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Assessment of intravascular volume status and volume responsiveness in critically ill patients

2013; Elsevier BV; Volume: 83; Issue: 6 Linguagem: Inglês

10.1038/ki.2012.424

1523-1755

Kambiz Kalantari, Jamison Chang, Claudio Ronco, Mitchell H. Rosner,

Cardiovascular Health and Disease Prevention

Accurate assessment of a patient's volume status, as well as whether they will respond to a fluid challenge with an increase in cardiac output, is a critical task in the care of critically ill patients. Despite this, most decisions regarding fluid therapy are made either empirically or with limited and poor data. Given recent data highlighting the negative impact of either inadequate or overaggressive fluid therapy, understanding the tools and techniques available for accurate volume assessment is critical. This review highlights both static and dynamic methods that can be utilized to help in the assessment of volume status. Accurate assessment of a patient's volume status, as well as whether they will respond to a fluid challenge with an increase in cardiac output, is a critical task in the care of critically ill patients. Despite this, most decisions regarding fluid therapy are made either empirically or with limited and poor data. Given recent data highlighting the negative impact of either inadequate or overaggressive fluid therapy, understanding the tools and techniques available for accurate volume assessment is critical. This review highlights both static and dynamic methods that can be utilized to help in the assessment of volume status. Accurate assessment of intravascular volume remains one of the most challenging and important tasks for clinicians. Rivers et al.1.Rivers E. Nguyen B. Havstad S. et al.Early goal-directed therapy in the treatment of severe sepsis and septic shock.N Engl J Med. 2001; 345: 1368-1377Crossref PubMed Scopus (5245) Google Scholar demonstrated that a protocol of early goal-directed therapy, which included aggressive fluid resuscitation targeted to central venous pressure (CVP) and physiological variables, reduced organ failure and improved survival in patients with severe sepsis and septic shock. However, more recent studies in critically ill patients have demonstrated that excessive fluid resuscitation and markedly positive net fluid balance is associated with higher rates of complications and increased mortality.2.Vincent J.L. Sakr Y. Sprung C.L. et al.Sepsis in European intensive care units: results of the SOAP study.Crit Care Med. 2006; 34: 344-353Crossref PubMed Scopus (1077) Google Scholar,3.RENAL Replacement Therapy Study Investigators Bellomo R. Cass A. Cole L. et al.An observational study fluid balance and patient outcomes in the Randomized Evaluation of Normal vs. Augmented Level of Replacement Therapy trial.Crit Care Med. 2012; 12: 1753-1760Google Scholar In a European multicenter observational study of patients admitted to the intensive care unit (ICU), each 1liter of positive fluid balance during the first 72h of ICU stay was associated with a 10% increase in mortality after adjustments for other risk factors.2.Vincent J.L. Sakr Y. Sprung C.L. et al.Sepsis in European intensive care units: results of the SOAP study.Crit Care Med. 2006; 34: 344-353Crossref PubMed Scopus (1077) Google Scholar Furthermore, in a landmark study of liberal versus conservative fluid management of patients with acute lung injury in the ICU, a more conservative fluid management strategy improved lung function and shortened the ICU stay, whereas there was no difference in the 60-day mortality between the two groups.4.Wiedemann H.P. Wheeler A.P. Bernard G.R. et al.Comparison of two fluid-managemet strategies in acute lung injury.N Engl J Med. 2006; 354: 2564-2575Crossref PubMed Scopus (1298) Google Scholar Thus, outcomes are clearly influenced by fluid balance with either inadequate or overly aggressive resuscitation associated with excess morbidity and mortality (Figure 1). Despite this, most decisions regarding fluid therapy are still made empirically. When dosing intravenous fluids, two key clinical questions are asked: (1) what is the current state of the patient's intravascular volume? and (2) if the patient receives continued fluid resuscitation