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J Appl Physiol 99: 814-821, 2005. First published May 12, 2005; doi:10.1152/japplphysiol.00816.2003
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Subcutaneous PO2 as an index of the physiological limits for hemodilution in the rat

Frederick J. Pearce,1 Christine Waasdorp,2 Howard Hufnagel,2 David Burris,2 Joseph DeFeo,2 Peter Soballe,2 and William R. Drucker1,2

1Department of Resuscitative Medicine, Walter Reed Army Institute of Research, Silver Spring; and 2Department of Surgery, Uniformed Services University of the Health Sciences, Bethesda, Maryland

Submitted 1 August 2003 ; accepted in final form 9 May 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
This study was designed to test the hypothesis that changes in subcutaneous PO2 (PscO2) during progressive hemodilution will reliably predict a "critical point" at which tissue O2 consumption (O2) becomes dependent on O2 delivery (O2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315–375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 ± 1 to 6 ± 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced (P < 0.05) after exchange of 45% of the blood volume (Hct = 16 ± 1%). O2 became delivery dependent when O2 fell below 21 ml·min–1·kg body wt–1 (mean Hct = 13 ± 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3–, or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of O2 during hemodilution.

optode; oxygen delivery; oxygen consumption; hemodynamics; critical hematocrit; colloid

HEMODILUTION IS NOW WELL ACCEPTED as an adjunct to protection of circulatory volume (5, 30, 38, 39, 54). It has particular interest for the military, because supplies of whole blood during combat often cannot meet the need. In addition to logistics, the rationale underlying use of hemodilution is reduction or elimination of the multiple dangers associated with administration of homologous blood (38). The physiological responses to hemodilution have been well characterized through numerous clinical and laboratory studies over the past three decades (16, 17, 22, 27, 31, 32). Tolerance of hemodilution has many facets, including the patient’s clinical condition, cause and rate of blood loss, extent and duration of hemodilution, use of pharmacological agents, ventilatory support, provision of supplemental O2, and use of anesthetic agents (6, 18, 19, 28, 47, 56). One significant factor compromising tolerance for hemodilution in early studies was the associated reduction in circulatory volume, which accompanied the fluid exchange procedure (3, 36, 38).

Abundant experience has demonstrated that an acute reduction of hematocrit (Hct) to 20–30% in an otherwise healthy individual is a safe procedure that does not compromise hemodynamics. However, at Hct <20%, it is well recognized clinically that blood pressure needs to be supported, usually with colloid infusions (33–35). Furthermore, a universally applicable "critical Hct," below which O2 supply becomes insufficient for tissue needs during hemodilution, has not been identified (36, 61, 64). In part, this reflects the multiple factors affecting the balance between O2 delivery (O2) and O2 consumption (O2), including the magnitude of the cardiac output (CO) reserve, the efficiency of pulmonary gas exchange, and the metabolic rate. Under normal conditions, the rate of O2 to tissue greatly exceeds the rate of O2, which is independent of delivery until O2 falls below the O2 demands of the tissue (2). However, under pathophysiological conditions, such as hemorrhagic shock or hemodilution, the excess of supply over demand is diminished and the O2 reserve is depleted. If O2 and/or O2 could be measured easily, clinical treatment decisions involving the necessity for cardiac (e.g., volume or inotropic) support or augmentation of O2 content (e.g., transfusions or O2 administration) could be facilitated. As long as O2 exceeds the O2 demands of the organism, no intervention should be required. Because of the difficulty involved in direct measurement of O2 clinically and evaluation of the adequacy of O2 supply, surrogate measurements have been sought that would allow the critical point of O2 supply to be determined dynamically.

