Fluid Therapy: Double-Edged Sword during Critical Care?

Identification and validation of sepsis subphenotypes using time-series data

Purpose

The recognition of sepsis as a heterogeneous syndrome necessitates identifying distinct subphenotypes to select targeted treatment.

Methods

Patients with sepsis from the MIMIC-IV database (2008-2019) were randomly divided into a development cohort (80%) and an internal validation cohort (20%). Patients with sepsis from the ICU database of Peking University People's Hospital (2008-2022) were included in the external validation cohort. Time-series k-means clustering analysis and dynamic time warping was performed to develop and validate sepsis subphenotypes by analyzing the trends of 21 vital signs and laboratory indicators within 24 h after sepsis onset. Inflammatory biomarkers were compared in the ICU database of Peking University People's Hospital, whereas treatment heterogeneity was compared in the MIMIC-IV database.

Findings

Three sub-phenotypes were identified in the development cohort. Type A patients (N = 2525, 47%) exhibited stable vital signs and fair organ function, type B (N = 1552, 29%) was exhibited an obvious inflammatory response and stable organ function, and type C (N = 1251, 24%) exhibited severely impaired organ function with a deteriorating tendency. Type C demonstrated the highest mortality rate (33%) and levels of inflammatory biomarkers, followed by type B (24%), whereas type A exhibited the lowest mortality rate (11%) and levels of inflammatory biomarkers. These subphenotypes were confirmed in both the internal and external cohorts, demonstrating similar features and comparable mortality rates. In type C patients, survivors had significantly lower fluid intake within 24 h after sepsis onset (median 2891 mL, interquartile range (IQR) 1530-5470 mL) than that in non-survivors (median 4342 mL, IQR 2189-7305 mL). For types B and C, survivors showed a higher proportion of indwelling central venous catheters (p < 0.05).

Conclusion

Three novel phenotypes of patients with sepsis were identified and validated using time-series data, revealing significant heterogeneity in inflammatory biomarkers, treatments, and consistency across cohorts.

Diagnostic accuracy of left ventricular outflow tract velocity time integral versus inferior vena cava collapsibility index in predicting post-induction hypotension during general anesthesia: an observational study

BACKGROUND

Point of care ultrasound (POCUS) is being explored for dynamic measurements like inferior vena cava collapsibility index (IVC-CI) and left ventricular outflow tract velocity time integral (LVOT-VTI) to guide anesthesiologists in predicting fluid responsiveness in the preoperative period and in treating post-induction hypotension (PIH) with varying accuracy.

METHODS

In this prospective, observational study on included 100 adult patients undergoing elective surgery under general anesthesia, the LVOT-VTI and IVC-CI measurements were performed in the preoperative room 15 minutes prior to surgery, and PIH was measured for 20 minutes in the post-induction period.

RESULTS

The incidence of PIH was 24%. The area under the curve, sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of the two techniques at 95% confidence interval was 0.613, 30.4%, 93.3%, 58.3%, 81.4%, 73.6% for IVC-CI and 0.853, 83.3%, 80.3%, 57.1%, 93.8%, 77.4% for LVOT-VTI, respectively. In multivariate analysis, the cutoff value for IVC-CI was >51.5 and for LVOT-VTI it was ≤17.45 for predicting PIH with odd ratio [OR] of 8.491 (P=0.025) for IVCCI and OR of 17.427 (P<0.001) for LVOT. LVOT-VTI assessment was possible in all the patients, while 10% of patients were having poor window for IVC measurements.

CONCLUSIONS

We recommend the use of POCUS using LVOT-VTI or IVC-CI to predict PIH, to decrease the morbidity of patients undergoing surgery. Out of these, we recommend LVOT-VTI measurements as it has showed a better diagnostic accuracy (77.4%) with no failure rate.

