Continuous fiberoptic arterial oxygen tension measurements in dogs

Continuous intra-arterial blood gas monitoring

OBJECTIVE

To review the technology, clinical trials and current status of continuous blood gas monitoring in intensive care.

DESIGN

The review describes the history, technology, various clinical trials on continuous blood gas monitoring and discusses the various factors which might affect their performance characteristics and outlines their potential role in intensive care and during anaesthesia.

CONCLUSIONS

Over the past 10 years a number of continuous intra-arterial blood gas monitoring systems have been developed. The performance characteristics of these systems are comparable. Their levels of accuracy as measured in bench tonometry are not consistently achieved in clinical trials. The potential usefulness of these monitors in various clinical situations has been described in case studies. Controlled studies demonstrating an improvement in outcome with the use of these monitors have not been published.

Optical sensors for clinical monitoring

Technical progresses make it now possible to monitor well known or new parameters in vivo or in the laboratory with high accuracy. Especially optical sensors can advantageously be used for many medical applications. To understand advantage and limitation of a measuring technique the basic processes will be shortly discussed. There are two types of optical sensors: 1) optical sensors which use intrinsic indicators (as for example haemoglobin or cytochromes). In this chapter tissue photometry and evaluation methods for multicomponent scattering systems are discussed; nearinfrared and NADH fluorescence measurements are shortly mentioned. 2) Optical sensors using extrinsic indicators (optodes). As extrinsic indicators absorbant as well as luminescent indicators are used. Luminescence indicators are especially sensitive. Microoptodes and two dimensional imaging is possible. From the basic molecular reactions of the sensing mechanisms follows that for most of the indicator reactions there is a non-linear, almost hyperbolic relationship between optical signal and concentration of the analyte. Consequently, accuracy as well as sensitivity of the optode is changing in a given measuring range. Therefore, the optical indicator must be carefully selected. Lifetime (or phase angle) measurements have the advantage that their accuracy is independent of indicator concentration, intensity of the light source and light transport between the sensing element and the photometric setup. Optodes can be manufactured as flexible membranes permeable for the analyte. This facilitates the construction of fibreoptic sensors. As practical examples oxygen optodes, ion optodes, optical pCO2 sensors, and bench-top as well as intra-arterial blood gas measurements are discussed in detail.

Clinical evaluation--continuous real-time intra-arterial blood gas monitoring during anesthesia and surgery by fiber optic sensor

A clinical evaluation of the clinical utility, techniques of use, durability, accuracy, and potential complications of a newly available system for the continuous real-time, intra-arterial monitoring of arterial blood gas and acid base status (ABG) has been studied (Optex BioSentry System, Optex Biomedical, Incorporated, The Woodlands, Texas, U.S.A.). The system consists of three separate fiber optic channels with contained fluorescent and absorbant chemical analytes imbedded in a single probe and capable of insertion inside of a twenty gauge indwelling arterial catheter (The Optex Optode Sensor), along with an external monitor. The system was utilized during anesthesia and surgery in the care of five informed patients. Constant trend monitoring of all three variables was deemed satisfactory in four of the patients. The fifth sensor was damaged by a surgical assistant while in place and ceased to function. Comparison of Optode sensor readings with standard clinical laboratory ABG analysis was excellent in three uncomplicated patients (pH: bias -0.0183 pH units, precision: 0.0237 pH units; PCO2: bias 3.22 mmHg, precision 2.04 mmHg; PO2: bias -5.94 mmHg, precision 11.74 mmHg). Postoperative study suggested that discrepancies in a fourth patient may have been due to an incorrect 'offset' applied to the Optode sensor yielding a constant error.

Continuous intra-arterial PO2 monitoring during thoracic surgery

Intermittent blood gas sampling has several disadvantages, the most important being that samples are usually taken at set intervals, or when changes in oxygenation are suspected--when it is too late. Another problem is inaccuracy caused by careless blood sample handling. Continuous intravascular PaO2 monitoring eliminates these problems. This study shows that the Continucath sensor is an easy-to-use and reliable monitor, with specific early warning capabilities for hypoxia, thereby improving anesthetic and intensive care management. Its characteristics are: a stabilization period of 10 minutes, a 90% response time of 90 seconds, temperature dependence of 4% per degree celsius, a flow dependence of less than 1% if the flow is more than 5 cm/sec, a drift of less than 0.7% per hour, a correlation coefficient of 0.92 when compared to blood gas analysis during surgery.

