Artículos de Reflexión - Comentarios

Jorge Andrés Rubio Romero*

* Profesor Asociado, Departamento de Obstetricia y Ginecología, Instituto de Investigaciones Clínicas, Facultad de Medicina, Universidad Nacional de Colombia. Correspondencia: Carrera 30 No. 45-03, Facultad de Medicina. Of. 205. Bogotá, Colombia. Correo electrónico: jarubior@unal.edu.co

Recibido: agosto 31 de 2011. Enviado para modificaciones: septiembre 30 de 2011. Aceptado: octubre 10 de 2011.

Introduction. To present some thoughts on the article “Regional subarachnoid anesthesia for C-section and post-partum Pomeroy procedure. Application of a local anesthetic at a rate lower or greater than 60 seconds” in order to provide readers with a different perspective regarding the results reported.

Methods. The paper was read critically in order to assess the quality of the methodology, the potential sources of bias and error, the confounding variables, and also to review the results from a fresh perspective.

Results. Hypotension occurs frequently when subarachnoid anesthesia is applied, and the rate of injection may explain it in part. The case series analyzed has a limited sample size, creating the possibility of information bias and confounding factors, not to mention that the potential interaction between the rate of infusion and the mix of anesthetics was not considered.

Conclusion. A cohort study with a larger sample size must be undertaken in order to control confusion by means of standardized procedures, an analysis stratified by subgroups, and mathematical modeling designed to identify the strength of association between the injection rate and the onset of hypotension, as potential sources of confusion and interaction.

Based the Departamento Administrativo Nacional de Estadística - DANE (Colombian National Statistics Department) unconsolidated data, there were 541,292 births in Colombia in 2010. Of these, 60 % were spontaneous deliveries and the other 40 % (215,321) were born by C-section.

C-section rates are variable throughout the territory of Colombia, with a high of approximately 70 % in the Caribbean region (1).

However, despite the fact that it is performed with a very high frequency, C-section is not a harmless procedure. It is estimated that the morbidity associated with it is twenty times higher than that associated with vaginal delivery.

The anesthetic and surgical techniques involved in a C-section have implications both for the mother as well as for the neonate, due to the physiological changes associated with the pregnancy, including increased cardiac output, aorto-caval compression by the pregnant uterus, and a placental blood flow that is dependent on the maternal arterial pressure.

Hypotension, regardless of its cause, exposes the fetus to impaired perfusion and secondary acidemia (2).

For the mother, C-section increases the risk of post-partum bleeding and infection, and of mortality associated with those events. It also increases the risk of thromboembolic disease, and of anesthesia-derived complications (3,4).

Aside from the hemodynamic effects produced by subarachnoid anesthesia, the delivery of the fetus, the exit of amniotic fluid, the reduced aorto-caval compression and the subsequent bleeding, ranging between 500 and 1000 cc, there are also dramatic hemodynamic changes in the mother that increase cardiac output and may last at least 2 hours after the delivery. These changes are more significant during the first 10 to 15 minutes post-partum, when cardiac output, heart frequency and systolic volume may rise 60-80 %. These changes return to ante-partum values within the first hour after delivery (5).

The presence of pregnancy-related diseases or comorbidities render these changes still more acute and dramatic.

Given this epidemic of obstetrical procedures, which is much greater than the 15 % proposed by the WHO as the optimal rate for C-section (6), the observational study undertaken by doctors Hoyos, Navarro and Eslava (7), published in this issue, describes de incidence of hypotension and other perioperative adverse effects associated with the rate of injection of the anesthetic agent in a group of women undergoing C-section or post-partum tubal ligation (Pomeroy).

The paper refers to differences between the anesthetic mixes and the injection rates, as well as to the preferred infusion rates by individual anesthesiologists. It also points out the various factors that influence injection rate depending on the anesthetic dose, the urgency for performing the procedure, the medical complications associated with gestation, and whether the woman is in gestation or in the post-partum period. Likewise, the paper presents a regression analysis designed to identify the confounding factor, between the anesthetic technique and the mix used, as the cause of hypotension during surgery.

