(Circulation. 1995;92:587-594.)
© 1995 American Heart Association, Inc.
Articles |
Electrical Resistances of Interstitial and Microvascular Space as Determinants of the Extracellular Electrical Field and Velocity of Propagation in Ventricular Myocardium
From the Department of Physiology, University of Bern, Switzerland.
Correspondence to A.G. Kléber, MD, Department of Physiology, University of Bern, Bühlplatz 5, CH-3012, Bern, Switzerland.
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Background In myocardial ventricular tissue, extracellular electrical resistance (ro) is an important determinant of propagation velocity (
) and the magnitude
of the extracellular bipolar electrogram (
Vo).
The extracellular space is composed of two compartments, the vascular
space and the interstitial space. To assess the electrical
equivalent of this compartmentation in the ventricular
myocardium and its effect on ro, ,
and Vo, electrical cable analysis was
performed in an arterially perfused rabbit papillary
muscle.
Methods and Results Vascular resistivity was changed from 75 to
86 to 143 and to 221 
/cm by variation of hematocrit in the perfusate
from 0% to 10% to 40% and to 60%. As a means to vary the volume of
the interstitial space and with this as its resistivity,
colloid osmotic pressure (COP) in the perfusate was changed from 9 to
36 and to 94 mm Hg by altering the dextran concentration in the
perfusate from 10 to 40 to 80 g/L. Decreasing COP had a marked effect
on ro (56% decrease), Vo (decrease from 61
to 42 mV), (increase from 48 to 59 cm/s), and the diameter of the
muscle fiber (increase of 12%). If COP was increased from 36 to 94
mm Hg, ro (by 35%) and Vo (from 62 to 75
mV) increased; and diameter showed no significant changes. In
contrast, alterations of intravascular electrical resistivity in a
range from 75 to 221 /cm did not induce any significant changes in
ro, Vo, , and diameter of
the preparations.
Conclusions We conclude from our data that (1) the microvascular tree in ventricular myocardium is electrically insulated to a large degree from the interstitial space and that (2) electrical current flow in the extracellular space during excitation is confined to the narrow, anisotropic interstitial space.
Key Words: electrophysiology • myocardium
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Myocardial ventricular tissue is composed of discrete cells interconnected by low-resistance gap junctions1 2 and an extracellular space that hosts the dense microvascular network.3 Morphometric measurements indicate that the extracellular compartment occupies approximately 20% to 25% of tissue volume3 4 and that within this compartment, the microvessels occupy a large volume fraction. Only approximately 5% to 10% of total tissue volume is taken by the interstitial space, which appears as narrow, irregular clefts in histological sections.5 6 7
Functionally, myocardial tissue can be represented by a "reaction-diffusion system" in which an impulse (reaction), generated by ionic current flow through activated membrane channels, propagates (diffuses) through an electrical medium. The electrical properties of this medium are a matter of ongoing investigation, and inhomogeneities in electrical resistance are likely to be present in both the intracellular and extracellular compartments. In the intracellular compartment, inhomogeneities of propagation due to the anisotropic cellular architecture occur during propagation in both transverse and longitudinal directions.8 9 10 11 Many of the simulations used for the analysis of propagation in normal and pathological settings involve a so-called single-domain model of cardiac tissue where intracellular and extracellular compartment resistances are added to form a single resistor. Theoretical12 and experimental13 studies have stressed the importance of the resistive properties of the extracellular space, ie, of the bidomain nature of cardiac tissue. Comparison of the amplitude of the extracellular wave front with the action potential in dog hearts14 as well as linear cable analysis in isolated arterially perfused tissue13 15 have shown that the lumped electrical resistances of the extracellular and intracellular compartments are of approximately equal magnitude. Therefore, changes in extracellular and intracellular resistivities are predicted to affect propagation velocity to an equal degree.
The resistive properties of the intracellular compartment have been analyzed in a large number of studies, mostly involving the effect of gap junctional resistance on electrical interaction between cells. By contrast, no information is available at present on the relative contributions of the components of the extracellular compartments, the interstitial and the vascular compartments, to the electrical extracellular resistance. Accordingly, it was the purpose of this work to assess the multicompartment nature of extracellular myocardial resistance. To this aim, we used the arterially perfused rabbit papillary muscle preparation,13 16 which allows for the simultaneous measurements of the electrical resistances in extracellular and intracellular compartments, conduction velocity, and the amplitude of the extracellular electrical field. Moreover, both the resistive properties of the intravascular and the interstitial spaces can be modified in this preparation independently via changes in the composition of the perfusate.
