To the Editor:
With interest we read the article by Zhang and Critchley1 and sincerely commend them for their effort to further expand the role of ultrasound in the perioperative setting. They find an association between a greater respiratory variation in the diameter of the inferior vena cava (IVC) and the collapsibility index (IVCCI), suggestive of low volume status and the occurrence of hypotension after induction of anesthesia. Clearly, this offers the anesthesiologist another tool to identify patients at potential risk for adverse outcomes during anesthesia. However, a few issues may warrant some consideration before extrapolation of the present results to clinical practice, as the question on the preferred treatment of these patients (e.g., fluid administration) in our opinion still remains.
First, transthoracic echocardiography was performed in spontaneously breathing patients. During respiration, the position of the IVC changes, possibly altering the angle at which the ultrasound plane intersects the vessel and thus altering the measured diameter.2 Kimura et al.3 showed that depth of inspiration as reflected by diaphragmatic displacement influenced IVCCI. Differences in breathing patterns may in this way be responsible for the moderate reliability of IVCCI to predict fluid responsiveness during spontaneous breathing. For standardization, guidelines from the American Society of Echocardiography4 recommend performance of a short sniff by patients during scanning of the IVC. In order to translate the current findings into clinical practice, it would be important to specify how spontaneous breathing was standardized between patients.
Another potential confounder in the current observations is the initiation of mechanical ventilation after the ultrasound examination. As airway pressures reverse as compared to normal physiologic conditions, positive intrathoracic pressure during inspiration causes a decrease in venous return that can lead to hypotension in not only hypovolemic but also normovolemic patients.5,6 Keeping the aforementioned in mind, it would be necessary to standardize ventilator settings and explore whether a relationship between pressure and the occurrence of hypotension in the study population could be substantiated.
Furthermore, the authors state that they did not perform another scan of the IVC after intubation because the change to positive pressure ventilation precludes reliable assessment of IVCCI. We agree with the authors that the collapsibility index cannot be used during mechanical ventilation. However, the distensibility index ([max IVC diameter – min IVC diameter]/max IVC diameter) can be used as an indicator of volume status and fluid responsiveness.7,8 In contrast, where IVCCI is conceivably an indicator of volume status, some recent evidence shows that it does not reliably predict fluid responsiveness in spontaneously breathing subjects, but has a moderate correlation at best.9–11 A recent meta-analysis showed that fluid responsiveness was predicted in a more reliable way in mechanically ventilated patients than in spontaneously breathing patients.12 Accordingly, we would like to argue that another scan would have been of additional value. Besides exploring the complex relationship between mechanical ventilation and the cardiovascular system, it could also indicate which patients would probably benefit from fluid administration.
Nevertheless, this well-designed study elegantly demonstrates another useful application of ultrasound in the perioperative setting. Follow-up studies will undoubtedly confirm the efficacy of this noninvasive measurement of volume status, thereby aiding identification of patients at risk for hypotension after induction. With attention to the above-mentioned points, hopefully this will also translate into improved outcomes by not only detecting these patients but also providing clues to the best possible (fluid) management.
Drs. Korsten and Bouwman have received consultant fees from Philips Medical Research, Eindhoven, The Netherlands, since 2016. The other authors declare no competing interests.