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Cardiovascular Research 1997 34(3):504-514; doi:10.1016/S0008-6363(97)00062-X
© 1997 by European Society of Cardiology
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Copyright © 1997, European Society of Cardiology

Red cell flux during the cardiac cycle in the rabbit myocardial microcirculation

Gerald A. Klassena,b,*, Katherine D. Barclayb, Rene Wonga, Barry Patonc and Alan Y.K. Wongb

aDepartment of Medicine, Halifax, N.S., Canada
bDepartment of Physiology and Biophysics, Halifax, N.S., Canada
cDepartment of Physics, Dalhousie University, Halifax, N.S., Canada

* Corresponding author. Department of Medicine, Division of Cardiology, Queen Elizabeth II Health Sciences Centre, New Halifax Infirmary, Room #2111, 1796 Summer Street, Halifax, N.S. B3H 3A7, Canada. Tel.: +1 (902) 473-6815; fax: +1 (902) 473-2434.

Received 29 August 1996; accepted 6 February 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: (1) To measure regional phasic myocardial red cell flux during a cardiac cycle using a laser Doppler velocimeter. (2) To test the responses of regional red cell flux to a vasodilator (adenosine), a vasoconstrictor (angiotensin II), and an inotrope (isoproterenol). Methods: Using an anaesthetised open-chest rabbit with the pericardium intact a 140-µm-tip fibre optic probe was placed in the left ventricular myocardium in various locations. With the fibre in place drugs were given to alter myocardial loading conditions while red cell flux was registered. Results: Phasic red cell flux was similar in the epicardium to endocardium giving an average endo/epi ratio of 1.14 in the rabbit heart. At least two peaks of increased red cell flux within a single cardiac cycle were observed. Some unique patterns for red cell flux were observed in specialised myocardial structures. Adenosine increased red cell flux but minimally changed the pattern of phasic flux throughout the cycle. Conclusions: Laser Doppler velocimetry permits the recording of phasic red cell flux during the cardiac cycle in the myocardial microcirculation. Its pattern is determined by both coronary arterial inflow and venous outflow. The pattern of red cell flux may be characteristic for a region—probably determined by difference in tissue pressure (attributable to the pattern of muscle fibre shortening and collagen tethering) and changes in capillary length and density.

KEYWORDS Myocardial red cell flux; Laser Doppler velocimetry; Adenosine; Angiotensin II; Isoproterenol; Rabbit, anesthetized


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
How the coronary microcirculation is regulated to deliver oxygen from red cell to myocyte remains unknown. Does systole interfere with coronary blood flow in arterioles and capillaries as it does in the epicardial coronary artery? Determination of the role of systolic extravascular compression on the pattern of blood flow in the microcirculation during the cardiac cycle has been limited to measurements made in those regions of the myocardium where direct observations of these microvessels is possible [1–3]. This has limited such observations to the epicardium and recently the endocardium [4]. Such observations have been compromised by the motion of the heart which has necessitated the use of restraining devices to permit the focusing of optical devices. Such restraints may alter the pattern of regional blood flow.

Laser Doppler velocimetry is a method for measurement of particle velocity [5]. Using a monochromatic laser, the velocity of red cells can be measured by registering the Doppler shift in the frequency of reflected light. We have developed such a device [6] and have applied it to measure red cell flux [the aggregate root mean squared (RMS) voltage] in large vessels [7] where it can be calibrated using a flowmeter. More recently we applied its use to measuring tissue red cell flux in adipose tissue [8] and malignant tumors undergoing radiation [9]. In this study we applied the methodology to measuring the pattern of red cell flux in the microcirculation during the cardiac cycle in a limited number of sites in the myocardium of the rabbit. In addition, we administered drugs which alter the loading conditions of the left ventricle to determine whether such changes in vascular and extravascular pressures alter the pattern of regional red cell flux during systole and diastole.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
The preparation used was the anaesthetised open-chest rabbit. The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1985). Rabbits (n=20) were anaesthetised with pentobarbital, intubated and respired using a Bird respirator (Bird Corporation, Palm Springs, CA, USA). The chest was opened in the midline, and the ribs reflected exposing the pericardium. A small incision was made in the pericardium at the cardiac apex. Through this opening a 2-French solid-state catheter pressure transducer (Millar Instruments, Houston, TX, USA) was inserted into the left ventricular cavity. A second 2-French-catheter pressure transducer was inserted via the femoral artery and advanced so that it lay at the aortic root. The femoral vein was cannulated for systemic delivery of drugs.