The need for a reliable and practical method to assess tissue oxygenation during hemodilution is clear, because extreme hemodilution is hazardous without dependable monitoring (24, 25, 29, 35). The advent of recent technology that utilizes minimally invasive serial measurements of subcutaneous PO2 (PscO2) has presented the possibility of detecting the onset of delivery-dependent O2 before the development of acidosis (7, 9). This study was based on the hypothesis that PscO2 would become significantly reduced before systemic O2 utilization became dependent on O2. To test this hypothesis, a new O2 probe was utilized to explore the possibility that serial measurements of PscO2 might be a prognostic and, perhaps, a more sensitive index of a critical reduction in O2 than changes in Hct, mixed venous O2 saturation, or systemic biochemical signs of hypoxia.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Animal preparation.   Male Sprague-Dawley rats (275–400 g) were anesthetized with pentobarbital sodium (35 mg/kg ip) and ketamine (60 mg/kg im). Polyethylene catheters (PE-50, Clay Adams, Piscataway, NJ) were aseptically introduced into both jugular veins. Anesthesia was maintained by a constant infusion of propofol into the left jugular vein at 1.08–1.10 mg·h–1·kg body wt–1 via an infusion pump (Harvard Apparatus, South Natick, MA). Rats were allowed to breathe spontaneously through a tube (PE-205, Clay Adams) inserted into the trachea through a tracheostomy that was left open to room air to avoid potential problems with upper airway congestion and to facilitate access for suction. CO was measured using a thermodilution method for small animals. The right jugular catheter was connected to a cooled liquid infuser for injection of 50 µl of 5°C saline for CO measurements using the Cardio Max system (Columbus Instruments, Columbus, OH). A thermistor probe (no. 1.5) was placed in the external carotid artery and advanced to the root of the ascending aorta for recording of thermodilution curves. CO was measured in duplicate at each sampling point and averaged. A tail artery catheter was connected to a pressure transducer and a computerized physiograph system (Buxco Electronics, Sharon, CT) for continuous monitoring of systolic blood pressure, diastolic blood pressure, mean arterial blood pressure (MABP), and heart rate (HR). The femoral artery catheter was used to obtain samples of blood. Cannulas were filled with saline containing heparin (1 U/ml). The cannulas were flushed with 0.2 ml of this saline after each withdrawal of blood or infusion of plasma or blood. The animals were placed on a thermostatically controlled heating pad, and core body temperature was continuously recorded with a rectal probe.

Tissue oximetry.   The system for obtaining measurements of PscO2 was developed by the Edwards Critical Care Division (formerly Inner-Space Medical) on the basis of the concepts and results of numerous studies primarily conducted by Hunt (4, 9, 14, 15, 20–22). The oximeter probe containing an integrated thermocouple (Inner-Space Medical, Irvine, CA) was passed through a 20-gauge Terumo introducer into a Silastic sheath filled with 0.9% saline (tonometer) and placed in the ventral (anterior) subcutaneous space between the abdominal muscle and skin of each rat for continuous monitoring of PscO2.

The principle of operation of the system is based on the O2-induced quenching of ruthenium red fluorescence. This optical probe is permanently bound within a silicone gel-sol matrix. Provision is also made to adjust the intensity of the excitation light to minimize the photobleaching effect. The dye matrix is connected to a calibrated light transmitter-receiver module by a small fiber-optic cable. The intensity of the fluorescence is inversely proportional to the PO2 in the silicone matrix at the tip of the catheter. A dedicated optical module and computer convert the intensity of the return analog optical signals to PO2 using the Stern-Volmer equation. PO2 readings were automatically corrected for probe temperature changes and displayed every 2 s (4, 9, 20).

Probes were calibrated before insertion by alternate exposure to saline equilibrated with room air and nitrogen. Calibration was also checked on completion of each experiment. The manufacturer claims that the measuring system has a linear response between 0 and 250 Torr, with an accuracy of ±5% (14, 21). Detailed laboratory studies by Drucker (8) and independent studies by Hopf et al. (20) confirmed these claims and demonstrated excellent congruence of results between the "gold-standard" Clark electrode and the optode. The sensitivity and response half times to changes in O2 concentration were identical (8, 20).

Measured parameters.   Baseline measurements included CO by thermodilution, PscO2, arterial blood temperature from the CO thermistor probe, subcutaneous temperature from the optode, and rectal temperature from a rectally placed thermistor. Physiograph readings of MABP and HR were monitored continuously, and arterial and venous blood samples (0.35 ml) were taken just before the start of each exchange for analysis of pH, HCO3–, arterial PO2, mixed venous PO2 (PO2), Hct, and electrolytes using the Stat Profile 2 Blood Gas and Electrolyte Analyzer (Nova Biomedical, Waltham, MA).

Exchange-transfusion procedure.   A significant criticism of early studies of hemodilution was failure to distinguish between the effects of inadequate maintenance of vascular volume and the effects of hemodilution (66). Preliminary attempts to create a stable hemodynamic state by replacing the withdrawn blood with a crystalloid solution proved inadequate. Other investigators using different experimental models, animals, and anesthetic agents experienced similar difficulty in stabilizing the circulatory volume when a crystalloid solution, saline, or Ringer solution was used to replace the blood lost (8, 12, 40, 41, 44, 55).

