So far, most studies have indicated that reducing the dose of local anesthetic in spinal anesthesia can effectively decrease the incidence of hypotension in parturients. In Van de Velde’s study, the parturients in the high-dose group were injected with 9.5 mg bupivacaine into the subarachnoid cavity, while the dosage of bupivacaine in the low-dose group was reduced to 6.5 mg. Additionally, 2.5 μg sulfentanyl was added in both groups. The result showed that the mean minimum systolic blood pressure (SBP) in the low-dose group was apparently higher than in the high-dose group, and the incidence of hypotension in the low-dose group was distinctly lower than in the high-dose group. Due to the use of opioid analgesic, all parturients achieved satisfactory anesthetic effect(8). Farzi analyzed the analgesia duration and spinal anesthesia complications due to the use of bupivacaine with fentanyl or sulfentanyl, and placebo. The result showed that the use of bupivacaine with 25 μg fentanyl or 2.5 μg sulfentanyl as intrathecal drug can increase the analgesia duration and hemodynamic stability, without causing serious complications. Therefore, this study adopted low dose of bupivacaine (9 mg) and sulfentanyl (2.5 μg) as the intrathecal drug(9), which not only reduced the incidence of hypotension but also ensured better analgesia effect.
According to statistics, the increase in caesarean section rates in Northern Europe is due to the increase in the caesarean delivery rate (CDR) of primiparas and the growing proportion of parturients with caesarean history(10). The uterus of a parturient with caesarean section history is already scarred, so caesarean section is the most likely choice for any subsequent delivery. The operation of parturient with scarred uterus is tough and complicated, and the difficulty of anesthesia is increased. Re-caesarean section obviously puts the parturient and fetus at higher risk. One of the essential factors for operation success is anesthesia. At present, single-shot spinal anesthesia can be safely used in re-caesarean parturients. Nonetheless, due to several physiological changes occurring in the expectant mothers, hypotension remains the most common complication in re-caesarean parturients(11-12).
In this study, the incidence of hypotension after spinal anesthesia in parturients with scarred uterus was 45.96%. The factors including height, post-pregnancy BMI, weight gain in gestation, fasting time, exercise in gestation and dermatomal level of analgesia were statistically related to hypotension after spinal anesthesia in caesarean section. Moreover, BMI (OR = 1.146), weight gain in gestation (OR = 1.127), exercise in gestation (OR = 0.399), and dermatomal level of analgesia (OR = 2.248) were predictors of hypotension after spinal anesthesia in caesarean section. Ohpasanon and Bernd found that age, pre-pregnancy BMI and weight gain during pregnancy were risk factors for hypotension in primiparas after spinal anesthesia. In this study, pregnant women with scarred uterus were selected as the research subjects. Different from primiparas, BMI after pregnancy was one of the strongest predictors of hypotension in women with scarred uterus after spinal anesthesia. Meanwhile, this study also included exercise during pregnancy as a factor, which was not examined in previous studies.
The high incidence of hypotension is relatively contributed by physiological changes during gestation. This study found that four independent variables were closely related to the high incidence of hypotension after caesarean spinal anesthesia.
Consistent with previous studies(13), this study confirmed that the increase of post-pregnancy BMI is a risk factor for hypotension triggered by spinal anesthesia in caesarean section. Pitkanen et al. indicated that the diffusion of bupivacaine of equal dose is closely correlated to post-pregnancy BMI(14-15). The increase of intra-abdominal pressure caused by cyesis or obesity can reduce the volume of cerebrospinal fluid and increase dermatomal level of analgesia. Therefore, the dose of local anesthetic to be injected into the subarachnoid cavity of parturients at full-term is one-third less than that of non-pregnant patients. Meanwhile, larger rump of obese patient may result in higher inclination of lumbar and thoracic vertebrae, and further accelerate the diffusion of local anesthetic from subarachnoid cavity to head. Under the combined effect of the two factors, the use of equal dose of local anesthetic probably leads to over-blockade and results in higher incidence of hypotension.