Associations between fluid overload and outcomes in critically ill patients with acute kidney injury: a retrospective observational study

Increased fluid overload (FO) is associated with poor outcomes in critically ill patients, especially in acute kidney injury (AKI). However, the exact timing from when FO influences outcomes remains unclear. We retrospectively screened intensive care unit (ICU) admitted patients with AKI between January 2011 and December 2015. Logistic or linear regression analyses were performed to determine when hourly %FO was significant on 90-day in-hospital mortality (primary outcome) or ventilator-free days (VFDs). In total, 1120 patients were enrolled in this study. Univariate analysis showed that a higher %FO was significantly associated with higher mortality from the first hour of ICU admission (odds ratio 1.34, 95% confidence interval 1.15-1.56, P < 0.001), whereas multivariate analysis adjusted with age, sex, APACHE II score, and sepsis etiology showed the association was significant from the 27th hour. Both univariate and multivariate analyses showed that a higher %FO was significantly associated with shorter VFDs from the 1st hour. The significant associations were retained during all following observation periods after they showed significance. In patients with AKI, a higher %FO was associated with higher mortality and shorter VFDs from the early phase after ICU admission. FO should be administered with a physiological target or goal in place from the initial phase of critical illness.

Usefulness of the velocity-time integral of the left ventricular outflow tract variability index to predict fluid responsiveness in patients undergoing cardiac surgery

BACKGROUND

Haemodynamic monitoring of patients after cardiac surgery using echocardiographic evaluation of fluid responsiveness is both challenging and increasingly popular. We evaluated fluid responsiveness in the first hours after surgery by determining the variability of the velocity-time integral of the left ventricular outflow tract (VTI-LVOT).

METHODS

We conducted a cross-sectional study of 50 consecutive adult patients who underwent cardiac surgery and in whom it was possible to obtain VTI-LVOT measurements. We then determined the variability and correlations with our pulse pressure variation (PPV) measurements to predict fluid responsiveness.

RESULTS

A strong positive correlation was seen between the VTI-LVOT variability index absolute values and PPV for predicting fluid responsiveness in the first hours after cardiac surgery. We also found that the VTI-LVOT variability index has high specificity and a high positive likelihood ratio compared with the gold standard using a cut-off point of ≥ 12%.

CONCLUSIONS

The VTI-LVOT variability index is a valuable tool for determining fluid responsiveness during the first 6 postoperative hours in patients undergoing cardiac surgery.

Effect of focused cardiopulmonary ultrasonography on clinical outcome of septic shock: a randomized study

Objective

To investigate the effect of focused ultrasonography on clinical outcomes of septic shock.

Methods

Patients with septic shock were randomized into an integrated cardiopulmonary ultrasonography (ICUS) group and conventional (CON) group. Within 1 hour of admission, the ICUS group underwent ICUS examination for hemodynamic decision-making, while the CON group received standard treatment. The primary endpoint was 28-day mortality after admission. The secondary endpoints were cumulative fluid administration in the first 6, 24, and 72 hours; use of vasoactive drugs; lactate clearance; duration of ventilation; and ICU stay.

Results

Ninety-four qualified patients were enrolled (ICUS group, 49; CON group, 45). ICUS showed no significant effect on 28-day mortality. Within the initial 6 hours, the ICUS group tended to have a higher fluid balance and fluid intake than the CON group. The duration of vasopressor support was shorter in the ICUS group. There were no differences in the cumulative fluid infusion within 24 or 72 hours, lactate clearance, ICU stay, or duration of ventilation.

Conclusions

The initially focused ICUS did not affect the clinical outcomes of septic shock, but it tended to be associated with a higher fluid balance within the initial 6 hours and shorter duration of vasopressor support.

Is restrictive fluid resuscitation beneficial not only for hemorrhagic shock but also for septic shock?: A meta-analysis

BACKGROUND

Whether to use limited fluid resuscitation (LFR) in patients with hemorrhagic shock or septic shock remains controversial. This research was aimed to assess the pros and cons of utilizing LFR in hemorrhagic shock or septic shock patients.

