Microvascular response in cancellous bone to halothane-induced hypotension in pigs

Int. J. Oral Surg. 1983: 12: 178-185 (Key word' :flolV, microoascular; hypotension; »asoregulation; bone, cancellous; mandible; laser Dopplerflowmetry ;

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Microvascular response in cancellous bone to halothane-induced hypotension in pigs S. HELLEM, L. S. JACOBSSON AND G. E. NILSSON Department of Oral Surgery, University Hospital, Linkiiplng, Departments of Pharmacology and Biomedical Engineering, Linkiiping University, Linkoping, Sweden

ABSTRACT - The microvascular response in cancellous bone to halothaneinduced hypotension was studied simultaneously using Laser Doppler Flowmetry (LDF) and clearance of locally injected 133Xe in the subapical mandibular bone of 16 young pigs. During halothane inhalation, a significant reduction in mean blood pressure to values ranging from 22 to 60 mm Hg was observed. The blood flow, as measured with LDF, increased in more than 60% of the animals with hypotension levels above 30 mm Hg , At hyp otension levels below 30 mm Hg, blood flow decreased in all cases. The individual regional vascular resistance, calculated as the quotient between blood pressure and recorded blood flow value, was significantly reduced (p < 0.00 1) during hypotension. Rhythmical flow variations, observed in some animals at normotension, disappeared immediately following the start of halothane inhalation, but returned when blood pressure was stabilized at a lower level. For recordings made with 133Xe-clearance, a significant reduction (p<0.001) in the logarithmic decay of the washout curves was observed during hypotension. This logarithmic decay was correlated to the reduced blood pressure levels (r=0.73). No average change in regional vascular resistance could, however, be demonstrated. The lack of correlation between results obtained by the 2 methods may be due to the fact that LDF is sensitive to the total blood flow in a limited tissue volume, while the 133Xeisotope seems to be washed out mainly by the capillary blood flow. It can be further concluded that halothane-induced hypotension levels, applicable to clinical routines, may produce an increase rather than a decrease in total blood flow in cancellous bone tissue.

(Receivedfor publication 26 January, accepted 10 March 1983)

The effect of controlled hypotension for limitation of operative blood loss has been studied in several surgical disciplines1 • 3 , 1 8 - 2 1 , The amount of operative bleeding is expressed in terms of estimated

whole blood loss or measured as decreased erythrocyte volume using radio isotope techniques 18. Existing methods for clinical induction of hypotension are normovolemic and are

VASCULAR RESPONSE TO HALOTHANE thought to be dependent on vasodilatation, reduction in cardiac output and redistribution of blood flow. Many reports on halothane as a hypotensive agent deal with the general cardiovascular effects in man and laboratory animals. According to some investigators 5,8,13,17,23, the hypotensive effect of halothane concentrations<2% is almost exclusively due to a dose-related decrease in cardiac output with only minor changes in the total vascular resistance. In small subcutaneous arteries and veins in bat wings, however, halothane concentrations of 0.8-1.4% produced significant vasodilatation as shown by direct microscopy observations 12. Different regional vascular responses to halothane-induced hypotension have also been observed in some other tissues 5,13,14-,22,23, but not in bone. An important regulatory factor determining blood flow in peripheral tissue is autoregulation through a local activity within the vessel wa11 10. In bone tissue, vessels down to capillary size are supplied with fine sympathetic nerve fibres, indicating a neural contribution to the control of the microvascular blood flow". According to Poiseuille's Law, the blood flow through a vascular bed equals the blood pressure divided by the vascular resistance. Consequently, regional vasodilatation under hypotension may produce either a reduced, an unchanged or an enhanced microvascular flow. The purpose of this study was to evaluate the hemodynamic changes in cancellous bone tissue during halothane-induced hypotension using laser Doppler flowmetry (LDF) and the 133Xe-clearance technique.

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performed at the time when probe-holders for blood flow measurements were implanted. In series II (n = 9), the probe-holders wereimplanted 5weeksprior to the experiment. The investigation included blood gas analyses and blood pressure and flow recordings under normotension and hypotension.

Preparation The pigs were sedated with azaperon (Stresnile) i.m.~4 mgfkg b.w. Atropine was given s.c, in a standard dose of 1 mg and general anesthesia was induced by metomidate (Hypnodilw) ~ 10 mgJkg b.w. i.v, The anesthesiawas maintainedby injecting small amounts of metomidate intravenously, as needed. The pigs were intubated endotracheally and breathed room air spontaneously. A femoral artery was cannulated and connected to a pressure transducer (Grass P23Dc). The blood pressure was recorded continuously on a multichannel recorder (Grass Polygraph 7) and arterial blood samples were analysed for pH, pC0 2 , base excess (BE), Hb, pOz and oxygen saturation (SAT)in an acid-base analyser (ABLI, Radiometer). To gain entrance to the cancellous bone of the mandible with minimal damage to the tissue, a canal was drilled through each of 2 contralateral incisors. Probe-holders for blood flow measurements were screwed into thesecanalsand brought in contact with the subapical bone. In series II, the probe-holders were sealed with a piston during the 5-weekpost-implantation period. Laser Doppler flowmetry An early version of the Periflux" laser Doppler flowmeter was used in the study. The flowmeter probe was inserted through the probe-holder and brought in contact with the subapical cancellous bone. A portion of the monochromatic light beam from a 2 mW He-Ne-Iaser was scattered by circulating blood cellsand underwent a frequency shift according to the Doppler effect. With a differential detector system and a signal processing unit (bandwidth 4 kHz), a low-noise output signal, linearly related to low and moderate flow rates, was obtained for continuous recording of the blood flow value (BFV).

Material and methods

133 Xe-clearance Local injection of 133Xe into the subapical cancellous bone was performed with a microsyringe inserted through the probe-holder. Approximate-

Animals 16 young pigs were divided randomly into 2 series. In series I (n=7), the experiment was

* Periflux. Perimed KB, PO Box 5607, S-11486 Stockholm, Sweden.

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ly 10 jtCi of the isotope in 10 It! sterile saline was injected and the needle was held in place for 30 s. The probe-holder was then flushed with saline to remove possible residual radioactive tracer. A collimated NaI-crystal detector with a 2 x 3 em window was placed in contact with a defined area of the mandible and the signal was fed into a spectrometer connected to a ratemeter and a logarithmic recorder (NUCAB)*. Details concerning the experimental model and the technical devices have been presented in a previous publication1 .

Preliminary studies To obtain information on the microvascular response to vasoactive agents in cancellous bone, as measured with LDF, some preliminary studies were performed. In pigs prepared for measurement as described above, 10 IU of Iyprcssin (Vasopressin Sandoz's) were intravenously injected. In other pigs 22.5 JLg epinephrine in a 2% lidocaine solution (Xylocain@) were injected into the soft tissue adjacent to the subapical bone area. The microvascular response was recorded with LDF. Blood flow measurements under normotension and hypotension Identical measurement sequences were designed for both series I and II . For simultaneous blood flow recordings with LDF and t33Xe-clearance, the flowmeter probe was placed at random in one probe-holder and the micro syringe in the other. The stability of the flow level was checked by a short recording with LDF from both SUbapical bone areas before the onset of each experimental sequence. After completion of the clearance period, the same check was performed on the bone area where 133Xe had been injected . Each simultaneous flow recording period for LDF and 133Xe was 15 min. Hypotension was induced by supplyin g a respiration mixture of 4% halothane (Fluothanee) in oxygen. When a 50% reduction in mean blood pressure et systolic-t-j diastolic) was achieved, a steady state in blood pressure was maintained by successively decreasing halothane in the respiration mixture to 2% . The sequence of measurements, described above , was then repeated . After completion of measurements, the hypotensive gas mixture was discontinued and normalization of blood pressure and blood gases was awaited. Finally, the animals were killed with an overdose of anesthesia, and the BFV baseline • NUCAB AB, Sweden.

corresponding to arrested blood flow conditions was recorded.

Data presentation and statistical methods BFV is presented as flowmeter output signals (preliminary study) and as a % of initial values (main study). 133Xe-clearance values are presented as normalized absolute values with the highest value set to 100. All other data are expressed as mean±S.D. Vascular resistance was calculated from the quotient between mean blood pressure and flow value and referred to as the regional vascular resistance. For statistical evaluation, the Student z-test for unpaired and paired data was used . Statistically significant differences are denoted by : .'" = p < 0.01 and "'** = p < 0.001.

Results Preliminary studies

In the preliminary studies, the vascular response in cancellous bone to vasoactive agents was recorded continuously by LDF. Intravenous injection oflypressin resulted in an 80% elevation of mean blood pressure and a reduction in BFV to about 25% of the initial value within a few minutes (Fig. 1). Local injection of epinephrine reduced BFV significantly while mean blood pressure was virtually unaffected (Fig. 2). Blood flow measurements during normotension and hypotension Mean blood pressure, pH, peo 2 , base

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Fig. 1. Effect of an intravenous injection of 10 IU lypressin on blood pressure and blood flow value in cancellous bone of the mandible as measured with laser Doppler flowmetry.

VASCULAR RESPONSE TO HALOTHANE 200

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Fig. 2. Microvascular response to a local injection

of 22.5 fLg epinephrine into soft tissue adjacent to mandibular cancellous bone as measured with laser Doppler flowmetry.

excess, Hb, p02 and oxygen saturation, monitored in parallel with all blood flow recordings, are shown in Table 1. No statistically significant difference between data from series I and series II (unpaired data) was found. During hypotension, there was ap-

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proximately a 5-fold increase in arterial p02 and, consequently, an oxygen saturation, approaching 100%, explained by the high inspiratory 02-concentration. The hemoglobin concentration, however, was slightly decreased. A significant rise in arterial pC0 2, due to a slight hypoventilation, was found, accompanied by decreased pH. A decrease in base excess indicated accumulation of acidic metabolites. Changes in mean blood pressure, flow value, as measured with LDF and 133Xe-clearance and regional vascular resistance are shown in Table 2. In all animals, reduction in mean blood pressure to values ranging from 22 to 60 mm Hg was observed within 5-10 min following the start of halothane inhalation. The blood flow, measured with LDF, decreased when mean blood pressure was reduced to 30 rom or below (5 animals). At

Table 1. Mean blood pressure (MBP), pH, pC0 2, base excess (BE), Hb, p02 and oxygen saturation (SAT) at normotension and during hypotension; the values are expressed as mean±S.D. (n= 16) Parameter MBP pH pC0 2 BE Hb p0 2 SAT

Normotension (mm Hg)

(kPa) (mmol/l) (mmol/l)

(kPa) (%)

93±25 7.32±0.06 5.9710.61 -2.23±3.90 12.3 ± 1.0 11.9± 1.4 94.5±2.0

Hypotension 40± 15*** 7.19±0.09*** 7.89±0.09*** -6.35 ± 5.03** 11.2± 1.3*** 49.2± 11.6*** 99.5± 0.4***

Statistically significant changes during hypotension are denoted by: **=p
Relative changes (%)

Mean blood pressure Blood flow value (LDF) Blood flow value 33Xe-c1earance) Regional vascular resistance (LDF) Regional vascular resistance (13 3Xe-c1earance)

e

-55.9± 14.9*** 9.2±46.7 -43.5±31.9*** - 56.4± 13.0*** 58.0 ± 220.0

Statistically significant changes are denoted by ***=p
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HELLEM, JACOBSSON AND NILSSON

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Fig. 3. Relative changes (%) in blood flow values (laser Doppler flowmetry) in cancellous bone as a function of mean blood pressure during hypotension. 0 = pigs from series I and • = pigs from series II, dotted line = unchanged flow.