Progress in the development of a fluorescent intravascular blood gas system in man

In vitro and in vivo animal studies have shown accurate measurements of arterial blood pH (pHa), carbon dioxide tension (PaCO2), and oxygen tension (PaO2) with small intravascular fluorescent probes. Initial human clinical studies showed unexplained intermittent large drops in sensor oxygen tension (PiO2). Normal volunteers were studied to elucidate this problem. In the first part of this study, the probe and cannula were manipulated and the probe configuration and its position within the cannula were varied. The decreases in PiO2 were judged to be primarily due to the sensor touching the arterial wall. Retraction of the sensor tip within the cannula eliminated the problem. In the second part of this study, the accuracy of the retracted probe was evaluated in 4 subjects who breathed varying fractions of inspired oxygen and carbon dioxide. The arterial ranges achieved were 7.20 to 7.59 for pH, 22 to 70 mm Hg for PaCO2, and 46 to 633 mm Hg for PaO2. Linear regression of 48 paired sensor (i) versus arterial values showed pHi = 0.896 pHa + 0.773 (r = 0.98, SEE = 0.017); PiCO2 = 1.05 PaCO2 - 1.33 (r = 0.98, SEE = 2.4 mm Hg); and PiO2 = 1.09 PaO2 - 20.6 (r = 0.99, SEE = 21.2 mm Hg). Bias (defined as the mean differences between sensor and arterial values) and precision (SD of differences) were, respectively, -0.003 and 0.02 for pHi, 0.77 and 2.44 mm Hg for PiCO2, and -2.9 and 25.4 mm Hg for PiO2. The mean in vivo 90% response times for step changes in inspired gas were 2.64, 3.88, and 2.60 minutes, respectively, for pHi, PiCO2, and PiO2.

Detection of venous air embolism by continuous intraarterial oxygen monitoring

In a recent study, we compared a new intraarterial fiberoptic "optode" probe to continuously measure arterial oxygen and carbon dioxide tensions and pH with intermittently drawn blood samples in patients undergoing surgery. In one patient with a diagnosis of Arnold-Chiari type I malformation with outflow obstruction of the fourth ventricle, a major pulmonary air embolism occurred while the patient was undergoing suboccipital craniectomy and cervical laminectomy in the prone position. Three hours after the incision the optode-displayed oxygen tension decreased from a stable value of 225 +/- 8 mm Hg to 63 mm Hg over a 10-minute period. During the same interval, carbon dioxide tension increased and end-tidal carbon dioxide decreased; shortly thereafter, transcutaneous oxygen tension decreased also. Within 20 minutes after the inspired gas mixture was changed to 100% oxygen, the patient's respiratory variables returned to near baseline. No further complications ensued. This is the first time continuously monitored arterial oxygen tension values during a pulmonary embolism have been reported. With further refinement, intraarterial optode probes will add another valuable method of detecting pulmonary air embolism.

The oxyhemoglobin dissociation curve before, during and after cardiac surgery

The oxyhemoglobin dissociation curve was quantified in 15 patients subjected to hypothermic cardiopulmonary bypass under opiate-benzodiazepine anesthesia using the alpha-stat approach to control blood acid-base status. The P50 was calculated from a single measurement of oxygen tension and hemoglobin saturation in blood obtained from the pulmonary artery or the venous line from the cardiopulmonary bypass circuit. In addition, the P50 was directly determined at the registered patient temperature. The P50 decreased from 3.87(+/- 0.15) kPa (mean, SEM) before anesthesia to 1.55(+/- 0.16) kPa during hypothermic (25.43 +/- 1.99 degrees C) cardiopulmonary bypass (p less than 0.001). On rewarming, the P50 increased to 4.89 +/- 0.27 kPa (at 36.14 +/- 0.14 degrees C, p less than 0.001 compared to the preinduction and hypothermic values). Eight hours after cardiopulmonary bypass the P50 returned to the preinduction value (3.72 +/- 0.22 kPa). The relationship between temperature and P50 is described by the regression equation: P50 = 0.22(+/- 0.02).Temperature--3.78(+/- 0.62). The correlation was 0.78 (p less than 0.001). It is concluded that (1) the leftward shift of the oxyhemoglobin dissociation curve during hypothermia may be detrimental to oxygen delivery and (2) the oxygen saturation of the venous blood should not be used indiscriminately to evaluate cellular oxygen status.

Respiratory monitoring in the intensive care unit

Continuous monitoring of important respiratory indices has the potential for predicting catastrophes and providing an opportunity for the timely institution of lifesaving measures. Arterial oxygenation can be monitored noninvasively using oximetry or transcutaneous oxygen electrodes, while mixed venous oxygenation can be recorded continuously with modified pulmonary artery catheters. A satisfactory method of monitoring carbon dioxide tension does not exist. Measurements of respiratory drive can be obtained at the bedside, but their clinical usefulness remains unknown. Assessment of respiratory muscle strength is helpful in determining the need for mechanical ventilation, but a practical method of diagnosing respiratory muscle fatigue remains elusive. Measurement of thoracic compliance and detailed examination of the breathing pattern, i.e., tidal volume, respiratory frequency, and the pattern of rib cage-abdominal motion, are helpful in assessing abnormal pulmonary mechanics. The detailed information provided by respiratory monitoring can complement but not replace careful bedside examination.

