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Department of Anesthesiology; Division of Cardiovascular Anesthesia; Mayo Clinic College of Medicine; Rochester, Minnesota; mauermann.william{at}mayo.edu
To the Editor:
The purpose of cardiopulmonary bypass (CPB) is to provide systemic oxygen delivery (DO2) adequate to meet systemic oxygen demand (
o2). Most useful, as a marker of an adequate balance between DO2 and o2, is mixed venous oxygen saturation (SVO2). We describe an unusual cause of low SVO2 during CPB.
A 64-yr-old man (180 cm, 100 Kg) presented for mitral valve repair and coronary artery bypass grafting. Past medical history was significant only for hypertension and he had not undergone any prior anesthetics. Anesthesia was induced with fentanyl, midazolam, sodium thiopental, and pancuronium. Central access was achieved and a pulmonary artery catheter and radial arterial catheter were placed. Maintenance of anesthesia was achieved with a total of 25 µg/kg of fentanyl and isoflurane.
The patient's course was unremarkable during chest opening, harvest of the internal mammary artery and aortic and right atrial cannulation. Cardiopulmonary bypass commenced with a membrane oxygenator, open venous reservoir and roller pump (Sarns 9000®, Sarns/3M, Ann Arbor, MI). Continuous monitoring of arterial blood gases, venous oxygen saturation, hemoglobin, and potassium was achieved using the Terumo CDI 500® monitoring system (Terumo Cardiovascular Systems, Tokyo Japan).
After initiation of CPB and cooling to 34°C, the monitored SVO2 decreased to 45%–50%. CPB machine flows were 2.6 L · min–1 m–2, hemoglobin was 9.8 mg/dL, and arterial Po2 was 290 mm Hg (DO2 756 mL/min) and the patient's calculated o2 was 378 mL/min [expected o2 under general anesthesia at 34°C for this patient would be <225 mL/min (1)]. An infusion of phenylephrine was required to maintain mean arterial pressure >60 mm Hg and the central venous pressure was 0 mm Hg.
With the higher than expected o2 in the face of decreased SVO2, a search for increased oxygen requirements was undertaken. Hyperthermia was quickly ruled out. There was no reason to suspect sepsis. Malignant hyperthermia was possible but unlikely. As a "light plane of anesthesia" was possible, a bolus of 250 mg of sodium thiopental and 250 µg of fentanyl was administered and decreased the BIS monitor reading from 40 to 28. Two milligrams of pancuronium were also given for the possibility of shivering. These interventions resulted in no change in the SVO2.
After approximately 30 min, it was noted that the patient was severely oliguric and the possibility of inadequate oxygen delivery was reconsidered. An examination of the CPB pump tubing revealed that the roller head pump tubing had become partially kinked just proximal to its entry point into the pump head. After un-kinking the intake line the patient quickly became hypertensive. Over the next few minutes the calculated o2 value decreased to <200 mL/min with a venous oxygen saturation of 75% and urine production resumed. The remainder of the patient's intraoperative and postoperative course was unremarkable and he was discharged from the hospital without neurologic deficit.
When using a CPB machine utilizing a roller pump, flow is calculated based on the number of roller pump head revolutions per minute and a constant, predetermined stroke volume for each revolution. A flowmeter is not present on CPB machines utilizing roller pump heads. In this case the roller pump was displacing less than the predicted stroke volume with each revolution. Thus, while the calculated flows were more than adequate, the actual flows being delivered were insufficient and it was not until severe oliguria was noted that the possibility of inadequate flow was considered.
This case emphasizes the importance of being familiar with how the monitoring systems calculate and produce the values that are often taken for granted when assessing patient homeostasis. In addition, we illustrate here the importance of SVO2 as a marker of adequate DO2 versus o2; a relationship that is of paramount importance during any anesthetic.
REFERENCE
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