Kitahara(16) believed that in supine position, the local anesthetic is concentrated in the lowest part of thoracic vertebrae, regardless of the height of patients. However, Greene(17) believed that because taller patients have larger space in subarachnoid cavity and larger volume of cerebrospinal fluid, the local anesthetic should reach lower dermatomal level of analgesia within the same distance from the injection site. In addition, cerebrospinal fluid can dilute the injection. The larger the volume of cerebrospinal fluid, the more diluted the injected drug will be. Hence, both factors limit the diffusion of local anesthetic in taller patients, which is consistent with the result of this study. This study found that taller parturients have lower risk of hypotension after spinal anesthesia in caesarean section. However, according to the result of multivariate logistic regression model, this variable was not significantly related to the incidence of hypotension after spinal anesthesia in caesarean section.
This study found that the risk of hypotension increases with the increase in weight gain during gestation. In contrast, a previous study found that weight gain <11 kg during gestation is a risk factor for hypotension triggered by spinal anesthesia(18). Meanwhile, another study found that weight gain >11 kg during gestation is a risk factor for hypotension after spinal anesthesia(19).
Normally, parturients are required to fast for caesarean section. Long-time fasting probably results in insufficient capacity, relative dehydration and leads to higher risk of hypotension. This study found that over-fasting was one of the risk factors leading to hypotension after spinal anesthesia. However, in the multivariate logistic regression, this variate was not significantly related to the incidence of hypotension after spinal anesthesia in caesarean section.
Buchheit et al. showed that exercise can intensify the overall heart rate variability and promote the activity of cardiac sympathetic nerve and parasympathetic nerve, facilitating the transmission from sympathetic-parasympathetic nerve balance to parasympathetic-enhanced nerve activity(20). This study revealed that the change of sympathetic nerve activity might be related to the reduction of hypotension incidence after spinal anesthesia in parturients with exercise history.
Intrathecal block usually causes varying degrees of blood pressure decline with heart rate slowing. These effects are proportional to the level and degree of dermatomal analgesia. Vessel tension is determined by arterial and venous smooth muscles controlled by sympathetic fibers in T5-L1. Blocking the nerves might cause venous capacitance expansion, vessel vasodilation, hypostasis, and venous return decrease. Arterial vasodilation can even reduce systemic vascular resistance. When dermatomal level of analgesia exceeds T4, cardioacceleration fibers become blocked, resulting in heart rate decline and cardiac output decrease. Arteriectasis and venectasia combined with bradycardia might cause severe hypotension. Arterial vasodilation might be affected by compensatory vasoconstriction above blockade, especially when the dermatomal analgesia is limited to the lower thoracic segment. The result of this study is consistent with previous study(21). Higher dermatomal level of analgesia results in higher risk of hypotension in parturients after anesthesia.
Chung(22) et al. evaluated the correlation of fundal height with dermatomal level of analgesia, and found no clear correlation between them. However, another study revealed that for parturients with higher fundal height and higher intra-abdominal pressure, the diffusion of local anesthetic in the subarachnoid cavity was accelerated in early stage, resulting in higher dermatomal level of analgesia(19). Univariate analysis showed that fundal height was significantly related to the incidence of hypotension after caesarean spinal anesthesia in this study. However, multivariate logistical regression model showed that fundal height was not a risk factor for hypotension after caesarean spinal anesthesia in parturients with scarred uterus.
This was a prospective study on hypotension after caesarean spinal anesthesia. The subjects were parturients with scarred uterus. This is the first study to discuss the risk factors for hypotension after spinal anesthesia when a low dose of local anesthetic was used with sulfentanyl. Moreover, exercise history in gestation was examined for the first time in this study. However, this study had some limitations, such as the time taken to collect data and the failure of including weight gain in the stratification study.