JOURNAL OF EVIDENCE BASED MEDICINE AND HEALTHCARE

Table of Contents

2019 Month : September Volume : 6 Issue : 38 Page : 2598-2602

A PROSPECTIVE STUDY TO IDENTIFY THE RISK FACTORS FOR HYPOTENSION AFTER SPINAL ANAESTHESIA IN PATIENTS UNDERGOING CAESAREAN SECTION

Sourav Mahajan1, Hemangi Karnik2, Yash Bajpai3

1. International Training Fellow, Department of Anaesthesia, Pinderfields General Hospitals, Wakefield, United Kingdom.
2. Professor, Department of Anaesthesia, Lokmanya Tilak Municipal Medical College and General Hospital, Sion, Mumbai, Maharashtra.
3. Senior Resident, Department of Anaesthesiology, Dr. Baba Saheb Ambedkar Hospital, Kandilvali, Mumbai, Maharashtra.

Corresponding Author:
Dr. Sourav Mahajan,
#60, Aberford Road, Wakefield,
WF1, 4AL, United Kingdom.
E-mail: mahajan.sourav@gmail.com
DOI: 10.18410/jebmh/2019/535

ABSTRACT
BACKGROUND
Most commonly used technique of anesthesia for caesarean section is spinal anesthesia and its most common side effect is hypotension. The aim of the study was to identify risk factors for developing hypotension after spinal anesthesia in patients undergoing caesarean section.

METHODS
A total of 12 variables were studied for correlation with hypotension following spinal anaesthesia. Each of the variables was divided into categories and Chi-square test was used to assess this association in a univariable analysis. p-Value of <0.05 was considered statistically significant. Multivariable analysis via multiple logistic regression was employed to determine the effect of each independent variable.

RESULTS
Incidence of hypotension was found out to be 61.5%. Sensory block height of T4 (as compared with T6) was identified as a strong risk factor. Age >35 years was found to be a negative risk factor in a small number of participants.

CONCLUSIONS
The incidence of hypotension after spinal anaesthesia is high in patients undergoing caesarean section. A high sensory block (T4) is a strong risk factor for developing hypotension. As there are many factors playing a role in spinal hypotension, no other single factor was found to be responsible for hypotension in the present study.

KEYWORDS
Spinal Hypotension, Spinal Anaesthesia, Caesarean Section

How to cite this article

Mahajan S, Karnik H, Bajpai Y. A prospective study to identify the risk factors for hypotension after spinal anaesthesia in patients undergoing caesarean section. J. Evid. Based Med. Healthc. 2019; 6(38), 2598-2602. DOI: 10.18410/jebmh/2019/535

BACKGROUND

Currently most anesthesiologists prefer providing Spinal anaesthesia/ Subarachnoid Block (SAB) for caesarean sections.1,2,3 Despite offering several advantages, SAB is associated with considerable side effects, maternal hypotension being the most common one.1,2,3,4 This is primarily due to decreased sympathetic blockade leading to a reduction in systemic vascular resistance.2 Maternal hypotension leads to uteroplacental hypoperfusion with the potential for foetal hypoxia, acidosis, and neonatal depression. It is also frequently associated with maternal nausea and vomiting.4 The ability to predict which patients will develop severe hypotension would help the anesthesiologist to prepare adequately in the perioperative 

phase and could potentially result in changed treatment regimens and thus helping in avoidance of maternal discomfort and foetal acidaemia.

