Spinal anesthesia spread: Factors


Most important factors:

  • Baricity of anesthetic solution (relative to CSF)

    • Hyperbaric (glucose added) – follows gravity, greater cephalad spread, normal dependent portion of spine in supine position is T4-T8
    • Hypobaric (dilute with sterile water) – spreads in nondependent fashion
    • Isobaric- behave clinically as slightly hyperbaric

  • Patient positioning (during and immediately after injection)
  • Drug dosage
  • Site of injection

Other factors:

  • Age: elderly patients tend to have decreased CSF volume and are prone to higher blocks
  • CSF: volume correlates inversely with level of anesthesia (i.e., increased intraabdominal pressure or conditions that cause engorgement of epidural veins (pregnancy, ascites, large abd tumors) decrease CSF volume and are associated with higher blocks)
  • Curvature of spine
  • Drug volume
  • Intraabdominal pressure: (see CSF section)
  • Needle direction: higher levels are achieved if injection directed cephalad
  • Patient height: taller patients require more drug to achieve a given level
  • Pregnancy: (see CSF section), also decreased dosage requirements for term parturient


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