A total of 20 rats divided into two groups were studied. Controls (n = 8) had no hemodilution and received an equal volume of blood in exchange for the blood removed. Animals in the experimental group (n = 12) underwent hemodilution by exchange of 1.5 ml of pooled rat plasma for each 1 ml of blood removed. Pooled rat plasma, derived from whole blood containing heparin (5 U/ml), was used to replace the withdrawn blood. The first blood-plasma exchange was undertaken after baseline measurements were completed. A volume of blood equivalent to 25% of the estimated blood volume (7% of body weight) was removed via the femoral artery at a rate of 1 ml/min, whereas a volume of pooled rat plasma equal to 1.5 times the volume of blood removed was infused over the same time period into the jugular vein in the experimental group of animals (n = 12). After exchange 1, the volume of blood removed for all subsequent exchanges was 10% of the blood volume. Control animals had a 1:1 volume replacement of pooled rat blood containing heparin (n = 8).

After each exchange, the animals were observed until MABP and PscO2 were stable. Stability was defined as no variation in MABP >5 mmHg or in PscO2>2 Torr over a period of ≥5 min. A 0.35-ml aliquot of blood was taken for biochemical analyses from the arterial and venous cannulas at the start of each exchange, and all physiological measurements were repeated. The first 0.35 ml of arterial and venous blood taken from the animal at the start of the exchange was used for blood analysis and counted as part of the blood volume removed for the exchange (i.e., 0.7 ml). After Hct fell below the predetermined level of 10%, usually after seven to nine exchanges, a final exchange was completed. The animal was then killed by an infusion of propofol followed by 1 ml of 30% KCl.

Derived parameters.   O2 was calculated according to the Fick equation

where O2 and CO were normalized to body weight (ml·min–1·kg body wt–1) and CaO2 and CO2 represent arterial and mixed venous O2 content (ml O2/100 ml blood), respectively.

CaO2 and CO2 were calculated as follows



where HbO2 is Hb O2 content, [Hb] is Hb concentration, TPR is total peripheral resistance, and OEF is O2 extraction fraction.

Data analysis.   The data were analyzed using Quattro Pro, Sigma Plot, and Sigma Stat. Statistical significance was assessed using repeated-measures analysis of variance. Statistical tests yielding P < 0.05 were considered significant. Where differences from the control level within a group were to be determined, a test of within-subject contrast, Dunnett’s t-test with multiple comparisons with a single control, was used (67). The baseline value was used as the control reference point for comparison in the latter tests. Values are means ± SE.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Hct fell promptly from 42 ± 1 to 28 ± 1% after withdrawal of 25% of the estimated circulatory volume with simultaneous infusion of pooled rat plasma at 1.5 ml/min. Hct continued to fall after each successive blood-plasma exchange in which the volume of blood removed was equal to 10% of the estimated circulatory volume. Control animals, subjected to similar hemorrhages with 1:1 replacement of the blood lost with donor rat blood, experienced no significant alterations in Hct over eight exchanges (Fig. 1). Hct decreased significantly for each blood-plasma exchange compared with baseline or with control animals at the same time intervals (P < 0.01).



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  		Fig. 1. Changes in hematocrit (Hct) during progressive hemodilution. {blacksquare}, Hct from control animals exchange transfused with whole blood (1:1); {circ}, Hct from animals exchange transfused with plasma (1:1.5); dashed line, Hct predicted from a closed-system, single-compartment dilution model; solid line, measured Hct. Values are means ± SE. *Significantly different from baseline (P < 0.05).

 
To determine the effectiveness of the experimental procedure to maintain circulatory volume during the course of the study, the expected degree of hemodilution was calculated after each exchange on the basis of a single-compartment, closed-system model. If all the volume infused stayed in the vascular compartment, the change in Hct can be calculated if the volume and Hct of the blood removed and the volume of plasma infused are known. This assumes that there are no other inputs or losses of red blood cells into or out of the vascular compartment. Figure 1 shows the comparison between the measured Hct and the hemodilution predicted from the closed-system, single-compartment model and the excellent agreement between observed and predicted changes. The congruence of these curves indicates that the transfused plasma remained within the circulatory volume throughout the course of the study. Because there was no loss of volume from the vascular compartment, this exchange-transfusion method caused a 50% increase in circulatory volume after nine exchanges of plasma for blood.

