Continuous cardiac output monitoring by the Fick method

Current methods for longitudinal assessment of cardiac output in severely ill patients are intermittent only and in many respects appear unsatisfactory. We have developed a computerized on-line system for continuous Fick cardiac output monitoring, utilizing fiberoptic arterial and pulmonary arterial probes with a metabolic analyzer for VO₂. In 15 patients, cardiac output ranged 1.9–6.8 L/min and 12 were within 5% of thermodilution values. Continuous output monitoring during interventions in two patients (saline infusion and coronary angioplasty) illustrate the utility of the technique. Two additional patients had unsatisfactory VO₂ data due to low airflow velocity. The results of this pilot study suggest that cardiac output monitoring by the Fick method may have clinical and investigational uses in intensive care units and during cardiac catheterization or surgical procedures. © 1993 Wiley-Liss, Inc.     

Automated non-invasive measurement of cardiac output by the carbon dioxide rebreathing method: comparisons with dye dilution and thermodilution.

The accuracy and reproducibility of indirect measurement of cardiac output at rest by the carbon dioxide rebreathing (indirect Fick) method with an automated respiratory analysis system (Gould 9000IV) were compared with simultaneous measurements made in duplicate by dye dilution and thermodilution in 25 patients having cardiac catheterisation studies. Measurements of cardiac output by the carbon dioxide rebreathing method were not significantly different from those obtained with dye dilution (mean difference -0.3 l/min, SD 0.76, 95% confidence interval -0.7 to 0.1). Thermodilution significantly over-estimated cardiac output by a mean of 2.2 l/min or 39% (SD 1.5, 95% confidence interval 1.6 to 2.8) compared with the carbon dioxide rebreathing method and significantly overestimated cardiac output by 1.9 l/min or 31% (SD 1.2, 95% confidence interval 1.2 to 2.5) compared with dye dilution. The reproducibility of measurements of cardiac output in individual patients was satisfactory with the dye dilution method but was poor with carbon dioxide rebreathing and thermodilution. Indirect measurement of resting cardiac output by the Gould 9000IV automated carbon dioxide rebreathing method is more accurate but the variability inherent with this method requires that multiple measurements be taken for each determination. Measurement of cardiac output by the thermodilution method by a commercially available cardiac output computer was not satisfactory because not only was there considerable variability between repeat measurements but the method also consistently overestimated cardiac output compared with the dye dilution method.     

Determination of cardiac output by thermodilution during hypothermia

The thermodilution method for estimating cardiac output was compared with the electromagnetic flowmeter technique in 10 mongrel dogs at normothermia and during surface-induced deep hypothermia. Thermodilution curves obtained during cooling or rewarming must be corrected for the baseline drift caused by changing core temperature. At normothermia, the correlation coefficient between the two methods was 0.96 and the reproducibility of the thermodilution technique was 5 percent. Comparable correlation was present during hypothermia. Curves corrected for baseline drift resulted in significantly different output values from those derived from uncorrected curves (p <0.05). The thermodilution method is valid at low body temperatures. Clinical confirmation of these results, particularly during open heart surgery in infants, is warranted.     

Automated non-invasive measurement of cardiac output by the carbon dioxide rebreathing method: comparisons with dye dilution and thermodilution

The accuracy and reproducibility of indirect measurement of cardiac output at rest by the carbon dioxide rebreathing (indirect Fick) method with an automated respiratory analysis system (Gould 9000IV) were compared with simultaneous measurements made in duplicate by dye dilution and thermodilution in 25 patients having cardiac catheterisation studies. Measurements of cardiac output by the carbon dioxide rebreathing method were not significantly different from those obtained with dye dilution (mean difference -0.3 l/min, SD 0.76, 95% confidence interval -0.7 to 0.1). Thermodilution significantly over-estimated cardiac output by a mean of 2.2 l/min or 39% (SD 1.5, 95% confidence interval 1.6 to 2.8) compared with the carbon dioxide rebreathing method and significantly overestimated cardiac output by 1.9 l/min or 31% (SD 1.2, 95% confidence interval 1.2 to 2.5) compared with dye dilution. The reproducibility of measurements of cardiac output in individual patients was satisfactory with the dye dilution method but was poor with carbon dioxide rebreathing and thermodilution. Indirect measurement of resting cardiac output by the Gould 9000IV automated carbon dioxide rebreathing method is more accurate but the variability inherent with this method requires that multiple measurements be taken for each determination. Measurement of cardiac output by the thermodilution method by a commercially available cardiac output computer was not satisfactory because not only was there considerable variability between repeat measurements but the method also consistently overestimated cardiac output compared with the dye dilution method.     

