During acute circulatory failure, infusing fluids as first line therapy is the source of a therapeutic dilemma [increase in cardiac output (CO) and improvement of tissue perfusion vs. inconsistent effectiveness and risk of fluid accumulation]. Predicting effectiveness of a fluid bolus on CO before infusing it avoids administering fluid to patients who do not require it [1]. The principle is simple: if some changes in cardiac preload, induced by external maneuvers or by the ventilator, change CO above a given diagnostic threshold, fluid infusion will likely have a similar effect [2].
The tests and indices using the ventilator (Supplementary Fig. 1) are underpinned by heart–lung interactions. Under mechanical ventilation, increased intrathoracic pressure reduces the pressure gradient of systemic venous return by increasing right atrial pressure [3]. This effect is exaggerated upon insufflation. The resulting drop in CO is larger if both ventricles are preload responsive. Ventilation also increases afterload of the right ventricle (RV) and decreases left-ventricular (LV) afterload [3], but these effects are independent of preload responsiveness and only relevant if the RV is failing.
Key Points
- Clinical Challenge: Predicting fluid responsiveness helps avoid unnecessary fluid administration, reducing risks of fluid overload and associated complications in critically ill patients.
- Heart-Lung Interactions: Mechanical ventilation-induced changes in intrathoracic pressure serve as a basis for assessing cardiac preload responsiveness.
- Pulse Pressure and Stroke Volume Variation: PPV and SVV are reliable in patients without spontaneous breathing or arrhythmias, with thresholds of ≥12% indicating fluid responsiveness.
- Limitations of PPV/SVV: Reduced reliability in patients with low tidal volumes, high respiratory rates, intra-abdominal hypertension, or acute cor pulmonale (ACP).
- Vena Cava Diameter Changes: Respiratory variations in the inferior and superior vena cava diameters are less reliable but remain useful adjuncts.
- Tidal Volume Challenge (Vt Challenge): Temporarily increasing tidal volume identifies preload responsiveness based on changes in PPV, with a threshold of ≥3.5%.
- End-Expiratory Occlusion Test: A 15-second pause at end-expiration reduces intrathoracic pressure and increases preload, with a cardiac output rise of ≥5% indicating responsiveness.
- PEEP Test: Lowering positive end-expiratory pressure (PEEP) by 5 cmH₂O tests preload responsiveness; a ≥9% increase in cardiac output is predictive, though more validation is needed.
- Integration into Practice: Combining tools and using dynamic tests tailored to patient conditions enhances diagnostic accuracy. Automation could further improve efficiency.
- Future Directions: Research should focus on standardizing thresholds and integrating ventilator-based monitoring with hemodynamic devices for personalized fluid management.
Open Access This article is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, which permits any non-commercial use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc/4.0/.
