While not used regularly in the ED, respiratory changes in aortic blood velocity as measured by transesophageal echocardiography (TEE) may predict fluid responsiveness in mechanically ventilated patients.34 Peak aortic blood flow velocity variation is measured by TEE. Similarly, ventilator-induced variation in descending aortic blood flow measured by esophageal Doppler monitoring may predict fluid responsiveness.34 However peak aortic blood flow velocity measurements determined by TEE may have limited utility because TEE is an invasive procedure. Similarly, esophageal Doppler monitors can be used but are limited because of low predictive value and rare usage in the emergency setting.35
Passive Leg Raise
In spontaneously breathing patients, passive leg raising (PLR) has been studied as a substitute for volume challenge due to the ease of performing PLR at the bedside and absence of adverse events such as volume overload. When performing PLR, the patient starts in a semirecumbent position and is repositioned supine with the legs raised to 45°. Blood transferred to the heart during PLR increases cardiac preload and tests preload responsiveness. The maximum hemodynamic response to PLR occurs within one minute of performing the maneuver.36 The effects of PLR are assessed by the changes in cardiac output or stroke volume after PLR, which are extrapolated from aortic blood flow measured by esophageal Doppler, velocity time integral measured by transthoracic echocardiography, and femoral artery flow measured by arterial Doppler.36 These modalities may provide additional data points in the evaluation of fluid responsiveness but is out of the scope of this review.
Data in mechanically ventilated patients with esophageal Doppler and arterial access demonstrated that an increase in aortic blood flow by 10% with PLR predicted a positive fluid response with sensitivity 97% and specificity 94%.37 However, in the same study, the specificity in spontaneously breathing patients was markedly reduced (46%).37,38
Another study used a more conventional noninvasive measurement with transthoracic echocardiography to determine whether PLR could predict fluid responsiveness in hemodynamically unstable patients. In this study, a PLR-induced increase in stroke volume greater than or equal to 12.5% predicted an increase in stroke volume by greater than or equal to 15% after fluid administration with specificity 100% and sensitivity 77%.38 This study included patients on mechanical ventilation with active inspiration, patients without mechanical support, and patients with atrial fibrillation, enabling better generalization of results than previous studies.39
Bioreactance Technology
Cardiac output measurement using bioreactance technology is an alternative noninvasive method to measure cardiac output using only four surface electrodes. This technology is based on an analysis of relative phase shifts of an oscillating current that occurs when the current traverses the thoracic cavity. The bioreactance device (NICOM, Cheetah Medical, Tel Aviv, Israel) is comprised of a high frequency (75 kHz) sine wave generator and four dual electrode stickers that are used to establish electrical contact with the body. The cardiac output measured by bioreactance correlates well with values measured by thermodilution and pulse contour analysis.40 Performing PLR and determining its response using a bioreactance machine may be appropriate in the ED, in the ward, or at the initial presentation to the ICU because it is noninvasive and less labor intensive than other methods. In postoperative cardiac surgery patients, PLR-induced changes in cardiac output measured by bioreactance had sensitivity 88% and specificity 100%.40 In hemodynamically unstable patients, the results were more encouraging with a sensitivity of 94% and a specificity of 100% in predicting fluid responsiveness (defined as greater than10% increase in stroke volume index).41 However, in a group of critically ill patients (83% septic, 10% hypovolemic, and 7% cardiogenic), bioreactance coupled with PLR was unable to measure cardiac index compared with transpulmonary thermodilution, and bioreactance failed to predict fluid responsiveness.42 More research on bioreactance technology is needed, and its noninvasive evaluation of critically ill patients who need cardiac output monitoring and fluid therapy.
Conclusion
There are many tools available to estimate the volume status and fluid responsiveness of the critically ill patient. One of these tools, CVP measurement, must be used cautiously as an assessment of fluid responsiveness. It is important to understand the limitations of this technology. While other more advanced tools, such as ultrasonography to measure the IVC at the bedside and assess IVC variation or TEE to assess LV diastolic size and contractility during fluid resuscitation, may provide a better diagnostic picture, these tools/devices are not always available at most community hospitals.
The authors do not recommend placing a CVC simply to measure CVP; however, when a CVC or peripherally inserted central catheter is medically needed for treatment, the catheter can be used to trend CVP since the value of CVP is greatest as a trend to guide resuscitation. Other minimally invasive and noninvasive diagnostic tools currently are available, such as bedside ultrasound, and enable clinicians to assess volume responsiveness using dynamic procedures that challenge the Frank-Starling curve.4 These technologies have a useful place in resuscitation but each has its own limitations. With an understanding of the tools available, with their strengths and limitations, physicians can better individualize intravascular volume resuscitation.