or a fluid bolus, will physiological variables such as blood pressure, tissue perfusion, and urine output improve? Fundamentally, the only reason to give a patient a fluid challenge is to increase the stroke volume (SV; by at least 10–15%) and improve organ perfusion. It is therefore crucial during the resuscitation phase of critically ill patients to determine not only the volume status but also whether the patient is fluid-responsive or not. In clinical practice, physical examination, radiography, laboratory parameters, and in case of the critically ill patients in the ICU, monitoring of central pressures and cardiac output are combined to assess the patient's intravascular volume and determine clinical interventions such as fluid or diuretic administration. As with any diagnostic tests, clinicians utilizing these volume assessment techniques need to understand their limitations and diagnostic accuracy. The intent of this review is to survey the literature and summarize the performance characteristics of tests commonly used for the assessment of both intravascular volume status and volume responsiveness in critically ill patients. It is noteworthy that laboratory assessments such as the mixed venous oxygen saturation, blood lactate, and others are not discussed in this paper. The earliest assessment of the patient is the history and physical examination (Table 1). The majority of studies assessing the utility of the history and physical examination in clinical volume assessment are derived from either the assessment of patients with heart failure (HF) or acute blood loss.Table 1Commonly used clinical and laboratory parameters in the assessment of volume status• Vital signs○ Blood pressure (mean arterial pressure)○ Pulse○ Orthostatic changes in blood pressure and pulse• Physical examination○ Mentation○ Capillary refill○ Skin turgor/dryness○ Skin perfusion (color/mottling, temperature)○ Temperature of extremities○ Urine output• Laboratory parameters○ Fractional excretion of sodium, urea○ Blood lactate○ Mixed venous oxygen saturation Open table in a new tab Wang et al.5.Wang C.S. FitzGerald J.M. Schulzer M. et al.Does this dyspneic patient in the emergency department have congestive heart failure?.JAMA. 2005; 294: 1944-1956Crossref PubMed Scopus (219) Google Scholar performed a meta-analysis of 18 studies that evaluated the utility of the history, physical examination, and diagnostic tests in diagnosing HF and volume overload in patients presenting to the emergency department with dyspnea. Among all presenting symptoms, paroxysmal nocturnal dyspnea was most helpful, if present (positive likelihood ratio: 2.6), followed closely by orthopnea and peripheral edema. In a review on the value of physical examination findings in the diagnosis of hypovolemia, it was shown that physical examination findings are dependent on the type and amount of fluid loss.6.McGee S. Abernethy W.B. Simel D.L. The rational clinical examination. Is this patient hypovolemic?.JAMA. 1999; 281: 1022-1029Crossref PubMed Scopus (285) Google Scholar The most useful physical findings are postural dizziness (preventing measurement of upright vital signs) or a postural pulse increment of 30beats/min or more. However, these findings had a high sensitivity for hypovolemia caused only by large blood losses (approximately 1liter) but a poor sensitivity for moderate blood losses (approximately 500ml). The authors point out that in states of volume depletion produced by non–blood loss states, very few findings have clinical utility, and ancillary lab/diagnostic testing is required.6.McGee S. Abernethy W.B. Simel D.L. The rational clinical examination. Is this patient hypovolemic?.JAMA. 1999; 281: 1022-1029Crossref PubMed Scopus (285) Google Scholar This was confirmed in another study that demonstrated that no single clinical sign was useful in identifying a low circulating blood volume.7.Stephan F. Flahault A. Dieudonne N. et al.Clinical evaluation of circulating blood volume in critically ill patients- contribution of a clinical scoring system.Br J Anaesth. 