METHODS

PubMed, Cochrane Library, Embase, Web of science, CNKI, VIP, and Wan Fang database searches included for articles published before December 15, 2020. Randomized controlled trials of LFR or adequate fluid resuscitation in hemorrhagic shock or septic shock patients were selected.

RESULT

This meta-analysis including 28 randomized controlled trials (RCTs) and registered 3288 patients. The 7 of 27 RCTs were the patients with septic shock. Others were traumatic hemorrhagic shock patients. Comparing LFR or adequate fluid resuscitation in hemorrhagic shock or septic shock patients, the summary odds ratio (OR) was 0.50 (95% confidence interval [CI] 0.42-0.60, P < .00001) for mortality, 0.46 (95% CI 0.31-0.70, P = .0002) for multiple organ dysfunction syndrome (MODS), 0.35 (95% CI 0.25-0.47) for acute respiratory distress syndrome (ARDS), and 0.33 (95% CI 0.20-0.56) for disseminated intravascular coagulation (DIC).

CONCLUSION

Limited fluid resuscitation is the benefit of both traumatic hemorrhagic shock patients and septic shock patients.

Incoherence between Systemic Hemodynamic and Microcirculatory Response to Fluid Challenge in Critically Ill Patients

Background: The aim of the study was to assess the coherence between systemic hemodynamic and microcirculatory response to a fluid challenge (FC) in critically ill patients. Methods: We prospectively collected data in patients requiring a FC whilst cardiac index (CI) and microcirculation were monitored. The sublingual microcirculation was assessed using the incident dark field (IDF) CytoCam device (Braedius Medical, Huizen, The Netherlands). The proportion of small perfused vessels (PPV) was calculated. Fluid responders were defined by at least a 10% increase in CI during FC. Responders according to changes in microcirculation were defined by at least 10% increase in PPV at the end of FC. Cohen’s kappa coefficient was measured to assess the agreement to categorize patients as “responders” to FC according to CI and PPV. Results: A total of 41 FC were performed in 38 patients, after a median time of 1 (0–1) days after ICU admission. Most of the fluid challenges (39/41, 95%) were performed using crystalloids and the median total amount of fluid was 500 (500–500) mL. The main reasons for fluid challenge were oliguria (n = 22) and hypotension (n = 10). After FC, CI significantly increased in 24 (58%) cases; a total of 19 (46%) FCs resulted in an increase in PPV. Both CI and PPV increased in 13 responders and neither in 11; the coefficient of agreement was only 0.21. We found no correlation between absolute changes in CI and PPV after fluid challenge. Conclusions: The results of this heterogenous population of critically ill patients suggest incoherence in fluid responsiveness between systemic and microvascular hemodynamics; larger cohort prospective studies with adequate a priori sample size calculations are needed to confirm these findings.

The Effect of Fluid Overload on Attributable Morbidity after Cardiac Surgery: A Retrospective Study

INTRODUCTION

Although the detrimental effects of excessive perioperative fluid administration are generally well established, data in the setting of cardiac surgery remain less robust.

METHODS

In this retrospective single-center observational study, the total fluid balance in the first 12 hours during and after surgery was evaluated. Primary endpoint was the relationship between total fluid balance and the incidence of prolonged mechanical ventilation. For this purpose, data were divided into quartiles (Q); prolonged mechanical ventilation and prolonged length of stay (LOS) in the ICU were defined as Q4. Secondary endpoints were prolonged LOS in the ICU, incidence of acute kidney injury (AKI; defined as a 1.5-fold increase in serum creatinine during, relative to baseline), and ICU mortality.

