One method of measuring the volume of a swollen limb is by measuring its circumference at various points and then calculating the volume as a series of truncated cones.
Patient selection: one limb normal, the opposite limb edematous
Equipment: flexible tape measure that does not expand under tension (usually metal)
Measurements on hand or foot:
(1) circumference at wrist or ankle
(2) circumference at midpoint of hand or foot
(3) circumference at digit tip
Measurement on upper or lower limb:
(2) circumference at regular intervals, usually 10 cm, until axilla or inguinal region is reached
Circumferences can be taken at closer distances depending on the length of the extremity. Some people use 4 cm as the interval.
volume of a truncated cone in mL =
= 0.333 * PI() * (height in cm) * (((radius one end)^2) + (radius one end) * (radius other end)) + ((radius other end)^2)) =
= (height in cm) / (12 * PI))) * (((circumference one end)^2) + ((circumference one end) * (circumference other end)) + ((circumference other end)^2))
fluid in mL =
= (volume in swollen limb) - (volume in normal limb)
• If the patient is going to be followed serially, then it is important to select the same points for the measurements.
• Multiple readings can be taken at each point to minimize variability.
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