2.1 Tissue blood flow
Tissue blood flow was measured using a laser Doppler velocimeter [10]. A 21-gauge intracath was used as an introducer so that the external plastic sheath could act as a guide for inserting a 140-µm-lensed fibreoptic probe. The needle was inserted through the apex into the myocardium so that it lay either deep or superficial within the myocardium, or in a specialized structure such as the septum or papillary muscle. In one instance the needle was inserted from the base in such a way that it lay within the apex of the myocardium. The inner metal needle was withdrawn when the appropriate site had been achieved. Usually no back bleeding into the sheath occurred unless the ventricular cavity or a major vessel was entered. A lensed (140 µm external diameter) fibreoptic probe sheathed in plastic except at its tip was inserted through the outer sheath of the intracath deep into the myocardium. A convex lens was created at the tip of the glass fibre using a microforge. Approximately 1 mm of the tip of the fibre was bare of plastic cladding. The approximate depth of the fibre could be recognized by the degree of transluminescence of the laser light through the wall of the myocardium. The plastic needle sheath was then withdrawn leaving the lensed fibre within the myocardium. It was held in place by the muscular contraction of the ventricle. If sufficient length of myocardium was not traversed by the fibre, it was ejected by the force of contraction. Because of the flexibility of the fibre and its low mass there was little tethering of myocardial action. Preliminary studies demonstrated that it was preferable that the pericardium remain intact if reproducible tissue flux patterns were to be achieved. Dose–response curves for adenosine, angiotensin II, and isoproterenol were performed. A dose which demonstrated a maximal haemodynamic effect was then selected. Adenosine in a dose of 1 mg/kg i.v. resulted in marked peripheral vasodilation with a fall in all pressures. Also, significant bradycardia occurred. Angiotensin II (0.5 µg/kg) caused significant hypertension and tachycardia. Isoproterenol (1 µg/kg) resulted in significant tachycardia and an outflow tract obstruction with LV systolic pressure greater than aortic pressure. These were the doses selected for comparison of effects on tissue red cell flux at a particular site. After completion of the measurements at one site, a second lensed fibre was inserted and the study repeated at the second site. With two fibres in place, and the study completed, the fibres were glued in situ with a cyanoacrylate glue, the fibres cleaved, and the heart removed en bloc. The cavity was distended with fibrous cotton and the heart preserved in formalin for 48 h. Following fixation transverse cross-sections were cut of the myocardium and the location of the fibre tip was determined. The site could usually be recognized by a very small area of haemorrhage. If the trauma to the myocardium was excessive in inserting the probe, bleeding occurred around the fibre tip: this would result in the loss of signal as no RBC movement would be present. In such instances the fibre was reinserted at a different site. As aortic pressure was recorded in 11 rabbits this report provides data on 20 sites in these rabbits. In 2 instances technical problems prevented adequate data from being recorded.

2.2 Measurement characteristics
The velocimeter used was of our own design and manufacture and uses the principle of the Doppler effect to determine the velocity of moving particles. The Doppler effect is caused by a shift in frequency of transmitted light reflected by the moving particles. Such frequency-shifted light is reflected back into the fibre. The Doppler shift depends on the velocity of the red cell as well as the cosine of the angle between the light scattering vector and the red cell velocity vector. Therefore only moving particles can impart a Doppler shift to the laser light [11]. In tissue, if transmitted light hits a stationary object, light will be reflected directly back to the receiver with only a minimal shift in frequency (the shift is caused by the difference in the refractive index between the fibre and tissue). By using a monochromatic laser (frequency=5x1014 Hz) to produce transmitted light with high spectral purity [5], small shifts in frequency (~3000 Hz) can be observed. A satisfactory positioning of the probe was accepted when a distinct consistent cardiac cycle pattern was observed which remained constant during control and persisted unchanged following an intervention.

