Academic Editor: Stefano Manodoro
Objective: The injection rate of the local anesthetic may affect the level and the time of onset of sensory block. The aim of this prospective study was to investigate the effects of two different injection rates of local anesthetic solution (0.5% heavy bupivacaine) on the onset of sensory block, and the incidence and the onset of hypotension in pregnant women undergoing spinal anesthesia for elective Cesarean delivery. Methods: A total of 67 patients were randomized into two groups: 120-second injection time (Group Slow; n = 33) and 15-second injection time (Group Fast; n = 34). Maximum level of sensory and motor block, time to sensory block at the level of T6, hemodynamic parameters, use of ephedrine and incidence of side effects were recorded at measurement time points. Results: Maximum level of the sensory block was similar in both groups. The time to achieve adequate and maximum sensory block level was shorter in Group Slow(S) (p = 0.004 and 0.037, respectively). Incidence of hypotension was similar, but hypotension occurred earlier in Group Fast(F) (p = 0.011). Requirement for ephedrine and incidence of nausea and vomiting was similar. Conclusions: This study reveals that 120-second injection duration during spinal anesthesia is associated with shorter time to achieve the maximum sensory block level and slower onset of hypotension. It means that prolonging the duration of local anesthetic injection to 120-seconds is advantageous compared with 15-seconds in caesarean section.
Today, spinal anesthesia is the most common anesthesia for caesarean section due to the lower exposure of the fetus to drugs, creating fast, profound sensory and motor block as well as low risk of pulmonary maternal aspiration [1, 2]. It is quite difficult to predict the level of the sensory block after spinal anesthesia during a caesarean section. The level of sensory block is important for preventing the development of complications from high-level spinal anesthesia and for a painless, comfortable intraoperative period. Many factors (such as the type of local anesthetic, dose, injection site, volume of the subarachonoid space, patient’s position and demographic characteristics) play a role for adequate sensory block level in cesarean section with spinal block [3, 4, 5, 6]. In addition, the injection rate of the local anesthetic affects the level and the time of onset of sensory block [7, 8].
Hypotension is the most common complication during spinal anesthesia. The most feared effect of hypotension in obstetric anesthesia is that it may lead to fetal hypoxia and acidosis by reducing uteroplacental perfusion if not treated in a timely or well [9, 10]. These effects are associated with the depth and duration of hypotension [11]. It is also important to prevent as well as to treat hypotension in pregnant women because of its negative effects on both mother and fetus. In the literature, techniques such as preoperative fluid replacement, different positioning techniques, leg wrapping with elastic bandages, and administration of prophylactic parenteral vasopressors have been used to prevent the development of hypotension during spinal anesthesia [12, 13, 14, 15]. Adjusting the rate and dose of intrathecal local anesthetic injection is may be a practical way to prevent maternal hypotension due to spinal anesthesia [4, 16, 17]. However, there are very few studies in the literature on what the injection speed of local anesthetic should be.
In the present study, we hypothesized that there may be a delay in reaching the level of sensory block with the turbulent flow caused by rapid local anesthetic injection. We thought that this delay in sensory block might have an effect on the development of maternal hypotension. For this reason, we aimed to investigate the effects of 2 injection times that not close the each other on the time to achieve a sensory block level and intraoperative maternal hypotension using height and weight-adjusted doses of local anesthetic in elective caesarean section operations.
This study is a prospective, randomized (computer-aided), double-blind study
conducted after obtaining the approval of the institutional ethics committee at a
tertiary health center. The trial has been registered at clinicaltrials.gov on
10/24/2021. Written informed consent was obtained from all participants. The
first participant was recruited on 16 July 2020, and the anticipated completion
date was January 2021. The study included a total of 76 female patients with
American Society of Anesthesiologists risk score (ASA) II risk and singleton
pregnancy aged between 18–40 years status undergoing elective Cesarean delivery.
Pregnant women with contraindications for spinal anesthesia; placental anomaly;
hypertensive, cardiac, metabolic, vascular, hepatic, renal disease; hemodynamic
instability and spinal deformity; severe mental retardation; weight
Before spinal anesthesia, ECG, peripheral oxygen saturation, non-invasive blood
pressure monitoring were performed as routine monitoring in all patients, and
baseline values were recorded. Basal blood pressure measured automatıcally and
non-invasively from upper right arm with air filled occluding cuff. All of the
patients were given a mixture of intravenous Ringer’s lactate and hydroxyethyl
starch (HES) solution at a dose of 10 mL/kg with the aid of a 22 G intravenous
cannula and were premedicated with 1 mg/kg ranitidine and 0.1 mg/kg ondansentron
IV before the induction of spinal anesthesia. The injection site of all of the
patients was disinfected using povidone iodine in the sitting position. The
patients’ L
Consort flow diagram.
