EFFECT OF METHYLENE BLUE ON THE ABSORBANCE OF SOLUTIONS OF HAEMOGLOBIN

Transcription

1 Br. J. Anaesth. (1988), 60, EFFECT OF METHYLENE BLUE ON THE ABSORBANCE OF SOLUTIONS OF HAEMOGLOBIN E. S. GORMAN AND M. R. SHNIDER Methods for the in vitro and in vivo measurement of Po 2 and haemoglobin oxygen saturation have been available for some time. Although oximetry has evolved since Joseph Priestly discovered oxygen, the development of methods and equipment has accelerated only in the past years, culminating in the latest device, the pulse oximeter. The in vitro determination of haemoglobin oxygen content began in 1900 when Haldane developed a chemical method using ferricyanide. Van Slyke developed a manometric method in Photometric methods were introduced in the 1930s, utilizing the absorbance of specific bands of light wavelengths by haemoglobin solutions or blood. The specimens were transilluminated, a photocell served as a receptor and a galvanometer quantitated the resultant current. Reflection oximeters utilized similar principles, but the specimen was not transilluminated but merely illuminated, reflecting the non-absorbed light back to a photocell. In 1949, Brinkman and Zijlstra introduced the Neon Tube Haemoreflector based on this principle. It used a filtered neon light source which emitted light between 600 nm and 700 nm and a Kipp galvanometer to measure the voltage across a selenium barrier layer photocell. The "Cyclops" was also developed in 1949 by Brinkman and served as a non-invasive means of quantitating the in vivo haemoglobin oxygen saturation during operation, thus eliminating the need for blood sampling. The apparatus was mounted in the centre of the forehead, thus EUGENE S. GORMAN, M.D. ; MARC R. SHNIDER, M.D. ; Department of Anesthesia, Harvard Medical School and Beth Israel Hospital at the Charles A. Dana Research Foundation. Accepted for Publication: September 21, Correspondence to E.S.G., Beth Israel Hospital, Department of Anesthesia, 330 Brookline Avenue, Boston, MA 02215, U.S.A. SUMMARY The effect of the i. v. administration of methylene blue dye on haemoglobin oxygen saturation, measured by a pulse oximeter was investigated. The absorption spectra of various concentrations of haemoglobin, oxyhaemoglobin and methylene blue were compared. The amount of 660-nm light absorbed by methylene blue was sufficient to cause a factitious haemoglobin desaturation as measured by the pulse oximeter. acquiring its name. Its construction was slightly different from the haemoreflector because nonblood tissue would be absorbing light as well, necessitating a reference or control. The Cyclops utilized three lamps. Green light is almost completely absorbed by blood, so all reflected light was attributable to the presence of other tissue. Thus, a green light was alternated with a red light and their respective voltages compared, the difference deriving from the absorbance of the red light by blood. A third lamp was used as a compensation cell. The Cyclops was small, quantitative and a means by which many intraoperative events were documented for later study. In 1956, Mook and Zijlstra adapted a cyclopslike oximeter to cuvettes connected to a cardiac catheter. This was called the CC Oximeter (" catheter cuvette") (Kipp and Sons, the Netherlands) and produced virtually instantaneous readings during cardiac catheterization [1-3]. Transcutaneous oxygen determination followed and utilized a modified Clark electrode which measured the oxygen which dissolved from the capillary bed through the skin. This did not measure the haemoglobin oxygen saturation, but rather an index of the oxygen tension of the patient. While this has been of some use in the paediatric patient population, there have been many drawbacks to its use [4,5].

