Segmental 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 segment. 

 

IPG 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 orthostasis. 

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 is:

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.

 

 

 


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Peripheral Vascular Properties Measured by Strain Gauge Plethysmography (SPG)
Laser Doppler Flowmetry (LDF)
Indicator Techniques to Measure Blood Volume and Cardiac Output
Heart Rate and Blood Pressure
Near Infrared Spectroscopy
Microneurography and Muscle Sympathetic Nerve Activity (MSNA)
Transcranial Doppler Ultrasound (TCD)
Impedance Plethysmography (IPG)
Skeletal Muscle Pump