The anesthetist was blinded to the groups after the intrathecal administration
of anesthesia and followed up with the patients and collected the data. All
spinal blocks were performed by one anesthesiologist. The other anesthesiologist
responsible for data collection after local anesthesic injection was unaware of
patient group allocation. After blocking was achieved, the patients were placed
in the supine position, and the uterus was transported to the left by tilting the
table 15
Considering the moment when the subarachnoid injection began as the onset time,
the time to reach the T
Modified Bromage Scale (0 = ability to raise and extended the leg; 1 = inability to raise and extended the leg, ability to flex the knee; 2 = inability to flex the knee, ability to flex the ankle; 3 = inability to flex the knee and ankle).
The peak heart rate (HR) and systolic blood pressure (SBP) values of all
patients were recorded before the procedure; immediately after spinal anesthesia;
and at 1, 3, 5, 7, 9, 11, 13, 15, 20, 25, 30, 35, 40, and 45 minutes after the
procedure. A decrease of systolic blood pressure (SBP) below 90 mmHg and more
than a 20% decline in baseline blood pressure were considered as hypotension
[11]. Any hypotension was treated with boluses of 5 mg ephedrine each time until
the systolic arterial pressure returned to normal ranges (
The sample size was calculated based on a pilot study (n = 12) which
demonstrated that with a standard deviation of 1.7 min the time to onset of
sensory block T
Statistical analyses were performed with SPSS version 17.0 (SPSS Inc., Chicago,
Illinois, USA). Descriptive statistics were given as median frequencies (ASA,
nausea and vomiting, and presence of maternal hypotension) and mean
The study included a total of 67 patients in Group F (n = 34) and Group
S (n = 33). The level of sensory block was below T
The demographic data of all patients included in the final analysis are summarized in Table 1. Briefly, patient characteristics were similar (Table 1).
Group | Slow (n = 30) | Fast (n = 30) | p value |
Age (year) | 31.1 |
31.1 |
0.981 |
Weight (kg) | 78.3 |
79.2 |
0.782 |
Height (cm) | 161.4 |
162.4 |
0.572 |
BMI (kg/m |
30 |
29.1 |
0.535 |
Gestation week | 38.1 |
38.4 |
0.726 |
Bupivacaine dose (mg) | 1.8 |
1.8 |
0.988 |
Injection rate (mL/seconds) | 0.015 |
0.131 |
|
Baseline sBP (mmHg) | 124.4 |
125.3 |
0.745 |
Baseline HR (batt/min) | 93.8 |
93.2 |
0.840 |
BMI, Body Mass Index; ASA, American Society of Anesthesiologist.
All values are presented as mean and standard deviation, number of cases. |
The time to reach the T
Group | Slow (n = 30) | Fast (n = 30) | p value |
Time to onset of sensory block T |
6.17 |
7.7 |
0.004 |
Max thoracal sensory block level |
2 | 2.25 | 0.064 |
Time to onset of max. sensory block (min) |
9 |
11 |
0.037 |
Max .motor block starting time (min) |
4.3 |
4.8 |
0.087 |
Maternal hypotension (number) |
11 (36.7%) | 15 (50%) | 0.297 |
Time to onset of hypotension (min) |
6.7 |
4.7 |
0.011 |
Ephedrine dose requirements (mg) |
4.3 |
6 |
0.34 |
Nausea and vomiting |
9 (30%) | 11 (36.7%) | 0.584 |
Values are presented; |
Incidence of hypotension was similar (p
In this prospective, double-blind, randomized study, we found that pregnant women undergoing spinal anesthesia with slow injection rate had an adequate level of sensory block, and the maximum level of sensory block occurred earlier. Although the incidence of hypotension was similar, hypotension occurred later with the slow injection rate.
In-vitro studies investigating the effect of local anesthetic injection
rate on the spread of local anesthetic solution proposed various mechanisms.