2 440 BRITISH JOURNAL OF ANAESTHESIA Pulse oximetry, the functioning of which is described in more detail in the discussion, has proven to be an invaluable asset for the management of a variety of patients during and after operation. It is reliable, accurate, non-invasive and applicable to both adults and children. In our experience, and that of others, oximetry has allowed detection of haemoglobin desaturation before that registered by other monitors or by changes in the vital signs [6]. It has been our practice to monitor haemoglobin saturation, using a pulse oximeter, during anaesthesia in patients at high risk for developing intraoperative hypoxaemia. Included in this group are patients with significant cardiopulmonary disease, those undergoing bronchoscopy or one-lung ventilation [7], and victims of severe trauma. While the monitor appears to be highly sensitive and specific to desaturation, false positive readings may occur. We report an example of such an occurrence. CASE REPORT A 65-yr-old woman with no pertinent medical history was admitted to the hospital via the emergency ward following a motor vehicle accident. Her ventilatory and haemodynamic status was satisfactory. A thorough evaluation was performed. Preoperative radiographic studies revealed fractures of the right leg, forearm and hand, and a defect in the right urinary collecting system which was believed to be located in the right ureter. No other abnormalities were evident. The patient was brought to the operating room for an exploratory laparotomy and concurrent reduction and fixation of several fractures. Monitoring for the procedure included an ECG, arterial pressure (direct), CVP, temperature, oesophageal stethoscope, and measurements of Fi 02 and endtidal carbon dioxide concentration. A Foley catheter was inserted and a pulse oximeter placed on the right index finger. Anesthesia was induced with ketamine 70 mg and maintained with fentanyl, 50 % nitrous oxide and isoflurane in oxygen. Vecuronium (total 10 mg) was given; neuromuscular blockade was also accomplished with pancuronium. An Ohio Modulus II ventilator was utilized. After the abdomen was opened, gross inspection failed to reveal the defect in the urinary collecting system. The surgical team then requested the i.v. administration of methylene blue to dye the patient's urine and thus locate the area of extravasation. Fifty milligram of 1 % methylene blue (5 ml) was chosen as the total dose. An initial 20 mg (2 ml) was administered as a single dose via a 14-gauge catheter placed in an antecubital vein while the patient was observed for adverse reactions; if none occurred, the remainder of the dye was to be given. Less than 1 min following this initial dose, the pulse oximeter began to display decreasing haemoglobin oxygen saturations from an initial 100% to a nadir of 1 %. This occurred over an interval of s then, just as rapidly, the value returned to 100%. A rapid and careful examination of the patient and equipment failed to reveal any defect in our ability to oxygenate the patient, nor any haemodynamic changes. A second decrease in displayed haemoglobin oxygen saturation followed several minutes later, this time to 65%. Subsequent doses of methylene blue yielded similar readings. The remainder of the procedure was uneventful. The patient's course revealed no evidence of intraoperative hypoxaemia. MATERIALS AND METHODS Because the hypothesis for the factitious haemoglobin desaturation was absorption of 660-nm light by methylene blue, the absorption spectra of haemoglobin, oxyhaemoglobin and methylene 2 0.5' FIG. 1. The absorbance spectra of haemoglobin 0.5 ing ml" 1 ( ), oxyhaemoglobin 0.5 mg ml" 1 ( ), and methylene blue mg ml" 1 (---) in bicarbonate buffer 0.05 mol litre" 1. Methylene blue has a significant absorbance at 660 nm, even in extremely small concentrations. Methylene blue may thus be identified as desaturated haemoglobin by the pulse oximeter.

3 I.V. DYE ALTERS MEASURED OXYGEN SATURATION 441 blue were determined (fig. 1). Haemoglobin solutions were chosen instead of whole blood because the effects of reflection, refraction and scattering caused by the non-haemoglobin components are eliminated in vivo by the pulse oximeter [8]. Dilutions of methylene blue were prepared containing between 1 and 10 ug ml" 1. The spectra were determined between 320 and 800 nm (fig. 2). The spectrum of methylene blue was very reproducible and has been documented many times in the past. A haemoglobin solution was prepared from fresh, whole human blood. The red cells were separated from 15 ml of blood by centrifugation at 2000 at 2 C for 10 min. The plasma was discarded and the red cells resuspended in 30 ml of saline and recentrifuged. This washing procedure was repeated four times. The packed red cells were then lysed in 75 ml of distilled water. In order to separate cell debris for removal, the lysate was centrifuged at at 2 C for 15 min. The haemoglobin concentration of this solution was determined by spectrophotometry as cyanmethaemoglobin at 540 nm and the solution used as the stock solution for the experiments. Desired" concentrations of haemoglobin were obtained by serial dilutions of the stock solution. Oxyhaemoglobin solutions were prepared by equilibrating the haemoglobin solutions with oxygen at 20 C for 30 min in an IL tonometer, model 237 (Instrumentation Laboratories Inc., Lexington, MA). Oxygen-free haemoglobin solutions were prepared by equilibrating the specimens with nitrogen for 60 min in the tonometer. We have found that such solutions prepared by equilibration with nitrogen have better spectral reproducibility than those obtained by chemical treatment using, for example, sodium hydrosulphate. The haemoglobin solutions were transferred anaerobically from the tonometer with a 10-ml glass syringe to a closed-system spectrophotometry cell. The spectrum of each specimen was determined three times with a dual beam Beckman DB-G spectrophotometer. Distilled water was used in the reference cell for all measurements. The coefficient of variation of the readings of the same specimen varied between 0 and at the various wavelengths and for the different specimens between and 0.063, indicating that the spectra of the solutions were reproducible lopgml" 1 8 Mgnil" 1 6 M9 ml " 1 4 M9 ml " 1 2 pgml" 1 1 ugmr 1 < FIG. 2. The absorbance spectra of various concentrations of methylene blue. The peak absorbance occurs at 660 nm.

