7.3 Factors influencing the level of block 

Distribution of local anesthetic solutions within the subarachnoid space determines the extent (height) of the neural blockade produced by spinal anesthesia.

The 3 most important factors in determining distribution of local anesthetics: 

i. Baricity of the local anesthetic solution: Migration of the local anesthetic cephalad in CSF depends on its baricity. A hyperbaric solution of local anesthetic is denser (heavier) than CSF, whereas a hypobaric solution is less dense (lighter) than CSF. With a head-down position, a hyperbaric solution spreads cephalad and a hypobaric anesthetic solution moves caudad. A head-up position causes a hyperbaric solution to settle caudad and a hypobaric solution to ascend cephalad. Similarly, in a lateral position, a hyperbaric spinal solution will have a greater effect on the dependent (down) side, whereas a hypobaric solution will achieve a higher level on the nondependent (up) side. The local anesthetic solutions can be made hyperbaric by the addition of glucose or hypobaric by the addition of sterile water. An isobaric solution tends to remain at the level of injection. Anesthetic agents are mixed with CSF (at least 1:1) to make their solutions isobaric. 

ii. Position of the patient during and just after injection- related with baricity of local anesthetics 

iii. Dose of the anesthetic injected: The higher the dosage or site of injection, the higher the level of anesthesia obtained. 

Other factor which affects height of block includes: 

• Age- plays a negligible role in block height. As we age there are anatomical changes in the subarachnoid area which may increase block height- require small dose. 
• Height- plays a minor role. However, for the very short the dose of local anesthetic should be decreased, and for the very tall it may need to be increased. 
• Intra-abdominal pressure: plays a role in relation to engorgement of epidural veins, decreasing CSF volume, resulting in a higher subarachnoid block. Conditions that increase intra-abdominal pressure include: pregnancy, obesity, ascites, large abdominal tumors, etc. They require relatively small dose 
• Site of injection: the level of injection will influence spread. For example, a greater spread of local anesthetic will occur if injected at L2-L3 or above, as opposed to L4-L5. 
• Direction of injection: if the local anesthetic is injected in a caudal direction, the spread of local anesthetic will be limited compared to injection in a cephalad direction. 
• Barbotage, the intentional creation of currents by the repeated aspiration and re-injection of CSF, may achieve block height quicker than the usual injection with isobaric solution. 
• CSF volume: It is inversely related to block height. This is the most significant physiologic factor. Obese patients have a smaller volume of CSF when compared to the non-obese patient. Decreased volumes of CSF result in a higher block, whereas increased volumes of CSF decrease the level of blockade. CSF volume is influenced by patient characteristics (i.e. abnormal spinal anatomy).