Bourke showed that a local anesthetic injection rate above 0.017 mL/sec may cause
a turbulent flow [19]. Serpell and Holman showed that in case of slower injection
rates, the local anesthetic solution tends to move more cephalad or in the
direction of the injection, in contrast to fast injection rates, which tend to
cause a disturbed and turbulent flow [20, 21]. Similarly, we observed that
the time to the level of sensory block at the level of T
Studies to date comparing the injection rates of the local anesthetic solution
for spinal anesthesia during caesarean sections, determined the appropriate
intervertebral space for injection via palpation. The correct identification rate
with this method varies between 29–41% [22]. It was also shown that the
intervertebral space determined by palpation is 1–2 spaces higher than the level
desired to be injected [23]. This may affect the distribution of local anesthetic
and the time to onset of the sensory block. Therefore, in our study, we performed
all injections at the same (L
There are controversial results in studies on the effects of local anesthetic injection times on maternal hypotension in pregnant women undergoing spinal anesthesia. While some authors have shown that maternal hypotension develops more frequently in patients receiving fast injections [16, 4, 24], some authors have shown that the injection times have no effect on maternal hypotension [8, 25, 26]. However, unlike our study, injection rates were not considered in these studies (Table 3, Ref. [4, 8, 16, 24, 25, 26]). We attributed the reason for the different results in these studies comparing slow and fast injection rates to the fact that the injection rates they used were above the rate (0.017 mL/s) that causes turbulent flow in in-vitro studies. In our study, we have shown that using an injection rate that does not cause turbulent flow in the slow injection group resulted in the delay of maternal hypotension. Although the exact reason for this is unknown, we conclude that the fast, turbulent flow reached the sympathetic efferent fibers originating from the anterior motor neurons that provide vasoconstriction, thus preventing the local anesthetic from spreading cephalad. We also thought that the patients in the fast injection group did not have time to compensate for the hemodynamic change that occurred after the sudden-onset sympathetic block. Contrary to the theory [27], which states that high block level is associated with high incidence of hypotension, hypotension has occurred later in the slow injection period, which reached the level of early sensory block in our study. This situation is more in line with the theory that hypotension after spinal block in pregnant women related to decrease in systhemic vascular rsistance secondary to artery vasodilation [28, 29] with a modest degree of venodilation [30].
Injection rate (mL/s) | Results | ||||
Author | Slow | Fast | Volume of local anesthetic (mL) | Incidence of hypotension | Onset of adequate level of sensory block |
Tugcugil | 0.015 |
0.13 |
1.8 |
No difference | Slow is better |
Chiang [26] | 0.04 | 0.15 | 2.3 | No difference | - |
Bouchna [4] | 0.06 | 0.18 | 3.5 | Slow is better | - |
Badheka [25] | - | - | 2.2 (2–2.6) | Slow is better | No difference |
Singh [8] | 0.06 | 0.55 | 2.2 | No difference | No difference |
Simon [16] | 0.03 | 0.27 | 4 | Slow is better | No difference |
Nugroho [24] | 0.027 | 0.2 | 2.5 | No difference | - |
All values are presented as mean and Standard deviation, median value (minimum–maximum). |
Our study has some limitations. First, the injection rate may not have been uniform because we performed the injection manually, rather than with an electronic pump. To facilitate slow and fast injection we used a 2.5 mL syringe (divided into 0.1 mL section). Secondly, we could not identify or change some factors (such as volume of subarachnoid space and intraabdominal pressure) in the development of sensory block level and maternal hypotension, which made it difficult for us to standardize these variables. Finally, only elective cesarean sections were included in our study. Because pregnant women in emergency cesarean section may not have taken enough fluid and adequate preoperative optimization may not have been provided, which may affect the hypotension and sensory block occurrence due to spinal block. We must also indicate that our study was registered in clinicaltrials.gov retrospectively. This was due to the limitation of forgetting to get registiration our study.
Slow injection rate of local anesthetic may allow the surgery to start earlier with the earlier onset of adequate level of sensory block. In addition, it may help to prevent potential complications by delaying the onset of hypotension.
ET—data collection, data analysis, manuscript writing, consultation, protocol development AB—data collection, data analysis, manuscript writing, consultation. All authors have been personally and actively involved in substantive work leading to the manuscript, and will hold themselves jointly and individually responsible for its content.
All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Institutional Review Board of Karadeniz Technical University Medicine Faculty (Trabzon, Turkey; approval on January 2020, 2019/357) and the clinical registration number of the trial is NCT05091294.
Thanks to all the peer reviewers for their opinions and suggestions.
This research received no external funding.
The authors declare no conflict of interest.
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