Our study evaluated the utility of Trendelenburg position combined with carotid ultrasound in predicting fluid responsiveness in VV-ECMO patients with ARDS in the prone position. The results showed that the optimal threshold for FTc induced by Trendelenburg position was 331.5ms, with a sensitivity of 84.85% and specificity of 83.33%, the AUC of the ROC curve was 0.866, the grey zone encompassed 29% of patients (15/51), indicating high diagnostic accuracy. The optimal threshold for △Vpeak was 10.1%, with sensitivity and specificity of 81.82% and 77.78% respectively, AUC was 0.833, grey zone encompassed 45% of patients (23/51), it`s diagnostic accuracy slightly lower than FTc. This study enables clinicians to predict fluid responsiveness in such patients non-invasively, facilitating the development of appropriate fluid management strategies without increasing the risk of fluid overload.
VV-ECMO is a valuable therapeutic option for severe acute respiratory distress syndrome (ARDS) with suboptimal response to protective lung ventilation[31, 32]. During this support, clinicians typically administer appropriate fluid infusion to stabilize ECMO flow and correct hypotension, thereby increasing oxygen delivery[33]. However, Positive fluid balance is associated with a higher risk of death in ECMO patients[34], Fluid overload can cause pulmonary edema and heart failure, exacerbating the condition of ARDS patients and ultimately leading to increased mortality[35].Therefore, fluid management in VV-ECMO patients is closely intertwined with their condition. Accurate prediction of fluid responsiveness is crucial to reduce fluid overload for such patients.
Fluid responsiveness refers to the physiological response of patients to fluid loading[12]. The evaluation of fluid responsiveness has always been a hot and challenging area of research. Clinical indicators are diverse, ranging from static parameters such as CVP and IVC to dynamic parameters such as stroke volume variation (SVV) and Pulse pressure variation (PPV). Evaluation indicators have undergone a transition from static to dynamic and from invasive to non-invasive. The purpose of our study of volume responsiveness was to improve organ tissue ischemia and hypoxia while avoiding volume overload, increasing mortality and hospitalization time.
Early CVP measurement can assess fluid responsiveness and improve the survival rates of critically ill patients with or at risk for ARDS [36]. CVP, serving as a pressure indicator, can reflect preload depending on cardiac compliance. However, various factors such as heart failure, volume expansion, the use of vasopressors, mechanical ventilation, or PEEP application can alter cardiac compliance, leading to inaccuracies in preload assessment via CVP [37]. Since our study population predominantly used a protective ventilation strategy with low tidal volume and high PEEP, CVP is not suitable for predicting fluid responsiveness in such patients.
PPV and SVV are dynamic parameters widely used to estimate cardiac preload and predict hemodynamic fluid responsiveness[38]. Nowadays, optimization strategies for fluid management are often based on these parameters, which can be easily obtained through monitoring devices such as non-invasive ultrasound, offering the advantages of safety, repeatability, real-time monitoring, and efficiency. However, some studies suggest that factors such as tidal volume (Vt) may influence the performance of PPV and SVV, especially in patients undergoing thoracoabdominal closure or positioned in the prone position[39, 40]. During the treatment of ARDS patients, mechanical ventilation can induce ventilator-induced lung injury (VILI) through various mechanisms, including volutrauma, barotrauma, and atelectrauma[41]. Therefore, guidelines from the Extracorporeal Life Support Organization (ELSO) recommend limiting tidal volume to less than 6 ml/kg to reduce the occurrence of VILI[42], and supplementing with prone positioning ventilation during ECMO to treat moderate to severe ARDS [43]. Multiple studies[44, 45] have demonstrated that prone positioning during ECMO is feasible, safe, and can enhance ECMO weaning and significantly improve outcomes. Constrained by the lung-protective ventilation strategy with low tidal volume(≤ 6 ml/kg), PPV and SVV are not suitable for predicting fluid responsiveness in such patients.
In contrast, the application of FTc and ΔVpeak derived from neck ultrasound has multiple advantages. Firstly, this technique is entirely non-invasive, allowing for easy assessment of a patient's hemodynamic status through carotid artery ultrasound. Secondly, these parameters are applicable to patients with low tidal volume(≤ 6ml/kg) and cardiac arrhythmias[39], unlike other dynamic markers such as PPV and SVV, which require higher tidal volumes and regular heart rhythms. Additionally, the measurement of FTc and ΔVpeak is not affected by changes in intrathoracic pressure during respiration[46].
Trendelenburg position or PLR are used either as a diagnostic tool to assess fluid responsiveness[19, 20]. The physiological mechanisms of both methods are similar, involving changes in body position or elevation of the lower limbs to promote blood flow toward the heart, resulting in "autotransfusion," increasing venous return to the heart, enhancing CO, and ultimately increasing organ perfusion. However, PLR requires the patient to maintain a supine position, VV-ECMO patients often undergo prone positioning ventilation. Therefore, our study ultimately chose the Trendelenburg position.
Limitations
Our study had several limitations. Firstly, it was conducted in a single center, which may limit its generalizability across different clinical settings. Secondly, all patients in this study were under deep sedation; therefore, the application of Trendelenburg position combined with carotid artery ultrasound in predicting fluid responsiveness needs further validation in non-sedated states. Finally, This study solely involved VV-ECMO patients, thus the predictive capability for other types of ECMO patients remains unclear. In the future, we will further explore combining FTc and ΔVpeak with other predictive tools to enhance the accuracy of fluid responsiveness assessment.