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Artifacts are a significant problem affecting the accurate display of information during surgery. They are also a source of false alarms. A secondary problem is the inadvertent recording of artifactual and inaccurate information in automated record keeping systems. Though most of the currently available patient monitors use techniques to minimize the effect of artifacts, their success is limited. We reviewed the problem of artifacts affecting patient monitor data during surgical cases. Methods adopted by currently marketed patient monitors to eliminate and minimize artifacts due to technical and environmental factors are reviewed and discussed. Also discussed are promising artifact detection and correction methods that are being investigated. These might be used to detect and eliminate artifacts with improved accuracy and specificity.

Leakage of electric current through cardiac structures surrounding the ventricle is a primary source of error during ventricular volume measurements using a conductance catheter. This error can be represented as a leakage volume, VL. VL is generally estimated by a saline-bolus method, and is assumed constant throughout the cardiac cycle. However, dynamic changes in ventricular volume and cardiac wall thickness could change VL. To estimate VL, a dynamic finite element model of the heart was developed based on MR images. Conductance measurements were simulated using a modeled conductance catheter, and true VL was calculated. VL varied from 22.7 ml (end-systole) to 26.4 ml (end-diastole) in the left ventricle and from 19.9 ml (end-systole) to 26.9 ml (end-diastole) in the right ventricle. The saline-bolus method underestimated VL in both the left (VL = 19.4 ml) and the right (VL = 4.1 ml) ventricular volume measurements. VL increased linearly with the ratio of blood to tissue resistivity, and changed minimally with catheter position. These results indicate that VL has to be estimated dynamically throughout the cardiac cycle to obtain accurate cardiac volume measurements. The results also show that the saline bolus method does not estimate current leakage accurately, especially in the right ventricular volume measurement.

Objective: To propose and verify a technique by which blood resistivity can be measured continuously and instantaneously with a conductance catheter used to measure ventricular volume by intracardiac impedance volumetry., Methods: Intracardiac impedance volumetry involves the measurement of ventricular blood volume using a multi-electrode conductance catheter. Ventricular volume measurement with the conductance catheter requires the value of blood resistivity. Previously, blood resistivity has been determined by drawing a sample of blood and measuring resistivity in a separate measuring cell. A new technique is proposed that allows the resistivity of blood to be measured with the conductance catheter itself. Two adjacent electrodes of the catheter are chosen to establish a localized electric field. With a localized field, the resistance measured between the adjacent electrodes bears a constant ratio (resistivity ratio) to the resistivity of blood. Finite element cylindrical models with exciting electrodes were created to determine the resistivity ratio. Blood resistivity was determined by dividing the resistance found due to the localized electric field by the resistivity ratio. The proposed scheme was verified in cylindrical physical models and in in vivo canine hearts., Results: Finite element simulations showed the resistivity ratio to be 1.30 and 1.43 for two custom-made catheters (Ohmeda Inc. and Biosensors Inc., respectively). The resistivity ratio remained constant as long as the cylindrical volume of blood around the adjacent electrodes had a radius larger than the electrode spacing. In addition, this ratio was found to be a function of electrode width. The new technique allowed us to measure saline resistivity with an error, -0.99+/-0.25% in a physical model, and blood resistivity with an error, -0.625+/-2.75% in an in vivo canine model., Conclusion: The new in vivo technique can be used to measure and track blood resistivity instantaneously and continuously without drawing blood samples.

The authors propose using a multi-electrode conductance catheter to measure continuous right ventricular volume. True ventricular volume measurements are affected by four main sources of error. 1) field non-uniformity, 2) catheter curvature, 3) blood conductivity changes, and 4) leakage of current through surrounding tissues. Three-dimensional finite-element models were developed to investigate the effects of these sources of error and to devise schemes for correcting them. The models include an axisymmetric cylindrical model, a rectangular block model, and a heart model with left and right ventricular chambers. The heart model is built from conical primitives, with major dimensions derived from the literature. Finite-element simulations showed that volume measurements were underestimated due to field nonuniformity to as much as 1/25th actual volume in segments near the exciting electrodes. The extent of underestimation in a segment decreased with increasing distance of the segment from the exciting electrodes and increased for larger segmental volumes. Catheter curvature overestimated measured volume by as much as 4.5 times when the curvature was increased from 0.0 to 1.25 (from a straight catheter to a very curved one). The leakage of current through surrounding tissues overestimated volume by nearly 30%. The sensitivity of volume measurement to blood resistivity changes was found to be very high, at 70%. Correction factors established with the computer models compensate for field nonuniformity. Mathematical mapping of the curved catheter onto a fictitious straight catheter corrects for the catheter curvature error. Correction for both nonuniform field and catheter curvature allowed measurement of total ventricular volume with an error of 7%. Leakage current is determined by using different frequencies to build the catheter electric field and to separate tissue and blood resistance paths. Using this scheme, the percentage overestimation in volume measurement due to leakage could be determined with an accuracy of 85%. The proposed correction scheme for blood conductivity changes involves the in-vivo measurement of blood conductivity with the catheter itself. It was found that blood conductivity could be determined with insignificant error (< 0.5%) so long as the blood volume around the exciting electrodes had a radius of more than the electrode spacing.

Hypertension after a cardiac operation is a frequent phenomenon. Complications resulting from this include bleeding, disruption of vascular suture lines, subendocardial ischemia, and possible cerebrovascular accidents. Treatment with sodium nitroprusside has become accepted practice to prevent these complications. To improve control of arterial blood pressure, a closed-loop system for sodium nitroprusside administration was developed. A prospective, randomized multicenter study was carried out postoperatively in 180 cardiac surgical patients to evaluate the performance of this system compared with manual control of infusion. Adherence of mean arterial blood pressure to +/- 10% of the target blood pressure occurred 85% of the time with the automatic system and 61% of the time with manual regulation (p less than 0.0001). With the automatic system, there was less hypertension (9% versus 22%; p less than 0.0001) and hypotension (6% versus 22%; p less than 0.0001). The superior control of hypertension was achieved more rapidly with less requirement for nurse regulation of infusion rate. The superior control of blood pressure resulted in less chest tube drainage in the automatic mode (720 mL versus 840 mL; p less than 0.05).