Our velocimeter has a limited region of interest with a light divergence of approximately 12 degrees at the tip of the fibre. Thus the measurement represents the aggregate motion of all red cells in the field at an instant [5]. The resulting RMS voltage is defined as red cell flux because red cells constitute the majority of the particles which display a velocity vector in the field of observation of the optical fibre. The system is relatively insensitive to fibre motion artifacts. The velocity profile was linear up to 30 cm/s as tested in a flow reference system and this velocity was not exceeded during the study. Statistics were performed on the haemodynamic data using ANOVA and Student's t-test. Results were compared for statistical significance using the computer program SYSTAT 5.1 (SYSTAT, Inc., Evanston, IL). A P-value of less than 0.05 was used to indicate significance. Data were collected in digital format using a MacScope A-D converter (Thornton Associates, Waltham, MA). It is comprised of a 12-bit A to D converter and software. A sampling rate of 6.25 kHz was used with no filters. A Macintosh IIcx computer was used to program the converter and to store and analyze the data.

2.3 Measurements
2.3.1 Systolic/diastolic flow ratio
Because of the unique characteristics of the coronary circulation (i.e., repetitive contraction of the myocardium) our first consideration was to determine the percentage distribution of red cell flux (as determined from the integral of the RMS voltage over time) occurring in the microcirculation during systole and diastole. The cardiac cycle was divided into 4 time periods: (1) Isovolumetric systole was defined as starting when left ventricular systolic pressure began to rise following atrial contraction and ending when the aortic valve opened. (2) Systole was defined as that part of the cycle between aortic valve opening and aortic valve closure as measured from the aortic and LV pressure tracings. 3) Isovolumetric diastole was defined as that part of the cycle following aortic valve closure until the flex point where the left ventricular pressure trace ceased to fall and started to rise. 4) Diastole was defined from this point until diastolic pressure began to rise following atrial systole. These points were defined from the left ventricular and aortic pressure tracings. The area under the velocity curve was integrated and measured as millivolt-seconds, a measure of relative red cell flux during that part of the cycle.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
Satisfactory results were obtained from 20 sites in 11 rabbits. Fig. 1 is a diagram of the location of the fibre tip at each site.


Figure 1
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  		Fig. 1 Cross-sections of the left ventricle of the rabbit demonstrating 19 locations where red cell flux measurements were accomplished.

 
3.1 Regional patterns of tissue red cell flux during the cardiac cycle
3.1.1 Control
Fig. 2 is representative and displays 3 cardiac cycles of red cell flux recorded from 5 regions of the left ventricle: epicardium, endocardium (inner half), septum, papillary muscle and apex. One cycle has been divided into the 4 phases, isovolumetric systole (1), systole (2), isovolumetric diastole (3) and diastole (4). Inspection of the figure demonstrates some of the unique patterns observed. Epicardial RBC flux tended to be greater in systole than diastole while the opposite pertained to the endocardium. A relatively high RBC flux was observed in the papillary muscle while it was somewhat continuous at the apex except during phase 3. As the number of sites we were able to observe was limited, we aggregated the data for comparison purposes into superficial (6 sites in 4 rabbits), deep (10 sites in 8 rabbits), septum (3 sites in 3 rabbits) and apex (1 site). Table 1 displays the control data for total RBC flux as means±s.e.m. No significant differences in RBC flux were observed between these values with wide standard errors indicative of regional heterogeneity. The haemodynamic values for the entire group are displayed in Table 2.


Figure 2
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  		Fig. 2 Representative regional patterns of myocardial red cell flux. Left ventricular pressure (LVP, mmHg) and aortic pressure (AP, mmHg) and myocardial red cell flux (mV) for 5 regions of the left ventricle. Four levels of shading are used to describe the parts of the cardiac cycle (from left to right): black, isovolumetric systole; dark grey, systole; black stippled, isovolumetric diastole; light grey, diastole.

 

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  		Table 1 Total RBC flux for the 4 regions as millivolt-seconds

 

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  		Table 2 Haemodynamic data for control and peak effects of drugs

 
Table 3 displays the total red cell flux (millivolt-seconds) for each of the 4 phases as means±standard deviation with their coefficient of variance. At these heart rates and in the rabbit heart systolic flux predominated (P≤0.05) followed by diastolic flux and with isovolumetric flux being less and similar in systole and diastole. No statistical differences in regions were present. Table 4 displays RBC flux distribution, under control conditions, for the 4 phases of the cardiac cycle as a percent of total cycle flux so as to normalize the data.


