ATS SEMINARS Intensive Care Ultrasound Series Editor: Gregory A. Schmidt, M.D.
Intensive Care Ultrasound: VI. Fluid Responsiveness and Shock Assessment Daniel De Backer and David Fagnoul Department of Intensive Care, Erasme Hospital, Universite´ libre de Bruxelles, Brussels, Belgium
Fluid therapy is one of the key components of hemodynamic resuscitation. Fluids are administered to increase cardiac output and, ultimately, tissue perfusion. However, not all patients respond to fluid administration, and a positive fluid balance is associated with a poor outcome (1). To optimize fluid administration, it is thus important to try to predict fluid responsiveness. During the echocardiographic assessment, several indices can be used to evaluate fluid responsiveness.
the probe on the chest over the sixth intercostal space in the midclavicular line, directed toward the right scapula, with its long dimension directed toward the left scapula. The long-axis parasternal view is obtained by placing the probe over the second or third intercostal space, about 2 cm lateral to the sternum, with its long axis directed toward the right scapula. The parasternal short axis is obtained by a 908
rotation of the probe on its axis at the level of the papillary muscles. Image quality should be optimized so that the endocardial border is clearly delineated.
Inferior Vena Cava Imaging and Variation Another way to address fluid responsiveness is to evaluate central venous pressure
Ventricular Size The size of the ventricles can be used as a gauge to the response to fluids. Although responders to fluids usually have smaller left ventricles than nonresponders (2–4), there is an important overlap between values in published studies so that no clear cut-off can be proposed for reliable prediction of fluid responsiveness. Nevertheless, the size of the ventricles can be used in extreme conditions, and this difference may be useful: response to fluids is often observed in patients with very small ventricular cavities, especially when associated with kissing papillary muscles (Figure 1, Video 1), whereas it is unlikely in patients with dilated left or right ventricles. The size of the ventricles can be estimated in apical four-chamber and parasternal views, whereas kissing papillary muscles are best estimated in parasternal views. The apical four-chamber view is obtained by placing
Figure 1. Kissing papillary muscles. Time-mode acquisition at the level of the papillary muscles in parasternal short-axis view. During systole, opposing walls touch, and the left ventricular cavity becomes very small.
(Received in original form September 21, 2013; accepted in final form October 26, 2013 ) This article has associated videos, which are accessible at www.atsjournals.org/doi/full/10.1513/AnnalsATS.201309-320OT. If you cannot view Flash videos on your device, please access original videos at www.atsjournals.org/doi/suppl/10.1513/AnnalsATS.201309-320OT. Correspondence and requests for reprints should be addressed to Daniel De Backer, M.D., Ph.D., Department of Intensive Care, Erasme University Hospital, Route de Lennik, 808, B-1070 Brussels, Belgium. E-mail:
[email protected] Ann Am Thorac Soc Vol 11, No 1, pp 129–136, Jan 2014 Copyright © 2014 by the American Thoracic Society DOI: 10.1513/AnnalsATS.201309-320OT Internet address: www.atsjournals.org
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Figure 2. Respiratory variations in inferior vena cava. (A) Normal diameter inferior vena cava (IVC) (18 mm) with marked respiratory variation in size, suggesting low central venous pressure (CVP). (B) Dilated IVC (25 mm) without respiratory variations in size, suggesting elevated CVP.
(CVP), which can be estimated using echocardiography (5). Several indexes, such as maximal IVC diameter, respiratory variations in IVC diameter, or a combination of both, have been used to evaluate CVP (Figure 2). At best, these can provide semiquantitative information (CVP low/high). In spontaneously breathing patients, an IVC diameter less than 20 mm was associated with a CVP less than 10 mm Hg (5). On the other hand, an IVC
diameter greater than 20 mm Hg with no respiratory variation is associated with an elevated CVP. Unfortunately, as with invasive measurements of CVP, maximal or minimal IVC diameters fail to predict fluid responsiveness (6). Of greater interest, respiratory variations in IVC can predict fluid responsiveness, both in mechanically ventilated and in spontaneously breathing patients. These indices were initially
Figure 3. Identification of the inferior vena cava (IVC) in transverse view. The probe is positioned at the subxiphoid level, perpendicular to the skin. This view is useful to localize the IVC. Ao = aorta.
