Cardiac output using an electrically calibrated flow-through conductivity cell

The cavity perturbation method for evaluating hematocrit via dielectric properties

The physical parameters of human blood (complex permittivity and conductivity) at microwave frequencies have been investigated to assess the hematocrit (HCT). The cavity perturbation method based on a rectangular cavity operated in TE101mode at frequency 4.212 GHz has been utilized to measure the permittivity of blood with different hematocrit % at a range of temperatures. According to the results, the dielectric constant, loss factor, and conductivity appeared to be influenced by HCT level. Though the dielectric constant is the only parameter that shows clear linear regression decreasing behavior with a correlation value around (R2= 0.93). For thirty healthy donors the dielectric constant decreases from (65.61 ± 1.4 to 44.64 ± 4.0) and from (65.3 ± 1.2 to 48.3 ± 1.88) for men and women, respectively, with increasing hematocrit percentage from 20% HCT up to 95% HCT. The temperature dependence of the dielectric constant is also examined in the temperature range 27 °C-50 °C and the results display a slight decrease in dielectric constant with elevation temperature. The temperature-dependence dielectric constant of water and blood samples were fitted to an empirical polynomial with temperature. A comparison of estimated HCT using the cavity technique based on dielectric properties shows a very good agreement with commercially standard HCT measurement methods. Finally, the cavity technique can be applied to measure the hematocrit up to high values based on the dielectric constant with high precision, simplicity, and low cost compared with traditional techniques.

Electrochemical Approach to Measure Physiological Fluid Flow Rates

In vivo measurement of the flow rate of physiological fluids such as the blood flow rate in the heart is vital in critically ill patients and for those undergoing surgical procedures. The reliability of these measurements is therefore quite crucial. However, current methods in practice for measuring flow rates of physiological fluids suffer from poor repeatability and reliability. Here, we assessed the feasibility of a flow rate measurement method that leverages time transient electrochemical behavior of a tracer that is injected directly into a medium (the electrochemical signal caused due to the tracer injectate will be diluted by the continued flow of the medium and the time response of the current-the electrodilution curve-will depend on the flow rate of the medium). In an experimental flow loop apparatus equipped with an electrochemical cell, we used the AC voltammetry technique and tested the feasibility of electrodilution-based measurement of the flow rate using two mediums-pure water and anticoagulated blood-with 0.9 wt% saline as the injectate. The electrodilution curve was quantified using three metrics-change in current amplitude, total time, and change in the total charge for a range of AC voltammetry settings (peak voltages and frequencies). All three metrics showed an inverse relationship with the flow rate of water and blood, with the strongest negative correlation obtained for change in current amplitude. The findings are a proof of concept for the electrodilution method of the flow rate measurement and offer the potential for physiological fluid flow rate measurement in vivo.

Parallel conductance determination in cardiac volumetry using dilution manoeuvres: theoretical analysis and practical implications

Left ventricular volume calibration based on the conductance catheter depends on the correct determination of the parallel conductance (Gp). Baan's saline manoeuvre procedure leads to Gp by finding the end-systolic (Ges) and end-diastolic (Ged) conductances, for each beat of the dilution curve rising limb. After plotting such values in an xy-system, their linear regression is back-projected to intersect the identity line, so yielding an estimated Gp. The objective is to theoretically analyse all possible lines, Ges = aGed + b (Baan's line) and, based on experimental results, to establish their limitations. This was attained by calculating the regression lines using, first Ged = f1(Ges) and thereafter, Ges = f2(Ged), which led to two values, Gp2 and Gp1, for the parallel conductance. The morphology of the saline curve was also modified to assess its effect on the extrapolation. Multiple dilutions were recorded in eight experimental dogs injecting different concentrations. Each curve was classified according to the maximum change (VAR) reached by the total average conductance. Over 138 manoeuvres, 276 regressions were processed yielding correlations higher than 0.65. Of this total, 92.4% gave positive parallel conductances. The rest produced negative values and, thus, were neglected. If the two (Ged, Ges) statistical relationships were ideal, they should yield Gp = Gp1 = Gp2; however, there were differences which, when Gp1 was studied against Gp2, led to: Gp1 = 0.97 Gp2 + 0.055, with r = 0.9476, and n = 85. The remaining 53 were discarded because either some Gp values were negative, or the correlation of Ges which Ged (or vice versa) was < 0.85, and/or VAR < 15%; the two latter conditions were found necessary for reliable calibration. Baan's line high correlation is not a unique condition to ensure the accuracy and precision of Gp determination because the slope a depends on VAR and, thus, different intersections with the identity line may be obtained. Its recommended that manoeuvres be used with at least eight data points, with VAR > 15% and, finally, with (Ges, Ged) correlation better than 0.85. Theoretical analysis of Baan's line offers a reference frame, which contains only a limited number of practical possibilities.