The hemodynamic status of the animals was significantly affected by the progressive hemodilution. Hemodilution produced a decline in the calculated TPR that became significant (P < 0.05) when Hct reached 24% (Fig. 2A). MABP remained unchanged until Hct fell to 20% after exchange of 45% of the circulatory blood volume for plasma. Further hemodilution resulted in a large decrease in MABP to 47 ± 3 mmHg after eight exchanges. Systolic pressure fell only modestly from 129 ± 8 to 115 ± 9 mmHg over the range of Hct values associated with O2-independent O2, whereas diastolic pressure decreased linearly with Hct from 95 ± 7 to 63 ± 7 mmHg over the same range (i.e., change in Hct from 42 to 13%). Systolic and diastolic pressures fell further to 85 ± 20 and 41 ± 8 mmHg, respectively, over the range of Hct associated with O2-dependent O2. Arterial PO2 was well maintained over the whole range of hemodilution, starting at 80 ± 3 Torr [arterial O2 saturation (SaO2) = 89 ± 1%] and ending at 118 ± 8 Torr (SaO2 = 96 ± 1%) after nine exchanges.



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  		Fig. 2. Changes in hemodynamic and metabolic variables and subcutaneous PO2 (PscO2) with progressive hemodilution in rats. Mean arterial blood pressure (MABP, {bullet}) and total peripheral resistance [TPR, expressed in peripheral resistance units (PRU), {blacksquare}], (A) O2 extraction fraction (OEF, {blacksquare}) and cardiac output ({triangleup}) (B), base deficit ({blacksquare}) and systemic O2 consumption (O2, {circ}) (C), and mixed venous PO2 (PO2, {blacksquare}) and PscO2({circ}) (D) are plotted against Hct. Values are means ± SE. BW, body weight. *Significantly different from baseline (P < 0.05).

 
CO rose with progressive hemodilution but was not statistically significant until CO had risen by 40% and Hct had fallen to 20% (Fig. 2B). The increase in CO was not maintained, however, and declined with exchanges 7 and 8, where Hct fell below 10%. OEF rose from 0.37 ± 0.03 to a maximum of 0.77 ± 0.06 at Hct of 8%. This rise became statistically significant (P < 0.05) after exchange 2, when 35% of the blood had been removed and the mean Hct had been reduced to 24% (Fig. 2B). The data show that the compensatory response to hemodilution to an Hct of ~30% was nearly totally accounted for by increases in CO, whereas OEF changed little. At Hct <30%, increased OEF played the major role in buffering the changes in systemic O2. Control animals showed no significant changes in MABP, CO, or TPR during eight blood-blood exchanges (data not shown).

Figure 2, C and D, illustrates the progressive fall in systemic O2 that occurred with this hemodilution protocol and the corresponding alterations in base deficit, PO2, and PscO2. O2 was significantly reduced (P < 0.05) from a control value of 15.9 ± 1.3 to 14.0 ± 1.0 ml·min–1·kg body wt–1 when Hct reached 13%. Study of the relation between changes in O2, base deficit, and PscO2 demonstrated that significant alterations from baseline values in base deficit and PscO2 developed at the same point, i.e., when mean Hct fell to 16%. These changes developed before a significant reduction in systemic O2, which did not occur until mean Hct was reduced to 13% (Fig. 2C). Figure 2D shows the relation of PO2, a commonly used clinical guide, and PscO2 to Hct. Baseline PO2 and PscO2 were 41 ± 1 and 70 ± 6 Torr, respectively, and both gradually declined with increase in hemodilution. However, PO2 dropped significantly to 36 ± 1 Torr by the time Hct reached 24% at exchange 2, whereas a significant drop in PscO2 to 57 ± 7 Torr was not seen until exchange 4.

Because delivery-limited O2 should be accompanied by evidence of cellular disruption, plasma electrolytes were also measured. Measurements of plasma Na+ revealed no changes during the progressive increase in hemodilution: 143.8 ± 0.9 and 144.9 ± 1.0 mM at baseline and after exchange 9, respectively. In contrast, plasma K+ fell during the early phase of hemodilution in control and hemodiluted animals from a control level of 4.3 ± 0.1 mM to a nadir of 3.7 ± 0.1 mM after exchange 2. As hemodilution was continued beyond this point, there was a progressive, but modest, rise of plasma K+, which reached a final mean level of 4.8 ± 0.1 mM, which was significantly above the prehemodilution level (P < 0.05; data not shown). No significant changes in O2, CO, or OEF were observed in the control group of animals.

