US Probe: Velocity Time Integral (VTI) in Sepsis Management

Author: Jude Luke (EM Resident Physician, NYU/Bellevue Hospital) and Jonathan Warren, MD (Clinical Ultrasound and EMS Fellow, Department of Emergency Medicine, Harbor-UCLA Medical Center)  // Reviewed By: Steve Field, MD; Brit Long, MD (@long_brit)

Case

A 65-year-old female with a history of heart failure with reduced ejection fraction, type 2 diabetes, and hypertension presents to the emergency department (ED) with fever, altered mental status, and weakness. She reports worsening fatigue and decreased oral intake over the past few days. On arrival, her vital signs are BP 85/50 mm Hg, HR 115 bpm, RR 22, temperature 101.6°F, and SpO₂ 92% on room air. She is confused and lethargic, with no pedal edema, cool extremities, and dry mucous membranes. Laboratory results demonstrate an elevated lactate, creatinine, and WBC count. Her BNP is slightly above her baseline. Bedside ultrasound reveals a collapsible inferior vena cava (IVC). After an initial 500cc fluid bolus, her blood pressure remains low. Repeat IVC measurement is now 2.3 cm with >50% respiratory variation. 

How would you determine if additional fluid resuscitation will benefit this patient?

Sepsis is a life-threatening condition that demands rapid diagnosis and effective management. In emergency departments, determining a septic patient’s fluid responsiveness can be challenging, as both under- and over-resuscitation can lead to poor outcomes. The 2021 Surviving Sepsis Campaign guidelines recommends high-volume administration of 30ml/kg of fluid during the initial resuscitation of patients in septic shock, however this recommendation is based on weak evidence with a “low-quality” recommendation from the guidelines.¹

The 2022 CLASSIC and 2023 CLOVERS trials are two landmark studies that further examine approaches to fluid resuscitation in septic patients. Both studies demonstrated no difference in their primary outcome of mortality before discharge home by day 90 when employing a ‘liberal’ or ‘restrictive’ approach to fluid resuscitation for patients in septic shock.^2,3 However, patient populations who are prone to volume overload or depletion require a tailored approach to the treatment of septic shock. In these patient populations, fluids should be used judiciously with specific attention to whether a patient will be responsive to further fluid resuscitation.

Left Ventricular Outflow Tract (LVOT) Velocity Time Integral (VTI) is one tool that can aid clinicians in guiding fluid resuscitation management. This non-invasive, ultrasound-driven measurement offers a practical, and validated, method to approximate cardiac output and monitor patient response to fluid resuscitation.^4-7

Understanding the Velocity Time Integral (VTI)

VTI is a measure, obtained by ultrasound, of the distance blood travels during one cardiac cycle. It is an essential metric for estimating cardiac output without the need for invasive procedures within the emergency department. LVOT VTI is particularly helpful in managing septic patients because it offers a reliable way to evaluate fluid responsiveness. By measuring how cardiac output changes in response to a fluid bolus, clinicians can better determine whether a patient will benefit from additional fluid resuscitation or if a patient should be started on vasopressors instead.

Calculating VTI: The Basics

VTI calculations assume that the left ventricular outflow tract (LVOT) behaves like a rigid cylinder and thus the equation to calculate stroke volume (SV) is derived from the basic formula for the volume of a cylinder:

LVOT Surface Area

LVOT surface area can be calculated by obtaining a measurement of the LVOT diameter and utilizing this within the formula for circular area. Often, ultrasound machines will perform this calculation for you. The measurement should be obtained from a parasternal long-axis view and by measuring at the base of the aortic leaflets. You can consider using the zoom function to obtain a more accurate measurement.

LVOT VTI

LVOT VTI is separately measured using pulse-wave doppler while in the apical 5-chamber view. The doppler gate should be placed just proximal to the aortic leaflets with care taken to ensure the doppler gate is as parallel to flow through the LVOT as possible. At times, this may require angle adjustment of the doppler gate. 

The VTI represents the area under the velocity curve generated via pulse-wave doppler. As you may remember in physics, this integral then correlates to the distance traveled over this set period of time (ie, “how far” the blood traveled during systole). The ideal VTI envelope should have a hyperechoic border with a hypoechoic center. This can be achieved with appropriate angle adjustment and optimization of your apical four chamber image.

Poor envelopes or incorrect angles may incorrectly underestimate the VTI and subsequently the cardiac output. 

Cardiac Output

Using this information and the equation detailed above, SV can be calculated, and ultimately cardiac output (CO) can be derived. A step further, an approximate cardiac index can also be determined:

VTI and Fluid Resuscitation in Sepsis

One of the key clinical applications of LVOT VTI is to assess whether a septic (or any other patient) patient will respond if given additional fluids. The big questions to answer in sepsis management are:

• Fluid Responsiveness: How does the patient’s cardiac output respond to a fluid bolus?
• Fluid Tolerance: Can the patient handle more fluids without developing pulmonary edema?

Calculating a VTI helps provide an answer to this first question. For instance, a passive leg raise may simulate a fluid bolus and determine whether patients demonstrate a significant increase in cardiac output that would indicate fluid responsiveness8. Conversely, a minimal change in cardiac output suggests that additional fluids are unlikely to improve the patient’s hemodynamic status and could potentially cause harm. 

Fluid responsiveness should always be interpreted in the context of a patient’s fluid tolerance, however, evaluation of this is beyond the scope of the current article. Often, evaluation of a patient’s fluid tolerance is determined using ultrasound to estimate central venous pressure via IVC and VEXUS measurements, in addition to the clinical exam. Consider reviewing the EMDocs article on the VEXUS exam for further information on the ultrasound guided evaluation of fluid tolerance.