RESULTS

In a 3-year period, 748 patients were included. In a univariate analysis, the median duration of mechanical ventilation was 2.9 h [2.4-3.7] in Q1 of the fluid balance and increased significantly to 4.7 h [3.1-9.2] in Q4 of the fluid balance (p < 0.001). In addition, patients in Q4 of the fluid balance had a significantly longer LOS in the ICU, as well as a higher incidence of AKI and ICU mortality. In a multivariate analysis, Q4 of the fluid balance was independently associated with prolonged mechanical ventilation (OR 4.9, CI 2.9-8.4, p < 0.001) and prolonged LOS in the ICU (OR 11.3 CI 6.1-20,9, p < 0.001), but not with the incidence of AKI or ICU mortality.

CONCLUSIONS

Perioperative fluid administration in cardiac surgery patients was independently associated with prolonged mechanical ventilation and prolonged LOS in the ICU.

Intravenous Fluid Management in Critically Ill Adults: A Review

TOPIC

This article reviews the management of intravenous fluids and the evaluation of volume status in critically ill adults.

CLINICAL RELEVANCE

Intravenous fluid administration is one of the most common interventions in the intensive care unit. Critically ill patients have dynamic fluid requirements, making the management of fluid therapy challenging. New literature suggests that balanced salt solutions may be preferred in some patient populations.

PURPOSE OF PAPER

The bedside critical care nurse must understand the properties of various intravenous fluids and their corresponding impact on human physiology. The nurse's clinical and laboratory assessments of each patient help define the goals of fluid therapy, which will in turn be used to determine the optimal patient-specific selection and dose of fluid for administration. Nurses serve a vital role in monitoring the safety and efficacy of intravenous fluid therapy. Although this intervention can be lifesaving, inappropriate use of fluids has the potential to yield detrimental effects.

CONTENT COVERED

This article discusses fluid physiology and the goals of intravenous fluid therapy, compares the types of intravenous fluids (isotonic crystalloids, including 0.9% sodium chloride and balanced salt solutions; hypotonic and hypertonic crystalloids; and colloids) and their adverse effects and impact on hemodynamics, and describes the critical care nurse's essential role in selecting and monitoring intravenous fluid therapy.

Change in Carotid Blood Flow and Carotid Corrected Flow Time Assessed by Novice Sonologists Fails to Determine Fluid Responsiveness in Spontaneously Breathing Intensive Care Unit Patients

Measurement of carotid blood flow (CBF) and corrected carotid flow time (ccFT) has been proposed as a non-invasive means of determining fluid responsiveness. We evaluated the ability of CBF and ccFT as assessed by novice sonologists to determine fluid responsiveness in intensive care unit patients. Three novice physician sonologists performed carotid ultrasounds before and after a fluid bolus and calculated changes in CBF and ccFT. Fluid responsiveness was defined as a ≥10% increase in cardiac index as measured using bioreactance. Of 112 participants, 56 (50%) were fluid responders. Changes in CBF and ccFT performed poorly at determining fluid responsiveness: 19 mL/min (area under the receiver operating characteristic curve: 0.58, 95% confidence interval: 0.47-0.68) and 6 ms (0.59, 0.46-0.65) respectively. Novice physician sonologists are unable to determine fluid responsiveness using CBF or ccFT. Further research is needed to identify the key limiting factors in using carotid ultrasound to determine fluid responsiveness.

Endothelial glycocalyx in acute care surgery - what anaesthesiologists need to know for clinical practice

The endothelial glycocalyx (EG) is the thin sugar-based lining on the apical surface of endothelial cells. It has been linked to the physiological functioning of the microcirculation and has been found to be damaged in critical illness and after acute care surgery. This review aims to describe the role of EG in severely injured patients undergoing surgery, discuss specific situations (e.G. major trauma, hemorrhagic shock, trauma induced coagulopathy) as well as specific interventions commonly applied in these patients (e.g. fluid therapy, transfusion) and specific drugs related to perioperative medicine with regard to their impact on EG.EG in acute care surgery is exposed to damage due to tissue trauma, inflammation, oxidative stress and inadequate fluid therapy. Even though some interventions (transfusion of plasma, human serum albumin, hydrocortisone, sevoflurane) are described as potentially EG protective there is still no specific treatment for EG protection and recovery in clinical medicine.The most important principle to be adopted in routine clinical practice at present is to acknowledge the fragile structure of the EG and avoid further damage which is potentially related to worsened clinical outcome.

