Oxygen saturation versus oxygen tension

Oxygen saturation versus oxygen tension A case report illustrating their relative values W. E. Martin, M.D., F. W. Cheney, Jr., M.D., D. H. Dillard, M.D., C. Johnson, Ph.D., and K. C. Wong, M.D., Ph.D., Seattle, Wash.

The measurement of arterial blood gases has markedly improved the intra- and postoperative management of cardiac surgery patients, but the intermittent sampling techniques widely practiced may on rare occasions lead to serious errors in patient management. In the case reported here, changing from intermittent sampling of arterial oxygen tension (Paa,) to continuous measurement radically of arterial oxygen saturation (Sao~) altered both the management of the patient and our interpretation of his condition.

Case report Cardiac catheterization of Patient S. L. at 2 days of age revealed pulmonic stenosis and a right-to-left shunt through an atrial septal defect. By the time he was 4 months old, clinical deterioration (including episodes of circulatory arrest) required total repair of the defect under deep hypothermia.' The intraoperative course (Fig. I) was unusual in that a metabolic acidosis developed and ventricular fibrillation occurred before we were ready From the Departments of Anesthesiology, Surgery, and Bioengineering and the Anesthesia Research Center, University of Washington School of Medicine, Seattle, Wash. 98195. Supported in part by U. S. Public Health Service Grant GM 15991-04, National Institute of General Medical Sciences, National Institute of Health, Bethesda, Md. Received for publication Sept. 1, 1972. Address for reprints: Wayne Martin, M.D., Department of Anesthesiology RN-10, University of Washington School of Medicine, Seattle, Wash. 98195.

to arrest the heart. A sharp pulmonic commissurotomy was done. The patent foramen ovale was left to serve as a decompression vent in case the small right ventricle was inadequate. Thirty minutes after successful cardiac resuscitation, the right ventricle became cyanotic and stiff, and cardiac arrest ensued. After 30 minutes of massage, cardiopulmonary bypass was instituted. The condition of the myocardium improved. Bypass was discontinued 45 minutes later, and the operation was successfully concluded. The right atrial pressure was higher than the left atrial pressure, although little shunt was apparent (Fig. 1). Unfortunately, by the time his condition had stabilized in the Intensive Care Unit, the Pas, had deteriorated (Table I). Over the next 22 hours many respiratory maneuvers were attempted in an effort to restore the Pas, to more viable levels. These included spontaneous ventilation with 100 per cent oxygen, 100 per cent oxygen on a Bird ventilator with both assisted and controIIed respirations, 100 per cent oxygen on a Bennett MA-I ventilator with various respiratory rates, and positive end-expiratory pressure. None of these maneuvers produced lasting improvement (Table I). At this time, Sao, measured directly (IL COOximeter) was much higher than the Sao, calculated from the Pas, (Severinghaus slide rule). This suggested a marked left shift in the oxygen dissociation curves (Table I). Because the cardiovascular system had been very stable, the arterial catheter was removed and a No. 4 Fr. fiberoptic oximeter catheter was placed in the femoral artery. The various respiratory maneuvers were tried in sequence to ascertain an optimum pattern of ventilation (Table II). Because it was apparent that ventilatory support did not significantly im-

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Volume 65 Number 3 Morch, 1973

Oxygen saturation versus oxygen tension

411

Table I. Summary of blood gas values Date Dec. 1, 1970

Dec. 2, 1970

Hour

Comment A rterial sample 21 67

5:30

P.M.

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39 43 26

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11:25

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42 57

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10:00

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Bird

80 75 54

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30 34

55 76

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42 37 33 28

32 18 28 28

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Left atrial sample 27 30 68 82

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30 47

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Endotracheal tube in place

SVC Po, 15 torr with calculated saturation of 17% Peripheral cyanosis unBird changed MA-1 Respiratory rate 40 breaths/min.; tidal volume 100 c.c.; peak pressure 40 em. H 2O MA-1 MA-1 30 min. after positive expiratory pressure of 5 em. H,O added MA-1 MA-1 MA-1 MA-1 Respiratory rate 10 breaths/min. MA-1 Respiratory rate 25 breaths/min. MA-1 Expiratory pressure 10 em. H,O MA-1 Expiratory pressure off

60

MA-1 Fiberoptic oximeter arterial catheter replaced arterial catheter at 3: 00

78 88

60 70

Breathing spontaneously Extubated at 6:30 P.M.; "doghouse" placed

64 62 57

78 78 75

70 60 60

P.M.

Dec. 3, 1970

Dec. 4, 1970

P.M.

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P.M. A.M.

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70 78 90

84

P.M.

P.M. P.M.

