Respiratory Variation of Superior Vena Cava in Transthoracic View as a Fluid Responsiveness Predictor

Prospective evaluation of diameter variability of the superior vena cava by transthoracic echocardiography as fluid responsiveness predictor in critically ill patients:

Background Fluid administration is frequent in critically ill patients, particularly during reanimation and its empirical use is recommended as an initial step during this phase.

However, even though excess fluid administration is associated to negative outcomes fluid use is still being empirical and with scarce application of responsiveness predictors before its indication.

Different measurements or maneuvers exist, that allow clinicians to predict if a patient would present a positive fluid response, usually defined as cardiac output increasing 15% when infusing 500ml of crystalloids, and application of these predictive parameters could have clinical benefits on outcomes by avoiding inappropriate fluid administration.

As for the variables used, different types stand out. In general, flow or pressure variations originated in cardiothoracic interaction, auto-infusion and reduced fluid test evaluations such as the "mini" o "micro" fluid tests, in addition venous diameter variations in ultrasound evaluation with variable accuracy depending on the context. However, cardiothoracic interaction variables might have false positive results when right ventricular failure is present, given the cyclic increment on its afterload induced by positive pressure ventilation, reducing right ventricle stroke volume limiting its application in established or unknown and probable right heart failure if and advanced hemodynamic monitoring has not been performed to rule it out.

In this context, venous evaluations and reduced volume fluid test have advantages, and particularly in patients in shock under positive pressure ventilation, superior vena cava variation, being a better predictor than inferior vena cava. However, traditionally, superior vena cava can only be observed with transesophageal echocardiography and that can be a limitation in resource limited settings. Recently a new transthoracic acoustic window has been described, using a vertical left parasternal approach that allows evaluation of the superior vena cave, and initially, this approach shows a good correlation with the transesophageal measurement, and acceptable feasibility in the pilot study patients.

In this way, it can be postulated as possible, to evaluate if the variability of superior vena cava diameter on a transthoracic approach can be used as a fluid responsiveness predictor in critically ill patients on positive pressure ventilation.

Hypothesis Superior vena cava diameter respiratory variation evaluated with transthoracic ultrasound can predict fluid responsiveness in critically ill patients in positive pressure ventilation.

Objective To evaluate if superior vena cava respiratory diameter variation is associated with fluid responsiveness, when compared with the mini fluid infusion of 100ml evaluated with expiratory left ventricle outflow tract velocity time integral (LVOT-VTI) before and after fluid administration.

Methods

After evaluation, inclusion and exclusion criteria checking:

A basal echocardiography is performed, main pattern and clinical data are recorded (age, sex, weight, height, main diagnosis, secondary diagnosis, length of stay, surgical procedures, sequential organ failure assesment (SOFA) score, renal replacement therapy), respiratory (ventilator mode, tidal volume, respiratory rate, positive end expiratory pressure (PEEP), plateau pressure, peak inspiratory pressure, autoPEEP, fraction of inspired oxygen (FiO2), I:E relation, total inspiratory time), hemodynamic data from unit monitors. (cardiac rate, systolic pressure, diastolic pressure, mean pressure, central venous pressure) and closest laboratory to evaluation (lactate, venous oxygen saturation (ScVO2), arterial partial pressure of oxygen (PaO2), arterial partial pressure of carbon dioxide (PaCO2), pH, base excess (BE), bicarbonate (HCO3), carbon dioxide (CO2) arterio-venous difference, )

• Superior vena cava evaluation, Respiratory variation observed in M mode and measured. Registry of expiratory and inspiratory diameter (evaluated in the same cardiac cycle phase)
• Using Apical 5 chamber view, basal LVOT-VTI would be obtained, 3 measurements in regular rhythms and 5 measurements in irregular rhythms such as atrial fibrillation. 100 ml will be infused without changing the view and end-expiratory LVOT-VTI would be obtained a minute after infusion in the same fashion as before.
• Considering ultrasound evaluation is part of standard patient care, informed consent was waived by the ethical board.
• For analysis, the proportion of variation in superior vena cava (SVC) and VTI variation would be calculated as percentage.
• All values will be registered in a google drive table and then included in microsoft excel for coding and analyzed with Stata 12.
• Fluid responsiveness will be defined as 10% increase in VTI with the "mini bolus" fluid test, considering the minimal detectable change.
• For analysis, receiver operator curve (ROC) statistic will be done in Stata 12.
• Summary variables of the population will be presented.
• For diagnostic evaluation, the tool "roctab" for receiver operating characteristic curve will be used, obtaining cut off values, sensitivity, specificity and gray area in addition to traditional area under curve (AUC). A dichotomic value for simple orientation will be selected with an ideally balanced sensitivity and specificity for general use. Linear correlation will be explored to detect a continuous relation between SVC variation and VTI increase.
• Window success will be recorded and proportion will be analysed during the months of the study to check for performance improvements that might be expected as a learning curve effect.
• Additional data might allow post hoc analysis including window feasibility in association with clinical variables and
• After the ultrasound evaluation, the result of the 100ml test will be informed to the treating physician.
• Sample size:

Considering a possible 50% loss with inadequate acoustic window, 100 evaluated patients would achieve at least 48 included to obtain AUC 0.7. (estimating 24 to be responders and 24 not responders)