2001; 86: 754-762Crossref PubMed Scopus (37) Google Scholar Furthermore, when clinicians were asked to predict hemodynamic parameters based only on history and physical examination, their performance was poor.8.Eisenberg P.R. Jaffe A.S. Schuster D.P. Clinical evaluation compared to pulmonary artery catheterization in the hemodynamic assessment of critically ill patients.Crit Care Med. 1984; 12: 549-553Crossref PubMed Google Scholar In this study, pulmonary artery occlusive (wedge) pressure was correctly predicted only 30% of the time. Cardiac output, systemic vascular resistance, and right atrial pressures were correctly predicted approximately 50% of the time. The daily chest X-ray (CXR) in the ICU is an established diagnostic tool to complement history and physical examination findings, and is commonly used to assess volume status. In a study of patients with systolic HF awaiting heart transplantation, CXR findings were correlated with measurement of pulmonary capillary wedge pressure (PCWP) as their gold standard for volume overload.9.Chakko S. Woska D. Martinez H. et al.Clinical, radiographic, and hemodynamic correlations in chronic congestive heart failure: conflicting results may lead to inappropriate care.Am J Med. 1991; 90: 353-359Abstract Full Text PDF PubMed Google Scholar Although venous redistribution and interstitial pulmonary edema were seen more commonly among subjects with high PCWP readings, there was a high degree of overlap among groups with different PCWP values. In addition, the absence of radiologic findings typically associated with volume overload did not ensure lower PCWP.9.Chakko S. Woska D. Martinez H. et al.Clinical, radiographic, and hemodynamic correlations in chronic congestive heart failure: conflicting results may lead to inappropriate care.Am J Med. 1991; 90: 353-359Abstract Full Text PDF PubMed Google Scholar Other studies have confirmed that the typical radiologic signs suggesting volume overload are highly variable and insensitive and that CXR findings cannot predict extravascular lung water.10.Ely E.W. Smith A.C. Chiles C. et al.Radiologic determination of intravascular volume status using portable, digital chest radiography: a prospective investigation in 100 patients.Crit Care Med. 2001; 29: 1502-1512Crossref PubMed Google Scholar, 11.Halperin B.D. Feeley T.W. Mihm F.G. et al.Evaluation of the portable chest roentgenogram for quantitating extravascular lung water in critically ill adults.Chest. 1985; 88: 649-652Crossref PubMed Google Scholar, 12.Saugel B. Ringmaier S. Holzapfel K. et al.Physical examination, central venous pressure, and chest radiography for the prediction of transpulmonary thermodilution-derived hemodynamic parameters in critically ill patients: A prospective trial.J Crit Care. 2011; 26: 402-410Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar, 13.Baudendistel L. Shields J.B. Kaminski D.L. Comparison of double indicator thermodilution measurements of extravascular lung water (EVLW) with radiographic estimation of lung water in trauma patients.J Trauma. 1982; 22: 983-988Crossref PubMed Google Scholar Another method derived from the CXR is to examine the vascular pedicle width (VPW). The VPW is measured by1.Rivers E. Nguyen B. Havstad S. et al.Early goal-directed therapy in the treatment of severe sepsis and septic shock.N Engl J Med. 2001; 345: 1368-1377Crossref PubMed Scopus (5245) Google Scholar dropping a perpendicular line from the point at which the left subclavian artery exits the aortic arch and2.Vincent J.L. Sakr Y. Sprung C.L. et al.Sepsis in European intensive care units: results of the SOAP study.Crit Care Med. 2006; 34: 344-353Crossref PubMed Scopus (1077) Google Scholar measuring across to the point at which the superior vena cava (SVC) crosses the right mainstem bronchus.14.Miller R.R. Ely E.W. Radiographic measures of intravascular volume status: the role of vascular pedicle width.Curr Opin Crit Care. 2006; 12: 255-262Crossref PubMed Scopus (9) Google Scholar Studies of critically ill patients have demonstrated correlations between serial changes in VPW and changes in volume status.15.Pistolesi M. Milne E.N. Miniati M. et al.The vascular pedicle of the heart and the vena azygos. Part II: Acquired heart disease.Radiology. 1984; 152: 9-17Crossref PubMed Google Scholar, 16.Haponik E.F. Adelman M. Munster A.M. et al.Increased vascular pedicle width preceding burn-related pulmonary edema.Chest. 