To our knowledge, this is the first study evaluating the diagnostic performance of IJVV in predicting fluid responsiveness in patients with AIS undergoing PSF surgeries with low Vt ventilation. The main finding of our study was that baseline IJVV was not a good predictor of fluid responsiveness in patients in the prone position under protective ventilation. Second, our findings indicated that the baseline value of PPV and SVV did not predict fluid responsiveness when a lung-protective ventilatory strategy and prone position were applied; moreover, the combination of IJVV and PPV was unreliable in predicting fluid responsiveness in this surgical setting.
AIS is a form of lateral curvature of the spine, which occurs in the absence of an explicit medical cause[25]. Adolescents found to have scoliosis of at least 40° typically undergo surgical correction with PSF to prevent neurologic deficits, further deformation, cardiopulmonary compromise, and resultant pain[26]. PSF surgery for the treatment of AIS has been reported to be associated with substantial perioperative bleeding, which often leads to massive blood transfusion and fluid administration[27]. Furthermore, adolescent patients have different fluid requirements from those of adults, for whom inappropriate use of intravenous fluids may lead to more serious consequences[28]. Therefore, predictors of fluid responsiveness or intravascular volume monitoring are greatly needed for hemodynamic optimization in adolescent patients.
Previous studies have demonstrated that prone position induces a lot of physiologic changes in the respiratory and cardiovascular system. Specifically, it may decrease cardiac index and the compliance of the respiratory system; and increase CVP, PPV, and SVV[29–31]. In agreement with previous findings, the prone position increased CVP in our study. However, we did not observe a decrease in cardiac index, and an increase in PPV and SVV. This discrepancy among studies might be due to differences in the level of cardiac preload and stroke volume. Typically, a higher intra-abdominal pressure (IAP) after prone positioning might collapse the inferior vena cava and lead to a decrease in the venous return and cardiac preload[32]. In the present study, the six pads allowed the abdomen to hang completely freely, and prevented abdominal compression and an influence of the abdominal viscera on the movement of the diaphragm during ventilation. Therefore, we speculate that the equipment used for prone positioning might have minimized the increase in IAP while having minimal effects on cardiac preload. Furthermore, continuous fluid administration during the study period might also have contributed to the minimal change in cardiac index. In addition, continuous fluid supplementation and increased cardiac preload after prone positioning[33] might ultimately lead to decreases in PPV and SVV.
Over the past several decades, enormous efforts have been made to discover non-invasive measures and alternatives to assess volume status. IJVV, a noninvasive and easily determined predictor, has recently been confirmed to accurately assess fluid responsiveness in multiple clinical settings[16–18, 34]. Nonetheless, our study did not demonstrate that baseline IJVV can be used to discriminate between responders and non-responders to fluid expansion among patients with low Vt ventilation in prone position. Notably, although the studies aforementioned showed a desirable predictive effect of IJVV on fluid responsiveness in various clinical settings, they did not include patients with low Vt ventilation under prone position who had different hemodynamic characteristics. We believe that the undesirable predictive effect of IJVV in our study might be explained by the following reasons. First, Vt < 8 ml·kg− 1 was used in this study, thus leading to small variations in intrathoracic pressure and preload[35]. Therefore, the IJVV in response to ventilation were theoretically expected to be small, regardless of volume status. Furthermore, prone position altered both CVP and the effective circulating blood volume, owing to the impeded venous return resulting from decreased respiratory compliance and increased intrathoracic pressure[36], which would decrease IJV distensibility; consequently, this predictor might have yielded false negative results.
Conflict
ing results have been found regarding the ability of PPV and SVV to predict fluid responsiveness in the prone position and under protective ventilation[22, 29, 30, 37–39]. The present study indicated that the baseline values of PPV and SVV could not be used to predict fluid responsiveness in conditions of low Vt ventilation in the prone position. This finding was not unexpected, given that the predictive value of PPV and SVV depends on not only ventilatory settings but also transmission of airway pressure to the intrathoracic structures[39–41]. This transmission is negatively associated with respiratory system resistance[40]. Therefore, under the conditions of low respiratory system compliance and low Vt ventilation in our study, the effect of stroke volume change caused by ventilation was weakened; thus, a high false negative rate was expected. Furthermore, in contrast to a previous study[16], we did not find that the combination of IJVV and PPV increased the sensitivity and specificity of fluid responsiveness prediction. This difference between studies reflects the complicated nature of heart-lung interactions, and implies that the results should be interpreted with caution, particularly under different clinical circumstances.
Collectively, our findings suggested that the development of more specific indicators is warranted. Interestingly, recent studies have demonstrated that tidal volume challenge (Vt adjustment from 6 to 8 ml·kg− 1) can compensate for the limitation of PPV under low Vt ventilation[22, 42, 43]. However, whether this method can be applied in prone position remains controversial and requires further investigation.
The present study has several limitations. First, all measurements were performed by the same operator, technical errors might have existed, because even slight pressure might have caused significant changes in IJV diameter and cross-sectional images during the measurement of IJVV. Second, because we did not measure IAP in this study, we could not estimate the effect of IAP on the predictive values of target hemodynamic parameters in the prone position. Third, the single-center nature of the present study may limit the generalizability of the findings. Fourth, we conducted this study on a limited number of patients, and type Ⅱ error might have been present; thus, additional studies with larger sample sizes might be necessary to identify statistical differences. Fifth, our results were based on patients in the prone position, in which six pads were used to allow the abdomen to hang completely freely. Because other positions can cause different alterations in hemodynamic and respiratory mechanics, the results might differ for various types of operating frames or tables used. Finally, because our results were acquired from serial measurements, errors in the precision of the system might have influenced the reliability of the measurements.