How to detect a positive response to a fluid bolus when cardiac output is not measured?

Background

Volume expansion is aimed at increasing cardiac output (CO), but this variable is not always directly measured. We assessed the ability of changes in arterial pressure, pulse pressure variation (PPV) and heart rate (HR) or of a combination of them to detect a positive response of cardiac output (CO) to fluid administration.

Methods

We retrospectively included 491 patients with circulatory failure. Before and after a 500-mL normal saline infusion, we measured CO (PiCCO device), HR, systolic (SAP), diastolic (DAP), mean (MAP) and pulse (PP) arterial pressure, PPV, shock index (HR/SAP) and the PP/HR ratio.

Results

The fluid-induced changes in HR were not correlated with the fluid-induced changes in CO. The area under the receiver operating characteristic curve (AUROC) for changes in HR as detectors of a positive fluid response (CO increase ≥ 15%) was not different from 0.5. The fluid-induced changes in SAP, MAP, PP, PPV, shock index (HR/SAP) and the PP/HR ratio were correlated with the fluid-induced changes in CO, but with r < 0.4. The best detection was provided by increases in PP, but it was rough (AUROC = 0.719 ± 0.023, best threshold: increase ≥ 10%, sensitivity = 72 [66–77]%, specificity = 64 [57–70]%). Neither the decrease in shock index nor the changes in other indices combining changes in HR, shock index, PPV and PP provided a better detection of a positive fluid response than changes in PP.

Conclusion

A positive response to fluid was roughly detected by changes in PP and not detected by changes in HR. Changes in combined indices including the shock index and the PP/HR ratio did not provide a better diagnostic accuracy.

Associated Mortality of Liberal Fluid Administration in Sepsis

Fluid resuscitation, to restore intravascular volume and improve oxygen delivery, is a crucial step in early resuscitation efforts of patients with sepsis or septic shock. The 2016 Surviving Sepsis Campaign guidelines suggest the use of dynamic versus static measures of fluid responsiveness and fluid resuscitation with at least 30 mL/kg of intravenous crystalloid within the first 3 hours followed by fluid administration if hemodynamic factors continue to improve. Despite these recommendations, risks to this practice may exist as multiple studies have demonstrated an association between a positive fluid balance and/or administration of large fluid volume and increase in mortality. These studies are limited by variations in their methodologic design; therefore, cause and effect cannot yet be determined. Future multicenter, randomized, controlled studies that evaluate fluid balance and fluid volume need to be conducted to clarify the role of fluid administration to patients with sepsis to maximize benefits and minimize risk.

Principles of fluid management and stewardship in septic shock: it is time to consider the four D's and the four phases of fluid therapy

In patients with septic shock, the administration of fluids during initial hemodynamic resuscitation remains a major therapeutic challenge. We are faced with many open questions regarding the type, dose and timing of intravenous fluid administration. There are only four major indications for intravenous fluid administration: aside from resuscitation, intravenous fluids have many other uses including maintenance and replacement of total body water and electrolytes, as carriers for medications and for parenteral nutrition. In this paradigm-shifting review, we discuss different fluid management strategies including early adequate goal-directed fluid management, late conservative fluid management and late goal-directed fluid removal. In addition, we expand on the concept of the "four D's" of fluid therapy, namely drug, dosing, duration and de-escalation. During the treatment of patients with septic shock, four phases of fluid therapy should be considered in order to provide answers to four basic questions. These four phases are the resuscitation phase, the optimization phase, the stabilization phase and the evacuation phase. The four questions are "When to start intravenous fluids?", "When to stop intravenous fluids?", "When to start de-resuscitation or active fluid removal?" and finally "When to stop de-resuscitation?" In analogy to the way we handle antibiotics in critically ill patients, it is time for fluid stewardship.

