BEAT- FR Beat-to-beat Evaluation of Atrial Pacing to Test Fluid Responsiveness

INTRODUCTION AND RATIONALE Fluid administration is still the cornerstone in the treatment of hemodynamically unstable patients to enhance oxygen delivery and ensure adequate tissue perfusion. The intended effect is an increase in cardiac output by increasing preload.

However, the effectiveness of fluid therapy depends heavily on the patient's fluid responsiveness, defined as the ability of the heart to increase stroke volume in response to a fluid challenge [1]. Despite its central role in intensive care, fluid management remains a challenge, with both under- and over-resuscitation posing risks to patient outcomes [2,3].

Excessive fluid resuscitation, commonly referred to as "fluid overload," has been linked to adverse outcomes, including increased risk of organ dysfunction, prolonged mechanical ventilation, and higher mortality rates [4,5]. On the other hand, insufficient fluid administration can exacerbate hypoperfusion and organ failure [6]. The need for accurate and timely assessment of fluid responsiveness has therefore become a priority in optimizing care for critically ill patients [7].

Challenges in current methods for fluid responsiveness assessment Current methods for evaluating fluid status have limitations concerning their invasiveness, applicability, and sensitivity and specificity. For example, static preload indicators such as central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP) have failed to reliably predict fluid responsiveness [8]. Dynamic variables such as pulse pressure variation (PPV) offer improved accuracy, but their predictive value is restricted to specific conditions [9], limiting their broader applicability [10]. Moreover, techniques like passive leg raising (PLR), though effective, are non-continuous, technically challenging, and require real-time stroke volume monitoring [11]. Premature ventricular complexes and preload variation From observational studies, preclinical data and modelling, the physiological concept of acutely varying preload and measuring the heart's contractile response has potential for assessing fluid status and predicting fluid responsiveness. A premature ventricular complex (PVC) causes such an acute preload change by causing a compensatory pause and increasing preload. Proof-of-concept studies in cardiac surgery patients [12] and critically ill populations [13] have demonstrated the potential of PVCs in predicting fluid responsiveness. However, since extrasystoles do not consistently occur in all patients, their usefulness for standardized fluid responsiveness evaluation is limited.

This limitation has led to interest in controlled methods for inducing acute preload variations. Advances in cardiac pacing technology have made it possible to manipulate heart rate and RR-intervals in a controlled manner, offering a potential solution.