Our results suggest that the microvascular tree in ventricular myocardium is electrically insulated from the interstitial space to a large degree. Consequently, extracellular electrical current flow during excitation is mostly confined to the narrow, interstitial clefts.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Preparation and Perfusion of Papillary Muscles
The method for the preparation of the isolated, arterially perfused rabbit papillary muscle has been described previously in detail.13 16 Rabbits of either sex weighing 2 to 3 kg were anticoagulated with heparin (200 IU/kg IV) and anesthetized with pentothal (50 mg/kg IV). The hearts were rapidly excised, placed in cold (4°C) Tyrode's solution, and transported to a dissection tray. The atria, the left ventricular wall, and the nonperfused part of the right ventricle were removed. The tissue was mounted with the left ventricular septal surface on a wax platform that contained the ground electrode (Fig 1
). The septal artery
(diameter, 150 to 200 µm) was cannulated and perfused with the
following solution (mmol/L): Na+ 149, K+ 4.5,
Ca2+ 1.8, Mg2+ 0.49,
Cl- 133,
HPO42-/H2PO4-
0.4, HCO3- 25, glucose 20 and insulin 1 IU/L,
heparin 400 IU/L, albumin 4 g/L, and dextran
(Mr, 70 000) 40 g/L. In some experiments, 10%
newborn calf serum (BioWhittaker) was added to the perfusate (see
"Results"). The total ischemic time measured from the
removal of the heart to cannulation was less than 4.5 minutes.
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After cannulation, the preparation was placed in the recording chamber, maintained at 37°C, and perfused with the solution previously described and washed bovine erythrocytes (hematocrit, 40%). A roller pump (Ismatec Instruments) was used to maintain a perfusion pressure of 35 to 45 mm Hg by adjustment of the perfusion flow rate (80 to 100 mL/min per 100 g tissue). Perfusion pressure in the septal artery was measured with a pressure transducer (P23 ID, Gould). This perfusion pressure is normal for arteries with a diameter of 120 to 160 µm because in the rabbit, about 40% to 50% of the peripheral coronary resistance is located in vessels with a diameter >150 µm.17 The blood perfusate was equilibrated with a mixture of N2, O2, and CO2 in a membrane gas exchanger. The relative amounts of N2, O2, and CO2 were adjusted to yield a pH of 7.35 to 7.45 and a PO2 of 120 to 160 mm Hg in the perfusate. The pH in the perfusate was continuously monitored throughout the experiment, and in addition, controlled at the beginning and at the end of each experiment by means of a blood gas analyzer (AVL 993S, AVL). Stainless steel tubing between the membrane gas exchanger and the recording chamber prevented diffusional gas losses. The preparation was surrounded by a humidified artificial gaseous atmosphere composed of 75% N2, 20% O2, and 5% CO2.13
Determination of the Amplitude of the Extracellular Field,
Propagation Velocity, and Resistances of the Extracellular and
Intracellular Spaces
Theory
The application of linear
cable analysis to
cylindrically shaped papillary muscles and the validation of the method
have been described previously in detail.13 16 In
brief,
presentation of a cardiac muscle cylinder as a linear
electrical cable assumes (1) a constant cross-sectional shape
(cylindrical or elliptical), (2) homogeneous distribution
of intracellular and extracellular resistivities along the cable axis
and in the direction of the fiber radius, and (3) uniform propagation
of the excitation wave front. In a uniform cable, no gradients of
potential exist within a cross-sectional area during excitation or flow
of subthreshold current. In comparison to the complex extracellular and
intracellular architecture, cable analysis will yield a value
for the lumped extracellular and intracellular resistances (see
"Discussion").