Anesthesia-Associated Relative Hypovolemia: Mechanisms, Monitoring, and Treatment Considerations

Although the utility and benefits of anesthesia and analgesia are irrefutable, their practice is not void of risks. Almost all drugs that produce anesthesia endanger cardiovascular stability by producing dose-dependent impairment of cardiac function, vascular reactivity, and compensatory autoregulatory responses. Whereas anesthesia-related depression of cardiac performance and arterial vasodilation are well recognized adverse effects contributing to anesthetic risk, far less emphasis has been placed on effects impacting venous physiology and venous return. The venous circulation, containing about 65–70% of the total blood volume, is a pivotal contributor to stroke volume and cardiac output. Vasodilation, particularly venodilation, is the primary cause of relative hypovolemia produced by anesthetic drugs and is often associated with increased venous compliance, decreased venous return, and reduced response to vasoactive substances. Depending on factors such as patient status and monitoring, a state of relative hypovolemia may remain clinically undetected, with impending consequences owing to impaired oxygen delivery and tissue perfusion. Concurrent processes related to comorbidities, hypothermia, inflammation, trauma, sepsis, or other causes of hemodynamic or metabolic compromise, may further exacerbate the condition. Despite scientific and technological advances, clinical monitoring and treatment of relative hypovolemia still pose relevant challenges to the anesthesiologist. This short perspective seeks to define relative hypovolemia, describe the venous system’s role in supporting normal cardiovascular function, characterize effects of anesthetic drugs on venous physiology, and address current considerations and challenges for monitoring and treatment of relative hypovolemia, with focus on insights for future therapies.

A Different Approach to the Nutritional Therapy in Intensive Care Units: Nutrition Software (ICNUS)

Critically ill patients receive nutritional support in addition to extensive organ support therapies in the intensive care units. The transcription of the data obtained by multiple devices from a wide spectrum of usage areas is a challenging process and is prone to errors. Software specifically designed for usage in intensive care units do offer numerous advantages. Primarily, it facilitates to improve the management of time and workflow for the benefit of patients. Therefore, we believed that a different approach was needed for the nutritional regime of our critically ill patients. Following nearly 2 years of research, we developed and implemented the Intensive Care Nutrition Software (ICNUS), which is explained in this article.

The utilization of critical care ultrasound to assess hemodynamics and lung pathology on ICU admission and the potential for predicting outcome

Aim

Critical care ultrasound (CCUS) has been used by many Intensive Care Units(ICUs) worldwide, so as to guiding the diagnosis and the treatment. However, none of the publications currently systematically describe the utilization of CCUS to analyze the characteristics of hemodynamics and lung pathology upon the new admission to ICU and its potential role in patients’ prognosis prediction. In this retrospective clinical study, we have demonstrated and analyzed the characteristics of hemodynamics and lung pathology assessed by CCUS and investigated its potential to predict patient outcome.

Methods

We have described and analyzed the epidemic characteristics of hemodynamics and lung pathology assessed by CCUS on ICU admission, which based on our database of 451 cases from the biggest medical center in Western China, between November 2014 and October 2015. The patients’ demographics, clinical characteristics, prognosis and ultrasonic pattern of hemodynamics and lung pathology had been analyzed. A bivariate logistic regression model was established to identify the correlation between the ultrasonic variables on admission and the ICU mortality.

