiology 128, 55–66 (2018), DOI: 10.1097/ALN.0000000000001936 

6. Miller, T. E. & Myles, P. S. Perioperative Fluid Therapy for Major Surgery. Anesthesiology 130, 825–832 (2019), DOI: 10.1097/ALN.0000000000002603 

7. Lewis, S. R. et al. Colloids versus crystalloids for fluid resuscitation in critically ill people. Cochrane Database Syst. Rev. 2018, CD000567 (2018), DOI: 10.1002/14651858.CD000567.pub7 

8. Annane, D. et al. Effects of fluid resuscitation with colloids vs crystalloids on mortality in critically ill patients presenting with hypovolemic shock: the CRISTAL randomized trial. JAMA 310, 1809–1817 (2013), DOI: 10.1001/jama.2013.280502