Separation of the extracellular resistance
from the intracellular
resistance necessitates two measurements18 : (1)
application of a subthreshold current pulse Is between an
extracellular electrode located at the tip of the muscle and a
corresponding electrode located at the interventricular
septum. The linear component of the resulting extracellular voltage
profile along the muscle reflects current flow through the longitudinal
tissue resistance, rt (/cm), which consists of
ro (extracellular longitudinal resistance in /cm) and
ri (intracellular longitudinal resistance in /cm) in
parallel. The longitudinal whole tissue resistance,
rt, is obtained from the subthreshold voltage,
Vs, recorded between two extracellular
electrodes (Fig 1
), the interelectrode distance, x, and
the
subthreshold current pulse strength, Is:
 | (1) |
(2)
During excitation and propagation of an active wave front,
local current within the wave front flows through a resistance network,
which, in a linear cable, consists of ro and ri
in series. The amplitude of transmembrane action potential
(Vm) and the amplitude of the bipolar extracellular
electrogram (Vo) determine the ratio (q) of the
extracellular (ro) and the intracellular
(ri) longitudinal electrical resistance. Vo
is a measure for the amplitude of the extracellular wave front voltage
and an important determinant of the amplitude of the ECG.
 | (2) |
The intracellular and the extracellular longitudinal resistances (ri and ro) can be calculated from Equations 1 and 2 using q and rt.
 | (3) |
 | (4) |
The
relation between longitudinal resistance,
rx, of a given tissue compartment x (in /cm) and
the specific resistance or resistivity, Rx, of this
compartment (in /cm) is given by
 | (5) |
where Ax (cm2) is the area of this compartment in a cross section.
Longitudinal propagation velocity
() is calculated from the
interelectrode distance (x) and the conduction time (tc)
measured as the time interval between the steepest part in the
deflection and inflection of the bipolar extracellular
electrogram.
 | (6) |
Recordings
Extracellular electrodes were made of polyethylene tubing
(diameter, 45 µm) backfilled with 150 mmol/L NaCl and contained a
fine silk thread. Electrical contact between the electrode and the
muscle was made only by the electrolyte-to-silk bridge. In such a way,
mechanical damage of the preparation by the electrodes was prevented
and DC stability was obtained.
The transmembrane action potential
(Vm) was recorded
with a conventional intracellular floating microelectrode (Fig
1). The
intracellular and extracellular microelectrodes were connected to
high-input impedance amplifiers (OPA 128 JM, Burr-Brown). The signals
were amplified in custom-built differential amplifiers and sampled on
an analog-to-digital converter data acquisition board (NB-MIO-16L,
National Instruments) installed on a personal computer (Macintosh IIfx,
Apple Computer Inc). The sampling rate for the signals was 25.0 kHz
with 12-bit resolution. Off-line data analysis was performed
using data analysis software (IGOR, Wavemetrics) on
a Macintosh IIfx computer. In addition, action potentials, perfusion
pressure, and contractile parameters were monitored on a
strip chart recorder (Linear Corder Mark IV, Watanabe Instruments
Corp). Subthreshold constant current strength, Is,
was measured in the feedback loop of an operational amplifier connected
between the preparation and ground.
The diameters of the preparations and the interelectrode distances were measured on video images acquired with a CCD video camera (Panasonic wv-BL202, Matsushita Co Ltd) and digitized on a frame grabber board (DT 2255, Data Translation Inc) installed on the personal computer. To analyze the exact fiber dimensions and interelectrode distances, images were acquired at a fixed interval during diastole. Off-line image analysis was done with an image processing and analysis software (IMAGE, NIH).
Experimental Protocol: Variation of Electrical Resistivity of the
Intravascular Space and the Interstitial
Space
It is well known that the specific resistance (resistivity) of
blood is a function of red blood cell content.19 20
This
property was used to produce marked and selective changes of the
electrical resistance of the intravascular space and to assess this
effect on the changes of extracellular and intracellular myocardial
longitudinal resistance, on the magnitude of the electrical
extracellular field (Vo), and on propagation
velocity (). This rationale assumes that the change in hematocrit
does not affect intravascular space volume and resistance. To our
knowledge, no such effects have been described. The resistivity of the
perfusates was measured in every experiment using a laboratory
conductivity meter (PW 9505, Philips) at a temperature of 25°C. The
dependence of perfusate resistivity on the red blood cell content is
shown in Fig 2. In absence of erythrocytes, the
resistivity of the perfusate was 74 /cm. This compared with 221
/cm with a hematocrit of 60% (threefold increase).
