We have read with interest an article by Lee et al. published in the August 2017 issue of Anesthesiology.1  We wish to congratulate the authors for evaluating the effects of supine positioning compared with a 15o left uterine displacement tilt on neonatal acid-base status in healthy, nonlaboring, term women scheduled for elective cesarean delivery under spinal anesthesia when systolic blood pressure was maintained using a crystalloid preemptive bolus and a phenylephrine infusion.

The authors found no effect of maternal positioning on neonatal acid-base status and concluded that the supine position was not inferior to the tilted left uterine displacement position. Because the study was conducted on nonlaboring healthy women, however, as stated in its limitations, we suggest that the tilted left uterine displacement position should not be abandoned despite the findings of this study. Even though there were no changes in neonatal acid-base status, the study’s results actually indicate the superiority of a 15o tilted left uterine displacement position as compared with the supine position. Patients who were in the supine position had statistically significant lower systolic blood pressures and cardiac outputs, and required significantly higher mean doses of phenylephrine during the first 15 min after placement of spinal anesthesia to maintain their blood pressure, as compared with the tilted left uterine displacement group. We believe that based on this study the supine position may serve as a safe alternative to the left uterine displacement position in above-mentioned patients only when 15o tilt is not feasible, which realistically should be extremely rare under elective conditions.

Furthermore, the authors emphasize the “disadvantage” of using base excess values because they are a “calculated value.” A clarification between estimated/approximated values versus calculated values should be addressed. A calculated value is deemed accurate, like any measured value, but estimated/approximated values may not be. The calculated value for bicarbonate/base excess concentration is derived from the Henderson-Hasselbalch equation using measured values for both the carbon dioxide pressure and hydrogen ion concentration. It is not an estimate, which could be inaccurate. Therefore, the calculated value for bicarbonate/base excess would only be inaccurate if the measured value for either the carbon dioxide pressure or the hydrogen ion concentration is incorrect, and thus there is no disadvantage to using a calculated value, despite this being incorrectly asserted by the authors.

Lastly, we wish to address some clerical/typographic errors. Among the study participants, the authors included parturients with American Society of Anesthesiologists Physical Status I and II and excluded patients with autonomic neuropathy (e.g., diabetes mellitus for greater than 10 yr). Generally, a healthy term pregnant patient is classified as no less than American Society of Anesthesiologists Physical Status II. The extent of time needed for a diabetic patient to become neuropathic is unknown and highly variable depending on many factors. We also believe the authors meant “LUD [left uterine displacement] placement is intended to reduce/prevent supine hypotensive syndrome in the pregnant patient”2  and not to prevent spinal-induced maternal hypotension, as the authors stated in their discussion. These two physiologic factors (sympathetic block induced by neuraxial anesthesia and aortocaval compression by the gravid uterus) are not synonymous, one with the other, and should not be confused as being related to each other. Neuraxial block causes hypotension by blocking preganglionic sympathetic fibers of which there are 14 pairs (T1–L2); the degree of hypotension is directly related to the number of segments blocked. In basic physiology, it has been known equally as long that reducing preload (by compressing the inferior vena cava) is a definite risk factor for developing hypotension even absent a pharmacologic blockade of preganglionic sympathetic fibers. Therefore, supine hypotension during pregnancy has no relationship to that induced by neuraxial anesthesia, albeit neuraxial anesthesia may worsen the consequences of reduced preload occurring by not adhering to the principles of the left uterine displacement position. Given that cardiac output is related to preload, afterload, contractility, and heart rate, any factor may independently compromise it, and combinations of factors are, of course, more likely to affect this response. Neuraxial blocks with local anesthetics can reduce cardiac output by reducing all four factors; aortocaval compression only affects preload. This is a critical difference that cannot, and must not, be confused.

The authors declare no competing interests.

Left lateral table tilt for elective cesarean delivery under spinal anesthesia has no effect on neonatal acid-base status: A randomized controlled trial.
Supine hypotensive syndrome.
Obstet Gynecol
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