Factors Influencing the Distribution of Local Anesthetics in Cerebrospinal Fluid
 

Twenty five hypothetical and demonstrable factors
Pateint characteristics Age Height Weight Gender
  Intraabdominal pressure Configuration of spinal column Position  
         
Technique of injection Site of injection Direction of Needle Direction of Bevel Rate of Injection
  Barbotage      
         
Diffusion        
         
Characteristics of CSF Composition Circulation Volume Pressure
  Density      
         
Characteristics of anesthetic solution Hypobaric Isobaric Hyperbaric Dose of anesthetic (amount)
  Concentration of anesthetic Volume Vasoconstrictor  
 

Factors having no clinically significant demonstrable effects
Weight Gender Bevel direction (standard needle) Rate of injection (turbulence) Barbotage (turbulence)
Diffusion CSF composition CSF circulation CSF pressure Isobaric solutions
Concentration of local anesthetic Vasoconstrictor      
 

Factors that demonstrably affect distribution, though of widely varying clinical significance
Age Height Bevel of directional needles Increased intraabdominal pressure (decreases CSF volume) Configuration of spinal column
Site of injection Direction of needle during injection CSF volume CSF density Hypobaric solutions (with position)
Hyperbaric solutions (with position) Position (with hypo- and hyper-baric solutions) Dosage of local anesthetic Volume of local anesthetic  
 

Notes
From:  Greene NM: Distribution of local anesthetic solutions within the subarachnoid space. Anesth Analg 1985; 64: 715-730

Solution injected through Whitacre needles leave the hole at a 90 degree angle.   Solution injected through Tuohy needles leave the bevel at a 45 degree angle. Both significantly affect the direction in which the anesthetic solution are injected into the CSF. Thus, both affect the distribution of spinal anesthetic solutions.

Increased intraabdominal pressure effects are most evident in term pregnancy, and in patients with ascites and large intraabdominal tumors.  This effect believed due to decreased CSF volume owing to engorged venous channels in the epidural space.

Direction of the needle during injection:  The initial distribution in CSF of anesthetic injected through a needle pointing cephalad would be likely to be greater above the site of injection than below it.

Cough/straining/bearing down, although  not mentioned per se in the article by Greene, do not appear to be factors that increase the cephalad spread of hyperbaric spinal anesthetics. In the section on intraabdominal pressure, Greene, states, "Chronic increases in intraabdominaal pressure have more effect on altering distribution of spinal anesthetic solutions than do acute increases in intraabdominal pressure."