 

METHODS

 

Study Design

It was a prospective observational study. After institutional Ethics committee approval, 200 patients undergoing lower segment Caesarean section surgery and willing to participate in the study were included. On the day of surgery, a written informed consent was obtained and each patient was thoroughly evaluated and the following parameters were noted down: Age, Weight, Height, Body mass index (BMI), Urgency of the surgery and Pre-existing hypertension. 3 After being taken on the operation table patient’s baseline vital parameters were checked and noted. Appropriate Intravenous Fluid (IV) was started. The level of patient’s anxiety was assessed with the help of 10 points Verbal Analogue Scale (VAS) with the scores of “zero? and “ten? taken as “No anxiety” and “worst anxiety” respectively. The amount of intravenous fluid received by the patient before receiving SAB was noted down. SAB was performed under strict aseptic precautions according to the standard protocol  

of the institution. All patients received Hyperbaric Bupivacaine 0.5% without any additives. The volume of the drug given was decided by the concerned anesthesiologist on individual basis. Immediately after SAB, patients were placed in supine position with a wedge placed under the right buttock. Surgery was allowed to begin when a sensory level of at least T6 was attained, as confirmed by pinprick method. Blood pressure (BP) was measured and noted down for the first 30 minutes after SAB, every two minutes for the first ten minutes and every five minutes for the remaining 20 minutes. Level of sensory block was checked every five minutes for 30 minutes and highest level was recorded. Hypotension was considered as a fall in Systolic blood pressure (SBP) of more than 20% of baseline value or SBP less than 90 mmHg & same was treated with IV fluids & vasopressors. Vasopressor of choice was Ephedrine and the total dose, if given, was noted. Single/Multiple birth and weight of the newborn were noted down. Patients were withdrawn from the study in case of failed spinal or conversion to general anaesthesia.

 

Statistical Analysis

Each of the variables was divided into categories:

Maternal variables (Age: <35 AND ≥35 years, Height: ≤1.5 meters AND >1.5 meters, Weight: <55 Kg AND ≥55 Kg, Body Mass Index: <29 Kg/m2 AND ≥29 Kg/m2, Pre-operative Anxiety Score: <5 AND ≥5, History of Hypertension: Yes AND No.)

Foetal variables (Multiple/Single Births: Single AND Twins, Weight of the new born: <1.5 Kg AND 1.5 to 3 Kg AND >3 Kg) Anaesthesia and Surgery related (Drug dose of Bupivacaine 0.5% Heavy: ≤2 mL AND >2 mL, Sensory block height: T4 AND T6, Urgency of the surgery: Elective AND Emergency, Pre-operative intravenous fluids received: ≤ 500 mL AND >500 mL.)

Data analysis was done with the help of SPSS Software version 15. To assess the association between two categorical variables in a univariable analysis, Chi-square test was used with P-value of <0.05 considered statistically significant. Multivariable analysis via multiple logistic regression was employed to determine the effect of each independent variable.

 

RESULTS

Hypotension was seen in total 123 patients (61.5%). 114 patients showed more than 20% fall in SBP from the baseline value. 83 patients had minimum SBP of less than 90 mmHg. Table 1 summarizes the demographical data of the patients included in the study.

Association between each of the variables with hypotension as assessed by Chi-square test has been detailed in Table 2. Only two variables showed statistically significant correlation with maternal hypotension with a p value of less than 0.05. Of these, though age >35 was associated with reduced risk of hypotension, the number of patients included in the group was very small for statistical

 

significance. Higher level of sensory block was associated with statistically significant increased risk of hypotension.

Table 3 gives the summary of the results of the multiple logistic regression which was employed to determine the effect of each independent variable. Graph 1 depicts percentage fall in SBP from baseline value. Graph 2 shows association between maternal age and spinal hypotension, whereas graph 3 illustrates the association of sensory block height with spinal hypotension.

 

N=200

Mean

Standard Deviation

Median

Range

Age (years)

26.32

4.34

25.00

18.39

Height (m)

1.54

0.06

1.55

1.33-1.68

Weight (kg)

62.87

13.25

61.50

42-130

BMI (Kg/m²)

26.48

5.32

25.62

17.94-53.42

Table 1. Demographical Data

 

 

Hypotension

{n (%)}

No Hypotension

{n (%)}

Total (n)

p- value

 

Maternal Age

< 35 years

119 (64%)

67 (36%)

186

0.009

≥35 years

4 (28.6%)

10 (71.4%)

14

Maternal height

≤ 1.5 m

40 (65.6%)

21 (34.4%)

