K. R. Visser

Electric properties of flowing blood and impedance cardiography.

An effective resistivity is defined for axisymmetric flow through a circular tube with a uniform electric field in the longitudinal direction. The resistivity of flowing blood is found to be a function of the shear rate profile. Under axisymmetric conditions shear rate profiles are a function of a single parameter: the reduced average velocity, which is the average velocity divided by the radius of the tube. The resistivity of human blood was investigated while the blood was in laminar flow in a circular tube with different constant flow rates. The relative change in resistivity in % is given by: −0.45·H·{1-exp[−0.26·(〈v〉/R)0.39]}; where H is the packed cell volume in % and 〈v〉/R is the reduced average velocity in s−1. In accelerating flow the resistivity change is synchronous with the change in flow rate, but in decelerating flow there is an exponential decay characterized by a relaxation time τ. For packed cell volumes of 36.4% and 47.5% τ was estimated to be 0.21 s, for a packed cell volume of 53.7% τ was estimated to be 0.29 s. The resistivity changes in elastic tubes are influenced by both velocity changes and changes in diameter, but in opposite directions.

OBSERVATIONS ON BLOOD-FLOW RELATED ELECTRICAL-IMPEDANCE CHANGES IN RIGID TUBES

SummaryMammalian blood in pulsatile flow through a rigid tube has been shown to be anisotropic with regard to its electrical conductivity. When flow increases there is a rise in conductivity in the longitudinal and a fall in the radial direction. These changes are caused by flow-dependent variations in the orientation of the disk-shaped erythrocytes.

Theory of the determination of systolic time intervals by impedance cardiography

It is possible to accurately measure the left ventricular ejection time from the dZ/dt signal of impedance cardiography. The pre-ejection period can be measured from simultaneous recordings of ECG and dZ/dt. The thoracic admittance (reciprocal value of thoracic impedance) is the sum of a constant tissue admittance and a varying blood conductance Gb (reciprocal value of the blood resistance). dZ/dt represents the changes in Gb. The notches of the dZ/dt signal, corresponding to opening and closing of the aortic valve, are due to conductivity changes of blood caused by changes in orientation of erythrocytes; near zero-velocity the first derivative with respect to time of these conductivity changes approaches infinity. Therefore, these notches coincide with the actual opening and closing of the valve, although different vessels, including the aorta, contribute to dZ/dt. Comparison of ejection times, simultaneously measured by impedance cardiography and aortic pressure recording, showed excellent agreement for the whole range of measured heart rates (maximum heart rate = 140 beats min-1, n = 70, r = 0.986).

Electric conductivity of stationary and flowing human blood at low frequencies

The conductivity of stationary and flowing blood was measured at different hematocrit values (H). For stationary blood with erythrocytes in random orientation, the conductivity in S/m is estimated to be 1.57/(1+CH/(1-H)), with C=1.91. Variations in orientation cause C to vary between 1.18 and 3.35. The relative percent change conductivity for laminar flow in a circular tube with constant flow rate is also given. The estimate of the conductivity with parallel orientation for stationary blood and flowing blood are shown to be in good agreement.<<ETX>>

Origin of the impedance cardiogram

The parallel conductor model for the thorax as used in impedance cardiography is extended to account for the influence on the thoracic heart-synchronous impedance variation of the packed cell volume and the shear-rate-dependent orientation of the erythrocytes in addition to the volume variation of the great intrathoracic blood vessels. The blood of four dogs was gradually replaced by a stroma-free hemoglobin solution, causing a decrease in resistivity as well as in resistivity variation of the blood, the latter due to variations in orientation of the erythrocytes. This was used to estimate the average value of the relative variations in the blood conductance at a packed cell volume of 40% to be 7.5%, 4.5% being caused by the variations in orientation of the erythrocytes and 3% being caused by the volume variations.<<ETX>>

Isolated systolic hypertension from a vascular point of view.

