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On-line version ISSN 1806-907X
Rev. Bras. Anestesiol. vol.54 no.1 Campinas Jan./Feb. 2004
Fluid preload in obstetric patients. How to do it?*
Expansión volémica en raquianestesia para cesárea. ¿Como realizarla?
Mônica Maria Siaulys Capel Cardoso, TSA, M.D.I; Márcio Martines Santos, M.D.II; Eduardo Tsuyoshi Yamaguchi, TSA, M.D.III; Jacqueline Toshiko Hirahara, M.D.IV; Antonio Rahme Amaro, M.D.V
IDoutora em Anestesia pela FMUSP;
Médica Supervisora da Anestesia Obstétrica do HC-FMUSP; Anestesiologista
do Hospital e Maternidade Santa Joana
IIME2 do HC-FMUSP
IIIMédico Colaborador da Disciplina de Anestesiologia do HC-FMUSP; Anestesiologista do Hospital e Maternidade Santa Joana
IVME3 do HC-FMUSP; Anestesiologista do Hospital e Maternidade Santa Joana
VAnestesiologista do Hospital e Maternidade Santa Joana; Diretor Clínico do Hospital e Maternidade Santa Joana
BACKGROUND AND OBJECTIVES: In has been
shown in non-obstetric patients, that a fast acute fluid preload immediately
after spinal anesthesia was more effective than a slow preload before regional
block to decrease the incidence and severity of arterial hypotension after spinal
anesthesia. This study aimed at comparing the incidence of arterial hypotension
and vasopressants consumption in parturients submitted to C-section under spinal
anesthesia with different fluid preload regimens.
METHODS: Sixty term pregnant women submitted to C-section under spinal anesthesia. Patients were randomly distributed in three groups receiving acute preload (10 ml.kg-1 lactated Ringer's) as follows: Group 1 (n = 20), before spinal anesthesia through 18G catheter; Group 2 (n = 20), after spinal anesthesia through 18G catheter and Group 3 (n = 20), after spinal anesthesia through 16G catheter with pressurizer. Blood pressure (BP) was monitored at 1-minute intervals until delivery and 0.2 mg bolus metaraminol was administered for any BP decrease from baseline values; 0.4 mg was administered for BP decrease > 20%. Control blood pressure was defined as the mean of three successive SBP values obtained before acute preload and spinal block. Variables studied were: preload rate, incidence of maternal arterial hypotension, nausea, vomiting, vasopressants consumption, Apgar scores and umbilical artery pH.
RESULTS: Acute preload was faster in Group 3 as compared to Groups 1 and 2 (201 ± 61 vs 56 ± 13 and 59 ± 21 ml.min-1, p < 0.05). Groups were similar regarding the incidence of arterial hypotension, nausea and vomiting, metaraminol consumption, Apgar scores and umbilical artery pH.
CONCLUSIONS: Acute preload before or after spinal anesthesia performance, in a slow or fast rate, does not change vasopressants consumption, the incidence of maternal arterial hypotension, nausea and vomiting, as well as fetal wellbeing.
Key Words: ANESTHETIC TECHNIQUES, Regional: spinal block; SURGERY, Obstetric: Cesarean section; VOLEMY: volemic expansion
JUSTIFICATIVA Y OBJETIVOS: En pacientes
no obstétricas, se demostró que la expansión volémica
rápida realizada inmediatamente después de la inyección
subaracnoidea del anestésico local era más efectiva que la expansión
volémica lenta, realizada previamente a la instalación del bloqueo
para reducir la incidencia y la gravedad de la hipotensión arterial después
de raquianestesia. El objetivo del estudio fue comparar la incidencia de hipotensión
arterial y el consumo de vasopresores en embarazadas sometidas a cesárea
bajo raquianestesia con diferentes regímenes de expansión volémica.
MÉTODO: Fueron evaluadas prospectivamente 60 embarazadas de término sometidas a cesárea bajo raquianestesia. Las pacientes recibieron expansión volémica con 10 ml.kg-1 de solución de Ringer con lactato como sigue: Grupo 1 (n = 20), antes de la raquianestesia a través de catéter 18G; Grupo 2 (n = 20), después de raquianestesia y con catéter 18G y Grupo 3 (n = 20) después de raquianestesia con catéter 16G bajo acción de un presurizador. La presión arterial (PA) fue monitorizada a cada minuto y se administró 0,2 mg de metaraminol para cualquier diminución de PA, a partir del valor de control y 0,4 mg para diminución > 20%. La PA control fue definida como la media de tres valores sucesivos de PAS obtenidos antes de la expansión volémica y del bloqueo. Se estudió la velocidad de infusión de fluidos, incidencia de hipotensión arterial materna, náusea y vómito, consumo de metaraminol, índice de Apgar y pH de la arteria umbilical.