Figure 3 shows the relation of systemic O2 to systemic O2. The mean baseline O2 and O2 were 43.5 ± 2.0 and 15.9 ± 1.3 ml·min–1·kg body wt–1, respectively. O2 did not become delivery limited until O2 fell below 21.4 ml·min–1·kg body wt–1 at Hct of 13%. This inflection point was determined by iterative fitting of a linear regression line to the mean data, progressing from the lowest O2 to the highest. Examination of the residual plots showed significant deviation from the linear fit at O2 >20 ml·min–1·kg body wt–1. The equations for the fit were as follows: O2 = 0.64(O2) + 1.1 (R = 0.996, P < 0.01) for the lowest five points (Hct = 6–13%) and O2 = 0.02(O2) + 14.4 (R = 0.365) for the highest five points (Hct = 16–42%). The slope of the latter equation was not significantly different from zero. Solving these two equations for their point of intersection yielded the transition point at O2 of 21.4 ml·min–1·kg body wt–1 and O2 of 14.9 ml·min–1·kg body wt–1.



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  		Fig. 3. Relation of systemic O2 to systemic O2 delivery (O2) in anesthetized rat during progressive hemodilution. Two least squares regression lines were fit to mean data for Hct ≤14% and Hct >14%. Point of intersection for the 2 separate fits was chosen from plots of residuals of the regression line fit iteratively over the increasing range of O2. Point of intersection of these 2 lines was calculated to occur at O2 = 21.4 ml·min–1·kg body wt–1 and O2 = 14.9 ml·min–1·kg body wt–1. This point fell between measured Hct of 13 and 16%. Linear interpolation between measured Hct and O2 measurements on either side of the point of intersection yielded a value for Hct at the critical O2 point of 14%. Hct values at each point are shown at the top of the frame. Values are means ± SE. *Significantly different from control O2 (P < 0.05).

 
Because PO2 has been proposed as a viable end point for determining the adequacy of systemic O2 supply in the clinical setting, the PscO2-PO2 relation was also examined. Figure 4 shows that PscO2 fell steeply once PO2 reached 35 Torr. Delivery-dependent O2 was associated with PO2 <26 Torr and PscO2<42 Torr.



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  		Fig. 4. Relation of PscO2 to PO2 during progressive hemodilution. Solid line represents fit to the mean data using a 3rd-order polynomial. Mean Hct values corresponding to each point in decreasing order are 42, 29, 24, 20, 16, 13, 10, 8, and 7%. Values are means ± SE. Significantly different from control (P < 0.05): *PscO2; +PO2.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The goal of this study was to test the hypothesis that serial measurements of PscO2 could provide adequate warning of an impending systemic mismatch between O2 supply and demand as reflected by the onset of O2-limited O2. Measurements of arterial lactic acid, pH, PCO2, HCO3, and PO2 are widely used clinically as systemic indicators of tissue hypoxia. However, these measurements require repeated blood sampling and cannot be made with sufficient frequency to allow timely identification of the onset of metabolic acidosis or to allow these measures to be used as end points for therapy. The method and rationale for using PscO2 as an index of tissue oxygenation is the culmination of a long series of studies in humans and animals primarily conducted by Hunt (4, 14, 21, 22, 26, 43). The subcutaneous implantation site, which encompasses skin and underlying muscle, was selected for analysis of peripheral tissue oxygenation, because it consumes a constant and relatively small amount of O2 and because skin and skeletal muscle microcirculatory beds contain the bulk of the resistance vessels that respond in a compensatory manner to vascular volume changes as part of a coordinated set of homeostatic mechanisms that act to maintain systemic blood pressure and preserve blood flow to the heart and brain. The vasoactivity of the resistance vessels of skeletal muscle and skin, the tissues in contact with the tonometer, represent a large proportion of the compensatory responses, primarily because of their mass, which represents ~60% of the total body weight. Because these vascular beds are recruited early in defense of central blood volume and arterial blood pressure, it has been presumed that normalization of perfusion and O2 in these beds in resuscitation settings would imply that all other tissue would also be adequately perfused, barring isolated vascular abnormalities (3, 22). This situation results in local PO2 changes that principally reflect changes in O2 (i.e., changes in arterial flow or O2 content) (15, 40, 61). This provides the rationale for continuous tracking of tissue PO2 as a measure of the systemic balance between O2 and O2 in the adjacent tissues.