61

0.433

> 1.5 m

83 (59.7%)

56 (40.3%)

139

Maternal weight

< 55 Kg

36 (60%)

24 (40%)

60

0.775

≥55 Kg

87 (62.1%)

53 (37.9%)

140

Maternal BMI

< 29 Kg/m²

89 (59.7%)

60 (40.3%)

149

0.380

≥29 Kg/m²

34 (66.7%)

17 (33.3%)

51

Maternal Pre-op Anxiety (VAS score)

< 5

27 (55.1%)

22(44.9%)

49

0.289

≥5

96 (63.6%)

55 (36.4%)

151

Maternal Past history of Hypertension

Yes

18 (62.1%)

11 (37.9%)

29

0.946

No

105 (61.4%)

66 (38.6%)

171

Multiple/Single

Births

Single

122 (61.6%)

76 (38.6%)

198

0.737

Twin

1 (50%)

1 (50%)

2

Weight of the new-born

< 1.5 Kg

6 (66.7%)

3 (33.3%)

9

0.783

1.5 to 3 Kg

90 (62.5%)

54 (37.5%)

144

Drug Dose of

Bupivacaine 0.5% Heavy

> 3 Kg

27 (57.4%)

20 (42.6%)

47

0.146

≤ 2 mL

96 (64.4%)

53 (35.6%0

149

>2mL

27 (52.9%)

24 (47.1%)

51

Sensory Block

Height

T4

41 (93.2%)

3 (6.8%)

44

0.000

T6

82 (52.6%)

74 (47.4%)

156

Urgency of the Surgery

Elective

26 (54.2%)

22 (45.8%)

48

0.231

Emergency

97 (63.8%)

55 (36.2%)

152

Preoperative Intravenous fluids received

≤ 500 mL

116 (61.1%)

74 (38.9%)

190

0.571

Table 2. Association Between Each of the Variables with Hypotension as Assessed by Chi-Square Test

 

Graph 3. Association of Sensory Block Height with Spinal Hypotension

Variable in the Equation

p

Value

Odds Ratio

95% C I for

Odds Ratio

Lower

Upper

Maternal Age <35 years

0.016

5.907

1.393

25.044

Maternal Height ≤ 1.5 m

0.671

1.177

0.555

2.497

Maternal Weight < 55 Kg

0.829

0.919

0.428

1.976

Maternal BMI < 29 Kg/m²

0.288

0.623

0.261

1.490

Pre-op Anxiety Score < 5

0.625

0.824

0.380

1.790

H/O Hypertension (Yes)

0.657

1.254

0.462

3.405

Single Birth

0.696

3.673

0.005

2,500.856

Weight of the newborn <1.5 Kg

0.296

2.590

0.435

15.414

Weight of the newborn 1.5 to 3 Kg

0.210

1.650

0.755

3.606

Dose of Bupivacaine 0.5% (H) ≤ 2mL

0.220

1.615

0.751

3.474

Sensory Block Height T4

0.000

14.434

4.048

51.471

Urgency of the surgery (Elective)

0.857

0.933

0.439

1.983

Pre-op IV fluids received ≤ 500 mL

0.849

1.184

0.207

6.760

Table 3. Multivariable Analysis

 