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  		Table 3 Control, adenosine, angiotensin II and isoproterenol total RBC flux (mV) during each of the 4 phases of the cardiac cycle: isovolumetric systole (1), systole (2), isovolumetric diastole (3) and diastole (4)

 

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  		Table 4 Normalized (as percentage of total flow per cycle) rbc flux for the 4 phases of the cardiac cycle

 
3.2 Effect of drugs
3.2.1 Adenosine
Systemic administration of adenosine resulted in a significant fall in systolic and diastolic aortic pressure and a decrease in heart rate (Table 2). Fig. 3 presents the raw data for one rabbit (a typical response). Table 1 displaying total RBC flux demonstrates that at this dose adenosine significantly increased RBC flux in the myocardium, consistent with vasodilation. Examining flux during the 4 phases (Table 3) demonstrates that in the superficial layer flux increased significantly during systole and diastole while in the deep layer it increased during all 4 phases.


Figure 3
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  		Fig. 3 Effect of adenosine, 1 mg/kg i.v., on red cell flux in the myocardium (mid-zone). Control is on the left and intervention on the right. Shading of the components of the cardiac cycle as in Fig. 2.

 
The percent of RBC (Table 4) flux decreased during isovolumetric diastole and increased during diastole. Thus adenosine increased RBC flux in most regions of the heart and increased diastolic flux as is apparent in Fig. 3.

3.2.2 Angiotensin II (Ang II)
Systemic administration of Ang II resulted in a significant increase in all pressures and heart rate when compared to control (Table 2). This is illustrated in Fig. 4, the raw data from a single animal. No regional variations in total flux were recorded (Table 1) nor were fluxes different from control. Deep regional RBC flux was greater during isovolumetric diastole compared to control. No percentage differences in RBC flux was observed for the 4 phases when compared to control (Table 4). The pattern illustrated in Fig. 4 was usually observed at the onset of peak pressure.


Figure 4
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  		Fig. 4 Effect of angiotensin II, 0.5 µg/kg i.v., on red cell flux in the myocardium (endocardium). Control is on the left and intervention on the right. Shading of the components of the cardiac cycle as in Fig. 2.

 
3.2.3 Isoproterenol
Systemic administration of isoproterenol caused a tachycardia (Table 2) with a fall in aortic pressures so that an outflow-tract dynamic gradient appeared (Fig. 5). Total flux increased significantly during phase 1 in the superficial layer and during phase 2 (systole) in the deep layer (Table 3). Table 4, the percent distribution of flux to the phases, shows a significant decrease in isovolumetric diastolic flux. The example (Fig. 5) demonstrates these changes.


Figure 5
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  		Fig. 5 Effect of isoproterenol, 1 µg/kg intravenously, on red cell flux in the myocardium (epicardial). Control is on the left and intervention on the right. Shading of the components of the cardiac cycle as in Fig. 2.

 
3.3 Effect of contractility on tissue red cell flux (Fig. 6)
At the end of the study we recorded the effects of euthanizing the animal with barbiturate on the pattern of tissue flux. As aortic and LV pressure fell, the phasic diastolic component of RBC flux ceased at an aortic perfusion pressure of 14±4 mmHg (n=5). Below this pressure phasic systolic movement of erythrocytes was observed even though no cavitary pressure was generated. The latter phenomenon presumably relates to some localized continuing myocyte contraction. In addition, persistent atrial contractions may have caused erythrocyte movement. However, phasic diastolic myocardial tissue flux was no longer present once aortic perfusion pressure fell below 14±4 mmHg presumably because critical vessel closure had occurred.


Figure 6
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  		Fig. 6 Effect of a lethal dose of barbiturate on myocardial red cell flux (deep), aortic pressure (AP) and left ventricular pressure (LVP). Note that phasic diastolic flux ceases when aortic pressure falls below 15 mmHg.

 

   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
To our knowledge these are the first reported illustrations of red cell flux during the cardiac cycle within the myocardium. Many of the data should be considered as preliminary given the small volume of microvasculature observed by our probe and the number of observations we were able to perform. What we observed was that within the cardiac cycle red cells move in a phasic fashion with at least 2 major velocity peaks. Also, while total flux per cycle varied somewhat from region to region (spatial heterogeneity), some unique regional patterns may be present. A systematic investigation of regional red cell flux may reveal valuable information concerning regional myocardial oxygen demand because red cell flux should parallel regional myocardial oxygen utilization where oxygen extraction is maximal. The fact that the endo/epi ratio of total RBC flux was approximately 1.14 between deep and superficial regions is consistent with microsphere data [16]. The distribution of RBC flux during the cardiac cycle demonstrated a greater preponderance of systolic flow (66%) compared to diastole (33%). This may in part be a reflection of the uniqueness of the rabbit heart with an intact pericardium, the rapid heart rate and low systemic pressures which characterize this anaesthetised preparation. Unfortunately, the use of coronary arterial or venous flow probes was precluded as reference tracings by the size of the heart. Further studies in larger hearts should be helpful. The use of 3 drugs to alter coronary tone and haemodynamics demonstrated that the pattern of RBC flux during the cycle can be altered in a predictable fashion.