Determination of stroke volume and cardiac output during exercise: comparison of two-dimensional and Doppler echocardiography, Fick oximetry, and thermodilution

Simultaneous estimates of cardiac output were made during graded upright maximal exercise in 10 male subjects by means of Doppler velocity spectrum of ascending aortic flow, apical two-dimensional echocardiograms, thermodilution, and Fick oximetry. In 15 subjects, aortic annular and root diameters were measured during similar exercise from parasternal two-dimensional echocardiograms. The linear correlation between Doppler, two-dimensional echocardiography, and the invasive estimates ranged from r = .78 to r = .92. Both echocardiographic techniques were able to predict changes in invasive flow estimates with reasonable accuracy. Two-dimensional echocardiographic flow estimates underestimated invasive values by about 60%. The accuracy of Doppler flow estimates varied with the method of estimating aortic cross-sectional area. Greatest accuracy was obtained with areas calculated from diameters measured at the aortic value anulus with the leading edge-to-leading edge method of measurement. Correlation coefficients comparing Doppler and thermodilution flow estimates were generally higher (r = .75 to .96, mean .86) for individuals than for the group, but accuracy of the Doppler estimates in single subjects was quite variable. Aortic diameters did not increase from rest to moderate levels of upright exercise. A 3% to 5% increase in resting aortic diameter was noted in the upright posture as compared with the supine. Doppler flow estimates were obtained in all subjects to maximal exertion but in only a minority of subjects with two-dimensional echocardiography or thermodilution. Thus two-dimensional and Doppler echocardiography offer a noninvasive means of estimating cardiac output during vigorous exercise. The Doppler technique is technically more suitable to the study of exercise than two-dimensional echocardiography.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of cardiac output during exercise by the thermodilution and direct Fick techniques in patients with chronic congestive heart failure

Nine men with chronic stable congestive heart failure (New York Heart Association class II or III) were studied. Oxygen consumption was measured continuously and cardiac output (CO) determined by thermodilution and from the Fick equation at the end of each stage of treadmill exercise. CO measured by the 2 techniques was similar (r = 0.98) over the range of 2.5 to 13 liters/min (43 separate estimations). Tricuspid regurgitation developed in 2 patients during exercise, which resulted in unphysiologic estimates of CO (more than 30 liters/min) by thermodilution. In these circumstances estimation of CO by the direct Fick technique is superior. With this exception, CO measured by thermodilution was accurate even during exercise and provided results similar to those using the direct Fick technique.     

Measurement of cardiac output by the thermodilution method in conscious spontaneously hypertensive rats

The usefulness of the thermodilution method for measuring cardiac output (CO) was evaluated in awake rats by comparison with electromagnetic flowmetry. CO was measured in 3- and 6-month-old conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The correlation co-efficient between CO obtained by the two methods was 0.66 (p less than 0.01). Although CO values obtained by the thermodilution technique tended to be overestimated in comparison with those determined by electromagnetic flowmetry, this method was shown to be useful for measuring CO in unanesthetized rats because of its technical simplicity. Left ventricular mass (LVM) and the ratio of CO to LVM were significantly greater in SHR than in WKY at both ages and CO/LVM increased with increasing age in SHR. The ratio of heart work (HW) to body weight (HW/BW) was increased only in 3-month-old SHR compared with WKY and there was no difference in HW/LVM in 3- and 6-month-old SHR compared with age-matched WKY. The present results suggest that the development of cardiac hypertrophy in SHR is an adaptation to the increased HW due to high afterload.     

Determination of cardiac output at rest and during exercise by carbon dioxide rebreathing method in obstructive airway disease

We evaluated the accuracy of the CO2 rebreathing method (CO2rb) for measuring cardiac output at rest and during steady-state exercise in 15 patients (mean +/- SD age, 59.7 +/- 7.5 yr) with obstructive airway disease. At rest, there was a significant correlation between direct Fick and CO2rb methods using measured arterial PCO2 (r = 0.70; p = 0.002), but not with using end-tidal PCO2 (r = 0.38; p = NS). During exercise, there was greater correlation with CO2rb using arterial PCO2 (r = 0.79; p = 0.001) than using end-tidal PCO2 (r = 0.63; p = 0.007) compared with the direct Fick determination. Correlation between the CO2rb and direct Fick methods was greater with moderate air-flow obstruction (n = 6) than with severe airway disease (n = 9), and the CO2rb method was more accurate during exercise than at rest. The CO2rb method using either end-tidal or arterial PCO2 underestimated the direct Fick measurement in 13 of 15 patients at rest, which may reflect inadequate equilibration between alveolar and oxygenated mixed venous PCO2. However, no consistent error was observed during exercise when higher CO2 production and an increased venoarterial PCO2 difference would diminish potential inaccuracies. We concluded that the CO2rb technique is an acceptable method for measuring cardiac output during exercise in patients with moderate and severe obstructive airway disease as long as arterial PCO2 is directly measured rather than estimated from end-tidal PCO2.     