Measurement of cardiac output in small laboratory animals using recordings of blood conductivity

No method exists which enables easy, frequent, and, at the same time, reliable cardiac output (CO) measurements in mice. To validate a simple indicator-dilution method suitable for frequent measurements of CO in small laboratory animals, a 5% glucose solution was injected as a bolus into femoral veins of mice and rats. The corresponding blood conductivity was measured continuously between an intra-aortic and a rectal electrode. The resulting conductivity-dilution curves were used to calculate CO in mice during hypervolemia and hypovolemia and in conscious as well as halothane-anesthetized mice and rats. In rats, conductivity-dilution curves and time courses of plasma glucose concentration were recorded simultaneously. Compared with CO in awake animals, CO in both species was slightly, but not significantly, reduced during halothane anesthesia. CO was significantly and gradually reduced in hypovolemic mice (up to 58 ml blood/kg body wt), whereas hypervolemia (23 ml saline/kg body wt) had no significant effect. Simultaneous recordings of conductivity-dilution curves and time courses of plasma glucose concentration yielded corresponding values of CO (P < 0.001). Measurement of blood conductivity appears to be a reliable, simple, and convenient method for quantification of CO in small animals.

The relationship between airway carbon dioxide excretion and cardiac output during cardiopulmonary resuscitation

There is currently no practical method for determining cardiopulmonary resuscitation (CPR) efficacy in the field. We investigated the relationship between the volume of carbon dioxide (CO2) excreted in the airway (CO2EX) when tidal volume and respiratory rate are controlled, and cardiac output (CO), an indicator of CPR efficacy, to determine the potential of CO2EX as a practical noninvasive field monitor of CPR efficacy. Thirteen mongrel dogs were anesthetized, instrumented and ventilated 13 times/min at a fixed tidal volume with an infrared airway CO2 sensor. CO2EX = (PCO2/bar. press) x (tidal vol) x (breaths/min), and expressed in ml/min per kg. Sequences of control, CPR with 3-4 different compression forces, and recovery measurements were recorded 10-15 times/animal. CO2EX and CO fell simultaneously with ventricular fibrillation. CPR immediately increased CO2EX and CO. Both changed consistently and in the same direction as compression force. Return of spontaneous circulation immediately increased CO2EX and CO above controls, with a gradual return to control levels. CO2EX was always below 8 ml-CO2/min/kg during CPR and above this during spontaneous circulation. With alveolar ventilation controlled, CO2 movement is regulated by CO, CO distribution and CO2 stores shifts. Normally, CO accounts for 15% of CO2EX variability. In this study CO accounted for > or = 65% of CO2EX variability during CPR, indicating CO2EX changes were primarily due to CO changes. When ventilation is controlled, CO2EX during CPR reliably tracks changes in CO. Therefore, CO2EX may provide a practical noninvasive method of determining CPR efficacy as the CPR is being performed.

Low cost, microprocessor-based system for flow evaluation in a hydraulic model using the dilution method

An Automatic Dye Dilution Processor (ADP) based on a microprocessor is applied to a hydraulic model to process on-line dilution curves, using saline or thermodilution methods. A low cost, F8 microcomputer with minimum parts configuration was used in an Automatic Dilution Processor (ADP) system. Software allows an operator to select methods, amount of indicator to be injected, cell calibration constant, and temperature step. Care has been taken in the curve data processing, to avoid errors when recirculation occurs. The system is used, together with an open hydraulic model, for flow measurement demonstration, and the estimation of cell calibration constant.