1986; 90: 649-655Crossref PubMed Google Scholar, 17.Don C. Burns K.D. Levine D.Z. Body fluid volume status in hemodialysis patients: the value of the chest radiograph.Can Assoc Radiol J. 1990; 41: 123-126PubMed Google Scholar, 18.Thomason J.W. Ely E.W. Chiles C. et al.Appraising pulmonary edema using supine chest roentgenograms in ventilated patients.Am J Resp Crit Care Med. 1998; 157: 1600-1608Crossref PubMed Google Scholar Ely et al.10.Ely E.W. Smith A.C. Chiles C. et al.Radiologic determination of intravascular volume status using portable, digital chest radiography: a prospective investigation in 100 patients.Crit Care Med. 2001; 29: 1502-1512Crossref PubMed Google Scholar demonstrated that using a VPW cutoff value of 70mm in addition to a cardiothoracic ratio (cardiac width divided by thoracic width) greater than 0.55 significantly improved the accuracy of CXR in determining volume status (gold standard determined by PCWP). The VPW is best utilized in a single patient on serial measurements, and changes in VPW show a high correlation with changes in volume status.19.Pistolesi M. Milne E.N. Miniati M. et al.The vascular pedicle of the heart and the vena azygous. Part II: Acquired heart disease.Radiology. 1984; 152: 9-17Crossref PubMed Google Scholar,20.Ely E.W. Haponik E.F. Using the chest radiograph to determine intravascular volume status: the role of vascular pedicle width.Chest. 2002; 121: 942-950Crossref PubMed Scopus (32) Google Scholar In clinical practice, VPW is not widely used by practitioners, owing to the fact that the position of the patient, patient's height and body build, technical factors, diseases of mediastinum, and prior trauma, surgery, or irradiation may all affect the validity of VPW measurements.20.Ely E.W. Haponik E.F. Using the chest radiograph to determine intravascular volume status: the role of vascular pedicle width.Chest. 2002; 121: 942-950Crossref PubMed Scopus (32) Google Scholar It is evident that both the physical examination and chest radiography provide limited value in the determination of a patient's volume status and that other more sensitive and specific techniques are needed. In theory, fluid resuscitation in the hypovolemic patient increases right ventricular end-diastolic volume, left ventricular end-diastolic volume, and depending upon the position on the Frank–Starling curve, SV and cardiac output (CO). If this holds true, measuring such parameters would be a useful tool in guiding decisions regarding fluid resuscitation. The most common of these parameters are CVP, pulmonary artery occlusion pressure, or PCWP, and right ventricular end-diastolic volume. Arguably, CVP is the most commonly used parameter for guiding fluid management in ICUs. Surveys of intensivists and anesthesiologists suggest that more than 90% use CVP to guide fluid management.21.Boldt J. Lenz M. Kumle B. et al.Volume replacement strategies on intensive care units: results from a postal survey.Intensive Care Med. 1998; 24: 147-151Crossref PubMed Scopus (107) Google Scholar,22.Kastrup M. Markewitz A. Spies C. et al.Current practice of hemodynamic monitoring and vasopressor and inotropic therapy in post-operative cardiac surgery patients in Germany: results from a postal survey.Acta Anaes Scand. 2007; 51: 347-358Crossref PubMed Scopus (0) Google Scholar In addition, guidelines have recommended the use of CVP in guiding fluid management in critically ill septic patients.23.Dellinger R.P. Carlet J.M. Masur H. et al.Surviving Sepsis Campaign guidelines for management of severe sepsis and septic shock.Crit Care Med. 2004; 32: 858-873Crossref PubMed Scopus (1977) Google Scholar In a meta-analysis of five studies, the pooled correlations between CVP and measured blood volume or change in cardiac index (CI) or SV in response to volume expansion were 0.16 and 0.18, respectively, and the pooled area under the ROC curve was only 0.56.24.Marik P.E. Baram M. Vahid B. Does central venous pressure predict fluid responsiveness? A systematic review of the literature and the tale of seven mares.Chest. 