Results

The mean age of the 451 patients was 56.7±18.7 years; the mean APACHE II score was 19.0±7.9, the ICU mortality was 30.6%. Patients received CCUS examination of pericardial, right ventricle (RV) wall motion, left ventricle (LV) wall motion, LV systolic function, LV diastolic function, lung and volume of inferior vena cava (IVC) were 423(93.8%), 418(92.7%), 392(86.9%), 389(86.3%), 383(84.9%), 440(97.6%), 336(74.5%), respectively; The univariate analysis revealed that length of mechanical ventilation was significantly correlated with the diameter of IVC, tricuspid annular plane systolic excursion(TAPSE), mitral annular plane systolic excursion(MAPSE), early diastolic transmitral velocity to early mitral annulus diastolic velocity(E/e’) (p = 0.016, 0.011, 0.000, 0.049, respectively); The TAPSE, ejection fraction(EF), MAPSE, lung ultrasound score (LUS score) (p = 0.000, 0.028, 0.000, 0.011, respectively) were significantly related to ICU mortality. The multivariate analysis demonstrated that APACHE II, age, TAPSE, E/e’ are the independent risk factors for ICU mortality in our study.

Conclusion

CCUS examination on ICU admission which performed by the experienced physician provide valuable information to assist the caregivers in understanding the comprehensive outlook of the characteristics of hemodynamics and lung pathology. Those key variables obtained by CCUS predict the possible prognosis of patients, hence deserve more attention in clinical decision making.

A review of hemodynamic monitoring techniques, methods and devices for the emergency physician

The emergency department (ED) is frequently the doorway to the intensive care unit (ICU) for a significant number of critically ill patients presenting to the hospital. Hemodynamic monitoring (HDM) which is a key component in the effective management of the critically ill patient presenting to the ED, is primarily concerned with assessing the performance of the cardiovascular system and determining the correct therapeutic intervention to optimise end-organ oxygen delivery. The spectrum of hemodynamic monitoring ranges from simple clinical assessment and routine bedside monitoring to point of care ultrasonography and various invasive monitoring devices. The clinician must be aware of the range of available techniques, methods, interventions and technological advances as well as possess a sound approach to basic hemodynamic monitoring prior to selecting the optimal modality. This article comprises an in depth discussion of an approach to hemodynamic monitoring techniques and principles as well as methods of predicting fluid responsiveness as it applies to the ED clinician. We review the role, applicability and validity of various methods and techniques that include; clinical assessment, passive leg raising, blood pressure, finger based monitoring devices, the mini-fluid challenge, the end-expiratory occlusion test, central venous pressure monitoring, the pulmonary artery catheter, ultrasonography, bioreactance and other modern invasive hemodynamic monitoring devices.

Prediction of fluid responsiveness: an update

In patients with acute circulatory failure, the decision to give fluids or not should not be taken lightly. The risk of overzealous fluid administration has been clearly established. Moreover, volume expansion does not always increase cardiac output as one expects. Thus, after the very initial phase and/or if fluid losses are not obvious, predicting fluid responsiveness should be the first step of fluid strategy. For this purpose, the central venous pressure as well as other “static” markers of preload has been used for decades, but they are not reliable. Robust evidence suggests that this traditional use should be abandoned. Over the last 15 years, a number of dynamic tests have been developed. These tests are based on the principle of inducing short-term changes in cardiac preload, using heart–lung interactions, the passive leg raise or by the infusion of small volumes of fluid, and to observe the resulting effect on cardiac output. Pulse pressure and stroke volume variations were first developed, but they are reliable only under strict conditions. The variations in vena caval diameters share many limitations of pulse pressure variations. The passive leg-raising test is now supported by solid evidence and is more frequently used. More recently, the end-expiratory occlusion test has been described, which is easily performed in ventilated patients. Unlike the traditional fluid challenge, these dynamic tests do not lead to fluid overload. The dynamic tests are complementary, and clinicians should choose between them based on the status of the patient and the cardiac output monitoring technique. Several methods and tests are currently available to identify preload responsiveness. All have some limitations, but they are frequently complementary. Along with elements indicating the risk of fluid administration, they should help clinicians to take the decision to administer fluids or not in a reasoned way.