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As a means to
vary the volume of the interstitial space, we
changed the colloid osmotic pressure (COP) of the perfusate and
controlled the resulting volume change of the preparation by measuring
the diameter of the whole papillary muscle. COP was measured with a
collodion bag technique (Sartorius collodion bags).21 The
effect of changing dextran concentration on perfusate resistivity was
small and not significant. As shown from a single experiment, COP
changed from 9 mm Hg with 10 g/L dextran to 94 mm Hg with 80 g/L
dextran (Fig 2). In previous work it was shown that
physiological extracellular electrical properties
are obtained with a perfusion pressure of 35 to 45 mm Hg and with a
dextran concentration of 40 g/L (COP, 36 mm Hg) in the presence of a
minimal amount of albumin.13 In such conditions,
the values obtained for extracellular resistance, extracellular wave
front voltage, and propagation velocity are very close to those
observed in whole hearts in vivo.14 Therefore, these
conditions were selected for control. In each experiment, one control
period of 30 to 60 minutes was followed by a test period (20 to 40
minutes) and a subsequent control period (30 minutes). In some hearts,
a second test period followed a by a further control period was
performed.
Statistical Analysis and Experimental Protcol
Statistical
comparisons of values during control and test
periods were made by ANOVA with a statistical analysis software
package (STATVIEW, Abacus Concepts Inc). Results are given
as mean±SD. Differences between groups were considered significant at
P<.05. The Bonferroni correction was used for multiple
comparisons.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Extracellular Resistance, Conduction Velocity, and Magnitude of Extracellular Wave Front Voltage
In the present work, tissue resistance (rt), propagation velocity (
), action potential amplitude, and the
magnitude of the extracellular electrical field
(Vo) were measured at fixed time intervals during
the control and test periods. In order to obtain a high degree of
accuracy, rt and were calculated from voltage and
conduction time profiles based on multiple measurements, as shown in
Fig 3. In Fig 3A, superimposed digital images
show the
five consecutive locations of the apical extracellular electrode. The
position of the extracellular electrode at the base of the papillary
muscle remained unchanged during the measurements, which were obtained
within 40 seconds. Fig 3B shows the superimposed original
signals used
to measure subthreshold voltage (Vs) and conduction time
(tc) as a function of interelectrode distance (x). The
profiles of subthreshold voltage and conduction time are depicted in
Figs 3C and 3D, respectively, and show a high
degree of linearity. The
slope on Fig 3C and the reciprocal slope on Fig
3D were taken to
calculate rt (20 /cm) and (63 cm/s), and they served
to validate the linear cable model in each individual
experiment.13 The high degree of linearity of the
extracellular subthreshold voltage profiles and of the longitudinal
propagation profiles was consistent in all experiments. The
mean coefficient of correlation,
r2, was .998±.010 (n=214
measurements) for the voltage profiles and .996±.009 (n=214) for
the
propagation profiles, respectively.
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During control and test conditions,
there were no significant
variations in the action potential amplitude (Vm) and
extracellular wave front voltage (Vo) used to
calculate the ratio of extracellular to intracellular resistance.
Changes of Colloid Osmotic Pressure in the Perfusate Affect
Extracellular Resistance, Extracellular Wave Front Voltage, Propagation
Velocity, and Diameter
The effects of decreasing COP from 36 mm Hg (40
g/L dextran) to 9
mm Hg (10 g/L dextran) on intracellular longitudinal resistance
(ri), extracellular longitudinal resistance
(ro), and fiber diameter in a single experiment are
shown in Fig 4. Decreasing COP in the perfusate induced
swelling of the preparation, as shown by an increase of fiber diameter
from 1.47 to 1.62 mm. Assuming an extracellular to intracellular space
ratio of 1:3 (References 4 and 5) and an interstitial space
volume of 5% of total tissue volume,7 this would
correspond to an 85% increase of the extracellular compartment volume
or to a 430% increase of the interstitial space volume.
Parallel to the volume increase of the interstitial space,
there was a marked decrease in ro to 53% of control,
whereas ri did not change significantly. Both
interstitial swelling and the change in ro were
reversible upon the reversal of perfusion to control conditions.
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The
effects of the decrease in ro on the amplitude of the
extracellular wave front voltage (Vo) and
conduction velocity () are shown in Fig 5. The
reduction of COP produced a 31% decrease of Vo from 62
to 43 mV. Conduction velocity increased 41% from the control value of
46 cm/s to a maximum of 65 cm/s.