The study assesses the presence of hypotension at 5, 10 and 15 minutes after anesthesia induction, but does not provide information regarding the level of motor and sensory block achieved at the time of fetal delivery. Moreover, it does not clarify whether that factor may have modified the hemodynamic response after the anesthetic injection or throughout the procedure.

Likewise, considering that these may be elective or emergency procedures, although the study mentions a standard loading dose for the patients, no reference is made to the length of labor or to any other circumstances that may affect the hydration status of the patients.

On the other hand, patients undergoing C-section and post-partum tubal ligation were analyzed under the same group without making a distinction based on the differences resulting from changing hemodynamic conditions during gestation, delivery and the immediate post-partum period that may have affected the observed outcome observed of hypotension.

Considering that there is no independence between the infusion rate and the anesthetic mix, that no preference for either variable is established, and that there are physiological differences among the women that underwent the procedures, the logistic regression analysis is inadequate for the purpose of determining the weight of each of the factors.

Moreover, the limited sample size does not allow a more in-depth analysis of the different subgroups and hinders the identification, not only of the confounding factors but also of the interaction, if any, among the different variables, which might explain the association found by the regression model of the study in question.

Finally, just like the authors state, this exploratory analysis seeks to identify the factors associated with the presence of hypotension in patients receiving subarachnoid anesthesia. In order to obtain more accurate results, there is a need to undertake a different study based on standardized preparations, anesthetic mixes, patient types and rates of infusion (like the one undertaken by Singh, et al. (8), which did not find differences in the incidence of hypotension and nausea on the basis of the rate of infusion of hyperbaric bupivacaine in women in labor).

Such a study should also indicate the length of the labor period as well as the timing of the delivery. It should also recruit a sufficient number of patients in order to conduct an analy sis by subgroups and allow modeling of the factors associated with hypotension and with other adverse effects related to the use of regional subarachnoid anesthesia. The sample size should be estimated in such a way as to include at least ten patients with hypotension, or with the specific outcome assessed, for every variable included in the logistic regression model.

1. DANE Colombia. Nacimientos por tipo de parto. Información estadística; 2010. Disponible en: http://www.dane.gov.co/daneweb_V09/index.php?. Consultado el 29 de agosto de 2011.

2. Reynolds F, Seed PT. Anaesthesia for caesarean section and neonatal acid-base status: a meta-analysis. Anaesthesia. 2005;60:636-53.

3. Rubio-Romero JA, Ángel-Müller E. Operación cesárea. En: Obstetricia integral siglo XXI. 1 ed, tomo II. Disponible en http://www.medicina.unal.edu.co/obstetricia_integral.

4. National Collaborating Centre for Women’s and Children’s Health Commissioned by the National Institute for Clinical Excellence. Caesarean section Clinical Guideline; 2004. Disponible en: http://guidance.nice.org.uk/CG13/Guidance/pdf/English. Consultado el 29 de agosto de 2011.

5. Monga, M. Maternal cardiovascular and renal adaptation to pregnancy. En: Creasy RK, Resnik R, Iams JD, editores. Maternal-fetal medicine: principles and practice. 5 ed, Philadelphia: Sanders; 2004.

6. Althabe F, Belizan JF. Caesarean section: The paradox. The Lancet 2006;368:1472-3.

7. Hoyos-Cerón DC, Navarro-Vargas JR, Eslava-Schmalbach J. Anestesia regional subaracnoidea para cesárea y Pomeroy postparto. Aplicación de anestésico local a una velocidad de inyección menor o mayor a 60 segundos. Rev. Colomb. Anestesiol. 2011 Aug-Oct;39(3):341-50.

8. Singh SI, Morley-Forster PK, Shamsah M, Butler R. Influence of injection rate of hyperbaric bupivacaine on spinal block in parturients: a randomized trial. Can J Anaesth. 2007 Apr;54(4):290-5.