4 442 BRITISH JOURNAL OF ANAESTHESIA Mblue 0.3 mgml" pgml' mg ml" pgml" FIG. 3. The absorbance spectrum of oxyhaemoglobin 0.3 mg ml" 1 (HbO 2 ) with various concentrations of methylene blue (MBlue). Note that the spectrum is essentially unchanged except in the nanometer region. Aliquots of oxyhaemoglobin 0.3 mg ml" 1 solution were mixed with various concentrations of methylene blue (0, 1.4, 2.8, 5.6, 7.0 ugml" 1 ) and the absorption spectra determined (fig. 3). A 6-ml aliquot containing oxyhaemoglobin 0.2 mg ml" 1 and methylene blue 2.8 ug ml" 1 was incubated at 37 C for 2 h in the presence of oxygen. It was then placed in a Spectrapor membrane tubing (mol.wt cut-off = 2000) and dialysed against three exchanges of distilled water 1 litre. The spectrum was then determined as above and compared with those of un-incubated oxyhaemoglobin 0.1, 0.2, 0.4 and 0.8 mgml- 1 (fig. 4). RESULTS The estimated peak methylene blue concentration delivered to the peripheral circulation of our patient was 2.8 ug ml" 1. This concentration of methylene blue absorbed significantly more 660 nm light than either haemoglobin or oxyhaemoglobin in vitro. From these experiments we estimate that sufficient concentrations of methylene blue were delivered to the upper extremity, where the pulse oximeter was situated, to cause a factitious desaturation measurement. It was also demonstrated that methylene blue in the concentration predicted in vivo does not alter the absorbance spectrum of haemoglobin the absorbance effects are merely additive. DISCUSSION Saturated and desaturated haemoglobin exhibit different characteristics of absorption of the visible and infra-red light spectra. This principle is utilized by the pulse oximeter to calculate haemoglobin oxygen saturation. The pulse oximeter has two monochromatic light emitting diodes (660 nm and 940 nm) and a transducer which measures the light transmitted through tissue. These are placed in opposition, commonly over a finger-tip as in our example, but may also

5 I.V. DYE ALTERS MEASURED OXYGEN SATURATION mgml" 1» 0.4 mg ml' mgml" 1 (Mblue 2.8pgm - 1 after 24 h of dialysis D 0.1mgmr 1 c.o FIG. 4. The absorbance spectra of oxyhaemoglobin 0.1, 0.2, 0.4 and 0.8 mg ml" 1. The 0.2-mg ml" 1 specimen was incubated with methylene blue 2.8 \ig ml" 1 for 2 h at 37 C then dialysed for 24 h to remove the methylene blue. The spectrum is coincident with that of oxyhaemoglobin 0.2 mg ml" 1 without incubation with methylene blue. be placed over the bridge of the nose or pinna to transilluminate the tissue. Only light received with periodicity, pulsatile blood flow for example, is used for computation, in order to eliminate the background effects of venous blood, tissue, etc. [9]. As can be appreciated from figure 1, oxyhaemoglobin has a lesser absorption of red (660 nm) light than has haemoglobin, and thus appears red to the eye. This differential transmission through the transilluminated tissue is used to compute the degree of oxygen saturation of the blood. This information, as well as the pulse rate, is displayed on the oximeter. Various alarm settings may also be set and utilized. Any substance that absorbs significant light of wavelength 660 or 940 nm may interfere with the accurate measurement of haemoglobin saturation by the pulse oximeter. Methylene blue absorbs significant light in the red (660) nm range. The concentration of methylene blue in the radial artery and hence the hand was calculated from studies performed by Hamilton and colleagues [10-12], who utilized the dye "Brilliant Vital Red" in quantitating cardiac output in man and animals. The profound effect of this concentration of methylene blue on oxy- and deoxyhaemoglobin solutions was demonstrated in our present study. A bolus of this dye may, therefore, be mistakenly identified as desaturated haemoglobin in arterial blood. This is the mechanism we believe to be responsible for the observed measurements in our patient. Other aetiologies of the apparent desaturation have been considered, but are improbable in our opinion. A true desaturation is unlikely to have occurred, because a thorough inspection of the patient and anaesthetic apparatus failed to reveal any change or defect in our ability to oxygenate the patient adequately, and the measured desaturations were closely and temporally related to the individual doses of methylene blue. In addition, the apparent desaturations occurred too rapidly, even if there had been a complete cessation of oxygen delivery to the lungs.

