Blood Flows and Blood volume distribution by impedance plethysmography
Impedance plethysmography (IPG) has been used to estimate blood flow and
quantify body fluid volumes. . Typically this has been employed to
estimate thoracic blood flow, designated "impedance cardiography.
However, the technique has far more potential. The technique is based upon
a simple model which regards the body as composed of various electrical
segments comprising simple resistors and capacitors as shown in the
figure. Measurements of baseline
resistance, R0, and pulsatile resistance changes, ΔR, are
made.. Disposable EKG electrodes will be attached as described above. The
IPG introduces a high frequency (50 kHz), low amperage (0.1 mA RMS)
constant current signal between the foot and hand.
Resistance changes normalized to segment length and cross section
yield relative volume changes. Pulsatile ΔR are used to obtain
relative blood flow (ml/100 ml of body tissue/min) of each body
can be used to detect internal volume shifts
including those produced during orthostatic stress. We used a Tetrapolar
High Resolution Impedance Monitor (THRIM) four-channel digital impedance
plethysmograph (UFI, Inc). to measure volume shifts in four anatomic
segments designated the thoracic segment, the splanchnic segment, the
pelvic segment incorporating lower pelvis to upper leg, and the leg
segment Ag/AgCl EKG electrodes were attached to the left foot and left
hand, which served as current injectors. Additional electrodes were placed
in pairs representing anatomic segments as follows: ankle-upper calf just
below the knee (the leg segment), knee-iliac crest (pelvic segment), iliac
crest-midline xyphoid process (the splanchnic segment), and midline
xyphoid process to supraclavicular area (the thoracic segment). The IPG
introduces a high frequency (50 kHz), low amperage (0.1 mA RMS) constant
current signal between the foot and hand electrodes.
This is completely insensible to the subjects. Electrical
resistance values are measured using the segmental pairs as sampling
electrodes. Anatomic features were selected as the most appropriate
locations for comparing changes within and across patients.
This combination of electrodes gives highly repeatable changes in
computed volume shifts and has been tested in a wide range of
experiments by our group
Δsegmental blood volume (ml) = ρ•(L2/R0R1) • ΔR
which is relatively model independent.
Where ρ is electrical
conductivity of blood estimated as 53.2*exp(hematocrit*.022) given by
Geddes and Sadler We measure hematocrit from a venous sample taken from
the antecubital vein. R0 is the resistance of a specific
segment prior to change in tilt angle, R1 is the resistance
after change in the tilt angle, and ΔR is the change in resistance (R1-R0)
in a specific segment during the each incremental tilt step.
Such IPG measurements allow us
to trace blood volume changes in the various segments during
The figure shows impedance changes with upright tilt in the top panels
for the thoracic (trunk), splanchnic, pelvic and leg segments listed
from top to bottom. Corresponding calculated volume changes are shown in
the bottom hand panels.
A variety of formulae can be used to estimate blood flow within a given
anatomic segment (e.g. leg, thorax). Segmental beat-to-beat changes in
impedance are shown in the figure.
One such estimate
Flow= [HR ∙ ρ ∙ L2
∙ T ∙ ∂R/∂tmax]/R02 ,
where HR is heart
rate, T is the ejection period, R is the pulsatile resistance and R0
is the baseline resistance. IPG flows are expressed in units of
ml/min for each defined anatomic segment. Normalization to tissue volume
can be performed by dividing by estimated segmental volume.