100

Reintubated; clot on fiberoptic catheter gave erroneous low reading; endotracheal tube reo moved

70 40 Left atrial catheter then removed

Legend: After 5 :00 P.M., Dec. 2, 1970, the blood gas values are left atrial samples which are presumed to equal arterial values. HCO", The bicarbonate radical. Sat. cal., Oxygen saturations calculated from the Severinghaus slide rule. Sat. meas., Measurements from the vivo fiberoptic catheter. SVC, Superior vena cava. MA-I, Bennett MA-I ventilator.

The Journal of

4 12

Martin et al.

Thoracic and Cardiovascular Surgery

Table II. Summary of ventilatory maneuvers and the resultant oxygen saturations Oxygen saturation Time 3:00-4:00

P.M.

4:00-4:30

P.M.

4:30-5:00

P.M.

5:00-6:00

P.M.

6:00-8:00

P.M.

8:00

P.M.

Maneuver

(%)

MA-I on 100% oxygen; respiratory rate 30 breaths/min.; tidal volume 100 c.c, MA-1 on 100% oxygen; respiratory rate 30 breaths/min.; tidal volume 100 c.c.; positive expiratory pressure 5 em. H,O MA-1 on 100% oxygen; respiratory rate 30 breaths/min.; tidal volume 100 c.c. Spontaneous respirations of 100% oxygen 60% oxygen; endotracheal tube in place; spontaneous respirations 60-70% oxygen; extubated; spontaneous respiration

70-85

Saturation unstable, varying without discernible pattern

Comment

65-78

Lowest saturation of the attempted patterns

70-85

70-83 68-80 68-78

Little change from positivepressure respiration No deterioration in patient's clinical appearance Patient appeared vigorous

Legend: Note that ventilatory support elicited little or no improvement.

prove gas exchange, the patient was extubated and placed in an incubator with 60 to 70 per cent oxygen and high humidity. The oximeter catheter was left in place. Nine hours later the oximeter became erratic, and by 12 hours it gave a very low reading. The infant was reintubated but subsequently extubated when a left atrial sample revealed a 74 per cent saturation. A final left atrial sample at 4:00 P.M. on Dec. 4, 1970, revealed a Po, of 55 torr with a calculated saturation of 90 per cent. The child's condition improved steadily. He was released from the Intensive Care Unit on Dec. 6, 1970, and was discharged to his home on Dec. 13, 1970.

Discussion Although many aspects of this case are interesting, e.g., the cause of the arrest following resuscitation, the disproportionate right ventricular failure, and so on, we would like to focus on the postoperative hypoxemia. Intermittent measurement of Pao2 seriously misled us both as to the severity of the problem and as to the effectiveness of our therapeutic manipulations. Although it is well known that the deficiency of 2,3-diphosphoglycerate in bank blood can shift the oxygen dissociation curve to the left,2-4 we were unaware of the magnitude that could occur. On one occasion the calculated Saoe was 54 per cent, whereas the

measured Sao2 was 78 per cent. Even more misleading was the superior vena cava POe of 15 torr with a calculated saturation of 17 per cent. In our experience, mixed venous saturations of this level are very ominous. Therefore, we resorted to multiple patterns of continuous artificial ventilation including positive end-expiratory pressure in an attempt to reduce the calculated 59 per cent right-to-left shunt and raise the Pao2 to a viable level. Each change resulted in a slight improvement in the Paoe followed by an apparent deterioration. Nineteen hours postoperatively, serious consideration was given to placing the child in a hyperbaric chamber. However, before the transfer was effected, direct measurement of Saoe revealed a saturation of 82 per cent when a simultaneous Pas, measurement was 27 torr with a calculated saturation of 68 per cent. At this point, the patient's course was re-evaluated in light of a probable marked shift of the oxygen dissociation curve to the left. Reviewing the Paoe and recalculating the oxygen saturations from a redrawn oxygen dissociation curve (assuming a shift similar to those reported by Broennle's group'), we estimated that the oxygen saturations had been 80 per cent or better throughout the evening. From the redrawn dissociation

