The elusive relationship between cardiac filling and fluid responsiveness

Excerpt

Munoz et al. [1] have recently published an important pilot study in Critical Care. In 90 mechanically ventilated patients with circulatory failure who were admitted to the intensive care unit (ICU) for less than 24 h, four measures of ‘venous congestion’ (VC) were related to fluid responsiveness (F_R) or unresponsiveness (F_UR). To score VC, each patient received 1 point for any of the following: (1) a central venous pressure (CVP) > 12 mmHg; (2) a lung ultrasound score (LUS) > 10; (3) a venous excess ultrasound score (VExUS) > 1; and (4) a lateral E/e’ > 10 for the left ventricle. Additionally, F_R was measured pragmatically based on either pulse pressure variation, stroke volume variation, passive leg raising, or end expiratory occlusion test (considering the presence of arrhythmias, spontaneous ventilation, and the availability of monitoring devices). The authors hypothesized that F_R patients would have fewer VC signals than F_UR patients. Contrary to these expectations, there was no statistically significant difference between F_R and F_UR patients with respect to number of VC signals. Fifty-three percent of F_R patients had at least one VC signal; only slightly more (57%) of F_UR showed at least one VC signal. From this, the authors correctly concluded that VC is present in a clinically significant fraction of F_R patients and clinicians should, therefore, be wary when resuscitating based only upon measures of F_R. However, we note that their results are equally true in the reverse sense which brings to mind the fundamental relationship between surrogates of cardiac filling (e.g., VC or lack thereof) and the response to additional preload (i.e., F_R). For example, the frequently used strategy of prescribing fluids based on a lower CVP or flat IVC [2]. …