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validated in patients receiving mechanical ventilation, with 12 and 18% cut-off values separating fluid nonresponders from responders (2, 6). It was later suggested that respiratory variations in IVC diameter can also be used in spontaneously breathing patients (7). Importantly, during spontaneous ventilation, the IVC collapses in inspiration (Figure 2), whereas during mechanical ventilation the IVC dilates (7). The value of respiratory variations in IVC diameter in spontaneously breathing patients has been challenged recently (8). In 40 spontaneously breathing patients with circulatory failure, respiratory variations performed no better than did static indices of preload, such as estimation of pulmonary artery occluded pressure using the mitral inflow profile (8). In that trial, high respiratory variability in IVC diameter (.40%) was usually associated with response to fluids, but lower values did not rule out fluid responsiveness. To measure the IVC diameter and its respiratory variations, the IVC should first be identified in a transverse plane (Figure 3, Video 2), with the cardiac probe in a subxiphoid position perpendicular to the skin. The probe is moved progressively to the right to visualize the IVC in the center of the field. The probe is then rotated by 908 to obtain a longitudinal plane. It is important to identify the hepatic veins and entrance of the IVC into the right atrium (Figure 4, Video 3). The beam should be perpendicular to the vessel wall, and care should be taken to keep the probe centered on the vessel in the same location during the entire respiratory cycle, preventing motion artifact. The IVC diameter is measured in time-motion mode using a moderate speed (25 mm/s), ideally with acquisition of respiratory traces (Figure 5). Three measurements should be averaged. The IVC diameter can be measured either close to its entrance to the right atrium or 1 to 2 cm caudal to the hepatic vein–IVC junction (approximately 3–4 cm from the junction of the IVC and the right atrium). Compared with the caudal location, measurement of IVC collapsibility close to the IVC–right atrium junction may be influenced more by contraction of the diaphragm (9), but this location is less susceptible to motion artifact. Whatever the location, the measurement is valid only when there are no active contractions of abdominal wall muscles or raised intraabdominal pressure (10).
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Aortic Flow Variations
Figure 4. Identification of inferior vena cava (IVC) in longitudinal view. From the previous view (Figure 3), the probe is rotated by 908 . The time-mode sampling cursor is positioned perpendicular to the IVC, either 2 cm caudal to the junction of the hepatic veins or just below the junction with right atrium (RA).
Measurements over one respiratory cycle should be obtained first in end-expiration and then in early inspiration (Figure 6). Agreement between observers was good, provided that examiners had performed at least five previous examinations of the IVC, and visual evaluation may provide good agreement with quantitative measurements (11).
Respiratory Variation in Superior Vena Cava Diameter (Transesophageal Route Only) Respiratory variations in superior vena cava diameter (Figure 7) can predict the response to fluids (12, 13) with excellent specificity and sensitivity. This approach is feasible only using transesophageal
Figure 5. Time-mode visualization of inferior vena cava respiratory variations (IVC). Time-mode acquisition of the IVC at a speed of 25 mm/s. The beam is positioned 2 cm caudal to the hepatic vein–IVC junction. Three respiratory cycles are visualized.
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Stroke volume variation and its surrogates have been shown to predict fluid responsiveness (14). Stroke volume can be measured using the apical five-chamber view with a pulsed-wave Doppler sample positioned in the left ventricular outflow tract area (Figure 8). To obtain this view, the probe is positioned as for the apical four-chamber view (see above) and angulated slightly to the left side of the patient to allow visualization of the left ventricular outflow tract. As left ventricular outflow tract dimensions do not change, changes in velocity-time integral (VTI) can be used to evaluate variations in stroke volume (Figure 9). Respiratory variations should be measured over one respiratory cycle, beginning at inspiration, and three measurements should be averaged. A moderate speed should be used (50–75 mm/s) to allow reliable measurements of VTI while covering at least one respiratory cycle (Figure 10). A simpler approach consists of measuring respiratory variations in peak aortic flow (Figure 11). In this approach, a lower speed (6.25 mm/s) can be used, and several respiratory cycles can then be visualized. Care should be taken to minimize changes in the angle between the beam and left ventricular outflow tract during the respiratory cycle. As measured velocity equals true velocity times the cosine of the angle, angulations of 308 can lead to a 13% underestimation of the flow. Movements of the probe and of the heart during ventilation may induce angulation of the probe, and the operator should try to minimize these effects. Respiratory variations in VTI predict fluid responsiveness in ventilated patients, at a threshold of 20% (3). Respiratory variations in peak aortic flow predict fluid responsiveness in patients with septic shock under mechanical ventilation at a threshold of 12% (15). These methods, nevertheless, have several limitations. In addition to the technical limitations related to echocardiography (angle of the probe), there are also several limitations related to factors influencing stroke volume variation. The patient needs to be ventilated (16) with a tidal volume of at least 8 ml/kg (17) and should not have arrhythmias or raised 131
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Figure 6. Measurement of inferior vena cava respiratory variations in time-mode. Measurements are performed at end-expiration (A = 19.8 mm) and during inspiration (B = 19.8 mm).
abdominal pressure. False positives may be observed in patients with right ventricular failure, but this factor is easily detected by echocardiography (18). Evaluation of respiratory variations in aortic flow is, therefore, a convenient way to assess fluid responsiveness in patients receiving
mechanical ventilation provided that certain prerequisites are met.
Passive Leg Raising Test The passive leg raising test is another way to assess fluid responsiveness (19). Changes in
stroke volume are determined during an endogenous preload challenge induced by changing the position of the patient (20). After measurements of VTI performed in a semirecumbent position with the trunk at 308 and legs in a horizontal position, the legs are elevated to 458 and the trunk of the patient placed at 08 . A second set of measurements should be obtained within 1 to 2 minutes of the change in position. Several trials have shown that this technique reliably predicts fluid responsiveness, with a threshold around 12% (21). This technique can be used in patients breathing spontaneously and even in patients receiving extracorporeal membrane oxygenation (22). However, this technique has several limitations. First, the angle between probe and the left outflow tract must be kept minimal in both positions, which is not always easy. Second, this technique is not reliable in the presence of raised abdominal pressure (23). Third, it is not always feasible in surgical patients because of pain (e.g., polytrauma), risk of aspiration (abdominal surgery), or increase in intracranial pressure (brain injury and neurosurgical patients). Finally, the technique is somewhat cumbersome when it has to be repeated.