Comparison of three oxygen monitors in detecting endobronchial intubation

Rapid and reliable detection of inadvertent endobronchial intubation is an essential function of oxygen monitoring. We have studied the detection of this event by using three oxygen monitoring techniques: pulse oximetry, transcutaneous measurement of oxygen tension, and intraarterial fiberoptic measurement of oxygen tension. Four dogs were anesthetized, intubated, and monitored with these three techniques and with arterial and central venous cannulas. Endotracheal tubes were moved from the trachea into the right mainstem bronchi at several inspired oxygen fraction (FIO2) values for each dog, and the responses of the oxygen monitors were recorded for 20 minutes thereafter. The pulse oximeter showed little change in oxygen saturation (SpO2) during endobronchial intubation at FIO2 values above 0.3. SpO2 decreased by an average of 1.3 +/- 2.1% at an FIO2 of 1.0 and by 4.0 +/- 4.1% at an FIO2 of 0.5. Simultaneously measured transcutaneous oxygen tensions decreased by 42 to 64% and the optode reading decreased by 64 to 79%. At lower FIO2 values, the changes in SpO2 were more significant: a decrease of 6.0 +/- 6.3% at an FIO2 of 0.3 and of 9.8 +/- 6.1% at an FIO2 of 0.2. The transcutaneous oxygen and optode readings decreased by 31 to 45% under these conditions. Endobronchial intubations at FIO2 values greater than 0.3 may not yield immediate decreases in arterial saturation and hence may go undetected by pulse oximetry. Transcutaneous oxygen tension decreases significantly with endobronchial intubation at any FIO2. The experimental, intraarterial optode consistently yielded the greatest changes with the fastest time response.

Clinical assessment of a flow-through fluorometric blood gas monitor

We performed an observational study to evaluate a flow-through fluorometric instrument (Gas-STAT) that continuously measures the carbon dioxide tension (PCO2), oxygen tension (PO2), and pH of blood in the cardiopulmonary bypass circuit. Setup and calibration of the instrument typically required 20 minutes. During bypass, 129 blood samples were drawn from 16 patients for comparison with conventional measurements obtained with a blood gas machine. Data for each variable, within each sensor, were analyzed by linear regression. The ranges of the standard errors of the estimate were 0.7 to 4.2 mm Hg for PCO2, 18.3 to 78.7 mm Hg for the high PO2 range, 1.4 to 7.1 mm Hg for the low PO2 range, and 0.008 to 0.049 for pH. The regression lines differed from the identity line (P less than 0.05) in at least one variable in most patients, and large deviations from the line of identity in both slope and intercept were common. Among 58 sensors evaluated, failures occurred in 5 (2.9%) of the 174 optodes, and minor leakage occurred in 2 (3.4%) of the flow-through cells. We conclude that although this flow-through fluorometric instrument is an adequate monitor of trends in blood gases during cardiopulmonary bypass, it is not accurate enough to supplant conventional laboratory measurements.

In vitro assessment of a flow-through fluorometric blood gas monitor

The Gas-STAT blood gas monitor uses fluorometric techniques to continuously monitor blood gas tensions and acid-base status in the extracorporeal perfusion circuit during cardiac surgery. We evaluated the in vitro performance of this instrument by using a tonometry loop to simulate the clinical environment and to provide controlled gas tensions and pH in the circulating fluid. In this article we report the in vitro study in which 35 Gas-STAT blood gas sensors were used to assess the precision, stability, response time, and specificity of the instrument and to confirm the sterile integrity of its flow-through cells. The blood gas monitor exhibited precision values for pH, carbon dioxide tension (PCO2), and oxygen tension (PO2) of 0.1%, 1.3%, and 1.0%, respectively; stabilities were 0.002 units/h for pH, 0.5 mm Hg/h for PCO2, and 1.4 mm Hg/h for PO2; time constants (tau, a response to within 1/e of a new gas tension, approximately 63%) were 81 seconds for PCO2 and 72 seconds for PO2. No significant interference was detected in in vitro tests of 30 drugs and metabolites typically encountered during cardiac surgery. Bacterial challenge of the flow-through cell membranes showed that they provide an effective barrier isolating the sensors from contaminants in the fluid path. Our quality control consisted of measurement of a midrange gas standard as an unknown immediately following sensor calibration; this simple program is proposed as a complement to the manufacturer's operating procedures.(ABSTRACT TRUNCATED AT 250 WORDS)