Evaluation of the CO2 rebreathing cardiac output method in seriously ill patients.

The CO2 rebreathing cardiac output method is a totally noninvasive Fick procedure needing validation in various disease states to become clinically applicable. Simultaneous measurements of cardiac output by CO2 rebreathing and dye-dilution or direct Fick techniques were performed in 53 patients. In nine patients with pulmonary disease rebreathing cardiac output averaged 4.85 L/min compared to 5.18 L/min by dye-dilution or Fick (r = 0.16). In 14 instances of acute myocardial infarction cardiac output was 5.53 L/min by rebreathing and 5.87 L/min by dye-dilution (r = 0.95), while in nine shock cases it averaged 3.98 L/min by dye-dilution or Fick and 3.75 L/min by CO2 rebreathing (r = 0.94). In five heart failure cases with mitral insufficiency, which may distort dye durves, correlation between standard and rebreathing methods was r = 0.09, but in 16 cases without mitral regurgitation, r = 0.89. Acute interventions in ten patients increased dye-dilution cardiac output by 0.92 L/min and rebreathing outputs by 0.60 L/min (r = 0.87). The data suggest that the CO2 rebreathing cardiac output method may be useful in the CCU-MICU setting.     

Determination of cardiac output by thoracic electric impedance: comparison with thermodilution

When an alternating current of high frequency is applied to the thorax, the first derivative of the impedance (dZ/dt) is affected by the cardiac cycle, resulting in a characteristic wave form. The maximum negative of this wave occurring during systole together with the length of ejection time (VET), the blood resistivity, the basal impedance (ZO) and the distance between the inner detecting electrodes (L) makes it possible to calculate stroke volume (SV) and related parameters, as cardiac output (CO) and cardiac index (Cl) by a formula developed by Kubicek. Thoracic electrical impedance has been proposed as a non invasive technique to evaluate cardiac emodinamics. In the present study we have evaluated thoracic electrical impedance by comparing it with thermodilution, simultaneously performed in 21 catheterized patients. Reproducibility was assessed by comparing Cl measured several times in the same patient during ten minutes of rest in the supine position: coefficient of variation, expressed as CV = SD/m X 100 was 8,5 +/- 4,2% and 9,4 +/- 3,2% (p = NS) for thermodilution and thoracic electrical impedance respectively. Cl values obtained by both methods correlated well, with little scatter either baseline (r = 0,784, n = 40, p less than 0,001), either after an handgrip manoeuvre (r = 0,629, n = 15, p less than 0,05). This degree of correlation is similar to that observed comparing invasive techniques (Fick, thermodilution, dye dilution) either among them, or with noninvasive methods (echocardiography, gated equilibrium blood pool scintigraphy and ultrasonic Doppler).(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of the cardiac output in small animals by thermodilution

Cardiac output (CO) measurements based on indicator dilution, microspheres, thermodilution and ultrasonic sensors are not suitable for small animals, because of limited blood volume, high heart rates and small caliber vessels that do not allow probe placement within the heart. We developed a modified thermodilution method to measure CO in awake animals weighing less than 100 g. Under anesthesia, the animal is instrumented with a jugular vein catheter placed proximal to the subclavian vein and a temperature probe in the carotid artery with the thermocouple positioned at the aortic arch. Two days after implantation, room temperature saline is injected (150 microl) into the jugular catheter and the temperature change recorded. This system uses the temperature probe as a digital feedback control: (1) to minimize recirculation error; (2) to adjust baseline temperature, thereby increasing sensitivity to small changes in temperature; and (3) to stabilize animal core temperature. The system was calibrated using a laboratory bench model with anatomically scaled components. CO was measured (n=29) in 16 hamsters (65-115 g), and was linearly related to body weight. Cardiac index (CI=CO/weight) was 197.0 +/- 18.8 (ml/min)/kg. Repeated measurements were made. This technique allows correlating systemic flow changes to be correlated to those measured in the microcirculation of window chamber preparations.