Mitral valve velocity time integral and passive leg raise as a measure of volume responsiveness

The results of this study have shown that in hemodialysis patients, non-invasive bedside echocardiography coupled with a passive leg raise is helpful in predicting volume responsiveness. In the pre-dialysis cohort of patients, the passive leg raise maneuver did not result in any VTI changes, whether at the level of the LVOT or at the mitral valve, whereas, after the fluid was removed, the delta LVOT VTI increased by 15% with PLR and the delta MV VTI increased by 12.95%. Our results also showed that the delta VTI increase was volume dependent as patients who had greater than 4 L removed had the greatest increase in delta VTI.

Fluid responsiveness and fluid management in shock are important topics for intensivists and emergency physicians. Aggressive hydration in the ED became the mainstay of septic shock therapy after the EGDT trial [1]. The Rivers protocol was recently criticized and challenged by several studies that showed that aggressive fluid therapy in sepsis can be dangerous and can lead to increased mortality [2–4, 23, 24]. Furthermore, the use of static measures such as the central venous pressure (CVP) in predicting fluid responsiveness has been put to question [5, 6]. Fluid responsiveness is an even more pivotal issue in patients who are chronically fluid overloaded such as ESRD patients. When these patients present hypotensive due to sepsis, it is difficult for the treating physician to gauge their intravascular volume status through the use of physical exam findings and through static measures (mean arterial pressure, CVP) [25, 26]. This has prompted researchers to look for new methods of assessment of volume responsive. One of the first methods looked at aortic flow changes using an esophageal Doppler. While it showed a direct correlation with intravascular volume changes, this technique remained highly invasive, as the patient needed to be intubated or sedated [12]. More recently, there has been a gradual shift towards non-invasive dynamic ways of assessing volume responsiveness. Physicians looked at the inferior vena cava (IVC) collapsibility as a surrogate marker of volume responsiveness. While early studies showed great correlation between IVC collapsibility and patient’s volume status, the majority of studies did not show a great correlation with fluid responsiveness in spontaneously breathing patients [8–10, 27–29]. More recently, a group lead by Lamia et al. [13] looked at the role of non-invasive echocardiography as a measure of volume responsiveness in ICU, and they showed that a 12.5% increase in VTI was 77% sensitive and 100% specific for detection of a > 15% in cardiac output following volume expansion. These similar findings were reproduced by Maizel et al. [14] who showed that in spontaneously breathing patients presenting in shock, they showed that a 12% increase in stroke volume after passive leg raise was 69% sensitive and 89% specific for response to 500 mL of crystalloid administration. To the best of our knowledge, our study is the first study looking at the role of non-invasive cardiac echocardiography in the emergency department for the evaluation of volume responsiveness. A study by Dinh et al. in 2012 showed that emergency physicians can accurately measure LVOT VTI and cardiac output [30]. Our study remains the only study that looked at the role of MV VTI as a predictor of volume responsiveness. MV VTI was found to be highly specific for fluid responsiveness as well as having a high positive predictive value for detecting a volume-responsive state. This technique can be an alternative for physicians to evaluate volume responsiveness in cases where patients’ body habitus prevents them from getting an adequate apical five-chamber view. It is important to note that there were four patients that had a MV VTI < 12% while having an LVOT VTI > 12%. Possible explanations for the discrepancy could be that the patients had diastolic dysfunction or mitral regurgitation, two conditions that could affect mitral valve VTI. We did not, however, check for diastolic dysfunction on our patients. Furthermore, one patient had a MV > 12% and an LVOT VTI < 12%. This particular patient had an aortic valve replacement and his low LVOT VTI could be due to the metallic valve.

In contrast to the prior VTI studies, we removed fluid via hemodialysis and we looked at the effect of the passive leg raise once these patients had this fluid taken off. The rationale was that in the pre-dialysis cohort, patients would be hypervolemic and their overstretched heart will not respond to the PLR maneuver and the preload bolus, in contrast to the post-hemodialysis cohort, which would represent a fluid responsive state and would allow the heart to respond to the preload challenge and increase its stroke volume. The passive leg raise maneuver by definition is a reversible auto-bolus of about 200–400 mL. It has been studied in both ventilated and spontaneously breathing patients, and several studies have shown that an increase in stroke volume of 15% following a PLR maneuver had a specificity of 93% in detecting volume responsiveness after receiving a 500-cc fluid bolus [21, 22, 31]. Furthermore, we chose to conduct this study on end-stage renal disease because this subset of patients represents a challenge while being evaluated for fluid responsiveness, as these patients are chronically fluid overloaded and still can present with hypotension and it is often unclear whether these patients require more fluid or vasopressor therapy.

It is interesting to note that there were no differences in vital signs before and after hemodialysis except for systolic blood pressure, which we noted to be higher by 8 mmHg in the post-dialysis cohort. This increase in systolic blood pressure after dialysis was described by Inrig et al. [32] who explained that intradialytic hypertension is multifactorial, and its causes include subclinical volume overload, sympathetic overactivity, activation of the renin angiotensin system, endothelial cell dysfunction, and specific dialytic techniques. It is important to note that this increase in blood pressure is only temporary. In the majority of patients, there is a drop in afterload post-dialysis coupled with an increase in their cardiac output [33]. This could explain why our VTI values increased in the post-hemodialysis cohort. However, it is more important to note that there were no changes in vital signs after the passive leg raise signs which further strengthens the idea that vital sign changes are not adequate enough for assessing volume responsiveness. This has been shown several times in the literature and in previous studies on bedside echocardiography and further strengthens the argument that VTI changes are more sensitive during volume changes than vital sign changes [12, 13, 22].

The greatest increase in the delta VTI for both the mitral valve and the LVOT in our population was seen in patients who had more than 4 L of fluid removed. This can probably be due to several reasons; first, the interval between dialysis sessions was longer in our patients (7.8 days) than regular hemodialysis patients. Our patients receive hemodialysis on a compassionate or emergent basis based on criteria of fluid overload and hypoxia, severe acidosis (serum bicarbonate < 10 mmol/L0 or severe hyperkalemia (Potassium > 6 mmol/L). As such, they usually go more than the standard 3 days without dialysis and might be more fluid overloaded than the scheduled hemodialysis patient. Another possible explanation is that hemodialysis is done over a period of 3–4 h which could allow the system time to slowly adapt to the volume loss.

The present study is an observational study performed on a specific group of ESRD patients with volume overload. Therefore, its findings should be interpreted cautiously as they are not applicable to the general ED population. Our results should be compared with the results of future studies on other populations with hypervolemia or hypovolemia. Furthermore, our study is limited by its small sample size; however, this sample size was calculated based on the existing literature on volume responsiveness and velocity time integral. Moreover, considering ESRD patients after hemodialysis as euvolemic may not be accurate. Our study is also limited by the lack of lung ultrasonography. Although our aim was to investigate the value of mitral valve VTI, several studies have looked at the role of lung ultrasound in ESRD patients as a surrogate for volume status [34–36]. Future studies need to combine cardiac echo with lung ultrasonography in an effort to best understand volume responsiveness. Finally, the ultrasound examination was performed by physicians with extensive training in bedside ultrasound and therefore cannot be generalized to all emergency physicians.

Volume status evaluation of hemodialysis patients can be tricky as reliance on vital signs and the physical exam are not very accurate. Bedside echocardiography in conjunction with a passive leg raise is a relatively simple, non-invasive method that could help in evaluating for volume responsiveness and, therefore, is a valuable tool for intensivists. Further large studies, however, are needed to corroborate our findings.