Volume 65 Number 3

Oxygen saturation versus oxygen tension

4 13

March, 1973

curve, the superior vena cava PO z of 15 torr yielded a saturation of 44 per cent. Therefore, the unloading of oxygen in the periphery was occurring in the usual part of the dissociation curve. This fact along with the lack of evidence of a metabolic acidosis led us to conclude that, although the peripheral tissues were forced to operate at a lower than normal POz, oxidative metabolism was not impaired. It is also possible that the patient's serious pre-existing hypoxia had caused adaptation to hypoxia. We then estimated that the hypoxemia, although serious and in need of correction, was not immediately life threatening. Even with directly measured saturations, the patient had a right-to-left shunt of 41 per cent. Because the patient still had a patent foramen ovale and a higher right atrial pressure than left atrial pressure, it was probable that an intracardiac right-toleft shunt contributed to the calculated shunt. In addition, miliary atelectasis with intrapulmonary shunt, which is virtually always present after this type of operation, probably also contributed to the shunt. Therapy for miliary atelectasis requires positive pressure, which tends to increase pulmonary resistance to flow and increase the right-to-left intracardiac shunt. When continuous measurement of Sao" indicated that spontaneous ventilation was as effective as ventilator support, the patient was extubated and did well. Without the documentation provided by the oximeter it is almost certain that intubation would have been prolonged for many hours or (more probably) days. Because prolonged endotracheal intubation may lead to serious complications in infants," the oximeter, by allowing safe extubation and clarifying the confusion associated with the changing Pas, values, contributed materially to this patient's survival. Summary

In conclusion, two points deserve reemphasis. First, patients who receive large amounts of bank blood may have a marked shift in the oxygen dissociation curve.' While this shift does not usually produce

confusion in the interpretation of Pao2, the shift that occurred in this patient complicated his management. With the increased use of Swan-Ganz catheters and the use of measurements of mixed venous blood for assessing cardiovascular Iunction.?" it is apparent that oxygen saturation should be measured directly and not calculated from a measurement of PO z in patients receiving large amounts of bank blood. Second, although intermittent arterial blood gas sampling is of great value in managing difficult patients, in a rare unstable case such as this one, a continuous measurement of either PO z or S02 is needed if the appropriate interpretation of the data is to be made. Hopefully, such instruments will become widely available in the near future. Addendum The fiberoptic oximeter utilized in this study was obtained from the Bioengineering Program and has been reported on previously.P- 10 In brief, the oximeter is a completely solid-state device that uses light-emitting diodes as light sources. Light from the diodes is optically coupled to fiberoptic bundles and transmitted to the blood; reflectance levels at two wave lengths (685 and 925 nanometers) are detected and compared with appropriate circuitry. The in vivo correlation between oxygen saturation determined by the Instrument Laboratories CO-Oximeter and the fiberoptic catheter is 0.989 for 65 samples with a standard deviation of ± 1.46 per cent over an oxygen saturation range from 22 to 96 per cent. As with any device in contact with blood, clotting is a problem. The catheter that we used did have a clot on the tip when removed. Subsequent versions of the catheter have a lumen to flush the tip, and the circuitry has been modified to detect clot formation. REFERENCES Mohri, H., Dillard, D. H., Crawford, E. W., Martin, W. E., and Merendino, K. A.: Method of Surface-Induced Deep Hypothermia for Open-Heart Surgery in Infants, 1. THORAC. CARDIOVASC. SURG. 58: 262, 1969. 2 Valtis, D. J., and Kennedy, A. C.: Defective Gas-Transport Function of Stored Red Blood Cells, Lancet 2: 119, 1954. 3 Benesch, R., and Benesch, R. E.: Intracellular Organic Phosphates as Regulators of Oxygen Release by Haemoglobin, Nature 221: 618, 1969. 4 Broennle, A. M., Laver, M. B., Huggins, C.,

The Journal of

4 14

Martin et at.

Thoracic and Cordiovascular Surgery

and Austin, W. G.: Oxygen Transport and Massive Transfusion: The Unsteady State, Surg. Forum 21: 52, 1970. 5 Abbott, T. R.: Complications of Prolonged Nasotracheal Intubation in Children, Br. J. Anaesth. 40: 347, 1968. 6 Goldman, R. H., Klughaupt, M., Metcalf, T., Spivack, A. P., and Harrison, D. C.: Measurement of Central Venous Oxygen Saturation in Patients With Myocardial Infarction, Circulation 38: 941, 1968. 7 Scheinman, M. M., Brown, M. A., and Rappaport, E.: Critical Assessment of Use of Cen-

tral Venous Oxygen Saturation as a Mirror of Mixed Venous Oxygen in Severely III Cardiac Patients, Circulation 40: 165, 1969. 8 Hutter, A. M., and Moss, A. J.: Central Venous Oxygen Saturations: Value of Serial Determinations in Patients With Acute Myocardial Infarction, J. A. M. A. 212: 299, 1970. 9 Johnson, C., Palm, R., Stewart, D., and Martin, W.: A Solid State Fiberoptic Oximeter, J. Assoc. Med. Instrumen. 5: 77, 1971. 10 Cole, J., Martin, W., Cheung, P., and Johnson, C.: Clinical Studies With a Solid-State Fiberoptic Oximeter, Am. J. Cardiol. 29: 383, 1972.