Using non invasive dynamic parameters of fluid responsiveness in children: there is still much to learn

In this issue of the Journal of Clinical Monitoring and Computing, Chandler et al. [1] investigated the relationship between respiratory variation in the amplitude of the plethysmographic waveform (DPOP) and in the arterial pulse pressure (PPV) in children. While the assessment of fluid responsiveness in mechanically ventilated adults is now pretty well defined [2, 3], its prediction in the pediatric population remains controversial and underexplored. In patients under general anesthesia, positive pressure ventilation induces respiratory changes in stroke volume which, in turn, results in respiratory changes in arterial pressure [4] and in the plethysmographic waveform [5]. When the heart is on the steep portion of the Frank Starling relationship, these respiratory variations are high (the heart is preload dependent and the patient is more like to be a fluid responder) while when it is on the plateau of this relationship these respiratory variations are low (the heart is preload independent and the patient is more likely to be a fluid non responder). This phenomenon relies mainly on two hypotheses. First, the changes in intra- thoracic pressure need to be significant enough to induce changes in venous return to the right ventricle (explaining why a tidal volume less than 7 ml/kg is a limitation to the use of PPVin adults [6]). Second, if PPVis to be used as a surrogate for the respiratory variations in stroke volume (SVV), we assume that pulse pressure (the difference between systolic pressure and diastolic pressure) depends on stroke volume and on arterial compliance. We also assume the arterial compliance remains stable during a single respiratory cycle so that changes in pulse pressure only reflect changes in stroke volume [7, 8].