Discussion
critically ill, end-inspiratory occlusion maneuver, EEOt, septic shock, arrhythmias
CARDIAC FAILURE, CRITICALLY ILL, FLUID OVERLOAD, FRANK, SEPTIC SHOCK, ATRIAL FIBRILLATION, ARRHYTHMIAS
Our results showed a significant correlation between changes in CPDV during EEOt and changes in CI during the subsequent fluid challenge. An increase in CDPV greater than 10.5% during an EEOt predicted fluid responsiveness in ICU patients with septic shock with high specificity. This may allow clinicians to non-invasively identify about 40% of fluid responders and safely administer them fluids without risking a fluid overload.The carotid artery Doppler assessment of fluid responsiveness coupled with an EEOt can be performed in mechanically ventilated patients in whom continuous cardiac monitoring is unavailable or the transthoracic echocardiographic windows are technically impaired. These conditions may be common not only in the ICU, but also in the operating room in patients undergoing major surgery whose chest wall is inaccessible to echocardiography. In this setting, the carotid artery might be the only accessible large-size arterial vessel for performing arterial Doppler. The position of the carotid artery, which is located just distally to the aortic outflow tract, makes its flow a good approximation of the aortic flow [To the best of our knowledge, this is the first study assessing fluid responsiveness using the carotid Doppler variations in association with an EEOt in critically ill patients. The potential usefulness of the changes in carotid Doppler during EEOt has been previously investigated in healthy volunteers [Two studies [EEOt and fluid challenge were not consistently associated with increased Doppler velocity. Namely, CDPV decreased in a minority of responders (3/19 measurements during the EEOt and 2/19 measurements after the fluid challenge) and almost half of the non-responders (11/25 measurements at either time point; Additional file Results of the repeated measures correlation clearly showed that the changes in CDPV and cFT both during EEOt and after fluid challenge were correlated with the parallel changes in CI or SVI in responders. This is consistent with the ability of EEOt-induced changes in CPDV to predict fluid responsiveness in our study. However, the accuracy of CDPV was lower than that of other indices, such as PLR. In fact, the gray zone of CPDV was 61%, meaning that only the CDPV values included in the remaining 39% range were sufficiently accurate to make a clinical decision. A possible explanation for this finding may be that the EEOt causes smaller changes in venous return compared to the PLR. The PLR in fact causes a rapid displacement of a high volume (about 300 mL) of venous blood from the lower body toward the right heart, mimicking an entire fluid challenge [The clinical concordance analysis of the percentage changes in Doppler variables vs. the percentage changes in CI showed that many of the concordant changes in cFT were ± 5% or less. Compared to cFT, CDPV had a greater proportion of clinically significant concordant changes (5–15% or > 15%; categories 2 and 3 in Additional file Despite a clear correlation between cFT and CI changes at EEOt and fluid challenge, cFT did not show a significant predictive ability for fluid responsiveness in our study. The reasons for this are unclear and deserve further investigation. Since the LSC of cFT was only about half the percentage increase in cFT during EEOt or after the fluid challenge in fluid responders, this result does not seem to be due to an insufficient precision of the Doppler technique. Because of the small AUROC of cFT, our study was not powered enough to assess its predictive ability with confidence.In our study, we defined fluid responsiveness as a ≥ 15% increase in CI. This choice was based on previous literature on EEOt. However, an SVI increase could have been an equally valuable target [We may have considered adding an end-inspiratory to the end-inspiratory occlusion maneuver for predicting fluid responsiveness. In a study by Jozwiak et al. [Fluid responsiveness depends on the slope of the Frank–Starling curve, which is critically affected by cardiac contractility. In our study, we excluded patients with cardiac failure, so that the role of cardiac contractility was not specifically assessed. However, previous observations made using aortic Doppler [Some limitations of our study must be acknowledged. Firstly, this study was conducted on adult patients with septic shock in controlled ventilation with 6 mL/kg of PBW with absent or minimal spontaneous breathing, and its findings are not necessarily generalizable to other patient populations. Secondly, this technique is based on manual detection and—as such—is prone to intra-observer variation. In our study, all carotid Doppler measurements were made by a single experienced operator. While this was essential to ensure the reproducibility of our measurements, it may have overestimated their precision and reduced the external validity of our findings. Thirdly, we used the EV1000™/Volume View device, which calculates SVI and CI on the average pulse contour recorded over 20 s. Inevitably, this time window only partially overlapped with those of the EEOt, which may have affected the accuracy of our findings. Fourthly, as specified in our Methods, we compared both the hemodynamic measurements made during EEOt and those made after the fluid challenge with the first baseline (T1), since on T3 the return to the baseline after the EEOt was assessed only visually. Although the effect of the EEOt is assumed to be transient, we cannot exclude that some residual effects may have persisted after the end of EEOt and have reduced the accuracy of the subsequent measurements. Fifthly, the 61% gray zone we found in our study suggests caution when using Doppler results to make clinical decisions. Lastly, carotid Doppler might be less accurate in patients affected by arrhythmias, such as atrial fibrillation, which are common in septic shock patients [