RESULTADOS: La velocidad de administración de fluidos fue mayor en el Grupo 3 de que en los Grupos 1 y 2 (201 ± 61 vs 56 ± 13 y 59 ± 21 ml.min-1, p < 0,05). Los grupos fueron semejantes en relación a la incidencia de hipotensión arterial, náusea, vómito, consumo de metaraminol, índice de Apgar y pH de la arteria umbilical.
CONCLUSIONES: La expansión volémica, realizada antes o después de la instalación de la raquianestesia, de manera rápida o lenta, no modifica el consumo de vasopresor, la ocurrencia de hipotensión arterial materna, náusea o vómitos, y tampoco el bien estar fetal.
Maternal arterial hypotension is the most common C-section spinal anesthesia complication. Most popular prophylactic and therapeutic measures to decrease arterial hypotension incidence and severity include uterine displacement, preload and vasopressants. Preload before spinal anesthesia has been questioned since some studies have shown minor or no decrease in maternal arterial hypotension or vasopressants consumption 1,2. Factors interfering with preload efficacy include: the type of administered solution, the moment of preload and fluid administration rate.
In non-obstetric patients, Ewaldsson et al. 2 have shown that a fast preload with crystalloids performed immediately after spinal anesthesia was more effective in decreasing arterial hypotension incidence and severity as compared to a slow preload immediately before spinal anesthesia.
This study aimed at comparing the incidence of arterial hypotension and vasopressants consumption in patients submitted to C-section under spinal anesthesia, in whom preload was performed in different infusion rates immediately before or after spinal anesthesia.
After the Hospital das Clínicas, FMUSP and Hospital Maternidade Santa Joana Ethics Committee for Research Projects Analysis approval, 80 term pregnant women, physical status ASA I or II with single fetus undergoing C-section under spinal anesthesia were prospectively evaluated.
Patients were monitored with ECG, noninvasive blood pressure and pulse oximetry, to be then randomly distributed in three groups of 20 patients receiving preload with 10 ml.kg-1 lactated Ringer's, as follows: Group 1 immediately before spinal puncture at the maximum rate allowed by cephalic vein catheterization with 18G catheter and using as standard serum height the maximum extension of the line; Group 2 immediately after spinal anesthesia at the maximum rate allowed by cephalic vein catheterization with 18G catheter and using as standard serum height the maximum extension of the line; and Group 3 immediately after spinal anesthesia at the maximum rate allowed by cephalic vein catheterization with 16G catheter and with the aid of a pressurizer. Time for preload was recorded at its completion. This was a double-blind study, being the anesthesiologist in charge of collecting data and administering vasopressants unaware of the group patients belonged to.
Spinal anesthesia was performed at L2-L3 or L3-L4 spaces with the patient in the sitting position with 15 mg of 0.5% hyperbaric bupivacaine associated to 28 µg morphine injected in approximately 60 seconds. Control blood pressure (BP) was defined as the mean of three consecutive systolic BP obtained immediately before preload and regional block BP was controlled at 1-minute intervals and any decrease would be corrected with 0.2 mg bolus metaraminol; decreases beyond 20% were corrected with 0.4 mg bolus metaraminol.
Uterus was displaced with modified Crawford's wedge and so maintained until fetal extraction. After delivery, ocytocin was administered (10 Ul in 500 ml lactated Ringer's at a rate of 30 drops per minute).
Immediately after fetal extraction, umbilical cord blood was collected for further blood gases analysis. Apgar scores at 1st and 5th minutes, new-born, weight, induction-delivery time and uterine incision-delivery time were recorded for further comparison between groups.
Maternal variables evaluated were: mean preload rate, incidence of arterial hypotension (decrease > 20% of control SBP), incidence of nausea and vomiting and metaraminol consumption before and after delivery. Fetal variables included umbilical artery pH and pCO2 analysis, as well as Apgar scores at 1st and 5th minutes.
BP means, heart rate and metaraminol consumption along time were compared between groups through analysis of variance for repeated measures. Remaining variables were compared by Student's t test and Fisher's Exact test, as applicable. Significance level was considered p < 0.05.
Groups were homogeneous in age, weight and height (Table I). There were no statistically significant differences in the incidence of nausea, vomiting, arterial hypotension and vasopressants consumption. Maternal variables before delivery are summarized in table II. Mean fluid administration rate was higher in Group 3 as compared to Groups 1 and 2. Fetal variables were similar between groups and are shown in table III.
Our data have shown that fast or slow preload performed before or after spinal anesthesia has not changed vasopressants consumption, maternal arterial hypotension, nausea and vomiting incidence or fetal wellbeing.
These data are in disagreement with Edwaldsson et al. 3 who have shown that fast preload, when performed simultaneously to regional anesthetic induction, was more effective as compared to slow preload before spinal anesthesia in decreasing arterial hypotension incidence and severity after regional block installation. However, it is worth reminding that the fluid administration scheme proposed by the authors as effective involves the infusion at an extremely high rate (approximately 800 ml.min 1). In addition to the potential risk of unacceptable central venous pressure increase (the administration of 20 ml.kg-1 in 10 minutes has led to major central venous pressure increases) 4, this practice requires very large catheters. In our study, cephalic vein catheterization with thick 16G catheter using the maximum extension of the line and with the aid of a pressurizer has allowed the administration of fluids at the maximum rate of 336 ml.min-1.