2008; 134: 172-178Crossref PubMed Scopus (548) Google Scholar Thus, the likelihood that CVP can accurately predict fluid responsiveness is only 56% (slightly better than flipping a coin). The same meta-analysis also investigated the utility of changes in CVP after volume expansion in predicting changes in CO or SV by including 19 additional studies. Pooled correlation coefficient between the change in CVP and changes in CI or SI were even worse at 0.11.24.Marik P.E. Baram M. Vahid B. Does central venous pressure predict fluid responsiveness? A systematic review of the literature and the tale of seven mares.Chest. 2008; 134: 172-178Crossref PubMed Scopus (548) Google Scholar Numerous other studies have come to similar conclusions.25.Michard F. Teboul J.L. Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence.Chest. 2002; 121: 2000-2008Crossref PubMed Scopus (637) Google Scholar, 26.Baek S.M. Makabaki G.G. Bryan-Brown C.W. et al.Plasma expansion in surgical patients with high central venous pressure (CVP): the relationship of blood volume to hematocrit, CVP, pulmonary wedge pressure, and cardiorespiratory changes.Surgery. 1975; 78: 304-315PubMed Google Scholar, 27.Hoeft A. Schorn B. Weyland A. et al.Bedside assessment of intravascular volume status in patients undergoing coronary bypass surgery.Anesthesiology. 1994; 81: 76-86Crossref PubMed Google Scholar, 28.Godje O. Peyerl M. Seebauer T. et al.Central venous pressure, pulmonary capillary wedge pressure and intrathoracic blood volumes as preload indicators in cardiac surgery patients.Eur J Cardiothorac Surg. 1998; 13: 533-539Crossref PubMed Scopus (141) Google Scholar CVP is dependent on venous return (VR) to the heart, right ventricular compliance, peripheral venous tone, and posture, and the CVP is particularly unreliable in pulmonary vascular disease, right ventricular disease, patients with tense ascites, isolated left ventricular failure, and valvular heart disease.24.Marik P.E. Baram M. Vahid B. Does central venous pressure predict fluid responsiveness? A systematic review of the literature and the tale of seven mares.Chest. 2008; 134: 172-178Crossref PubMed Scopus (548) Google Scholar,29.Marik P.E. Baram M. Non-invasive hemodynamic monitoring in the intensive care unit.Crit Care Clin. 2007; 23: 383-400Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar In patients with an intact sympathetic response to hypovolemia, the CVP may actually fall in response to fluid, as compensatory venoconstriction is reduced.29.Marik P.E. Baram M. Non-invasive hemodynamic monitoring in the intensive care unit.Crit Care Clin. 2007; 23: 383-400Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar Thus, it is possible to have a low CVP and not be volume responsive, as well as have a high CVP and be volume responsive. Pulmonary artery occlusion pressure or PCWP has been widely used to assess volume status and fluid responsiveness in the ICU and operating room. Ideally, the PCWP is proportional to left -ventricular end-diastolic volume/preload and would seem to be an ideal parameter for monitoring volume status. However, the vast majority of studies have demonstrated a poor correlation between PCWP, volume status, and responsiveness to fluid resuscitation.25.Michard F. Teboul J.L. Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence.Chest. 2002; 121: 2000-2008Crossref PubMed Scopus (637) Google Scholar, 30.Diebel L.N. Wilson R.F. Tagett M.G. et al.End-diastolic volume: a better indicator of pre-load in the critically ill.Arch Surg. 1992; 127: 817-822Crossref PubMed Google Scholar, 31.Wagner J.G. Leatherman J.W. Right ventricular end-diastolic volume as a predictor of the hemodynamic response to a fluid challenge.Chest. 1998; 113: 1048-1054Crossref PubMed Google Scholar, 32.Tousignant C.P. Walsh F. Mazer C.D. The use of transesophageal echocardiography for preload assessment in critically ill patients.Anesth Analg. 2000; 90: 351-355PubMed Google Scholar, 33.Osman D. Ridel C. Ray P. et al.Cardiac Filling Pressures are not appropriate to predict hemodynamic response to volume challenges.Crit Care Med. 2007; 35: 64-68Crossref PubMed Scopus (309) Google Scholar Michard and Teboul25.Michard F. Teboul J.L. Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence.Chest. 