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Anaesthesia and circulating blood volume

Sano, Y.*; Sakamoto, A.*; Oi, Y.; Ogawa, R.*

European Journal of Anaesthesiology: April 2005 - Volume 22 - Issue 4 - p 258–262
doi: 10.1017/S0265021505000438
Original Article
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Background and objective: The exact change in circulating blood volume (BV) during general anaesthesia is still unknown because there is no standard method of evaluating BV. We evaluated the changes in BV by general anaesthesia using simple and easy estimation methods.

Methods: Fourteen patients scheduled for minor surgery under general anaesthesia were enrolled. Propofol and vecuronium bromide were used for the induction of anaesthesia, and anaesthesia was maintained with sevoflurane and nitrous oxide. Haematocrit (Hct), total protein concentration (TP), as well as colloid osmotic pressure (COP) measured using a colloid osmometer, were determined before anaesthesia, and 30, 60 and 90 min after the induction of general anaesthesia. BV was calculated using Allen's formula and the changes in Hct, TP and COP. The estimated BV was compared with directly measured BV using indocyanine green dilution method (BVICG).

Results: Hct, TP and COP significantly decreased after the induction of anaesthesia (Hct: 42.1-39.4%; TP: 7.3-6.9 g dL−1; COP: 23-19 mmHg). The calculated BV as well as BVICG significantly increased after induction of anaesthesia (calculated by COP: 4.13-5.03 L; BVICG: 4.54-5.56 L). The change rate in BV calculated by the change of COP was larger than other calculated BVs, and was approximated to the change rate in BVICG. After emergence from anaesthesia, all values tended to return to baseline.

Conclusions: General anaesthesia increases BV. The value of BV calculated from the change in COP was most changeable.

*Nippon Medical School, Department of Anaesthesiology, Tokyo, Japan

Nihon University School of Dentistry, Department of Anaesthesiology, Tokyo, Japan

Correspondence to: Yuka Sano, Department of Anaesthesiology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo 113 8603, Japan. E-mail: yuka-ki@bd5.so-net.ne.jp; Tel: +81 3 3822 2131 ext 6748, 6367; Fax: +81 3 5685 3077

Accepted for publication September 2004

It is well known that general anaesthesia decreases blood pressure (BP) through vasodilatory as well as negative inotropic and chronotropic effects. Vasodilatation results in insufficient circulating blood volume (BV) to maintain BP (relative hypovolaemia). However, few reports have observed the actual changes in BV induced by general anaesthesia [1-3].

The standard method for measuring BV has been indicator dilution, using either a radioisotope or dye [4-8]. It is difficult to measure BV serially with these methods because these tracers are retained in the blood for days [9-11]. Furthermore, minor risks associated with the use of radioactive iodine and mutagenicity of Evans blue dye have been reported [12-16].

In 1956, Fox and colleagues [17] introduced indocyanine green (ICG) dye, which is now the pulse indicator dye of choice for determining BV. ICG has no known side-effects, other than a rare iodine-induced allergic reaction. However, this method requires a long interval between each measurement point, and cannot be used in patients with hepatic failure. Therefore, we used changes in haematocrit (Hct), total serum protein concentration (TP), and plasma colloid osmotic pressure (COP) in patients undergoing minor surgery to estimate the BV changes induced by general anaesthesia.

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Methods

Fourteen patients undergoing minor surgery were included in this study. All patients were ASA I, and had no history of cardiovascular, pulmonary, or neurological disorders, arteriosclerosis or allergies. No patients had taken medication during the 2 weeks prior to the study. The study protocol was approved by the Ethics Committee of the Nippon Medical School, and written informed consent was obtained from all patients the day before surgery.

All patients fasted from 9 o'clock the night before surgery, and no patient was premedicated or given any intravenous (i.v.) fluid before the induction of anaesthesia. When the patients arrived in the operating theatre, a catheter was placed into the dorsalis pedis artery because the upper body was draped during the procedure. Blood samples were obtained through this line during the study period. Before anaesthesia, systolic blood pressure (SBP), Hct, TP and COP were measured and defined as the baseline values (‘Awake’), and estimated BV value of each patient was calculated from the formula (BVAllen) of Allen and colleagues [18].

