THE INFLUENCE OF BODY MASS INDEX ON SENSORIMOTOR BLOCK AND VASOPRESSOR REQUIREMENT DURING SPINAL ANESTHESIA FOR ELECTIVE CEASAREAN DELIVERY

Abstract

Background: Many factors influence CSF volume which have a crucial effect on the intrathecal spread of drug. Detailed studies are lacking in this regard because of the fact that CSF volume is difficult to measure even with radiological imaging. Many patient variables therefore have been suggested as influencing the ultimate spread of sensory blockade following sub arachnoid injection of local anesthetics, one among which is body mass index of patient. In this prospective observational study , we tested the hypothesis that obesity(class I and II) would not have a clinically important effect on vasopressor requirement or block height.

Objectives: The primary outcome were maximum block height at 30 min and total vasopressor dose required after spinal anesthesia. Secondary outcomes were changes in peak expiratory flow and time of regression of block.

 Participants: Two groups of 30 parturients , Group 1 (BMI 25-29.9 kg/m² ) and Group 2 (BMI 30-39.9 kg/m² ) requiring elective caesarean delivery were recruited.Morbid obese parturients(BMI >40 kg/m²⁾ were excluded in our study.

Design: Prospective observational study

Participants: Two group of 30 parturients, Group 1 (BMI 25-29.9kg/m²) and Group 2 (BMI 30-39.9 kg/m²) requiring caesarean delivery were recruited. Morbidly obese parturients ( BMI> 40 kg/m²) were excluded.

Methods:All participants received 12.5 mg of intrathecal hyperbaric bupivacaine coadministered with 25 micrograms of fentanyl. Dermatomal levels were assessed at 5 mins and 30 mins after spinal anesthesia and at completion of surgery , using light touch and cold sensation . Hemodynamic parametres were recorded at an interval of 2 mins for 20 mins followed by 5 mins till end of surgery.

Results: There were no significant between group differences in median block height as assessed by touch at 5 or 30 mins or by temperature at 30 mins. At 5 mins, there was a 2 dermatomal difference in median block height for loss of temperature sensation between group1 and group 2( T7 vs T9 p value <0.05). No blocks extended to cervical dermatomes . The mean phenylephrine dose was 46.67 micrograms in group 1 and 48.25 micrograms in group 2 and therefore there was no significant difference in vasopressor requirement between the two groups. There were no differences in mean percentage reduction in peak expiratory flow rate after spinal anesthesia. Mean surgical time was longer in group 2 than in group 1(34.03 mins vs 48.53 mins, pvalue <0.05). The mean time for recovery of touch sensation to T10 was longer in group 2 than in group 1 (195.9 mins vs 185.22 mins ). No analgesic supplementation was required in either of the groups.

Conclusion: The intrathecal dose of bupivacaine 12.5 mg and fentanyl 25 microgram results in clinically equivalent effects in two groups of parturients with widely differing BMI (25-39.9 kg.m²). However,there was increased time to regression of the block and longer surgical time in group 2.

Implication:The dose of intrathecal local anesthetic single shot spinal anesthesia for caesarean section should not be reduced in obese(class I and II) parturients.