Echocardiography-guided fluid resuscitation in critical illness: from fluid responsiveness to fluid tolerance

Fluid administration remains a cornerstone of shock resuscitation, but only a subset of critically ill patients increases stroke volume after a fluid bolus, and indiscriminate fluid loading may worsen pulmonary oedema, venous congestion, and organ dysfunction. Echocardiography has evolved from a diagnostic tool into a bedside haemodynamic monitor that can support individualised fluid decisions. This narrative review synthesises contemporary evidence on the role of transthoracic and transoesophageal echocardiography in guiding fluid resuscitation in adult critical illness, with emphasis on literature published after the major 2016 reviews and on the emerging concept of fluid tolerance. Static echocardiographic variables, including chamber size, inferior vena cava dimensions, and diastolic indices, remain useful for phenotyping shock and identifying patients at risk of fluid intolerance, but they do not reliably predict fluid responsiveness. By contrast, dynamic assessment of stroke volume surrogates is more informative. The best validated echocardiographic strategy is passive leg raising combined with real-time measurement of left ventricular outflow tract velocity-time integral, which shows a pooled sensitivity of around 85% and specificity of around 91% when preload responsiveness is assessed with any cardiac output monitoring technique, and remains applicable in both spontaneously breathing and mechanically ventilated patients, including those with arrhythmias. Mini-fluid challenge and end-expiratory occlusion testing are useful alternatives in selected settings. However, these approaches remain operator-dependent and have not yet been shown to improve patient-centred outcomes. Echocardiography should therefore guide serial, physiology-based fluid decisions rather than serve as a stand-alone protocol.