A peripheral i.v. cannula (18-G) was inserted under local anaesthesia, and a solution of 5% glucose was administered using an infusion pump at a rate of 2 mL kg−1 h−1 throughout the study period. The choice of a 5% glucose solution was made to prevent dehydration, while having less of an effect, and to affect on BV than colloid or electrolyte solutions. SBP was monitored via an automated non-invasive sphygmomanometer every 5 min. Other monitors included an electrocardiogram, pulse oxymeter and capnometer. Hct and TP were measured using standard techniques. COP was measured with a colloid osmometer (Wescor-4420; Wescor Inc, Logan, UT, USA). In brief, the operating principle of this colloid osmometer is based upon the movement of water molecules and diffusible solute particles through a synthetic semi-permeable membrane which has a diffusion cut-off of 30 000 Da the phenomenon known as transudation [19]. The membrane separates the specimen solution from a reference solution. After a sample is injected to the reference chamber, fluid moves through the membrane and into the sample chamber until the hydrostatic pressure reaches equilibrium. This pressure is measured by a piezoelectric pressure transducer.

General anaesthesia was induced in all patients with propofol (2.0 mg kg−1) and vecuronium bromide (0.15 mg kg−1) as a rapid i.v. bolus injection to facilitate endotracheal intubation using 5% glucose (5 mL) to wash through the induction agents. Intubation was performed about 5 min after induction, and an HME filter (heat-moisture exchanger; Nercore Hygroback S filter, Nercore Puritan Bennett Inc., Pleasanton, CA, USA) was connected to the endotracheal tube. The patient was mechanically ventilated with a tidal volume of 10-12 mL kg−1 at a respiratory rate of 8-10 breaths min−1 to maintain an end-tidal CO2 of 35-40 mmHg during the anaesthetic period. Anaesthesia was maintained with sevoflurane supplemented with nitrous oxide (67%) in oxygen. The sevoflurane concentration was controlled to maintain the SBP to within 20% change of the ‘Awake’ SBP. A urethral catheter was inserted immediately after induction for urine collection. The blood loss was calculated by weighing the surgical sponges every 30 min during the study period. SBP, Hct, TP and COP were measured 30, 60 and 90 min after the induction of anaesthesia (‘Anaesth-30’, ‘-60’ and ‘-90’, respectively). At the end of surgery, the sevoflurane was discontinued, and atropine (1.0 mg) and neostigmine (2.5 mg) were injected to reverse the effect of vecuronium. The patients were left undisturbed breathing supplemental oxygen (6L min−1) administered via face mask. The last measurement was performed 30 min after extubation (‘Recovery’).

BV at each sampling time was estimated from the baseline BVAllen and the subsequent changes of Hct, TP and COP, respectively. For example, BV estimated from the change in Hct at ‘Anaesth-30’ was calculated by the following formula:

Furthermore, BV measurement using the ICG dilution method (BVICG) was performed at ‘Awake’, ‘Anaesth-30’, ‘Anaesth-60’, ‘Anaesth-90’ and ‘Awake’ to compare with the calculated BV values mentioned above. A finger probe, which is connected to integrated pulse-spectrophotometry monitoring system (DDG 1001; Nihon Kohden, Tokyo, Japan) was applied to the left index finger to detect the blood concentration of ICG based on pulse spectrophotometry [20]. After the blood sampling in each time, 20 mg of ICG with 5% glucose (5 mL) was administered i.v.

All data are expressed as the mean ± standard deviation (SD) unless otherwise stated. Correlation between BV values calculated from Allen's formula and BV values estimated using BVICG was analysed by the Pearson correlative coefficient test. The agreement between two BV measurements was assessed by Bland-Altman analysis. A two-way analysis of variance (repeated-measures ANOVA) was performed comparing measured and calculated variables at each time point. A P value <0.05 was considered significant. When a significant difference was found, Tukey's multiple comparison test was performed to compare other values with the ‘Awake’ value.

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Results

Table 1 summarizes the clinical characteristics of the patients in this study. The significant difference arising from sex difference was seen with the mean BVAllen values at ‘Awake’ (F: M, 3.12: 5.49 L, respectively). Types of surgery included tympanoplasty (n = 3), sinus surgery (n = 6), parotidectomy (n = 4) and skin graft (n = 1).

