There are few teachings more deeply embedded in acute trauma care than this: establish two large-bore intravenous cannulae and administer a warm crystalloid bolus. For generations of doctors trained in the ATLS system, this instruction has been axiomatic - the first reflexive action in the C of ABCDE when a bleeding patient arrives in the resuscitation bay. The logic seemed unassailable: haemorrhagic shock is a problem of inadequate circulating volume, so the treatment is to replace that volume. Yet over the past decade, a growing body of evidence has converged on a deeply uncomfortable conclusion - that aggressive crystalloid resuscitation in the bleeding trauma patient may not merely be unhelpful, but actively harmful. The emergence of damage control resuscitation as a paradigm has fundamentally challenged the volume-replacement model, and a landmark 2025 study from the Israeli National Trauma Registry has now provided some of the clearest data yet that the very threshold ATLS has long taught - two litres - may be an independent risk factor for death.
The Traditional Teaching and Its Origins
The ATLS programme was developed in the late 1970s and first delivered in 1980. Its approach to haemorrhagic shock was built on the physiology of volume depletion: a patient who has lost blood has lost circulating volume, and isotonic crystalloid - typically normal saline or Hartmann’s solution - can temporarily expand the intravascular compartment until definitive haemorrhage control is achieved. The original teaching advocated a rapid infusion of one to two litres of warmed crystalloid as an initial resuscitation bolus, with the patient’s response (vital signs, urine output, mental status) used to classify the degree of shock and guide further management. This model was intuitive, practical, and universally available: crystalloid is cheap, does not require cross-matching, carries no infectious risk, and can be stored at room temperature. It became the backbone of trauma resuscitation worldwide.
The problem is that crystalloid is not blood. It carries no oxygen, contains no clotting factors, and distributes rapidly out of the intravascular space into the interstitium - approximately two-thirds of an infused volume of isotonic crystalloid leaves the circulation within an hour. The clinical consequence is that achieving haemodynamic targets with crystalloid alone requires large volumes, and those large volumes bring their own pathophysiology. This was recognised early in the damage control surgery literature of the 1990s and 2000s, but it has taken considerably longer for the evidence to translate into changes in resuscitation practice - and longer still for it to penetrate the ATLS curriculum.
How Crystalloid Harms the Bleeding Patient
The mechanisms by which aggressive crystalloid resuscitation worsens outcomes in haemorrhagic shock are multiple, synergistic, and well-characterised. The most immediately dangerous is dilutional coagulopathy. Crystalloid dilutes circulating clotting factors and platelets, impairing the already-compromised coagulation cascade in the bleeding patient. This effect compounds the acute traumatic coagulopathy (ATC) that develops independently as a result of tissue injury and hypoperfusion - creating a vicious cycle in which the treatment of haemorrhage actively promotes further haemorrhage.
Normal saline, the most widely used crystalloid worldwide, has additional specific harms. Its supraphysiological chloride concentration (154 mmol/L, compared with the plasma concentration of approximately 100 mmol/L) causes a hyperchloraemic metabolic acidosis that compounds the lactic acidosis of shock. This acidosis further impairs clotting factor function and may itself contribute to mortality. The SMART trial in critically ill adults, published in the New England Journal of Medicine in 2018, demonstrated that balanced crystalloids (such as Ringer’s lactate or Plasma-Lyte) reduced the composite of death, new renal replacement therapy, and persistent renal dysfunction compared with normal saline - a finding with direct implications for trauma practice, although the trial was conducted in a mixed ICU population.
Beyond coagulopathy and acidosis, large-volume crystalloid causes tissue oedema that impairs oxygen delivery to end organs, promotes abdominal compartment syndrome (a recognised cause of secondary organ failure in the resuscitated trauma patient), disrupts the endothelial glycocalyx (a critical regulator of vascular permeability), and produces hypothermia even when warmed fluids are used if infusion rates exceed the body’s capacity to maintain core temperature. These effects collectively constitute a iatrogenic assault on the very physiology the clinician is trying to restore. The concept of the ‘lethal triad’ - hypothermia, acidosis, and coagulopathy - was originally described as a consequence of injury, but it is now clear that aggressive crystalloid resuscitation actively accelerates all three components.