Studies by Hunt et al. (4, 14, 21, 22, 26, 43) utilized a polarographic Clark-type electrode. The O2 sensor, termed an optode, used in the present study represents an improvement over the earlier polarographic techniques, because much less O2 is consumed in the measurement process and the method has a greater signal-to-noise ratio at low PO2. This optode system utilizes the natural phenomenon of O2-induced fluorescence quenching, in which a fluorescent ruthenium compound, contained at the tip of the optode, emits light of a known wavelength when excited by light of a second wavelength. This quenching phenomenon results in an emitted fluorescence signal that is inversely proportional to PO2.

The optode has been used extensively in laboratory and clinical studies in >500 patients, some extending over several days, without ill effects. Clinical application, however, requires close attention to minimize stimulation of the sympathetic nervous system (8, 18–21, 45, 46). Detailed laboratory studies by Drucker et al. (9) and independent studies by Hopf et al. (20, 21) demonstrated excellent congruence of results between the gold-standard Clark electrode used in the initial studies of tissue PO2 and the optode. Identical results for the Clark electrode and the optode were obtained when the decline of PO2 from 150 to 0 Torr was tracked in a stirred solution containing glucose oxidase-catalase to remove all O2 from the solution. Similar results were obtained at solution temperatures of 24°C and 35°C. Mean half times for the response to O2 were identical for the two sensor modalities studied without the Silastic sheath at 0.5 Torr PO2/s. When the same assessment was done with the optode placed within the Silastic sheath, the half times of the response increased from ~22 s to 2.5 min at 35°C. The significantly slower half time of the response compared with without the sheath was apparently due to the diffusion lag caused by addition of the Silastic sheath and the saline contained within the sheath. These studies suggest, therefore, that equilibrium with the surrounding muscle and subcutaneous tissues would essentially be attained in less than four half times or <10 min.

To properly view the PscO2 measurement within a systemic context with hemodilution, O2 and O2 were measured to discover the "critical point" at which O2 became O2 dependent in the rats. The critical O2 of 21.4 ml·min–1·kg body wt–1 is in excellent agreement with 23 ml·min–1·kg body wt–1 reported by Adams et al. (1) for rats under somewhat different experimental conditions. This point was determined in both studies from the intersection of the two best-fit regression lines determined by a least sum of squares technique over the O2-independent and -dependent regions of the relation (50) (Fig. 3). Baseline O2 measurements agreed well with our study, with Adams et al. reporting 17.9 ± 1.3 ml·min–1·kg body wt–1 compared with 14.9 ± 1.3 ml·min–1·kg body wt–1 in the present study. The agreement is particularly striking, because pentobarbital anesthesia was used in the study of Adams et al. and a much wider dynamic range of O2 (5–85 ml·min–1·kg body wt–1) was obtained by changing the fraction of inspired O2, rather than Hb levels. These results indicate that the critical transition point for delivery-dependent O2 does not depend on the means used to alter O2 content in rats. In larger animals, the critical point for O2 is generally reported to be 8–10 ml·min–1·kg–1 in dogs (42), sheep (10), and pigs (57). Lieberman et al. (34) defined the point of critical O2 for unanesthetized humans to be <7.3 ml O2·min–1·kg body wt–1, whereas Shibutani et al. (51) found a similar critical level for O2 (8.2 ml O2·min–1·kg body wt–1) in anesthetized humans. On the basis of their study of a critically ill 84-yr-old Jehovah’s Witness who tolerated a chronic Hb level as low as 4 g/dl (37), van Woerkens et al. (64) set the critical limit for O2 at 184 ml·min–1·m–2 (4.9 ml·min–1·kg body wt–1). During acute normovolemic hemodilution with 5% albumin in children undergoing scoliosis surgery, Fontana et al. (11) were able to reduce O2 to 7.4 ml O2·min–1·kg body wt–1 at mean Hct of 9% (Hb = 3 g/dl) without evidence of systemic acidosis or complications. Patients were maintained in normocarbia on mechanical ventilation and 100% O2. These studies show remarkable agreement for the critical point of O2, when O2 becomes dependent on O2 given the large variety of study conditions in animals and humans. These results suggest that a systemic O2 threshold would be a good alternative for a therapeutic guideline if CO, Hct, and SaO2 could be continuously and reliably assessed. Biochemical markers of ischemia, such as lactate or base deficit, are less desirable if one is attempting to avoid metabolic acidosis, because the point of O2 inadequacy must be reached before the biochemical marker changes.