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The results obtained from 23
experiments in 13 different hearts are
listed in the Table. Reducing COP from 36 to 9 mm Hg
(n=9) decreased ro by 54% and increased fiber diameter by
12%. Assuming that the interstitial space occupies 5% of
total tissue volume, this corresponds to a 120% increase of the
interstitial space volume. The increase in
interstitial space volume was associated with a 31%
decrease of the extracellular wave front voltage
(Vo) and a 31% increase in conduction velocity.
When COP was increased from 36 to 93 mm Hg, the small increase in
fiber diameter and decrease in conduction velocity were not
significant. However, the extracellular longitudinal resistance
increased by 35%, and the extracellular wave front voltage
(Vo) increased by 22%.
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The amplitude of the transmembrane action potential amounted to 104±2.3 mV during the control state and showed no significant variations among the three groups.
Alterations of Hematocrit in the Perfusate and Influence on the
Extracellular Wave Front Amplitude, Propagation Velocity, and
Diameter
The effect of an increase of the electrical resistivity of
the
intravascular space (from 143 to 221 /cm) by increasing hematocrit
from 40% to 60% on cable parameters is shown from a
single experiment in Fig 6. The results from 30
different perfusions in 15 hearts are summarized in the Table.
Hematocrit was varied in four steps from 60% to 40%, 10%, and 0%.
This corresponded to resistivities in the perfusates of 221, 143, 86,
and 74 /cm. For the measurements in absence of red blood cells, 10%
calf serum was added to the perfusate (see below). As shown in Fig
6
and the Table, changing the intravascular space resistance from
75 to
221 /cm did not influence the electrical parameters
significantly or produce a change in fiber diameter.
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It has been shown
in myocardial tissue and mesenterium that the
physiological perfusion with crystalloid solution
containing macromolecules for exertion of osmotic pressure requires a
minimal amount of albumin22 or albumin and
hemoglobin23 for normal capillary permeability and
prevention of osmotic swelling. In our preparation, in absence of red
blood cells, albumin (4 g/L) did not suffice to prevent rapid
accumulation of interstitial water. By contrast, normal
perfusion conditions were maintained with adding 10% calf serum to the
perfusate. The effect of adding 10% calf serum to the perfusate is
shown in Fig 7. In the absence of calf serum, the
perfusion with a solution devoid of red blood cells produced a rapid
and partially irreversible increase of fiber diameter (swelling) and a
marked decrease of ro to 20% of control. Accordingly,
there was a decrease of the extracellular wave front voltage from 60 to
30 mV and a transient increase of velocity from 45 to 75 cm/s. In the
presence of calf serum, neither ro, nor diameter,
nor Vo, nor changed significantly during
perfusion in absence of red blood cells. Closely corresponding results
were obtained in three other experiments.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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In the present study we describe the contribution of the extracellular resistance to impulse propagation and passive electrical properties of the myocardium in the isolated, arterially perfused rabbit papillary muscle. Our results show that myocardial extracellular electrical resistance, conduction velocity, and the magnitude of the extracellular electrical field are sensitive to changes in the volume of the interstitial space. By contrast, alterations in the electrical resistivity of the intravascular space do not affect the electrical parameters.
The use of the so-called core conductor model of the papillary muscle assumes that all the intracellular and extracellular spaces distributed within a muscle's cross section are assembled into single extracellular and intracellular compartments.12 13 In the presence of a gaseous atmosphere surrounding the muscle, electrical current flow during application of a subthreshold pulse or during excitation is confined to these two compartments, and no short circuit by a surrounding bulk solution is present. In such a case, both intracellular and extracellular longitudinal resistances can be obtained in a relatively simple way from the measurement of the amplitudes of extracellular and intracellular potentials during propagation and the extracellular voltage during subthreshold current flow (see "Methods").12 13 16 The electrical shunting by a subendocardial space can be neglected because the radial width of this space, as determined by laser confocal microscopy during perfusion, amounts to only 6 to 8 µm (J. Fleischhauer, A.G. Kléber, unpublished observation). One advantage of this method is that its validity can be tested in each individual experiment. It was validated previously for the measurement of electrical tissue properties during normal perfusion and during acute ischemia.13 16 The very high degree of linearity of subthreshold current flow and propagation during the various experimental interventions demonstrate its applicability in this study. The values obtained for both intracellular and extracellular resistances can provide information only on the average or macroscopic electrical properties of the respective tissue compartments, however. In the intracellular compartment, small local inhomogeneities in the extracellular electrogram, probably due to cell borders or borders between fiber bundles, have been described.24 25 In the extracellular compartment, the vascular network as well as the interstitial clefts between single cells and between bundles6 26 of cells are highly nonhomogeneous. This inhomogeneity may affect the relation between compartment resistances and conduction velocity in the presence of a very high COP of the perfusate, as discussed below.