The 2025 Israeli Registry Study: Two Litres as a Mortality Threshold
The most significant recent contribution to this debate was published in the American Journal of Emergency Medicine in early 2025 by Goldman, Radomislensky, and colleagues from the Israel National Center for Trauma and Emergency Medicine Research. Using data from the Israeli National Trauma Registry spanning a decade (January 2013 to December 2022), the study included 10,707 haemodynamically compromised trauma patients - defined by a systolic blood pressure below 90 mmHg or a shock index above 1.0 either in the field or in the emergency department. After multivariable logistic regression adjusting for clinical variables including injury severity, the authors found that the administration of two or more litres of crystalloid during the initial phase of care (prehospital and emergency department combined) was independently associated with increased in-hospital mortality.
Crucially, the study also documented a clear temporal trend: over the ten-year study period, the volume of crystalloid administered to trauma patients at risk of shock declined steadily, reflecting the growing adoption of damage control resuscitation principles. The median age of the cohort was 39 years, 63.5% were male, and blunt trauma (predominantly motor vehicle collisions and falls) accounted for 81.4% of injuries. This is a population that closely mirrors the typical ATLS teaching case. The finding that the two-litre mark - the very volume historically advocated as an initial resuscitation bolus - represents a threshold for harm is a direct challenge to the traditional ATLS algorithm.
Consistent Findings from Other Datasets
The Israeli data are not isolated. Analysis of the Glue Grant database - a prospectively collected multicentre cohort of severely injured blunt trauma patients in haemorrhagic shock - by Kasotakis and colleagues found significant associations between crystalloid volume infused in the first 24 hours and the development of acute lung injury, acute respiratory distress syndrome, multi-organ failure, abdominal compartment syndrome, and surgical site infections. A study from St Michael’s Hospital in Toronto found that administration of five or more litres of crystalloid within 24 hours of injury was independently associated with increased mortality and prolonged mechanical ventilation. A cross-national multicenter cohort from the Pan-Asia Trauma Outcomes Study (PATOS) similarly found that prehospital crystalloid resuscitation was associated with adverse outcomes in multiple Asian trauma populations, leading the authors to discourage its routine use.
A 2024 perspective published in Critical Care by an international group of authors asked the question directly: are crystalloid-based fluid expansion strategies still relevant in the first hours of trauma-induced haemorrhagic shock? The review concluded that while crystalloid-based resuscitation remains relevant, its benefit is increasingly questioned because it is suspected to increase bleeding and worsen coagulopathy. The authors emphasised that current guidelines recommending isotonic crystalloid as first-line are supported by a weak evidence base, and that several fundamental aspects - including the optimal type of fluid, the volume, and the administration protocol - remain subjects of active scientific debate.
Damage Control Resuscitation: The New Paradigm
The alternative to aggressive crystalloid resuscitation is the damage control resuscitation (DCR) paradigm, which has emerged from military trauma experience and progressively migrated into civilian practice. DCR is built on several interconnected principles: permissive hypotension (accepting a lower-than-normal blood pressure until surgical haemorrhage control is achieved), minimisation of crystalloid use, early use of blood products in balanced ratios, early administration of tranexamic acid, aggressive correction of coagulopathy, and rapid progression to definitive haemorrhage control. The European guideline on management of major bleeding and coagulopathy following trauma, updated regularly, now recommends restricting crystalloid volumes and using vasopressors rather than pursuing volume-based normalisation of blood pressure. Current European recommendations suggest titrating fluids and vasopressors to a target systolic blood pressure of 80 to 90 mmHg until haemorrhage control in patients without traumatic brain injury, or above 110 mmHg when significant TBI coexists.