higher blood pressure levels (11 animals) BFV increased in 7 animals, decreased in 3 and remained unchanged in 1 animal (Fig. 3). Although BFV was generally higher in series II, no correlation was found between the relative vascular response to hypotension and the initial flow level. The individual regional vascular resistance, however, was significantly reduced during hypotension with LDF (Table 2). For recordings made with 1 33Xe_ clearance, a significant reduction in the logarithmic decay (first slope) of the washout curves was observed during hypotension. Average changes in the regional vascular resistance were not significant (Table 2). The correlation between logarithmic decay and reduced mean blood pressure is shown in Fig. 4. The linear regression line intersects the X-axis at a point corresponding to a mean blood pressure level of 13.7 mm Hg (p
Discussion In recent years, increased clinical experience has been gained concerning hypotensive anesthesia in bone surgery. However, little information is available on the microvascular response to hypotension in bone. In this study, the microcirculation in cancellous bone during halothane-induced hypotension has been studied using 2 different methods simultaneously. Laser Doppler flowmetry'" has recently been adapted for blood flow measurements in cancellous bone 7 . This method enables real time recording of blood flow in bone with good reproducibility from a limited tissue volume. The lack of a gold standard for quantitative determination of microvascular flow, however, has so far made true in vivo calibration of the method impossible. Clearance of locally-injected 133Xe has been suggested as reliable for the assessment of blood flow in bone tissue1\ but was estimated as uncorrelated to LDF under simultaneous flow recordings 7 • In preliminary studies with LDF, the microvascular response in cancellous bone to regionally or generally acting vasoconstrictors could be followed continuously, showing expected changes in blood flow.

VASCULAR RESPONSE TO HALOTHANE

Halothane caused a significant reduction in mean blood pressure to hypotensive levels similar to those noted by other investigators 5,14,22. Decreased cardiac output has been consistently reported as an effect of halothane inhalation and will not be further discussed in this paper. Increased arterial p02 in dogs, approximating 65 kPa, has been shown to cause only a minor rise in p02 in medullary bone", Moreover, no changes in vascular diameter in small subcutaneous vessels could be demonstrated following inhalation of 100% oxygen 12. The increased arterial p02 during hypotension probably had no influence on regional vascular resistance in our investigation. Carbon dioxide is known to act as a vasodilator '". Since pC0 2 was elevated during halothane-induced hypotension, the vasodilatory effect may well be added to that of halothane. Increased BFV in 50% of the animals during reduced blood pressure indicates that halothane-induced hypotension is associated with substantial vasodilatation in cancellous bone. Hypotension levels below 30 rom Hg always reduced flow, indicating that the flow-pressure relationship is out of the autoregulation range. At any higher hypotension level, the fall in blood pressure was balanced by lowering the regional vascular resistance and an increase as well as a decrease in BFV was recorded. Rhythmical flow variations, observed by LDF, were still persistent during hypotension and marked vasodilatation. This indicates that, although the vascular tonus may be significantly decreased, local vasoregulation is continuous. In contrast to the results obtained by LDF, blood pressure reduction and logarithmic decay of locally-injected 133Xe were found to be correlated. This observation suggests that the 133Xe-clearance method reveals blood flow in the minute vessels of the microvascular bed in bone, through

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which the flow is controlled by the pressure gradient rather than by active vasoregulation. The extrapolated zero decay rate at a low, but still non-zero blood pressure can be attributed to the non-Newtonian behaviour of capillary flow, resulting in arrested flow conduction even though a low pressure gradient along the vessel still persists. In the previously mentioned investigations on halothane-induced hypotension, different methods for blood flow measurements were used. Vasodilatation was associated with increased flow in skin and with decreased flow in intestine, kidney and cerebral white matter. In skeletal muscle, decreased flow and inconstant regional vascular resistance were reported. The divergent results may be explained not only by the specific vascular reaction in different tissues studied, but also by the use of varying methods, each having its specific sensitivity to different parameters associated with flow in the microvascular bed". The uncorrelated results obtained by the two different methods are in agreement with our previous findings 7 and can be explained by the fact that LDF is sensitive to the total blood flow in the tissue matrix, while the 133Xe isotope appears to be washed out mainly by the capillary flow. It can be further concluded that reduced blood pressure by halothane to hypotension levels applicable to clinical routines may produce an increased rather than a decreased total flow in cancellous bone. The capillary flow is suggested as being correlated with the existing pressure gradient, and an active vasoregulation has been demonstrated in this bone tissue. Acknowledgement - The authors would like to

thank Dr. D. Lewis (Head of the Clinical Research Center, University Hospital, Linkoping) for his critical review of the text and for making his laboratory facilities available for this study, Dr. B.-B. Dahlgren (Consultant, Department of Anesthesiology, University Hospital, Linkoping)

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for constructive discussions and valuable assistance and Mr. E. Leander (Linkoping University) for reviewing the statistical analysis. This study was supported by grants from the National Swedish Board for Technical Development (81-4048), the Swedish Medical Research Council project nos. 5219 and 04X-IOI, Ostergotlands Lans Landsting Medical Research Council project no. 32/79, the Swedish Research Defence Institute project no. H-564 and by Linkoping University.