The mechanisms involved in how the coronary circulation achieves an adequate distribution of blood flow to all functioning myocardial cells remains largely unknown. Scaramucci [12] in 1687 following observations of the still beating heart proposed that during systole blood was squeezed out of the myocardium into the surface veins and then during diastole the space so emptied was replenished by arterial blood delivered by the coronary artery. This observation as reported by Porter [13] was consistent with the observations subsequently made by Gregg [14] using electromagnetic flow meters which demonstrated that phasic coronary flow was predominately diastolic in the coronary arteries and systolic in coronary veins. Kirk and Honig [15] measured intramyocardial pressure and demonstrated a transmural pressure gradient during systole, being greatest in the endocardium and lowest in the epicardium. From this they surmised an endocardial vulnerability for blood flow. Using radioactive microspheres Buckberg and Hoffman [16] showed that with normal haemodynamics transmural blood flow tended to be even across the myocardium and decreased in the endocardium when diastolic pressure fell below a critical level. The question remained as to whether intramyocardial pressure was equivalent to extravascular compressive pressure and whether or not segments of the coronary circulation were either squeezed or even collapsed during the cardiac cycle [17]. Recent observations on the structure of myocardial capillaries has suggested that collagen tethers attach the capillary or vessel wall to the myocyte [18] so that as deformation of the contracting myocyte occurs it actually holds the vessel open rather than squeezing it shut [19]. Direct observation of the endocardial surface vessels demonstrates that some changes in diameter of intramyocardial veins occur but collapse does not appear [4]. We have reported the diameter changes of epicardial surface veins [20] which change minimally between systole and diastole. Also, we have observed discrete areas of collapse of surface veins during diastole [20]. What has been missing has been a consideration of the patterns of tissue blood flow throughout the cardiac cycle within the contracting myocardium. The laser Doppler technique as developed in our laboratory has the potential to make such observations of phasic myocardial tissue RBC flux. Our observations are consistent with capillary flow being continuous with at least 2 peaks, one related to coronary inflow and a second to coronary venous outflow.

We observed that the LDV can measure the pattern of myocardial red cell flux continuously for several hours without substantial change. In this series of observations we addressed the issue of geographic differences in the pattern of tissue red cell flux. In previous studies we have demonstrated the degree of myocardial blood flow variation, spatially and temporally using microspheres [21] and mechanically using lead beads [22]. This study demonstrates that myocardial tissue red cell flux, as one might predict is both systolic and diastolic (i.e., the composite of arterial inflow and venous outflow). It is however somewhat more complex than this presumably because of regional differences in structure and function. The epicardium tended to have more systolic red cell flux than diastolic with lowest flux occurring during the isovolumetric part of the cardiac cycle. This is consistent with Tillmann's observations by direct microscopy of red cells in coronary surface vessels [1]. Deeper in the endocardium (lateral wall) a more predominantly diastolic flux pattern was observed—however, some systolic flow was always present similar to the observation of others (2). Specialized areas of the heart such as the papillary muscle appeared to have higher red cell flux with both systolic and diastolic peaks. This may relate to their unique mechanical function of anchoring the valve leaflets. Similarly, the upper septum demonstrated a unique pattern of decreased flux during late systole. This haemodynamic observation may relate to reversed coronary arterial systolic blood flow in the septal artery [23] and may be in part the consequence of the septum being stabilized as systole is completed. Certainty concerning the patterns of regional coronary blood flow awaits improvements in this technology which will permit a larger number of sites to be examined simultaneously with regional mechanics and registration of coronary artery inflow and coronary vein outflow.

4.1 Effect of drugs
4.1.1 Adenosine
Our observations concerning adenosine effects on coronary arterial and venous flow patterns and on pressure gradients are consistent with those we have previously reported [24, 25]. Despite a marked fall in coronary perfusion pressure both systolic and particularly diastolic flux increased. No increase in isovolumetric flux as percent of total flux probably reflects a lower aortic perfusion pressure and bradycardia which would relatively shorten this phase of the cycle.

