Factors Affecting Block Height

More than 20 factors may alter spinal anesthetic block height ( Table 43-4 ).^{[100] [101]} Table 43-5 lists the most important documented factors. Age has a statistically significant effect on block height, but when examined, the difference in block height using isobaric bupivacaine and comparing the third to ninth decades is small (i.e., T9 for those 20 to 28 years old and T6 for those older than 80 years).^{[102] [103]} Unlike epidural dose requirements, weight is not related to block height during spinal anesthesia. Patient height is related, although the contribution is minor compared with more important factors. Similarly, injection rate and barbotage of isobaric and hyperbaric solutions have not been shown to affect block height,

TABLE 43-4 -- Factors postulated to be related to spinal anesthetic block height

Patient characteristics

Age

Height

Weight

Gender

Intra-abdominal pressure

Anatomic configuration of spinal column

Position

Technique of injection

Site of injection

Direction of injection (needle)

Direction of bevel

Use of barbotage

Rate of injection

Characteristics of spinal fluid

Volume

Pressure (cough, strain, Valsalva)

Density

Characteristics of anesthetic solution

Density

Amount (mass)

Concentration

Temperature

Volume

Vasoconstrictors

TABLE 43-5 -- Factors influencing block height

Controllable factors

Dose (volume × concentration)

Site of injection along neuraxis

Baricity of local anesthetic solution

Posture of patient

Factors not controllable

Volume of cerebrospinal fluid

Density of cerebrospinal fluid

Adapted from Stienstra R, Veering BT: Intrathecal drug spread: Is it controllable? Reg Anesth Pain Med 23:347, 1998.

although injection rates in these studies have been above 0.1 to 0.2 mL/sec.^[104] It is becoming clear that the direction of spinal needle lateral-facing openings affect block height levels, even with isobaric spinal solutions.^{[105] [106]} Other maneuvers that do not appear to affect block height are coughing and straining after local anesthetic injection. This is related to the physics of injecting drugs into a closed column of CSF, which instantaneously transmits pressure changes throughout the CSF column, such as those that occur with coughing or straining ( Table 43-6 ). ^[100]

When more global factors affecting block quality are examined, at least in some teaching programs, the spinal failure rate may be as high as one in six blocks.^[107] To understand failure rate fully, the definition must be examined. Investigators^[107] considered the need to supplement the block with any inhaled anesthetic as failure. This is a key point because to provide comprehensive regional anesthesia supplementation of the block must be considered appropriate rather than a de facto failure.^[108] These data emphasize an important contributor to failed blocks, the lack of free flow of CSF after needle placement. There is obviously some level of experience necessary to carry out spinal anesthesia successfully, but even in a teaching program, this should not prohibit the successful use of spinal anesthesia because the failure rate can be as low as 1%.^{[51] [81]}