2002; 121: 2000-2008Crossref PubMed Scopus (637) Google Scholar determined that seven out of nine studies demonstrated no significant association between a low starting PCWP and subsequent fluid responsiveness as defined by either an increase in SV or CO after a bolus of intravenous fluids. In one of the studies, responders (an increase in SV of at least 15% in association with a fluid challenge) had a higher baseline PCWP than nonresponders.30.Diebel L.N. Wilson R.F. Tagett M.G. et al.End-diastolic volume: a better indicator of pre-load in the critically ill.Arch Surg. 1992; 127: 817-822Crossref PubMed Google Scholar In two other studies, there was an association between lower baseline PCWP and fluid responsiveness, but the authors were unable to develop a threshold that would differentiate responders from nonresponders.31.Wagner J.G. Leatherman J.W. Right ventricular end-diastolic volume as a predictor of the hemodynamic response to a fluid challenge.Chest. 1998; 113: 1048-1054Crossref PubMed Google Scholar,32.Tousignant C.P. Walsh F. Mazer C.D. The use of transesophageal echocardiography for preload assessment in critically ill patients.Anesth Analg. 2000; 90: 351-355PubMed Google Scholar In addition, in a study of 100 ICU patients, Osman et al.33.Osman D. Ridel C. Ray P. et al.Cardiac Filling Pressures are not appropriate to predict hemodynamic response to volume challenges.Crit Care Med. 2007; 35: 64-68Crossref PubMed Scopus (309) Google Scholar determined that a baseline PCWP cutoff value of 15%. The sensitivity of this cutoff value was 77%, with a specificity of 51%. They further examined a combination of CVP<8mmHg and PCWP<12mmHg in predicting response to fluid challenge. This combination performed poorly as well, with a sensitivity of 35% and specificity of 71%.33.Osman D. Ridel C. Ray P. et al.Cardiac Filling Pressures are not appropriate to predict hemodynamic response to volume challenges.Crit Care Med. 2007; 35: 64-68Crossref PubMed Scopus (309) Google Scholar Furthermore, several studies have demonstrated no benefit of pulmonary artery catheters in improving ICU outcomes and maybe even harmful for these patients.34.Richard C. Warszawski J. Anquel N. et al.Early use of pulmonary artery catheters and outcomes in patients with ARDS and septic Shock: a randomized controlled trial.JAMA. 2003; 290: 2732-2734Crossref PubMed Scopus (346) Google Scholar, 35.Shah M.R. Hasselblad V. Stevenson L.W. et al.Impact of pulmonary artery catheters in critically ill patients: meta-analysis of randomized controlled trials.JAMA. 2005; 294: 1664-1670Crossref PubMed Scopus (352) Google Scholar, 36.Cheatham M.L. Nelson L.D. Chang M.C. et al.Right ventricular end-diastolic volume index as a predictor of preload status in patients on positive end-expiratory pressure.Crit Care Med. 1998; 26: 1801-1811Crossref PubMed Google Scholar The weak relationship between surrogate indicators of preload and volume responsiveness/intravascular volume status is due to several factors. First, the relationship between volume and pressure is not constant and is modified by changes in vascular and cardiac compliance. Depending on the compliance of a system, a given volume and volume change may correspond to widely varying pressures. For example, ventricular compliance is altered in critically ill patients owing to processes such as sepsis and ischemia, and cannot be easily predicted. Michard and Teboul25.Michard F. Teboul J.L. Predicting fluid responsiveness in ICU patients: a critical analysis of the evidence.Chest. 2002; 121: 2000-2008Crossref PubMed Scopus (637) Google Scholar highlighted two additional issues: (1) changes in the cardiovascular system depend on the proportional portioning of fluids between the various compartments of the cardiovascular system, and (2) the increase in SV with increased end-diastolic volume depends on preserved cardiac function, which is often not the case in critically ill patients. Viewed from this perspective, a patient may be a nonresponder to a fluid challenge because of high venous compliance, low ventricular function, and/or compliance or third-spacing of administered fluids out of the vascular space. Unfortunately, no current measure of cardiac preload can capture all this information. Furthermore, a given preload does not correlate with fluid responsiveness (Figure 2).37.Pinsky M.R. Teboul J.L. Assessment of indices of preload and volume responsiveness.Curr Opin Crit Care. 2005; 11: 235-239Crossref PubMed Scopus (81) Google Scholar This suggests a need for caution in using