Due to the results of antihypertensive intervention studies, isolated systolic hypertension (ISH) has gained new interest lately. Yet, apart from increased aortic stiffness, the specific pathophysiological features of ISH have remained largely undetermined. Therefore, we investigated the elastic properties of the vascular bed of an upper arm segment in uncomplicated ISH patients and matched normotensive controls using an electrical bioimpedance technique. Compared with the controls, the compliance of the arterial bed as a whole at normotensive blood pressure level was on the average 108.0% higher (p < 0.005) in the hypertensive patients. The blood volume of the arterial bed as a whole at operating blood pressure level and that of the larger arteries were significantly higher (40.5%, p < 0.05, and 40.5%, p < 0.01, respectively). The same held true for the venous blood volume (64.4%, p < 0.05), and for the width of the arterial compliance-pressure relation (34.6%, p < 0.01). We concluded that ISH is a separate pathophysiological entity in which all parts of a peripheral vascular bed are changed and the decreased buffering function of the aorta and large arteries is partly compensated for by an increase in small artery compliance.

Systolic Time Intervals By Impedance Cardiography

The systolic time intervals are preejection period (PEP) and left ventricular ejection time (LVET). Together they form the electromechanical systole (QS2). From the dZ/dt signal of the impedance cardiogram, the LVET can be measured. This is possible because the notches corresponding with the opening and closing of the aortic valve are due to conductivity changes of blood caused by changes in orientation of erythrocytes: near zero velocity the first derivative of these conductivity changes approaches infinity. Because of this, the notches coincide with the actual opening and closing of the valve, although different vessels contribute to the dZ/dt signal. Comparison with simultaneously and invasively measured ejection times showed excellent agreement up to about 140 beats per minute (n=70, r=0.986).

Blood Pressure Estimation Investigated By Electric Impedance Measurement

The electrical impedance of the left upper arm at changing cuff pressure was measured, together with the finger arterial blood pressure of the other arm at 13 healthy volunteers (66 f 5 yr). Apart from systolic, diastolic and mean arterial pressure, the arterial blood volume per centimetre length (1.4 f 0.3 ml/cm), the venous blood volume as a percentage of the total blood compartment (49.2 f 12.6 %), and the total arterial compliance as a function of mean arterial transmural pressure were estimated, based on a model for the impedance response. The effective physiological arterial compliance amounted to 2.0 2 1.3, the maximum compliance to 33.4 2 12.0 pl.(mmHg)-l.cm-l and the extravascular fluid volume expelled by the cuff to 0.3 2 0.3 ml/cm. These quantities have a close relation with patient-related sources of unreliable blood pressure measurements.

ORIGIN OF IMPEDANCE CARDIOGRAM INVESTIGATED IN DOG BY EXCHANGE-TRANSFUSION WITH A STROMA-FREE HEMOGLOBIN SOLUTION

SummaryIn an anaesthetized dog an exchange transfusion was carried out with stroma-free haemoglobin solution. The total circulating blood volume was kept constant. The heart-synchronous changes in the thoracic electrical impedance (Zo) were measured before and after the exchange transfusion.Zo consists of a parallel connection of a tissue impedance (Zt) and a blood impedance (Zb). With the aid of this model forZo the relative variations inZb (ΔZb/Zb) were calculated from the relative variations inZo (ΔZo/Zo). The marked decrease of ΔZb/Zb during the experiment can only be explained by the fact that apart from the heart-synchronous changes in vascular volume the impedance changes are also caused by flow-dependent changes in the electrical conductivity of blood caused by variations in orientation of the erythrocytes.

Properties of the upper arm vascular bed investigated by electrical impedance measurements in healthy subjects

The fluid shifts of an upper arm tissue segment under an occluding cuff were estimated from electrical impedance measurements. This was done using parameter estimation with a model. The model included parameters describing the arterial and venous pressure-volume relationship. To gain insight into the precision of the method, repeated measurements were made on healthy volunteers. Most parameters showed a large variation (13–44 %). This variation could only partly be explained by variations in the measured quantities.