Table 1

Table 1

The changes in SBP, Hct, TP, COP and BVICG during the study are summarized in Table 2. The significant difference arising from sex difference was seen only with the Hct values, however, the ratios of changes in Hct did not differ between female and male. The mean SBP was decreased at ‘Anaesth-30’, and remained stable thereafter. At the ‘Recovery’ time point, the SBP was recovered to the ‘Awake’ value. The Hct, TP, COP and BVICG were lower at ‘Anaesth-30’, and remained stable during anaesthesia. Only the COP was not returned to the ‘Awake’ value by the ‘Recovery’ time point.

Table 2

Table 2

The BVAllen and BVICG at ‘Awake’ in 14 patients had a significant correlation (r2 = 0.775; P < 0.01). Bland-Altman analysis resulted in a bias of −0.195 L with limits of agreement from −1.309 L to 1.061 L (Fig. 1). The BV changes by four kinds of methods during the study were shown in Figure 2. The mean increase rates of BVICG and BV estimated by COP were higher than those estimated by the other markers at Anaesth-30, Anaesth-60 and Anaesth-90.

Figure 1.

Figure 1.

Figure 2.

Figure 2.

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Discussion

Fluid is infused during surgery to correct dehydration due to preoperative fasting, transpiration, metabolism and blood loss to maintain the BV. Vasodilatation caused by general anaesthesia results in a relative decrease in BV and requires a larger infusion volume to maintain tissue perfusion. Determining the effect of general anaesthesia on BV is an essential component of perioperative fluid management [21]. However, no consensus exists as to how to measure the BV accurately intraoperatively.

Allen's equation has been used as a basic model for normovolaemic haemodilution, and many studies used this equation as an index of baseline blood volume [10,22]. In 1989, Hahn [11] gave similar results to measurement of the blood volume with the 131I-RISA technique in 10 patients scheduled for transurethral resection of prostate. In our study, BVICG in 14 patients was significantly correlated with that calculated from Allen's formula before induction of anaesthesia, although the confidence interval (CI) was wide which might reflect the small sample size.

The major finding of the present study is that the Hct, TP and COP decrease during general anaesthesia. Given the nature of the surgical procedures, there was probably little change in BV due to transpiration or blood loss. Furthermore, it is unlikely that appreciable numbers of erythrocytes or amounts of albumin, which determines the COP, leaked out of the intravascular space. Therefore, the continuous measurement of changes in Hct, TP and COP is an easy and reliable method for estimating the BV based on the dilution or concentration of the blood.

Plasma COP is an important determinate blood volume [23-25]. Furthermore, a low COP can cause pulmonary oedema [26]. Although changes in COP have been studied under different conditions, the changes induced by anaesthesia itself have not been studied. In this study, the COP decreased during anaesthesia. The mechanism responsible for this change is not well understood. However, Starling forces causing capillary exchange are thought to play an important role [18,27-29]. Red cell and colloid molecules are sufficiently large that they normally do not cross capillary membranes. Therefore, under normal conditions, most administered colloid remains in the intravascular space. The distribution of fluid throughout the body is dependent on the forces represented by the Starling equation:

in which Jv represents the rate of filtration of fluid across the capillary; K is the ultrafiltration coefficient (a measure of permeability); PMV is the hydrostatic pressure within the capillary; PT is the hydrostatic pressure in the interstitial space; d is the reflection coefficient representing the ability of a semipermeable membrane to prevent movement of a given solute; COPMV is the COP in the capillary; and COPT is the COP in the tissue.

The induction of anaesthesia causes a decrease in the arterial and capillary pressures. As a result, less fluid diffuses through the capillary membranes into the intravascular space, and the BV increases. Hct, TP and COP decreased during anaesthesia, but returned to baseline after anaesthesia. It is thought that the forces determining fluid exchange gradually recover to their pre-anaesthetic state. The BV value calculated from the COP increased more than that calculated from the Hct and TP. We are unable to account for this phenomenon, because we do not know what changes in the production and consumption of osmotically active colloids occurred during surgery, nor what the changes in erythrocyte size accrue as a result of osmotic changes. Further study is necessary to resolve these issues.

In conclusion, we found that the Hct, TP and COP all decreased during general anaesthesia in patients undergoing minor surgery and remained depressed throughout the anaesthesia period. Both BVs calculated by the above three kinds of materials and BVICG increased during general anaesthesia. The value of BV calculated from the change in COP was most changeable.

These results indicate that the absolute BV increases during general anaesthesia.

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Keywords:

GENERAL ANAESTHESIA; CIRCULATING BLOOD VOLUME; COLLOID OSMOTIC PRESSURE

© 2005 European Society of Anaesthesiology