DISCUSSION

SAB is preferred by most anesthetists for the majority of caesarean sections at present. Hypotension is the commonest adverse effect of SAB. The present study was conducted in an attempt to identify the risk factors which could be associated with increased incidence of hypotension in patients undergoing caesarean section after receiving SAB. Various studies have investigated the incidence of hypotension after SAB, the results have been variable. The incidence of hypotension in our study was 61.5%. Other studies have reported varying incidences ranging from 52.6 to 76.7%.5,6,7,8 The different incidences may be due to varying definitions of hypotension as there is no universal definition for hypotension9 and the incidence depends upon the chosen definition. In our study the cut offs were decided as per routine practice of treating hypotension with fluids and vasopressors and the observation period was limited to 30 minutes after blockade to prevent perioperative factors, such as bleeding, to affect the prevalence or intensity of fall in BP. 6 In the univariate analysis done by Chi Square test only 2 variables were found out to have a statistically significant association with spinal hypotension. They were maternal age <35 years and a spinal block height of Thoracic level 4 (T4). After the univariate analysis, a multiple logistic regression which is a statistical technique that brings every significant factor into the same equation for the explanation the degree of association was done. It enables the building of predicting models of the probability of a certain event to occur in the presence of risk factors. Spinal block height was found to have the strongest correlation with the hypotension. The odds ratio calculated was 14.434 meaning that having a spinal block height of T4 has around 14 times higher chances of having hypotension as compared with that of T6. The correlation of circulatory instability with higher cephalic levels of neuraxial blockade has already been proven in previous studies5,6,8,10,11,12 with odds ratios ranging from only 1.615 to 13.12 The explainable physiology is that circulatory regulation is affected by the blockade of the sympathetic nervous system, with resulting vasodilatation which leads to reductions in both venous return and systemic vascular resistance. Furthermore, when the level of analgesia exceeds T4, cardio-acceleratory fibres are blocked, leading to negative inotropic and chronotropic effects on heart function. The other variable found with significant correlation was maternal age <35 years. We considered age 35 as cut off level for risk, as in our country, parturients >35 years are considered as high risk cases for pregnancy as against 40 years in some other studies. Although all the previous similar studies5,8,10,11,12,13 have found increasing age to be the risk factor associated with hypotension after SAB, in our study we got opposite results. It is very important to enlighten the fact that number of patients aged >35 years included in the study 7 was only 14, while there were 186 patients of age <35 years. Hence, we postulate that the results of our study in case of maternal age might not stand true given the fact that such a small number of patients were included in the category. There also may be a bias involved as the older patients are cared for in a more aggressive manner or closely monitored and optimised as compared to younger ones. We believe that to find out the true association of maternal age with spinal hypotension a study with much larger sample size needs to be done with both types of patients receiving near identical or equal management. In our study we did not find maternal height as a risk factor for developing spinal hypotension as in a few other studies.5,8 However, in the study conducted by Chumpathong S et al6 patient?s height <155 cm was found out to be a risk factor for hypotension. Theoretically, the amount of local anaesthetic agents in the sense of milligram and total volume should affect the severity of hypotension. The authors of that study concluded that because at their institute, the patients mostly received 11 mg or 2.2 mL of 0.5% heavy bupivacaine and the drug dose was not adjusted according to patients? height, hypotension was seen more frequently in shorter patients. In our study as the amount of the drug to be given was adjusted according to the patients? height as per institute practice. Most patients <1.5 meters tall received 1.8 ml of Bupivacaine 0.5%. Patients having height >1.5 meters received 2.0 ml of the drug and taller patients received 2.2 ml. In our study we did not find maternal weight as a risk factor. Thus, our results were in concordance with other studies.5,6,8

BMI in our study was not identified as a positive risk factor. Many cut off points for BMI are found in current literature. The cut-off point in our study was 29 Kg/m2, being 8 slightly below the WHO definition of obesity grade 1. Our limits were set after taking into consideration similar study conducted by F. Brenck et al5 and also the fact that not many patients had their BMI above 35. Even after changing our limits, the association between BMI and spinal hypotension was not found to be significant. Similar results were seen in some previous studies.12,13 On the other hand, Ohpasanon P et al8 and F. Brenck et al5 found BMI >35 and ≥29 Kg/m2 respectively to be associated with maternal hypotension after SAB in their studies. Kyokong O et al11 found BMI of ≥30 as a risk factor for hypotension after SAB. However, this study included surgeries performed after SAB other than caesarean sections also. The studies that found positive correlation between patients? BMI and spinal hypotension had postulated that the higher incidence of hypotension might be due to increased abdominal pressure with compression of the subarachnoid space and a reduction of the cerebrospinal fluid. They also postulated that it might be due to the fact that obese women have a higher 24-hour BP profile and a higher night/day ratio with a blunted increase in systolic and diastolic nocturnal BP.5 Though, such effect was not seen in our cases with higher BMI. The variable preoperative anxiety was assessed by VAS score which is a direct psychological measure that captures the essence of the subjective experience. As it directly addresses subjective feelings, it usually provides a good estimate of how anxious participants truly are. The association was not found to be significant. In the study conducted by S. Orbach-Zinger et al14 the authors found a significant difference between low and high anxiety groups, but the differences between low and medium anxiety and the medium and high anxiety groups were not significant. The authors of that study had speculated that the results were associated with an anxiety-mediated increase in baseline sympathetic activation. As hypotension induced by SAB is mediated by sympatholytic, it seemed plausible that the higher the baseline sympathetic activation, the more dramatic will be the hemodynamic effect of SAB. In the present study as majority of the cases had intermediate levels of anxiety we also did not find statistically significant association.

