Basic concepts of fluid responsiveness

Predicting fluid responsiveness, the response of stroke volume to fluid loading, is a relatively novel concept that aims to optimise circulation, and as such organ perfusion, while avoiding futile and potentially deleterious fluid administrations in critically ill patients. Dynamic parameters have shown to be superior in predicting the response to fluid loading compared with static cardiac filling pressures. However, in routine clinical practice the conditions necessary for dynamic parameters to predict fluid responsiveness are frequently not met. Passive leg raising as a means to alter biventricular preload in combination with subsequent measurement of the change in stroke volume can provide a fast and accurate way to guide fluid management in a broad population of critically ill patients

Predicting Fluid Responsiveness by Passive Leg Raising: A Systematic Review and Meta-Analysis of 23 Clinical Trials

Objective: Passive leg raising creates a reversible increase in venous return allowing for the prediction of fluid responsiveness. However, the amount of venous return may vary in various clinical settings potentially affecting the diagnostic performance of passive leg raising. Therefore we performed a systematic meta-analysis determining the diagnostic performance of passive leg raising in different clinical settings with exploration of patient characteristics, measurement techniques, and outcome variables. Data Sources: PubMed, EMBASE, the Cochrane Database of Systematic Reviews, and citation tracking of relevant articles. Study Selection: Clinical trials were selected when passive leg raising was performed in combination with a fluid challenge as gold standard to define fluid responders and non-responders. Data Extraction: Trials were included if data were reported allowing the extraction of sensitivity, specificity, and area under the receiver operating characteristic curve. Data Synthesis: Twenty-three studies with a total of 1,013 patients and 1,034 fluid challenges were included. The analysis demonstrated a pooled sensitivity of 86% (95% CI, 79-92), pooled specificity of 92% (95% CI, 88-96), and a summary area under the receiver operating characteristic curve of 0.95 (95% CI, 0.92-0.98). Mode of ventilation, type of fluid used, passive leg raising starting position, and measurement technique did not affect the diagnostic performance of passive leg raising. The use of changes in pulse pressure on passive leg raising showed a lower diagnostic performance when compared with passive leg raising induced changes in flow variables, such as cardiac output or its direct derivatives (sensitivity of 58% [95% CI, 44-70] and specificity of 83% [95% CI, 68-92] vs sensitivity of 85% [95% CI, 78-90] and specificity of 92% [95% CI, 87-94], respectively; p < 0.001). Conclusions: Passive leg raising retains a high diagnostic performance in various clinical settings and patient groups. The predictive value of a change in pulse pressure on passive leg raising is inferior to a passive leg raising induced change in a flow variable

Multi-site and multi-depth near-infrared spectroscopy in a model of simulated (central) hypovolemia: lower body negative pressure

Purpose: To test the hypothesis that the sensitivity of near-infrared spectroscopy (NIRS) in reflecting the degree of (compensated) hypovolemia would be affected by the application site and probing depth. We simultaneously applied multi-site (thenar and forearm) and multi-depth (15-2.5 and 25-2.5 mm probe distance) NIRS in a model of simulated hypovolemia: lower body negative pressure (LBNP). Methods: The study group comprised 24 healthy male volunteers who were subjected to an LBNP protocol in which a baseline period of 30 min was followed by a step-wise manipulation of negative pressure in the following steps: 0, -20, -40, -60, -80 and -100 mmHg. Stroke volume and heart rate were measured using volume-clamp finger plethysmography. Two multi-depth NIRS devices were used to measure tissue oxygen saturation (StO2) and tissue hemoglobin index (THI) continuously in the thenar and the forearm. To monitor the shift of blood volume towards the lower extremities, calf THI was measured by single-depth NIRS. Results: The main findings were that the application of LBNP resulted in a significant reduction in stroke volume which was accompanied by a reduction in forearm StO2 and THI. Conclusions: NIRS can be used to detect changes in StO2 and THI consequent upon central hypovolemia. Forearm NIRS measurements reflect hypovolemia more sensitively than thenar NIRS measurements. The sensitivity of these NIRS measurements does not depend on NIRS probing depth. The LBNP-induced shift in blood volume is reflected by a decreased THI in the forearm and an increased THI in the calf

Myocardial function during ventilation with lower versus higher positive end-expiratory pressure in patients without ARDS

The aim of this study was to investigate whether lower PEEP (positive end-expiratory pressure) had beneficial effects on myocardial function among intensive care unit (ICU) patients without acute respiratory distress syndrome (ARDS) compared to higher PEEP. In this pre-planned substudy of a randomized controlled trial (RELAx), comparing lower to higher PEEP, 44 patients underwent transthoracic echocardiography. The exclusion criteria were known poor left ventricular function and severe shock requiring high dosages of norepinephrine. To create contrast, we also excluded patients who received PEEP between 2 cmH2O and 7 cmH2O in the two randomization arms of the study. The primary outcome was the right ventricular myocardial performance index (MPI), a measure of systolic and diastolic function. The secondary outcomes included systolic and diastolic function parameters. A total of 20 patients were ventilated with lower PEEP (mean ± SD, 0 ± 1 cmH2O), and 24 patients, with higher PEEP (8 ± 1 cmH2O) (mean difference, −8 cmH2O; 95% CI: −8.1 to −7.9 cmH2O; p = 0.01). The tidal volume size was low in both groups (median (IQR), 7.2 (6.3 to 8.1) versus 7.0 (5.3 to 9.1) ml/kg PBW; p = 0.97). The median right ventricular MPI was 0.32 (IQR, 0.26 to 0.39) in the lower-PEEP group versus 0.38 (0.32 to 0.41) in the higher-PEEP group; the median difference was –0.03; 95% CI: −0.11 to 0.03; p = 0.33. The other systolic and diastolic parameters were similar. In patients without ARDS ventilated with a low tidal volume, a lower PEEP had no beneficial effects on the right ventricular MPI