Patients Under Spontaneous Breathing

One of the most frequent interventions in an intensive care setting is fluid replacement. Recent trials emphasize that excessive volume, given unnecessarily, may be harmful to the patient, and that assessment of volume responsiveness is fundamental for intensivists. Volume responsiveness may be defined as increased systolic volume (SV) with consequent increased cardiac output (CO) from an established volume infusion which would provide better oxygen supply to the tissue. However, this response to volume testing will only take place when both ventricles operate in the ascending phase of the Frank-Starling curve, i.e., in a preload dependence status. In the last decade, with improved knowledge and practical application of physiology and heart-lung interaction, along with critical patient monitoring techniques, new volume responsiveness assessment methods were described, called dynamic methods.

Described as such are pulse pressure variation (PPV), systolic pressure variation (SPV), systolic volume variation (SVV), in addition to techniques using echocardiography to evaluate superior and inferior vena cava collapsibility. The dynamic evaluation methods have good accuracy to predict fluid responsiveness, with much higher predictive values than static measurements. However, an important limitation of these methods is that indexes and measurements were validated for specific groups of patients under sedation and volume controlled mechanical ventilation, with no respiratory effort and no arrhythmias. Other studies that tried to reproduce these results in different settings, did not reach the same results. In spontaneous breathing patients, or in those under mechanical ventilation with respiratory effort, fluid responsiveness assessment still requires additional studies, as the current intensive care trend is to maintain the patient with the mildest sedation and weaning from mechanical ventilation as soon as possible. This review aims to summarize the main evidences on fluid responsiveness assessment in the spontaneous breathing patient, didactically dividing the static measurement studies from those with dynamic methods.

referance:
Magder S, Georgiadis G, Cheong T. Respiratory variations in right atrial pressure predict the response to fluid challenge. J Crit Care. 1992;7(2):76-85.