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10. JORFELDT, L. & PERNOW, B.: Fysiologiska och patofysiologiska aspekter. In: PERNOW, B. (ed.): Perifer cirkulation. Kliniskt fysiologiska undersiikningsmetoder. Almqvist & Wiksell Forlag AB, Stockholm 1978, pp. 11-24. 11. LAHTINEN, T., ALHAVA, E. M., HAKUMAEKI, M., JAEAESKELABINEN, J., VAEAENABNEN, A. & KARJALAlNEN, P.: Blood flow in proximal femur of the dog determined by the local 133Xe injection method. Acta Physiol. Scand. 1979: 106: 437-440. 12. LONGNECKER, D. E. & HARRIS, P. D.: Dilatation of small arteries and veins in the bat during halothane anesthesia. Anesthesiology 1972: 37: 423-429. 13. MARCUS, CH. & KALFF, G.: Der heutige Stand der Kontrollierten Blutdrucksenkung. Anaesthesist 1977: 26: 212-219. 14. MILETICH, D . J., GIL, K. S. 1., ALBRECHT, R. F. & ZARED, B.: Intracerebral blood flow distribution during hypotensive anesthesia in the goat. Anesthesiology 1980: 53: 210-214. 15. MILETICH, D. J., IVANKOVICH, A. D., ALBRECHT, R. F., REIMANN, C. R., ROSENBERG, R. & MCKISSIC, E. D.: Absence of autoregulation of cerebral blood flow during halothane and enflurane anesthesia. Anesth, Analg , 1976: 55: 100-109. 16. NILSSON, G. E., TENLAND, T . & OEBERG, P. A.: Evaluation of a laser Doppler flowmeter for measurement of tissue blood flow. IEEE Trans. Biomed. Eng. 1980: 27: 597-604. 17. SAWYER, D. C., LUMB, W. V. & STONE, H. 1. : Cardio vascular effects of halothane, methoxyflurane, pentobarbital and thiamylal. J. Appl. Physiol. 1971 : 30: 36-43. 18. SCHABERG, S. J., KELLY, J. F., TERRY, B. C ., POSNER, M. A . & ANDERSON, E. F.: Blood loss and hypotensive anesthesia in oral-facial corrective surgery. J. Oral Surg. 1976: 34: 147156. 19. THOMPSON, G . E., MILLER, R. D., STEVENS, W. C . & MURRAY, W. R.: Hypotensive anesthesia for total hip arthroplasty: a study of blood loss and organ function (brain, heart, liver and kidney). Anesthesiology 1978: 48: 91-96. 20. WASHBURN, M . C. & HYER, R. 1. : Deliberate hypotension for elective major maxillofacial surgery: a balanced halothane and morphine technique. J. Maxillofac. Surg. 1982: 10: 5055. 21. WARD, C. F. , ALFERY, D. D., SAIDMAN, 1. J. & WALDMAN, J.: Deliberate hypotension in head and neck surgery. Head & Neck Surg. 1980: 2: 185-195 .

VASCULAR RESPONSE TO HALOTHANE 12.

L.: Blood circulation in the skeletal muscles and the skin of the cat under halothane anaesthesia. Acta Anaesthesiol. Scand. 1969: 13: 209-227. B. WYLER, F. & WEISSER, K.: Effect of halothane anaesthesia on distribution of cardiac output and organ blood flow in the rabbit. Br. J. Anaesth. 1972: 44: 551-556. WESTERMARK,

Address:

Solve Hellem Department of Oral Surgery University Hospital S-581 85 Linkiiping Sweden

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