This represents a philosophical reversal: from ‘fill the tank’ to ‘plug the hole first, then cautiously restore the volume’. The practical implications are enormous. In a DCR-oriented system, the first action in haemorrhagic shock is not necessarily to open the fluid warmer but to activate the massive haemorrhage protocol, apply direct haemorrhage control, and prepare for emergency surgery or interventional radiology. Crystalloid has not been eliminated - it remains the only universally available volume expander and has a role in maintaining vascular access and delivering medications - but its use as a primary resuscitation tool for major haemorrhage is increasingly viewed as a legacy practice that the evidence no longer supports.
The Tension with ATLS Teaching
The 10th edition of ATLS, published in 2018, reflects an evolving position. It acknowledges the concept of damage control resuscitation and the harms of aggressive fluid administration. It notes that an initial bolus of one litre of isotonic crystalloid may be appropriate, with the patient’s response guiding further management, and it recognises that massive transfusion protocols should be activated early in patients with suspected severe haemorrhage. However, the fundamental framework - two large-bore IVs, an initial crystalloid bolus, classification of shock by estimated blood loss and response to fluid - remains the structural backbone of the circulation chapter. The tension between the course’s legacy teaching and the direction of contemporary evidence is palpable.
For ATLS candidates, this creates a nuanced examination landscape. The core teaching remains examinable: candidates must understand the classification of haemorrhagic shock (Classes I through IV), the principles of initial fluid resuscitation, and the assessment of response to treatment. However, candidates who demonstrate awareness of the limitations of crystalloid resuscitation - who can articulate the mechanisms by which large-volume crystalloid worsens coagulopathy, acidosis, and oedema - will be better positioned in both written and viva examinations. Understanding why the trend in trauma care is towards smaller crystalloid volumes, earlier blood product administration, and permissive hypotension is now expected of any well-prepared candidate.
What Remains Unknown
Despite the accumulating evidence against aggressive crystalloid use, important questions remain. The Israeli registry study, like most evidence in this area, is observational: patients who received more crystalloid may have been sicker, bled more, or had longer transport times, and despite statistical adjustment, residual confounding cannot be excluded. No large randomised controlled trial has directly compared restrictive versus liberal crystalloid resuscitation in trauma patients as a primary intervention - designing such a trial would be ethically and logistically formidable. The optimal volume of crystalloid (if any) in the initial resuscitation remains undefined. The role of crystalloid in settings where blood products are not immediately available - rural and prehospital environments, resource-limited health systems - is particularly unclear, as withholding fluid entirely in a patient with uncontrolled haemorrhage and no access to blood products may be worse than the complications of crystalloid. And the relative merits of balanced crystalloids (Hartmann’s, Plasma-Lyte) versus normal saline in trauma specifically are still being investigated, though the balance of evidence favours balanced solutions.
Synthesis
The two-litre crystalloid bolus, once the unchallenged cornerstone of trauma resuscitation, is now one of the most contested teachings in acute care. A decade of registry data, cohort analyses, and mechanistic evidence has converged on the conclusion that aggressive crystalloid administration in the bleeding patient dilutes clotting factors, worsens acidosis, promotes tissue oedema, and may independently increase mortality. The 2025 Israeli registry study - with over 10,000 patients and a clear dose-response threshold at two litres - provides the most compelling population-level data to date. Damage control resuscitation, with its emphasis on crystalloid minimisation, permissive hypotension, and early haemostatic therapy, has replaced the volume-replacement model in leading trauma systems worldwide. For the ATLS candidate, the traditional classification of shock and the principles of initial assessment remain essential and examinable. But the candidate who understands that the evidence base beneath the crystalloid bolus has eroded - and who can articulate what is replacing it and why - will demonstrate the kind of critical engagement with evolving evidence that examiners and clinical supervisors increasingly expect.