Clinical Application in the ED

Incorporating VTI into sepsis management requires minimal additional equipment beyond the ultrasound already present in many emergency departments. The process typically involves:

1. Obtaining Baseline Measurements: Measure the LVOT diameter in the parasternal long axis and VTI using pulse-wave Doppler in the apical 5-chamber view.
2. Performing a Fluid Challenge or Passive Leg Raise: After establishing baseline VTI, either give the patient a fluid bolus or simulate one using a passive leg raise.
3. Reassessing VTI: Repeat your VTI measurement to evaluate for a response in cardiac output.

Fluid responsiveness is variably defined as an increase in cardiac output of 10-15% after a patient receives a 500mL fluid bolus.^9-11 Since a patient’s LVOT diameter should not change, the VTI before and after fluid administration can exclusively be reassessed to determine how cardiac output has responded. This assumption expedites the evaluation of a patient’s volume responsiveness and facilitates implementation in the ED setting. However, if a true cardiac output measurement is required, the LVOT should be measured again.

Using VTI as a guide, emergency physicians can and should make more informed decisions about fluid management in sepsis, ensuring that interventions are tailored to the individual patient’s physiology. It is important to evaluate how practical the use of VTI in the ED setting is, particularly when considering factors like time consumption, variability of operator skill, patient habitus, and resource allocation. With improving ultrasound technologies that integrate artificial intelligence to reliably facilitate VTI calculation^12,13 and growing training in ultrasound, using VTI to guide fluid resuscitation may become more accessible in the ED and potentially evolve into an essential tool in the emergency physician’s arsenal. However, existing knowledge and manual expertise in these ultrasound-guided calculations remain necessary for quality assurance.

Conclusion

VTI is a valuable tool for emergency physicians, offering a non-invasive and validated method for estimating cardiac output and assessing fluid responsiveness in septic patients. Its integration into sepsis management can improve decision-making regarding fluid resuscitation, potentially leading to better outcomes in this critically ill patient population and enhancing communication with intensivists.

Case Conclusion

You utilize your knowledge of VTI and use ultrasound to calculate a baseline LVOT VTI and LVOT diameter before you perform a passive leg raise. On reassessment of the patient’s VTI after the passive leg raise, you determine there is minimal change. You conclude that the patient is not fluid responsive and decide to start vasopressors early.

References

1. Evans L, Rhodes A, Alhazzani W, et al. Surviving sepsis campaign: International guidelines for management of sepsis and septic shock 2021. Crit Care Med. 2021;49(11). https://journals.lww.com/ccmjournal/fulltext/2021/11000/surviving_sepsis_campaign__international.21.aspx.
2. Meyhoff TS, Hjortrup PB, Jørn Wetterslev, et al. Restriction of intravenous fluid in ICU patients with septic shock. N Engl J Med. 2022;386(26):2459–2470. https://doi.org/10.1056/NEJMoa2202707. 
3. Early restrictive or liberal fluid management for sepsis-induced hypotension. N Engl J Med. 2023;388(6):499–510. https://doi.org/10.1056/NEJMoa2212663. 
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6. Chandraratna PA, Nanna M, McKay C, et al. Determination of cardiac output by transcutaneous continuous-wave ultrasonic doppler computer. The American journal of cardiology. 1984;53(1):234-237. https://dx.doi.org/10.1016/0002-9149(84)90718-5. 
7. Huntsman LL, Stewart DK, Barnes SR, Franklin SB, Colocousis JS, Hessel EA. Noninvasive doppler determination of cardiac output in man. clinical validation. Circulation. 1983;67(3):593-602. http://circ.ahajournals.org/cgi/content/abstract/67/3/593. doi: 10.1161/01.cir.67.3.593.
8. Monnet X, Rienzo M, Osman D, et al. Passive leg raising predicts fluid responsiveness in the critically ill. Critical Care Medicine. 2006;34(5):1402-1407. http://ovidsp.ovid.com/ovidweb.cgi?T=JS&NEWS=n&CSC=Y&PAGE=fulltext&D=ovft&AN=00003246-200605000-00016. 
9. [Peer-Reviewed, Web Publication] Cohen B, Wilson D. (2019, Aug 5). Fluid Responsiveness. [NUEM Blog. Expert Commentary by Morales-Nebreda L]. Retrieved from http://www.nuemblog.com/blog/fluid-responsiveness.
10. [Peer-Reviewed, Web Publication] Nickson C. (2015, Dec 16). Fluid Responsiveness. [Life In The Fast Lane Blog.]. Retrieved from http://litfl.com/fluid-responsiveness/.
11. Teboul J, Monnet X, Chemla D, Michard F. Arterial pulse pressure variation with mechanical ventilation. Am J Respir Crit Care Med. 2019;199(1):22–31. https://doi.org/10.1164/rccm.201801-0088CI. 
12. Gonzalez FA, Varudo R, Leote J, et al. Automation of sub-aortic velocity time integral measurements by transthoracic echocardiography: Clinical evaluation of an artificial intelligence-enabled tool in critically ill patients. British journal of anaesthesia : BJA. 2022;129(5):e116–e119. https://dx.doi.org/10.1016/j.bja.2022.07.037. 
13. Zhai S, Wang H, Sun L, et al. Artificial intelligence (AI) versus expert: A comparison of left ventricular outflow tract velocity time integral (LVOT‐VTI) assessment between ICU doctors and an AI tool. Journal of applied clinical medical physics. 2022;23(8):e13724–n/a. https://onlinelibrary.wiley.com/doi/abs/10.1002%2Facm2.13724.