Thermal dilution measurement of cardiac output in dogs using an analog computer

Semicontinuous cardiac output monitoring using a neural network

OBJECTIVES

This study compared 2-mL bolus thermodilution cardiac output measurements with standard 10-mL bolus measurements.

DESIGN

Cardiac output was measured with the new 2-mL bolus technique and the 10-mL standard thermodilution technique in a perspective series. We describe a system that automatically cools and injects 2-mL boluses of saline into a standard pulmonary artery catheter. It uses a Peltier effect solid-state cooler and pneumatically driven syringe injector to measure cardiac output once per minute.

SETTING

Animal laboratory.

ANIMALS

Eight adult Duroc swine weighing between 38.0 and 57.5 kg.

INTERVENTIONS

Once each minute, 2 mL of cooled 5% dextrose was injected through the pulmonary catheter. Once every 8 mins, four sequential measurements of cardiac output were made using 10-mL injections.

MEASUREMENTS AND MAIN RESULTS

A total of 1249 paired waveforms were processed with both a conventional algorithm and with a neural network. For the conventional algorithm, the correlation coefficient was r2 = .92 and the SD of the difference was 1.30 L/min. For the neural network, the correlation coefficient was r2 = .94 and the SD of the difference was 0.88 L/min. Output filtering improved the results in both cases.

CONCLUSION

Neural networks accurately derive cardiac output from 2-mL bolus thermodilution injections, allowing cardiac output to be monitored automatically once per minute in many patients. The technique is convenient and uses standard low-cost catheters.

Errors in the measurement of cardiac output by thermodilution

Cardiac output (CO) determination by thermodilution, which was introduced by Fegler in 1954, has gained wide acceptance in clinical medicine and animal experiments because it has several advantages over other methods with respect to simplicity, accuracy, reproducibility, repeated measurements at short intervals, and because there is no need for blood withdrawal. However, errors in determination of CO by thermodilution may be introduced by technical factors and the patients' pathological conditions. The current review summarizes these issues and provides our recommendations, based on the medical literature published between 1954-1992. To obtain more reproducible and accurate CO values by thermodilution, one should make several determinations (1) by using 10 ml injectate at room temperature for adults and 0.15 ml.kg-1 injectate for infants and children; (2) at evenly spaced intervals of the ventilation cycle; (3) when rapid intravenous fluid administration is discontinued; (4) by observing thermodilution curves so that baseline pulmonary artery temperature drift or the existence of intra- and extracardiac shunts are noticed. Finally, CO determination by thermodilution may be unreliable or impossible in patients with low CO states and tricuspid or pulmonary regurgitation. Since non-invasive CO monitoring has not replaced CO determination by thermodilution, intimate knowledge of this method is crucial for anaesthetists to prevent errors in the management of patients.

An evaluation of thermodilution cardiac output measurement using the Swan-Ganz catheter

Errors in thermodilution cardiac output measurement were quantitated to determine the order of accuracy of routine measurements performed by unskilled personnel. In vitro and in vivo studies were undertaken to examine factors affecting the volume and temperature of the injectate, catheter thermistor and computer performance, effect of respiration, use of cold (0-4 degrees C) versus ambient temperature (20-25 degrees C) injectate, and the interpretation of measurements. Ambient temperature injectate incurred unacceptably large errors; cold injectate (injections were timed with respiration) produced variations in performance by equipment and personnel which accounted for only 2% of the variation between successive measurements. Real changes in cardiac output and inherent variability of the downslope of the thermal curve, necessitating an empirically based calculation, account for up to 10% variation between successive measurements. When cold injectate was used, and the average of three corrected measurements taken, thermodilution cardiac output measurements were within 10% of a simultaneous dye dilution measurement.

Thermodilution cardiac output--a systematic error

The purpose of this study was to examine the mechanism and magnitude of a systematic error in thermodilution cardiac output measurement. One hundred and seventy-one thermodilution cardiac output measurements in dogs using a Swan-Ganz catheter were compared with simultaneously made dye dilution measurements under different conditions over a wide range of cardiac outputs. A systematic error with the thermodilution technique was confirmed and was almost identical to that observed in the literature. It is proposed that its mechanism is loss of thermal indicator between the injectate orifice and detection. Application of a further correction factor for thermal indicator loss is suggested.

Pulmonary gas exchange during acute myocardial ischaemia a study in the closed chest anaesthetized dog

The occurrence and the cause of arterial hypoxaemia were studied in spontaneously breathing dogs in which myocardial ischaemia was induced during anaesthesia. Changes in several circulatory and ventilatory variables and in parameters of gas exchange were assessed in twelve dogs in the two 1st h after the intervention; these included changes in the distribution of ventilation-perfusion ratios determined with an inert tracer gas method. Eight out of 12 dogs developed haemodynamic signs of an acute myocardial infarction after occlusion of a branch of the left coronary artery; the circulatory changes were moderate in 6 and severe in 2 animals. A drop in Pa,O2 occurred in 5 dogs. In one without significant haemodynamic changes it was largely due to a decrease in alveolar ventilation. The ventilation-perfusion ratio distribution in the other 4 dogs did not change in a consistent way; appreciable shunt circulation (6.6%) developed in one dog. Neither ventilation-perfusion mismatch nor shunt circulation contributed much to the hypoxaemia. The most consistent finding in the 5 dogs in whom Pa,O2 fell, was a drop in Pv,O2 (mean 1.3 kPa); it accounted for 86% of the drop in Pa,O2.