Patients with history of Hypertension were not found to have a higher incidence of post spinal hypotension in our study as compared to patients without such a history. As in our study only 29 patients were found to have pre-existing Hypertension as compared with 171 who did not. Thus, the number of hypertensive patients was not large enough to make a statistical significant inference. The results were

similar in a few previous studies.5,11,12 similarly, no statistical conclusion could be drawn for Single/ Multiple Birth category where only 2 parturients had delivered twins and with the preoperative intravenous fluids category where only 10 patients had received fluids more than 500 mL and 190 patients had received less than that.

Intravenous prehydration is the most popular non-pharmacological method employed for prevention of hypotension. The rationale for prehydration is to maintain or augment cardiac preload and Cardiac Output and thus prevent or attenuate hypotension. But results of most of the studies have not found crystalloid preloading to significantly decrease the incidence of hypotension after spinal anaesthesia.6,8,13,15,16 Poor efficacy of crystalloids in decreasing incidence of hypotension significantly probably reflects rapid redistribution and short clinical half-life. Neonatal weight was not identified as a risk factor with comparable incidences in all the 3 categories. The results were similar to the study conducted by F. Brenck et al.5,10

We found that the incidence of hypotension was relatively higher in the emergency surgeries (63.8%) as compared to the elective surgeries (54.2%), though this association was found to be statistically not significant. A few studies did not find any significant association between urgency of the section and incidence of hypotension after SAB,5,6,8,13 while there are other studies in which elective caesarean section was identified as a risk factor.17,18 We believe that higher incidence of spinal hypotension in emergency surgeries in our study might be due to lack of time to optimally prepare and prehydrate the patients before the spinal block. Also, generally the patients posted for emergency surgeries are seen to be more anxious as compared to the patients posted for elective surgeries given the lack of time for psychological preparation to undergo a surgery. Higher anxiety is associated with higher sympathetic activity as explained earlier, sympathetic blockade after SAB may lead to more fall in BP in such patients.

Most recent studies related to the topic have mainly focused their attention on the treatment part of the SAB induced hypotension. Even almost all of the studies done on the Indian population have been on the strategies on treatment. However, our main intent with this study was to make an attempt to identify the risk factors so that it might increase the ability of the attending anaesthesiologist to predict which patients will develop severe hypotension in order to decide on the need for various prophylactic and/or therapeutic techniques to prevent prolonged episodes of maternal hypotension and thus, decreasing the potential risk for the mother and the neonate. Our study will be one of the few first such studies to identify the associated risk factors in Indian population. The limitation of our study was the sample size. Though we conclude that apart from the sensory level of block, other parameters do not pose a risk factor for spinal hypotension,11 there is a need for a much larger metacentric study to confirm the association of the variables which our study could not.

 

CONCLUSIONS

The incidence of hypotension after spinal anaesthesia is high in patients undergoing caesarean section. A high sensory block (T4) is a strong risk factor for developing hypotension. As there are many factors playing a role in spinal hypotension, no other single factor was found to be responsible for hypotension in the present study.

 

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