1Department of Emergency and Critical Care Medicine and 2Department of Anesthesiology, Niigata University Faculty of Medicine, 1-757 Asahimachi, Niigata 951-8150, Japan*Corresponding author
Presented, in part, at the annual meeting of the American Society of Anesthesiologists, Dallas, USA, October 1822, 1999.
Accepted for publication: March 3, 2001
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Abstract |
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Br J Anaesth 2001; 87: 2238
Keywords: equipment, blood-gas monitors; surgery, cardiac; monitoring, jugular venous gas tension; brain, injury; brain, ischaemia
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Introduction |
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When increased oxygen extraction by the brain can no longer completely compensate for decreased oxygen supply, cerebral oxygen consumption will decrease and anaerobic metabolism with lactate production will result.4 These features may not be detected adequately by measuring changes in oxygen saturation or oxygen tension (PO2); measurements of pH and carbon dioxide tension (PCO2) in the jugular venous blood may more completely indicate changes in brain oxygenation.
The Paratrend 7 system (PT7) (Biomedical Sensors, High Wycombe, UK) incorporates four different sensors: PO2 is measured with a miniaturized Clark-type electrode, PCO2 and pH are measured with two optical fibres, and blood temperature is determined by a thermocouple. In addition, oxygen saturation, bicarbonate, and base excess are automatically computed from these variables. The four sensor elements are housed in a heparin-coated microporous polyethylene tubing approximately 0.5 mm in diameter, which can be passed through a 20-gauge catheter.
Acceptable accuracy and good clinical performance of the PT7 sensors in arterial blood have been demonstrated during surgery510 and in the intensive care unit.1115 We know of only one anecdotal report that describes the use of the PT7 in the jugular bulb of a patient with subarachnoid haemorrhage.16 We evaluated the accuracy and clinical feasibility of continuous intra-jugular blood-gas monitoring with the PT7 in adult patients undergoing cardiac or aortic surgery with hypothermic CPB.
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Methods |
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After induction of anaesthesia, a 8.0Fr introducer sheath (Arrow International, Inc., Reading, PA, USA) for a 7.5Fr pulmonary artery catheter (Arrow International, Inc.) and a 7Fr triple lumen catheter (Arrow International, Inc.) were inserted through the right internal jugular vein. In addition, a 5.1 cm long, 20-gauge venous catheter (Insyte, Becton Dickinson Inc., Franklin Lakes, NJ, USA) was inserted cephalad more than 2 cm apart from the insertion sites of the above two catheters. For continuous monitoring of jugular venous blood, the PT7 sensor was advanced more than 15 cm through the catheter into the jugular bulb, and adjusted to lie in the jugular bulb using fluoroscopy. In vitro calibration was performed before the insertion. After stabilization of the sensor in the jugular bulb, in vivo calibration between the sensor and the blood gas analyser (BGA) was carried out, and no further calibrations were done. Blood samples (0.5 ml) were drawn via a Y-connection attached to the venous catheter at a rate of 1 ml min1 regardless of body temperature or the status of CPB and measured with a blood gas analyser (Corning 280 with Corning 2500 co-oximetry, Bayer Medical Ltd, Tokyo, Japan) within 2 min. According to the manufacturer, when used with the usual calibration frequencies (one-point calibrations every 30 min and two-point calibrations every 2 h) the precision expected is 0.002 pH units, 0.25 kPa, 0.17 kPa, and 0.58% for pH, PCO2, PO2, and oxygen saturation, respectively, over the following range of measurements: pH 7.07.6 pH units, PCO2 2.713.2 kPa, and PO2 2.720.0 kPa.
All data are presented at 37°C without temperature correction, and the PT7 data were stored in a personal computer (Macintosh PowerBook 5300cs, Apple Computer, Tokyo, Japan) via a RS-232C port every 1 min for subsequent analysis.