Which Index in Which Setting? The different indices have not been compared head-to-head in a large population of patients. Nevertheless, treatment algorithms based on static and dynamic indices lead to different fluid management (24). Considering the literature and the corresponding indices measured invasively, the following recommendations can, therefore, be made: d
d
Figure 7. Respiratory variations in superior vena cava (SVC). Time-mode acquisition of the inferior vena cava at a speed of 25 mm/s. The beam is positioned 2 to 3 cm above the junction of the SVC and the right atrium. In this patient, the respiratory variation in SVC is 36%. Please note that the diameter at expiration (14 mm) is measured just before the diameter at inspiration (9 mm).
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Except for very low and very high values, all static indices (left ventricular and IVC sizes) poorly predict fluid responsiveness. If the patient is receiving mechanical ventilation with a tidal volume of at least 8 ml/kg and has no significant arrhythmia, measurement of respiratory variations in aortic flow is the most robust index of fluid responsiveness. If the patient has limitations to the use of respiratory variations in aortic flow, respiratory variations in IVC diameter should be measured. Passive leg raising
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Figure 8. Measurement of aortic flow. Pulsed-wave Doppler of aortic flow sampled at the level of the left ventricular outflow tract in an apical five-chamber view.
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test may also be considered, but this test is somewhat cumbersome and is seldom applied in the clinical practice of the authors. Finally, if transesophageal examination is performed, respiratory variations in superior vena cava should be measured.
Case Illustration A 52-year-old man was admitted with septic shock due to severe pneumonia. After initial fluid resuscitation, the patient remained hypotensive and oliguric with hyperlactatemia. An echocardiographic
evaluation was performed. The twodimensional evaluation showed excellent contractility and small cavities (Video 4 and Video 5). Left ventricular filling pressures were considered to be low, as evaluated by mitral inflow pulsed-wave Doppler (Figure 12) and tissue Doppler at the level of the mitral annulus (Figure 13) (25). On the basis of these findings, administration of additional fluid therapy was considered. Looking at the aortic VTI (Figure 14) showed that it was already quite elevated (VTI, 26.7 cm, which yielded a cardiac output of 13.7 L/min) without respiratory fluctuation. However, respiratory variations in VTI should not be evaluated in spontaneously breathing patients, because stroke volume variation fails to predict fluid responsiveness in these patients (16). Analysis of the IVC (Figure 15) showed a large (28 mm) IVC diameter without significant respiratory variations (7%). Hence, doses of norepinephrine were increased instead of giving further volume.
Conclusions Evaluation of fluid responsiveness using echocardiography is easily and rapidly achieved at the bedside using simple indices. Physicians need to understand the physiological background and technical limitations of the different tests and indices to optimize their use and interpretation. n Author disclosures are available with the text of this article at www.atsjournals.org.
Figure 9. Respiratory variations in aortic flow. The low speed (25 mm/s) allows visualization of several respiratory cycles, but this speed is too low to allow reliable measurements of the velocity-time integral. Exp = expiration; Insp = inspiration.
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Figure 10. Respiratory variations in aortic velocity-time integral (VTI). Respiratory variations in aortic VTI recorded at 50 mm/s allowing visualization of one respiratory cycle and reliable determination of VTI. Maximal aortic VTI was 19.6 cm and minimal aortic VTI 14.9 cm, yielding a respiratory variation of 27% (19.6 – 14.9/17.25)
Figure 11. Respiratory variations in aortic flow. Over one respiratory cycle, the maximum aortic flow velocity was 132 cm/s and minimum 111 cm/s. Respiratory aortic flow variation was ([136 – 111]/[(136 1 111)/2]) = 20%
Figure 12. Illustrative clinical case: mitral inflow pulsed-wave Doppler. The E wave was 103 cm/s and A wave 72 cm/s, with an E/A at 1.4, suggesting a nonelevated pulmonary artery occluded pressure.
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Figure 13. Illustrative clinical case: tissue Doppler at the level of the mitral annulus. The E wave at the level of the mitral annulus (Ea) was 16.5 cm/s, which, combined with measurement of E wave at 103 cm/s (as shown in Figure 12), yielded an E/Ea of 6.2, suggesting that pulmonary artery occluded pressure was low (25).
Figure 14. Illustrative clinical case: measurement of cardiac output. Aortic velocity recorded at the level of the left ventricular outflow tract. Aortic velocity-time integral (VTI) was 26.7 cm, which yielded a cardiac output of 13 L/min.
Figure 15. Illustrative clinical case: inferior vena cava. The inferior vena cava was large (28 mm), without significant respiratory variations.
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