6 444 BRITISH JOURNAL OF ANAESTHESIA Another possibility considered was the conversion of oxyhaemoglobin to another species with a different absorbance spectrum, by the high local concentration of methylene blue in each bolus. We showed, however, that the addition of various concentrations of methylene blue to oxyhaemoglobin (fig. 3) does not alter its absorbance spectrum, but merely contributes its absorbance in an additive manner. As can be appreciated, extremely small concentrations of methylene blue absorb significantly in the 660-nm range. A final possibility is that the observed phenomenon was a result of signal pick-up error. This is very common in pulse oximetry, but doubtful in our case because of the temporal relationship of the boluses to the readings described above. The presence of a second-pass effect further supports our conclusions. Neither human nor animal experimentation has yet been undertaken to repeat these findings. It would be useful and interesting to know the effect of methylene blue and other medically useful dyes on pulse oximetry. Methylene blue is commonly utilized during surgery to locate portions of the urinary tract or to treat methaemoglobinemia, for example that resulting from prilocaine toxicity. It is also administered before surgery to patients undergoing thyroid surgery, to highlight the parathyroid glands. Its use demands an understanding of its effects on our monitoring systems. We strongly advocate continuous intraoperative pulse oximetry in certain populations during anaesthesia; various artefactual readings, however, should be investigated and recognized in order to improve our utilization of this mode of monitoring. REFERENCES 1. Payne JP, Hill DW. Oxygen Measurements in Biology and Medicine. London: Butterworth, Zijlstra WG. A Manual of Reflection Oximetry. Assen, The Netherlands: Van Gorcum, Ltd, Zijlstra WG, Mook GA. Medical Reflection Photometry. Assen, The Netherlands: Van Gorcum, Ltd, Tremper KK, Shoemaker WC. Transcutaneous oxygen monitoring of critically ill adults, with and without low flow shock. Critical Care Medicine 1981; 9: Tremper KK, Waxman K, Bowman R, Shoemaker WC. Continuous transcutaneous oxygen monitoring during respiratory failure, cardiac decompensation, cardiac arrest, and CPR. Critical Care Medicine 1980; 8: Mihm FG, Halperin BD. Noninvasive detection of profound arterial desaturations using a pulse oximetry device. Anesthesiology 1985; 62: Brodsky JB, Shulman MS, Swan M, Mark JB. Pulse oximetry during one-lung ventilation. Anesthesiology 1985; 63: Shimada Y, Yoshiya I, Oka N, Homaguri I. Effects of multiple scattering and peripheral circulation on arterial oxygen saturation measured with a pulse-type oximeter. Medical and Biological Engineering and Computing 1984; 22: 475-^ Yoshiya I, Shimada Y, Tanaka K. Spectrophotometric monitoring of arterial oxygen saturation in the fingertip. Medical and Biological Engineering and Computing 1980; 18: Hamilton WF. Some mechanisms in the regulation of circulation. American Journal of Physiology 1932; 102: Hamilton WF, Moore JW, Kinsman JM, Spurling RG. Simultaneous determination of the pulmonary and systemic circulation times in man and of afigurerelated to the cardiac output. American Journal of Physiology 1928; 84: Hamilton WF, Remington JW. Comparison of the time concentration curves in arterial blood of diffusible and non-diffusible substances when injected at a constant rate and when injected instantaneously. American Journal of Physiology 1947; 148: