Anesth Analg 2006;103:484-487
© 2006 International Anesthesia Research Society
doi: 10.1213/01.ane.0000226108.81213.a3
REGIONAL ANESTHESIA
Section Editor:
Terese T. Horlocker
Orthogonal Polarization Spectral Imaging of the Microcirculation During Acute Hypervolemic Hemodilution and Epidural Lidocaine Injection
Huub L.A. van den Oever, MD*,
Misa Dzoljic, MD, PhD*,
Can Ince, PhD
,
Markus W. Hollmann, MD, PhD*, and
Fleur C. Mokken, MD, PhD*
From the *Department of Anesthesiology; Department of Physiology; Academic Medical Center, University of Amsterdam, The Netherlands.
Address correspondence and reprint requests to Misa Dzoljic, MD, PhD, Academic Medical Center, University of Amsterdam, Department of Anesthesiology, P.O. Box 22700, 1100 DE Amsterdam, The Netherlands. Address e-mail to m.dzoljic{at}amc.uva.nl.
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Abstract
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We used Orthogonal Polarization Spectral Imaging to examine the microcirculation of the vaginal mucosa in nine anesthetized patients during two consecutive anesthetic interventions: hypervolemic hemodilution using hydroxyethyl starch followed by thoracic epidural lidocaine. Images taken before and after each intervention were compared. During hypervolemic hemodilution, systolic blood pressures increased significantly, but functional capillary density remained unchanged. Epidural anesthesia decreased systolic and diastolic blood pressures, but there was no change in capillary density, venular diameter, or flow velocity. We concluded that when using Orthogonal Polarization Spectral imaging, no consistent effects on the microcirculation of the vaginal wall can be detected.
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Introduction
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Hypervolemic hemodilution and thoracic epidural block are commonly used anesthetic techniques that both have profound effects on systemic hemodynamics, but the effects on the microcirculation are not well known. Orthogonal Polarization Spectral (OPS) imaging is a new method to assess the microcirculation (1,2). We have previously validated OPS imaging in humans by comparing it to conventional intravital microscopy at the nail bed (3) and introduced its use in anesthetized patients during neurosurgery (4,5).
Hypervolemic hemodilution has been shown to decrease vascular resistance in humans (6). Furthermore, infusion of hydroxyethyl starches (HES) has been associated with increased tissue Po2 in humans (7) and with enhanced capillary flow by intravital microscopy in rats (8). Therefore, OPS imaging was expected to show generalized dilation and increased density of microvessels during hypervolemic hemodilution with HES.
Thoracic epidurals have been associated with decreased sympathetic tone (9,10) with a negative influence on cardiac output (11,12) but enhanced blood flow to the blocked region (13). Blood flow to the vagina is regulated by thoracolumbar sympathetic nuclei via the hypogastric nerve (14). We therefore hypothesized that blocking the sympathetic nerves with low thoracic epidural lidocaine would promote blood flow to the pelvis, and that increases of vessel diameter, flow velocity, and capillary density of the vaginal mucosa would be visible by OPS imaging.
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METHODS
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After Ethical Board approval and informed consent, patients (ASA physical status I–II) scheduled for abdominal gynecological surgery under combined general and epidural anesthesia were included.
After premedication with lorazepam and IV cannulation, a multiorifice catheter was introduced 4–6 cm into the epidural space between T9 and T11 using loss-of-resistance with normal saline, under electrocardiogram, pulse oximetry, and noninvasive arterial blood pressure monitoring. No anesthetic was injected at this point. Anesthesia was induced with fentanyl 3 µg/kg, propofol 2 mg/kg, and rocuronium 0.6 mg/kg, and, after tracheal intubation, maintained with sevoflurane 1.0%–1.4% end-tidal in oxygen/air. Patients were ventilated to normocapnia and placed in lithotomy position, warmed by a hot air blanket.
An OPS camera (Cytoscan, Cytometrics, Philadelphia) (1,2) was placed on the distal one third of the vaginal wall to scan a section of the capillary network, containing at least one straight postcapillary venule (blood column width 5–50 µm), and fixed for the duration of the study.
Two consecutive interventions took place: 1) Acute hypervolemic hemodilution by IV infusion of 15 mL/kg of a warm 6% polyhydroxyethyl starch (HES) solution (eloHAES® Fresenius, Netherlands); and 2) epidural injection of 5 mL of 20 mg/mL lidocaine. Recordings of the microcirculation were made before and 10 min after each intervention.
For each measurement 3 5-s video files were randomly coded and analyzed using CapiScope software (KK Technology, Bridleways, UK), by an observer blinded as to the file order. Functional capillary density was measured as the total length of all capillaries containing moving erythrocytes (in mm) in one field of view (approximately 1 mm2). No distinction was made between rapid or sluggish movement. When one venule remained in focus, changes in diameter and flow velocity were determined. Finally, the coding key was applied, the triplicates averaged, and values before and after each intervention were analyzed with Wilcoxon’s test (two-tailed), with 
0.05.
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RESULTS
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Eleven patients, aged 27 to 58 yr, were included in the study. No vasopressor drugs were administered, and body temperature and end-tidal CO2 concentration remained unchanged during the study period. All patients had bilateral sensory blocks after postoperative epidural injection.
Typical OPS images of the vaginal mucosa are demonstrated in Figure 1. There were marked differences in microcirculatory patterns among patients, varying from dense capillary networks with abundant flow to networks containing well defined but hardly moving trains of erythrocytes. Arterioles were not observed. One patient had total stasis of flow in all capillaries and venules in all four quadrants of the vaginal wall. Nine patients were suitable for OPS analysis. One was excluded because of vaginal blood and one because pressure from the camera disturbed capillary flow.
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Figure 1. Orthogonal Polarization Spectral imaging of vaginal mucosa under general anesthesia. The same field of view is shown at baseline (A), after hypervolemic hemodilution (B), and after epidural anesthesia (C). Some vessels that were flowing at baseline (e.g., the capillary at the arrow in A) were no longer visible after hemodilution (B). However, another blood vessel appeared (projecting over a vein at the black arrow in B) that was not visible at baseline. Decreased optical density of the background and some of the vessels after hemodilution was observed in all patients. After epidural anesthesia (C) no flow was observed in the vessel at arrow in B. The black bar in C represents 100 µm.
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After hypervolemic hemodilution systolic blood pressures increased significantly from baseline (P = 0.032) (Fig. 2), whereas diastolic blood pressure and heart rate remained unchanged. On OPS imaging, all patients showed a decrease in the intensity of background scattering. In three cases an index venule could be followed throughout the intervention: diameter and flow velocity increased in two patients, but in one patient the flow velocity decreased, along with the disappearance of some capillaries, without any signs of erythrocyte aggregation. Functional capillary density increased in six, decreased in two, and remained the same in one patient (P = 0.327) (Fig. 2).
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Figure 2. Systolic and diastolic blood pressure (in mm Hg) and functional capillary density (in mm per field of view) before and after acute hypervolemic hemodilution. Error bars indicate mean and sd.
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After epidural lidocaine injection both systolic and diastolic blood pressures decreased (P = 0.003 and P = 0.01, respectively) (Fig. 3). Heart rate remained unchanged. In seven patients an index venule remained in focus throughout the intervention. There were no consistent changes in diameter and flow velocity of these venules. In some cases, new capillaries appeared, but in others smooth capillary flow stopped. Functional capillary density increased in four, decreased in four, and remained unchanged in one patient (Fig. 3), resulting in no overall effect.
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Figure 3. Systolic and diastolic blood pressure (in mm Hg) and functional capillary density (in mm per field of view) before and after epidural lidocaine. Error bars indicate mean and sd.
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DISCUSSION
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In the first part of this study, the macrodynamic and microcirculatory changes of hypervolemic hemodilution were examined. We estimated that acute infusion of 15 mg/kg of HES would increase the patients’ circulating intravascular volumes by approximately one quarter. In a previous study, this level of hypervolemia was associated with an increase in systolic blood pressure, stroke volume, and cardiac output, whereas heart rate and diastolic blood pressure remained the same (6). The increase in systolic blood pressure observed and the absence of changes in diastolic blood pressure and heart rate therefore confirmed our expectations.
The OPS images did not show any consistent changes in microcirculatory flow patterns after HES infusion, although previous studies had suggested increased capillary flow (8) and increased tissue Po2 (7). In some of our patients, microcirculatory flow even deteriorated. Acute dilation of the central venous compartment and right atrium may have provoked reflex inhibition of sympathetic output, with altered microcirculatory flow in some patients. However, no changes in heart rate were observed. Alternatively, HES molecules have been noted to catalyze the aggregation of erythrocytes (15), which would impair flow through small vessels. Though no such aggregates were seen with OPS imaging in our study, this mechanism requires further clarification.
Earlier publications focusing on the effect of epidural analgesia on the microcirculation have yielded mixed results (9,16), but in most studies microcirculatory variables improved (13,17,18). However, the present study failed to demonstrate any overall changes. To avoid the use of vasopressors during the microscopy, a relatively small volume of lidocaine was used. Likewise, because patients were anesthetized, the extent of the block at the time of study could not be tested. It is possible that the degree of pelvic sympathectomy varied among patients, which might account for the variable results of the OPS analysis. Although the expiratory concentrations of sevoflurane and CO2, the dose of fentanyl, and patient temperature and position (lithotomy) did not change during the study, the possible impact of these factors on microcirculation must also be considered. There are no studies available on the influence of general anesthesia on flow as seen by OPS imaging, but large changes in capillary perfusion have been observed, for instance, during hyperventilation (5), and infusion of small-dose nitroglycerin (19).
In summary, acute hypervolemic hemodilution and epidural analgesia produced consistent changes in systemic hemodynamics but not in the OPS images of the microcirculation. Until additional studies are performed that directly focus on the microcirculation under different conditions, extrapolation from systemic hemodynamics deserves caution.
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Footnotes
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Accepted for publication April 10, 2006.
Support was provided solely from institutional and/or departmental sources. None of the authors have financial interests in the company producing OPS technology.
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Abstract
Introduction