The local circuit current, generated at the wave front of propagation by the difference in membrane potential between excited and resting cells and flowing through the intracellular and extracellular compartments, is a major determinant of propagation velocity. Moreover, the electrical field created by flow of local circuit current through the resistance of the extracellular space forms the basis of the ECG.27 This local circuit current during excitation is determined by the electrical membrane properties as well as the complex electrical impedance formed by both the intracellular and the extracellular compartments. Many reports at the level of cell pairs28 and whole tissue18 have characterized the electrical properties of gap junctions and of the intracellular compartment. However, experimental reports on the effects of the extracellular space resistance on impulse propagation and the genesis of the ECG are scarce. In the isolated, arterially perfused papillary muscle, the extracellular longitudinal resistance is of about equal magnitude to the intracellular longitudinal resistance.13 15 16 Therefore, the extracellular voltage across a homogeneous wave front of excitation is about 50% of the amplitude of the transmembrane action potential. Measurements of the wave front amplitude in whole heart indicate that such a high value for extracellular resistance is physiological.14
In the present study, we assessed the roles of the vascular and the
interstitial space on the extracellular resistance.
Information on the relative contributions of these subcompartments to
the extracellular resistance has not been reported previously.
Reduction of COP of the perfusate resulted in an increase in fiber
diameter that most likely was due to interstitial swelling.
Morphometric analysis of cardiac tissue suggests that the
extracellular compartment comprises 20% to 25% of total tissue
volume,3 4 and about 5% (References 5 and 7) is
interstitial space. In a study using confocal microscopy,
the intravascular space was 20% (inner vessel diameter) to 25% (outer
vessel diameter) of total tissue volume.29 Taking average
values of these data, the observed increase of papillary muscle
diameter of 12% (see Table) would correspond to an increase of
the
total interstitial space volume by about 2- to 2.4-fold. An
increase in intracellular volume concomitantly with the reduction in
COP and with interstitial swelling is unlikely. This is
because no changes in intracellular resistance (ri) were
observed. In contrast to the change in COP, a small alteration of
osmolality (by ±10% variation in extracellular [NaCl]) leads
to a
significant change in intracellular resistance, demonstrating that
ri is a sensitive indicator of changes in intracellular
volume (Yan Gan-Xin and A.G. Kléber, unpublished observation). As
expected, the increase of interstitial fluid was associated
with a marked decrease of the extracellular resistance,
ro, a concomitant increase in propagation velocity,
and a decrease of the extracellular wave front voltage. In the
theoretical case, this change in conduction velocity is indirectly
proportional to (ri+ro)1/2
during propagation along a linear cable.30 Comparison of
the calculated change of by 20% with the measured value of 23%
indicates that the effect of the decrease of ro is closely
predicted by linear cable theory.
In the case of the diminution of the extracellular space by an increase
in COP in the perfusate, quantitative application of linear cable
theory for the prediction of changes in velocity () from changes in
ro does not seem possible. Experimentally, increasing COP
to 90 mm Hg was followed by a marked increase of ro and
the extracellular field magnitude Vo but only a small,
nonsignificant decrease in fiber diameter and no change in . At
present, we have no straightforward explanation for the apparent
discrepancy between the increases of ro and Vo
and the unchanged at very high osmotic pressure. One possible
explanation could be related to the heterogeneous structure
of the extracellular space.5 A small shrinkage of the
interstitial space might first affect the very narrow
intercellular clefts. A reduction in volume of these very narrow clefts
might result in a extremely high cleft resistance, which would exclude
them from participation in local current flow and propagation. Only the
membrane parts adjacent to the larger clefts (with a larger volume and
consequently lower resistance) would take part in propagation. A very
similar explanation was given by Fozzard31 to explain
measurements of cell membrane capacity (in series with cleft
resistances) and propagation in Purkinje fibers. In all, our results
suggest that the changes in propagation velocity are not linearly
related to (ro+ro)1/2
(Reference 30) within the whole range of ro tested.