Relationship of blood pressure and flow during CPR to chest compression amplitude: evidence for an effective compression threshold

This study was conducted to investigate the importance of the depth of chest compression in producing effective cardiopulmonary resuscitation (CPR) in animals, as indicated by cardiac output and mean arterial blood pressure. Cardiac output was measured by a modified indicator dilution technique in 8 anesthetized dogs, 6 to 12 kg body weight, during repeated 2-minute episodes of electrically induced ventricular fibrillation and CPR provided by a mechanical chest compressor and ventilator (Thumper). Chest compression exceeding a threshold value (xo) between 1.5 and 3.0 cm was required in each animal to produce measurable cardiac output. In particular, cardiac output (CO) was linearly related to chest compression depth (x) by an expression of the form CO = a(x-xo) for x greater than xo. The mean value of xo was 2.3 cm. A similar threshold for measurable blood pressure was observed in 7 of the 8 dogs, with a mean value of 1.8 cm. For chest compression of 2.5 cm or greater, relatively modest increases in chest compression depth caused relatively large changes in cardiac output.

Comparison of sodium and urea as indicators of pulmonary vascular permeability

The objective of this work was to compare the blood-tissue transport properties of 14C-urea and 24Na in the lung circulation. The extraction of both substances was measured relative to intravascular reference tracers (125I-albumin for 24Na, 51Cr red cells and 125I-albumin for 14C-urea) in single-pass transpulmonary multiple indicator curves measured in awake sheep. Sheep were studied in baseline condition, after infusion of histamine solution (4 micrograms/kg-min for 4 hr), and after microvascular pressure was elevated. Permeability-surface area was computed for both sodium (PSn) and urea (PSu) by the Crone extraction model and by a mathematical model. In spite of the fact that the free diffusion coefficients of sodium and urea are approximately equal, the mean ratio of Crone PSn/PSu for baseline studies was 0.76. PSu was significantly increased by histamine infusion but PSn was not. The variation in PSn with histamine and increased pressure was marked. Thus, neither manipulation altered the ratio PSn/PSu in a consistent fashion. Correction of PS for extravascular distribution volumes of urea and Na did not change the PS ratio significantly. We concluded that the diffusional resistance to sodium is higher than expected from its diffusivity, possibly because of charge, and that PS for urea is more sensitive to changes in lung vascular permeability.

Saline and thermal dilution flow measurements: evaluation by means of an open hydraulic model

A simple open hydraulic model was constructed with the main objective of making comparative evaluations of flow rate measurements, with saline and thermal dilution. In the range of 0.5 to 3.5 l/min, it was found that saline indicator gave a slight underestimation (in the order of 2%) while the thermal indicator showed a minor overestimation of approximately 1%. These values wee taken from the regression equations F (saline) = 0.945 F (real) + 0.0351 and F (thermo) = 1.020 F (real) - 0.0144, both in l/min. Correlations were higher than 0.99 in the two cases. Considering the overall error range, the thermodilution produced a value approximately twice that of saline (-21 to 25% versus -12 to +11%). However, this is not necessarily extendable to living organisms. The model was found useful for test and calibration of detecting cells and for teaching purposes.

Protection of ischemic myocardium by whole-body hypothermia after coronary artery occlusion in dogs

Anesthetized dogs were cooled to a core body temperature of 26 degree C. or maintained at a body temperature of 37 degree C. during periods of 5 and 10 hours of LAD coronary artery occlusion. Subsequent macroscopic dehydrogenase enzyme mapping showed that ischemic injury was 25 per cent less after 5 hours of coronary occlusion and 20 per cent less after 10 hours of occlusion in hypothermic dogs than in normothermic controls. The heart rate and left ventricular minute work in hypothermic dogs decreased to roughly half the levels measured in normothermic animals, while left ventricular contractility was 10 to 40 per cent lower in hypothermic dogs than in normothermic dogs. However, cardiac index and left ventricular end-diastolic pressure were unchanged by whole-body cooling. Thus, hypothermia appeared to diminish the oxygen requirements of the ischemic myocardium without reducing the performance of the heart as a pump. Hypothermia may be useful as a therapeutic adjunct to myocardial revascularization or pharmacologic interventions.