At induction, six patients in group O and four in group L required at least one dose of ephedrine. During maintenance, one patient required ephedrine in group L and none required ephedrine in group O. The volume of lactated Ringer's solution administered during surgery, estimated blood loss, and urine output are shown in Table 3. No patient received red blood cells. The rate of lactated Ringer's solution administration during surgery was 5.9 ± 2.0 mL · kg−1 · h−1 in group O and 3.4 ± 0.8 mL · kg−1 · h−1 in group L, a statistically significant difference. By random chance, the difference in average rate of administration was exactly offset by the difference in average duration, so that the average volume of lactated Ringer's solution, 1217 mL, was exactly the same in both groups.
Baseline cardiac index and LVEDVI values were similar between groups and were kept within normal limits during surgery (Figs. 2–4). No differences within and between groups were found when baseline LVEDVI and cardiac index values were compared with the ones obtained at the end of surgery (Figs. 3 and 4, respectively). Postoperative hospital length of stay was 4.5 ± 1.3 days in group L and 10.1 ± 4.5 days in group O (P < 0.05).
The amount of fluid required to maintain LVEDVI and cardiac index was a remarkably modest: 1.2 L in both groups, based on TEE-guided replacement. Although the administration rate during open surgery was nearly twice the rate of laparoscopic surgery, the longer duration of laparoscopic surgery exactly compensated for the difference in rate, resulting in nearly identical requirements for total crystalloid replacement. The decreased rate of crystalloid replacement for laparoscopy has also been demonstrated for laparoscopic cholecystectomy.19 Some studies suggest that laparoscopic surgery may actually increase the neuroendocrine response to surgical trauma when compared to open surgery.20,21
The effects of increased IAP and sometimes extreme Trendelenburg or reverse Trendelenburg positions on cardiac index and LVEDVI are not easy to explain because, except for the one measurement made 5 min after establishing of pneumoperitoneum, measurements were made without considering the patient's position. It has been described that an IAP of 15 mm Hg induces minimal cardiovascular changes.22 The reverse Trendelenburg position causes a reduction of left ventricular end-diastolic area23 and, consequently, of LVEDVI, without reduction of MAP. This reduction of LVEDVI could not have been detected if there was no TEE measurement scheduled at that time or it did not generate a reduction of MAP that was greater than 20%.
Some studies suggest that excess intravascular volume replacement worsens surgical outcomes.3,5,6 However, other studies have shown a better postoperative fitness when large amounts of crystalloids are used.22,24 Our results suggest that volume replacement therapy must be adapted to the specific requirements of each patient. Current guidelines17,18 may recommend excessive replacement.
Our results show that cardiac index can be maintained in the presence of a reduced LVEDVI in patients in good cardiovascular condition. This likely results from compensatory mechanisms or a reduction in afterload from isoflurane anesthesia. This shows one of the pitfalls of using LVEDVI as a physiologic target for intraoperative fluid replacement. Perhaps baseline, rather than normal LVEDVI under a stable level of anesthesia, represents a more suitable objective.
We chose to replace intravascular volume loss with crystalloids, as this reflects our clinical practice. We cannot extrapolate from these results to recommendations for replacing volume loss with colloidal solutions.
In both groups, urine output was marginal and probably inadequate according to the standard recommendation.23 The study design cannot exclude regional blood flow redistribution that could reduce renal blood flow and glomerular filtration rate. However, maintenance of a baseline and normal cardiac index during the study period allow us to think that renal perfusion was normal, and that the urine output reduction observed was a physiological response to surgery.25 Renal function and perfusion under these study conditions must be evaluated to exclude the possibility of renal hypoperfusion and its consequences.
A possible limitation of our study is that patients could not be randomized to open or laparoscopic surgery, which was determined by surgeon based on the underlying pathology. As a result, there were more sigmoidectomies in group L and more hemicolectomies in group O. This unequal distribution of operations may have influenced our findings.
In addition the isoflurane concentrations were between 1 and 1.5 minimum alveolar concentration during maintenance of anesthesia in both groups. Since we did not use intraoperative electroencephalogram monitoring, there may have been differences in anesthetic depth. Since the range of isoflurane concentration was relatively narrow, and patients in both groups had similar demographic characteristics, we think that it was unlikely that differences in isoflurane dose affected our results.
Although there were no formal contraindication, the addition of nitrous oxide for anesthesia with bowel surgery might be controversial due to its accumulation in the gut. This constitutes a normal practice in our department and we have not seen technical difficulties for surgeons due to gut distension.
In summary, the rate of crystalloid administration required to maintain baseline LVEDVI and cardiac index was 5.9 ± 2 mL · kg−1 · h−1 for open surgery and 3.4 ± 0.8 mL · kg−1 · h−1 for laparoscopic surgery (P < 0.01). This slower rate for laparoscopic surgery was offset by the longer surgical duration. The rate of crystalloid solution to maintain baseline LVEDVI and cardiac index is lower than commonly recommended for colorectal surgery.
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