An unexpected and new finding was that a 3-fold increase in the
electrical resistance of the perfusate did not affect
ro, Vo, or . The vascular
space has been reported to occupy about 16% of cardiac volume in the
ventricle, or about 85% of the extracellular space.7
Consequently, a marked change in resistance of the major extracellular
compartment should have easily been detected. In the simplest possible
model of electrical current flow, the intravascular (rvasc)
and the interstitial (rint) resistances are
located in parallel
(1/ro=1/rint+1/rvasc). With about
equal interstitial and intravascular resistivities (70
/cm, absence of red blood cells) and for an intravascular to
interstitial space ratio of 5:1 (Reference 7), the
resistance of the intravascular space is about 5 times smaller than the
interstitial space resistance (see "Methods").
Increasing the intravascular space resistivity to 220 /cm
(Table)
would have changed the intravascular to interstitial
resistance ratio from 0.2 to 0.63 and would have increased
ro by 2.3-fold, that is, to an extent detectable by our
method. The discussion about the absence of this effect is speculative.
On one hand, an argument for a relatively small effect of hematocrit on
ro might reside in the different rheological behavior of
blood in the microcirculation versus the macrocirculation. In vessels
with a diameter of >0.5 mm, blood flows as a homogeneous
solution, whereas a dissociation of plasma from red blood cells occurs
in the microcirculation with a decrease of the effective viscosity
(Fahraeus-Lindquist effect).32 Accordingly, the change in
effective electrical resistivity of blood with hematocrit might be
smaller than anticipated from the measurements of blood resistivity in
vitro. On the other hand, an argument speaking in favor of an effect of
red blood cells on intravascular electrical resistance in capillaries
relates to the apposition of the red blood cells to capillary walls.
Since an average red blood cell is of larger diameter than an average
myocardial capillary, one might argue that the close contact of the
(deformed) red blood cell wall with the endothelium
during capillary passage might form an electrical seal to current flow.
The reason that the change in intravascular resistivity with hematocrit
was not at all reflected in a change of ro can be best
explained by an insulating effect of the capillary
endothelium, however. As an important finding,
extracellular electrical resistance, extracellular wave front voltage,
and conduction velocity remained constant even when blood cells were
completely removed from the perfusate. This indicates that the
endothelial layer of the microvasculature in
ventricular myocardium represents an
electrical seal to current flow during excitation. The high series
resistance provided by the endothelial barrier would
then mask the effect of the changes of hematocrit. Our results
therefore suggest that local current flow during excitation and
repolarization will be confined to the interstitial
space.
As shown previously in heart muscle and mesentery, prevention of interstitial swelling during arterial perfusion requires the presence of a minimal amount of albumin and/or red blood cells, in addition to a normal COP.22 23 Most likely, albumin determines vascular permeability by electrostatic apposition to subendothelial collagen, thereby sealing postulated "pores" in the microvasculature.22 In the present experiments, interstitial swelling occurred if erythrocytes were withdrawn from the perfusate even in the presence of 4 g bovine albumin per liter and a normal COP. Normal steady-state perfusion conditions in the absence of red blood cells (normal electrical parameters, no change in fiber diameter) were achieved by adding newborn calf serum to the perfusate, however. The explanation for this effect remains unknown.
An extrapolation of our results to the interpretation of the ECG in conditions of changed hematocrit or changed COP is complicated by the fact that changing blood composition appears to affect the ECG not only by modifying the intracardiac source but also by changing the complex resistances between the heart and the body surface. Thus, several clinical reports have shown that increasing hematocrit decreased QRS amplitude, and decreasing hematocrit had the opposite effect.33 34 This phenomenon, however, is explained by the so-called Brody effect,35 that is, by extracardiac resistances changing with hematocrit. A direct influence of blood protein concentration was demonstrated by Heaf,36 who showed a positive correlation between blood albumin concentration and QRS amplitude upon albumin infusion. This effect might well have been caused by the change of the intracardiac electrical field described in this study.
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Acknowledgments |
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This work was supported by the Swiss National Science Foundation and the Swiss Heart Foundation. We thank Dr S. Rohr for his help with some of the experiments and H.U. Schweizer, J. Burkhalter, and D. de Limoges for technical assistance.
Received December 20, 1994; revision received January 11, 1995; accepted January 16, 1995.
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References |
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TopAbstractIntroductionMethods Results Discussion References |
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