So, in the infusion rate range of our study (56.6 to 210 ml.min-1), we could not show any benefit of fast crystalloids administration. The administration of crystalloids through 18G catheter without pressurizer has determined arterial hypotension and vasopressants consumption similar to those found with fast preload through 16G catheter with the aid of a pressurizer. These data suggest that fluid administration rate is not significant in decreasing arterial hypotension incidence and severity. Another possibility would be that maternal arterial hypotension in obstetric patients is a phenomenon more complex than simply preload decrease, thus making it more difficult to control with a single prophylactic or therapeutic measure 1.
The idea of comparing preload efficacy in different moments related to spinal anesthesia would be based on the fact that the rat of fluid excretion decreases after regional anesthesia installation. This fact is translated into a higher capacity of the body to preserve fluids administered in the central compartment, making preload more effective 5. However, it has to be highlighted that, especially in obstetric patients, what is questioned here is just the moment in which preload should be performed (before or after spinal anesthesia) and not the practice of inducing anesthesia with or without fluid administration. Spinal anesthesia without previous fluid administration leads to fetal suffering. Mean umbilical artery pH was significantly lower in groups not receiving crystalloids before delivery 6.
We decided for limiting preload to 10 ml.kg-1 since the practice of administering large amounts of fluids before regional anesthesia is no longer accepted.
Carvalho et al. 7 have shown that a high number of patients have become anemic with crystalloids infusion above 10 ml.kg-1.
It is also questioned that the administration of large amount of fluids before spinal anesthesia for C-section for being useful only during a limited period of time (arterial hypotension is especially frequent before delivery) for not being effective for all cases, that is, although decreasing the incidence and severity of such complication, it does nor eliminate the problem. Moreover the effect on oxygen transportation to fetal tissues may be erratic: althoug increasing systolic volume and cardiac output there is a parallel hemodilution decreasing hemoglobin level and arterial O2 content. In addition, pregnant women have overloaded cardiovascular system and, due to lower oncotic pressure, have lower intravascular fluid retention ability.
Although preload with colloids is better than with crystalloids to prevent arterial hypotension or to decreasing its severity after extensive sympathetic block, we decided to use crystalloids alone for being a lower cost solution 8.
Uterine displacement to the left was achieved with a modified Crawford's wedge, which is smaller than traditional wedges (24.5 cm length, 16 cm width, 5 cm internal thickness, determining and angle of 15º with horizontal plane). In a previous study, it has been shown to be as effective as the manual uterine displacement to the left. Manual displacement sometimes creates a practical problem for the anesthesiologist, requiring concentration in this activity and impairing other cares to patients 9.
Traditionally, arterial hypotension during regional anesthesia for C-section is defined as SBP decrease equal to or above 20% of baseline values, or also, SBP below 100 mmHg, with SBP measured at 3-minute intervals. In our study, BP was monitored at shorter intervals (every minute until birth) and vasopressants therapy was started earlier, after any BP decrease from baseline values. Control BP was defined as the mean of the three last SBP values obtained immediately before spinal anesthesia. It is important to highlight that, even with the aggressive arterial hypotension treatment, approximately 15% of patients still develop blood pressure decrease above 20% of baseline value. This shows the need for accurate and almost continuous monitoring of BP evolution.
We decided to correct any BP decrease with bolus metaraminol, since the use of ephedrine as vasopressants of choice to prevent and treat arterial hypotension during spinal anesthesia for C-section has been recently questioned. Alpha-agonists, such as phenylephrine, have been shown to be superior to ephedrine in terms of maternal and fetal wellbeing 10. Metaraminol, another a-adrenergic agonist, may be also an advantageous alternative to ephedrine since, in addition to increasing afterload and having a positive inotropic effect, it also increases preload, especially during pregnancy 11. Our routine has been to administer 0.05 to 0.2 mg.ml-1 bolus metaraminol to treat any BP decrease as compared to baseline values, with BP measured at 1-minute intervals until delivery.
Data collection was ended with delivery, since the incidence of maternal arterial hypotension after birth is a much less frequent phenomenon. Contributing factors to this are vena cava decompression allowing better venous return, uterine auto-transfusion determining the acute infusion of approximately 500 ml of blood in maternal circulation and also the removal from circulation of a low resistance bed (uterus-placenta bed).
In conclusion, slow or fast preload before or after spinal anesthesia for C-section does not change vasopressants consumption, the incidence of maternal arterial hypotension, nausea, vomiting or fetal wellbeing.
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Submitted for publication February 11, 2003
Accepted for publication May 28, 2003
* Received from Hospital e Maternidade Santa Joana, São Paulo, SP