The key question that prompted this study was answered clearly: PscO2 declined significantly before progressive hemodilution reduced O2 sufficiently to make O2 dependent on it. Serial measurements of PscO2 showed a significant decline only after 55% of the estimated blood volume had been exchanged. At this point, mean Hct was reduced to 16.4 ± 0.6%. A significant fall in O2 utilization was not observed, however, until mean Hct fell to 13 ± 0.5% when 65% of the estimated circulatory volume had been exchanged (Fig. 2C). The base deficit, a measure of the extent of the onset of metabolic acidosis, increased simultaneously with the fall in PscO2 but before any significant change in systemic O2. Because plasma lactate concentration was not measured, it is not clear whether the early changes in base deficit were due to the onset of tissue ischemia and increased H+ production or the large diminution of H+ buffering by Hb in the face of normal, baseline rates of H+ production. Although PscO2 did not show greater sensitivity to decreasing O2 compared with monitoring the base deficit, there was a clear advantage with the optode, in that the measurement was made continuously, with online results updated every 2 s, and did not require repeated blood sampling. With care taken to protect against extraneous factors that affect the recordings, measurements of PscO2 proved to be a dependable and sensitive technique for assessing physiological adaptation to the stress of incremental hemodilution in this model.

The decline of MABP in this model when Hct fell below 25% requires examination, because a significant fall in perfusion pressure and flow will also contribute to limitation of O2 at the tissue level. Although surprising, our finding that MABP progressively fell when Hct dropped below 24% is consistent with observations in other animal species and humans. In humans (11, 40, 64) and animals (2, 48, 63), MABP rarely falls until Hct drops below 20–25%. Because MABP is normally a regulated variable, it would be expected that the normal homeostatic response would result in an increase in peripheral resistance and/or CO to maintain MABP, the driving force for organ blood flow. Because it is well known that CO during exercise is capable of increasing by as much as fivefold, the small 40% increase observed in the present study may reflect an intrinsic limitation of cardiac function. The adverse influence of hypovolemia on tolerance for hemodilution is well documented (3, 30, 48) and is a complication in studies that do not accomplish maintenance of the vascular volume in the exchange-transfusion process, because CO may be preload limited. It is unlikely that the increase in CO was limited by inadequate circulatory volume and preload in the present study, because the exchange of plasma was 1.5 times the volume of blood removed. Furthermore, the data in Fig. 1 verify that this entire volume remained in the vascular compartment, because the observed dilution closely matched the dilution predicted by a closed-system, one-compartment model. Prior studies in humans and animals that electively used hypervolemia to protect circulatory volume during presurgical hemodilution found no indication that it was deleterious (33, 53, 59, 65).

Other factors causing depression of CO have been observed to diminish the tolerance for hemodilution. Isoflurane, an anesthetic agent known to promote vasodilatation, causes a depression of CO and O2 and an earlier fall in MAP during extreme hemodilution (49). Increased depth of anesthesia, with halothane or ketamine, also reduces the tolerance for hemodilution in rats (62). Also, dobutamine exaggerates the drop in MABP and systemic vascular resistance found in rats during hemodilution after the Hct reached 20% (52). Although it is well known that hyperkalemia causes cardiac depression, it is unlikely that the relatively small rise in plasma K+ observed in this study (to 4.8 mM) could have significantly influenced myocardial performance. Because the eight control animals had no significant reduction in blood pressure throughout the course of eight exchanges of blood for blood in equal volumes, it is unlikely that the anesthetic agent or the technique of exchange was detrimental to hemodynamic stability. If there were some agent harmful to blood pressure in the infused rat plasma, it would be reasonable to expect that a similar effect would be manifest when pooled rat whole blood was used in the control animals. This did not occur.

Although CO increased by 40% as Hct declined from 41 to 20% in the present study, MABP fell from a mean of 110 to 88 mmHg. TPR at this point fell to ~50% of its initial value, and diastolic pressure was 40 mmHg. At Hct <20%, CO showed no further increase and actually returned toward baseline levels at Hct <10%, whereas TPR continued to fall, reaching 40% of control values. Therefore, because diastolic pressure and time are major determinants of coronary flow, the large decrease in diastolic pressure observed with hemodilution may have also played a role in decreasing coronary O2 to the point of limiting function.