these measures for assessing and managing volume therapy in patients. There are several two-dimensional and Doppler flow measurements that can be obtained from trans-thoracic or transesophageal echocardiography (TTE or TEE) that provide cardiac chamber volume assessment.38.Josephs S. The use of current hemodynamic monitors and echocardiography in resuscitation of the critically ill or injured patient.Int Anesth Clin. 2007; 45: 31-59Crossref PubMed Scopus (4) Google Scholar For example, TEE has been used to measure left ventricular dimensions in mechanically ventilated patients.38.Josephs S. The use of current hemodynamic monitors and echocardiography in resuscitation of the critically ill or injured patient.Int Anesth Clin. 2007; 45: 31-59Crossref PubMed Scopus (4) Google Scholar Specifically, Reuter et al.39.Reuter D.A. Felbinger T.W. Schmidt C. et al.Stroke volume variations for the assessment of cardiac responsiveness to volume loading in mechanically ventilated patients.Intensive Care Med. 2002; 28: 392-398Crossref PubMed Scopus (242) Google Scholar demonstrated that left ventricular end-diastolic area (LVEDA) is a good predictor of volume responsiveness. Other studies have failed to replicate this finding.40.Feissel M. Michard F. Mangin I. et al.Respiratory changes in aortic blood velocity as an indicator of fluid responsiveness in patients with septic shock.Chest. 2001; 119: 867-873Crossref PubMed Scopus (289) Google Scholar,41.Rex S. Brose S. Metzelder S. et al.Prediction of fluid responsiveness during cardiac surgery.Br J Anesth. 2004; 93: 782-788Crossref PubMed Scopus (0) Google Scholar One major issue with this parameter is that in the absence of baseline echocardiographic data absolute levels of LVEDA are hard to interpret and vary from patient to patient, depending upon baseline cardiac anatomy and physiology.38.Josephs S. The use of current hemodynamic monitors and echocardiography in resuscitation of the critically ill or injured patient.Int Anesth Clin. 2007; 45: 31-59Crossref PubMed Scopus (4) Google Scholar Thus, static, single echocardiographic measurements are of limited value in the assessment of preload and volume responsiveness. Given the limited ability of static measures to predict volume status in critically ill patients, research has targeted measures that better discriminate those patients who increase SV with fluid therapy ('fluid responders') from those who do not ('nonresponders'). As these measures look at changes in cardiovascular parameters in real time, they have been deemed 'dynamic' measures as opposed to the static or snapshot measures discussed above. The degree of variation in parameters such as SV (SV variation, SVV), pulse pressure (pulse pressure variation, PPV), changes in aortic flow velocity, and the diameter of inferior vena cava (IVC) or SVC as a result of changes in intrathoracic pressure induced by spontaneous respiration or by positive pressure ventilation are among the tools to assess volume responsiveness. Although in the normal breathing cycle the VR and right heart preload increase during inspiration and decrease at the end of inspiration, these changes are reversed during positive pressure ventilation (Figure 3).42.Morgan B.C. Martin W.E. Hornbein T.F. et al.Hemodynamic effects on intermittent positive pressure ventilation.Anesthesiology. 1966; 27: 584-590Crossref PubMed Google Scholar With mechanical insufflation, the rise in pleural pressure leads to reduction of VR and RV preload. At the same time, the RV afterload increases because of an increase in transpulmonary pressure (Figure 3).43.Monnet X. Bleibtreu A. Ferre A. et al.Passive leg raising and end-expiratory occlusion tests perform better than pulse pressure variation in patients with low respiratory system compliance.Crit Care Med. 2012; 40: 152-157Crossref PubMed Scopus (54) Google Scholar Left ventricular preload increases during mechanical inspiration due to the increase in transpulmonary pressure. Left ventricular afterload decreases during mechanical inspiration because the positive pleural pressure increases systolic extracardiac pressure. Importantly, the codependence of the LV output on RV output is seen because of the relatively long (approximately 2s) transit time of blood through the alveolar ca