Cardiopulmonary bypass
A non-pulsatile pump flow rate of 2.22.6 litres min1 m2 was maintained using a membrane oxygenator and a 40-µm arterial filter. Perfusion pressure was maintained at 5090 mm Hg using either chlorpromazine 0.31.0 mg kg1 i.v. or phenylephrine 0.10.5 mg i.v. Oesophageal temperature during aortic cross-clamp was maintained in the range of 25 to 30°C for CABG surgery or 15 to 20°C for aortic surgery. PaCO2 was adjusted to 4.55.3 kPa without temperature correction. Selective cerebral perfusion was maintained at a flow rate of 510 ml kg1 min1 for all patients who underwent aortic surgery.
Statistical analysis
We used a statistical package (SPSS 9.0 for Windows, Base and Advanced models; SPSS Inc., Chicago, IL, USA). All data are presented as mean and SD. Hydrogen ion concentration ([H+] nmol litre1) was calculated from the value of pH. Bias (mean of the differences of the values measured by the PT7 minus the blood sample values) and precision values (1.96 SD of the differences) for each variable were calculated for each patient by the Bland-Altman method.17 Simple linear regression analysis of each variable was made by the least-squares method. The paired data were then divided into three subgroups according to the value of bladder temperature or haematocrit. Bias and precision for each variable were also calculated in each of the three subgroups, and the bias values were compared using one-way analysis of variance. When significance was found, Fishers protected least significant difference test was used as a post hoc comparison procedure. P values <0.05 were considered significant.
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Results |
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Five to eight paired measurements with the PT7 and blood samples were made on each patient, with a total of 101 paired measurements. The ranges of each variable measured from the blood sample were: pH 7.12 to 7.59, PCO2 3.7 to 9.6 kPa, PO2 3.5 to 16.0 kPa, oxygen saturation 4099%, bicarbonate 18.6 to 34.4 mmol litre1, and base excess 7.8 to 12.5 mmol litre1. Bias and precision values for each variable in each patient and overall bias, precision, and regression values are given in Table 1.
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Discussion |
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The threshold value for the jugular venous PO2 has not yet been determined. In volunteers breathing 100% nitrogen, slowing of EEG was observed when the jugular venous PO2 decreased to 2.5 kPa.18 Taking the difference between normal jugular venous PO2 (4.76.2 kPa) and the critical level described above (2.2 kPa), the bias, precision, and 95% CI for PO2 presented in this study are acceptable for clinical decision making. Similarly, there are no available human data describing critical values for jugular venous pH and PCO2. However, in several human studies when the PT7 was inserted into brain tissue19 20 or in cerebrospinal fluid,21 trends showed a concomitant increase in PCO2 and decrease in pH, along with a decrease in PO2 during sustained brain ischemia19 20 or after brain death.21 Continuous measurement of jugular venous pH and PCO2 together with PO2 using the PT7 may give more information on the balance between brain oxygen supply and demand than the currently used fibreoptic oximetric catheter.
Sensor malfunction occurred in two of eight patients who underwent aortic surgery with selective cerebral perfusion, but not in 12 patients who underwent CABG surgery. Postoperative fluoroscopy showed kinking of the sensors, which had not been seen preoperatively. All malfunctions occurred before the start of selective cerebral perfusion, and perfusion itself did not contribute to malfunctions. However, the patients head was moved for surgical positioning, presumably resulting in deflection of the sensors.
In conclusion, we describe good accuracy of the pH, PO2, and PCO2 sensors of the PT7 in the jugular venous bulb, comparable to previous findings with arterial blood. Mild to deep hypothermia and marked haemodilution during CPB did not affect the reliability of the sensors. There were no complications attributable to the insertion of the sensor. Thus, continuous jugular venous blood-gas monitoring is clinically feasible using the PT7 and may provide valuable information during hypothermic CPB. However, large studies would be needed to demonstrate clinical improvement.
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Acknowledgements |
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References |
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