The present study showed that the mean values of IVCDmax observed with both LLT and LUD were significantly larger than that in SPN. Based on reports that IVCDmax measured by ultrasonography is a reliable index of IVCD [12–14], the present results suggest that both LLT and LUD can effectively relieve compression of the IVCD by the gravid uterus that occurs in SPN in full-term pregnant patients. Additionally, the fact that a few patients (10%) demonstrated the largest IVCDmax in SPN in the present study cautions us that both LLT and LUD are not necessarily appropriate treatment for relieving IVCD compression in pregnant women.
Based on previous reports that showed that application of LLT or the LUD maneuver ameliorated the decrease in maternal blood pressure and enhanced vasopressor requirements observed in the supine position [5, 6, 10], it is believed that both LLT and LUD lead to an increase in venous return to the right atrium via relief of the SPN-induced IVC compression. A previous report showed that IVCDmax measured by ultrasonography might be a reliable indicator of intravascular volume status, since it is almost directly proportional to central venous pressure [11, 15]. A previous study that quantitatively evaluated IVCD using ultrasonography, as in our study, demonstrated that the IVCDmax observed with both LLT and right lateral tilt positioning were significantly larger than that observed in SPN [9]. Conversely, several studies showed that the angle of LLT by 15° and 20° had no effect on IVCD enlargement [16, 17]. Another study also showed that the IVCD measured by magnetic resonance imaging significantly increased with LLT by 30° and 45° compared with that in SPN, although a 15° LLT had no significant effect [18]. Additionally, it has been demonstrated that LLT might lead to narrowing of the IVCD due to compression of the IVC by the liver [19]. However, while there is no report evaluating the effect of LUD on enlargement of the IVCD using quantitative measurement methods, some studies demonstrated that LUD was more effective in maintaining hemodynamics during spinal anesthesia than LLT [10]. Considering these above facts, the present results showing that IVCDmax was significantly larger with LLT and LUD than that in SPN suggested that both LLT and LUD are effective in increasing blood flow volume in the IVC due to enlargement of the IVCD. Also, in the present study, more patients had the largest IVCDmax with LUD as compared to LLT although the mean values of IVCDmax with LLT and LUD were statistically equivalent, suggesting that LUD might be a more reliable method to relieve IVCD compression than LLT.
There were no statistically significant differences in measured IVCD parameters other than IVCDmax in the three postures in this study. In ultrasonography for the IVC, the combined evaluation of IVCDmax and CI value, calculated as the difference between IVCDmax and IVCDmin, which denotes respiratory variation in IVCD, can estimate the status of circulating blood volume and right atrial pressure (RAP), as below [11]. It is estimated that RAP might decrease to < 5 mmHg and hypovolemia may develop when IVCDmax ≤ 21 mm and CI > 50%, and RAP may increase to > 15 mmHg and hypervolemia may develop when IVCDmax > 21 mm and CI < 50%. Therefore, since the IVCDmax and CI in each posture of all patients observed in the present study did not fulfill the above condition, circulating blood volume in all the patients might have been within the normal range and their RAP might have been suitably maintained in each posture. Also considering the fact that there were no significant differences in values of BP and HR among the three postures, the effect of both LLT and LUD on increase in IVC blood flow secondary to enlargement of IVCDmax might be slight, which would explain the lack of significant improvement in the hemodynamic decline seen in the SPN by adoption of LLT or LUD. However, the measured values in this study were not obtained under spinal anesthesia and were evaluated in healthy parturients. After induction of spinal anesthesia, blood pressure often suddenly decreases due to decrease in central blood volume [20, 21], and both LLT and LUD suppress this decrease in blood pressure in clinical settings [5, 6, 10]. Additionally, there exists a phenomenon whereby parturients with pre-existing and pregnancy-induced hypertension have a greater tendency to develop hypovolemia [22]. Considering the above facts, it is easy to assume that the effectiveness of LLT and LUD on enlargement of IVCD observed in this study would be more remarkable in anesthetized and/or parturients with complications. Therefore, further studies are needed in order to define the real effect of LLT and the LUD maneuver on IVCD measured using ultrasonography in actual clinical settings.
The present study also showed that more patients had the largest IVCDmax with LUD as compared to LLT and SPN, as seen in the individual evaluations of patients. This order of incidences of the largest observed values of IVCDmax were not inconsistent with the results of the mean values of IVCDmax in the respective postures. Meanwhile, it should be noted that the effectiveness of the three postures on IVCDmax enlargement was not uniform between patients, as indicated by the fact that there were two patients (10%) in whom the largest IVCDmax was found in SPN. This suggests that both LLT and LUD are not necessarily appropriate treatment to relieve compression of the IVC in some cases. Although the reason for the individual differences in the effects of each posture on IVCD is still unclear, this could be due to variations in certain factors, such as placental site, size of the uterus and uterine adnexa, and collateral circulation via the vertebral venous system [9, 23]. Ultrasonography for evaluation of the IVC can be performed non-invasively and comparatively easily, and economically.12 In fact, we were able to measure IVCDmax and IVCDmin in all postures within a total of about 20 min in almost all our patients, although we were unable to visualize the IVC clearly in two patients. Therefore, ultrasonography for assessment of the IVC before induction of spinal anesthesia can be easily performed not only in the operating room, but also at the patient’s bedside in the ward as a point-of-care method [24, 25]. Preanesthetic ultrasonography assessment of the IVC might help us to preoperatively estimate which is the best posture to maintain IVCD during spinal anesthesia for cesarean section in individual patients.
There are several limitations to this study. First, the present data were obtained from patients who were not under spinal anesthesia. Since it is possible that the effect of each posture on IVCD, as described in the study, might be different in patients under spinal anesthesia, the part of our discussion related to the effects of these postures on IVCD under spinal anesthesia is only speculative. Second, it is desirable that ultrasonography for IVC assessment should be performed under standardized respiratory conditions in order to obtain accurate values of IVCD [26, 27]. The measured values of IVCD parameters in this study might have been affected by variations in respiratory conditions in the three postures in each patient, even though we urged all patients to hold their steady breathing during ultrasonography in all three postures.