4.1.2 Angiotensin II
The response to angiotensin II was somewhat variable. Measurements were made at the time of peak pressure when function was well preserved. The increase in RBC flux during isovolumetric diastole may reflect early heart failure as the relaxation phase of the cardiac cycle was prolonged. As one might predict, the combination of increased systemic pressure with no increase in coronary RBC flux resulted, over time, in acute heart failure as indicated by an increase in end-diastolic pressure and heart rate. In preliminary experiments when the pericardium had been removed the ventricle dilated and contraction ceased. With the pericardium intact there was less evidence of left ventricular dilation. Myocardial red cell flux was the last parameter to return to control values following the injection of this peptide.

4.1.3 Isoproterenol
As with angiotensin II, only minor changes in the pattern of myocardial tissue red cell flux were observed. With increases in heart rate, peak RBC velocities were greater. However, the greatest effect appeared to be a reduction in RBC flux late in systole when a gradient across the outflow tract was noted.

4.2 Critical closing pressure
There has been considerable speculation concerning what constitutes critical closing pressure in the coronary circulation. If critical closing pressure is defined as that pressure which, during diastole, is required to induce red cell movement, our data suggest a value of approximately 14±4 mmHg. It was at this pressure that we no longer observed any diastolic tissue RBC flux following euthanasia. This value is not very different from values proposed from extrapolation of pressure flow data to a 0-flow state [26].

4.3 Limitations of the methodology
Ours is not the first group to attempt to make such tissue measurements. Ahn et al. [27, 28] using a laser Doppler flowmeter demonstrated that their device accurately reflected coronary sinus blood flow in the pig heart on bypass, but that motion artifact excessively distorted the detail of their signal. They used 2 types of probes, one sutured to the epicardium and the second a needle probe. They also used a ‘blind probe’ which registered the movement of the fibre, which they estimated contributed 2% of the Doppler signal. Their methodology differed from ours in that they used a 2-fibre system, one for transmission of light and the second for receiving while our system uses a single fibre. This has permitted us to use a fibre of significantly smaller mass and flexibility which could be threaded into the myocardium. We presume that in our system the fibre tip moves in concert with the contracting myocytes registering movement of red cells rather than muscle contraction. In the rabbit heart in situ with pericardium intact it was not possible to simultaneously measure coronary arterial or venous blood flow using a flowmeter. Proof that the tissue Doppler device measured tissue vascular events would be that perturbing arterial inflow would result in a delayed RBC flux response in tissue equal to the transit time between artery and tissue. Our test for an artifact-free signal was a stable reproducible pattern which returned to control after an intervention and responses to drugs which were predictable. It should also be noted that we cannot analyse the spectrum of the Doppler shift as had been done by Kajiya et al. [29] to detect back flow. This is because he has available a 3-spectrum-peak system while with our system the spectrum is represented by a single broad peak. A 3-peak system would be ideal for measuring flow in vessels where flow is either all antegrade or retrograde. In tissue our system is superior as tissue flow of RBC will be directional (i.e., from arterial to venule) but because of the small size of microvessels a device which can examine the aggregate RBC Doppler shift provides the requisite information. As our system measures aggregate red cell velocity, if tissue haematocrit changes (Fahraeus effect), red cell flux and total blood flow may differ [30].

4.4 Summary
We have demonstrated using laser Doppler velocimetry that it is possible to measure regional phasic myocardial red cell flux. This flux occurs during both systole and diastole and represents red cell movement in the dominant coronary microvessels (i.e., capillaries and venules). Minimal transmural differences in this pattern were observed. Unique patterns of RBC flux were observed in the upper septum and papillary muscle. Drugs which changed loading conditions had only minor effects upon the pattern of flux in a region. These data suggest that regional tissue myocardial red cell flux patterns are mainly determined by structural and functional characteristics of the coronary circulation in a region. Dynamic changes in tissue haematocrit may influence our observations, in which instances red cell flux may not be equivalent to blood flow. The observations add to our understanding of how contracting myocytes interact with red cells during the cardiac cycle.

Time for primary review 21 days.


   Acknowledgements
 
The technical assistance of Maria Dorey and Jonathan Trites was much appreciated. Supported by a grant from the Heart and Stroke Foundation of Nova Scotia.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

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