With these considerations in mind, it is reasonable to ascribe the fall in MABP to an inadequate increase in CO as compensation for the incremental fall in peripheral resistance (64) that resulted primarily from the large changes in blood viscosity with hemodilution. This possibility is consistent with the results of Tsai et al. (60), who showed that extreme hemodilution is accompanied by a decrease in functional capillary density. Their results indicate that modest increases in blood viscosity alone with very-high-molecular-weight (i.e. 500,000) dextran improved functional capillary density, tissue blood flow, and PO2 without adding any additional O2-carrying capacity. This may allow a lower O2 content to be tolerated, shifting the critical Hct to even lower values (60). This hypothesis has yet to be tested experimentally but provides an intriguing alternative to artificial Hb solutions. These studies underscore the important role of local viscosity factors in potentially limiting the TPR impact on blood pressure homeostasis. If MABP had been maintained by a greater CO response or an increase in plasma viscosity, it would be expected that the flow would have been higher, O2 would have been higher for a given degree of hemodilution, and, therefore, PscO2 may not have fallen to such an extent. If the diastolic pressure was better maintained, cardiac function may not have become a limiting factor and a lower Hct would have been tolerated. However, maintenance of MABP and, perhaps, a greater CO would probably not have affected the critical point for flow-dependent O2 but would have lowered the Hct at which it was observed.

The need to discover impending onset of dependency of O2 on O2 remains an important challenge for the application of hemodilution to a variety of clinical conditions. However, progressive hemodilution without supplemental O2 or respiratory support eventually impairs circulatory dynamics, and the resulting fall in blood pressure further compromises tissue oxygenation induced by hemodilution. Because of the concern of producing hypoxia during restoration of circulatory volume with erythrocyte-free solutions, the use of hemodilution is restricted to Hct of 20–30% (13, 58). However, successful case reports of extreme hemodilution indicate a wide margin between current practice and more extensive, but still safe, hemodilution (11, 17). This disparity could be reduced by accurate and continuous information about tissue oxygenation during hemodilution (34). More use of hemodilution and an improved cost-to-benefit ratio are realistic potential outcomes if a reliable physiologically relevant end point can be developed to prevent the onset of dangerous tissue hypoxia as O2-carrying capacity is reduced (36). The present study indicates that subcutaneous O2 monitoring shows excellent potential for this application, exhibiting a significant fall just before the fall in systemic O2.

In summary, this study determined that the critical point at which O2 became limiting to O2 was 21.4 ml·min–1·kg–1 for the rat under the hypervolemic hemodilution conditions of this model. Furthermore, the transition to O2-dependent O2 was associated with O2 <21 ml·min–1·kg body wt–1, Hct <14%, PO2 <28 Torr, mixed venous O2 saturation <50%, CaO2 <7 vol%, OEF >0.65, and PscO2<50 Torr. Continuous monitoring of PscO2 can provide a minimally invasive and reliable guide for evaluation of the adequacy of systemic O2 during intentional hypervolemic or normovolemic hemodilution and may have potential as an end point for resuscitation from shock as well.


   GRANTS
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This study was supported by a grant from the US Navy Research and Development Command under Uniformed Services University of the Health Sciences Grant G1-90DV.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
Dr. David Cruess contributed able dedicated assistance for statistical analyses. Three first-year medical students, ENS David Murray, 2LT Michael Nelson, and 2LT Romney Anderson, assisted with the pilot studies. ENS Jay Krishnan contributed editorial assistance.

The results were presented in abstract form at the Federation of American Societies for Experimental Biology Meeting in April 1995.

Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation and/or publication. The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting true views of the Department of the Army, the Uniformed Services University of the Health Sciences, or the Department of Defense.


   FOOTNOTES
 

Address for reprint requests and other correspondence: F. J. Pearce, Walter Reed Army Institute of Research, Dept. of Resuscitative Medicine, Rm. 1A34, 503 Robert Grant Ave., Silver Spring, MD 20910 (e-mail: frederick.pearce{at}na.amedd.army.mil)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 RESULTS
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 ACKNOWLEDGMENTS
 REFERENCES
 

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