Determination of cardiac output using acoustic quantification in critically ill patients

Cardiac output measured by transthoracic echocardiography and Swan-Ganz catheter. A comparative study in mechanically ventilated patients with high positive end-expiratory pressure

Objective

To compare cardiac output measurements by transthoracic echocardiography and a pulmonary artery catheter in mechanically ventilated patients with high positive end-expiratory pressure. To evaluate the effect of tricuspid regurgitation.

Methods

Sixteen mechanically ventilated patients were studied. Cardiac output was measured by pulmonary artery catheterization and transthoracic echocardiography. Measurements were performed at different levels of positive end-expiratory pressure (10cmH₂O, 15cmH₂O, and 20cmH₂O). The effect of tricuspid regurgitation on cardiac output measurement was evaluated. The intraclass correlation coefficient was studied; the mean error and limits of agreement were studied with the Bland-Altman plot. The error rate was calculated.

Results

Forty-four pairs of cardiac output measurements were obtained. An intraclass correlation coefficient of 0.908 was found (p < 0.001). The mean error was 0.44L/min for cardiac output values between 5 and 13L/min. The limits of agreement were 3.25L/min and -2.37L/min. With tricuspid insufficiency, the intraclass correlation coefficient was 0.791, and without tricuspid insufficiency, 0.935. Tricuspid insufficiency increased the error rate from 32% to 52%.

Conclusions

In patients with high positive end-expiratory pressure, cardiac output measurement by transthoracic echocardiography is comparable to that with a pulmonary artery catheter. Tricuspid regurgitation influences the intraclass correlation coefficient. In patients with high positive end-expiratory pressure, the use of transthoracic echocardiography to measure cardiac output is comparable to invasive measures.

Intramyocardial transplantation of autologous endothelial progenitor cells for therapeutic neovascularization of myocardial ischemia

BACKGROUND

We investigated whether catheter-based, intramyocardial transplantation of autologous endothelial progenitor cells can enhance neovascularization in myocardial ischemia.

METHODS AND RESULTS

Myocardial ischemia was induced by placement of an ameroid constrictor around swine left circumflex artery. Four weeks after constrictor placement, CD31+ mononuclear cells (MNCs) were freshly isolated from the peripheral blood of each animal. After overnight incubation of CD31+ MNCs in noncoated plates, nonadhesive cells (NA/CD31+ MNCs) were harvested as the endothelial progenitor cell-enriched fraction. Nonadhesive CD31- cells (NA/CD31- MNCs) were also prepared. Autologous transplantation of 10(7) NA/CD31+ MNCs, 10(7) NA/CD31- MNCs, or PBS was performed with a NOGA mapping injection catheter to target ischemic myocardium. In a parallel study, 10(5) human CD34+ MNCs, 10(5) human CD34- MNCs, or PBS was transplanted into ischemic myocardium of nude rats 10 minutes after ligation of the left anterior descending coronary artery. In the swine study, ischemic area by NOGA mapping, Rentrop grade angiographic collateral development, and echocardiographic left ventricular ejection fraction improved significantly 4 weeks after transplantation of NA/CD31+ MNCs but not after injection of NA/CD31- MNCs or PBS. Capillary density in ischemic myocardium 4 weeks after transplantation was significantly greater in the NA/CD31+ MNC group than the control groups. In the rat study, echocardiographic left ventricular systolic function and capillary density were significantly better preserved in the CD34+ MNC group than in the control groups 4 weeks after myocardial ischemia.

CONCLUSIONS

These favorable outcomes encourage future clinical trials of catheter-based, intramyocardial transplantation of autologous CD34+ MNCs in the setting of chronic myocardial ischemia.

Recent advances in echocardiographic evaluation of left ventricular anatomy, perfusion, and function

This article provides a brief overview of several recently developed, emerging technologies and discusses their potential uses on clinical grounds. These new technologies include three-dimensional imaging, objective automated evaluation of ventricular function with acoustic quantification, assessment of regional ventricular performance using color kinesis and tissue Doppler imaging, harmonic imaging, and power Doppler imaging. Our hope is that readers will gain a better understanding of the principles underlying these technological advances, which will help them to integrate these new techniques efficiently into their clinical practices.

The future of bedside monitoring

Today's intensivists are provided with more information than ever before, yet current monitors present data from multiple sources in a relatively raw form with virtually no intelligent data integration and processing. In the next century, technological advances in miniaturization, biosensors and computer processing, coupled with an improved understanding of critical illnesses at the molecular level, will lead to the development of a new generation of monitors. Monitoring will move from the traditional macroscopic invasive approach to a noninvasive, molecular analysis of evolving critical disease processes. It is likely that disturbances in homeostasis will become known immediately or before they would otherwise be manifest clinically. Nanotechnology will permit monitoring of critical changes in the intracellular environment or the by-products of cellular metabolism and signal messaging. This article discusses monitoring technologies that hold promise for further development in the next century and point out techniques likely to be abandoned.

Validity and reproducibility of echocardiographic measurement of left ventricular ejection fraction by acoustic quantification with tissue harmonic imaging technique

The tissue harmonic imaging technique can enhance detection of the cardiac endocardial border. When combined with an acoustic quantification (AQ) method, an improvement of accuracy and reproducibility of real-time measurement of left ventricular (LV) function might be expected. However, few data exist regarding the measurement of LV function by AQ with the harmonic imaging technique. Therefore, we evaluated the validity and reproducibility of AQ measurement of LV ejection fraction with or without harmonic imaging technique. A total of 50 patients (mean age 58 +/- 10 years) who underwent left ventriculography were included in our study. The LV end-diastolic and end-systolic volumes by ventriculography were 131 +/- 52 mL and 72 +/- 43 mL, respectively, and were underestimated by both conventional (70 +/- 32 mL and 36 +/- 25 mL) and harmonic (67 +/- 30 mL and 34 +/- 22 mL) AQ obtained in the apical 4-chamber view. The calculated ejection fraction by ventriculography was 0.49 +/- 0. 11 and correlated with that by conventional AQ (0.51 +/- 0.11; y = 0. 72x + 0.152; r = 0.73). This was a marked improvement when compared with the ejection fraction by harmonic AQ (0.50 +/- 0.11; y = 0.89x + 0.065; r = 0.91). Interestingly, interobserver and intraobserver variabilities of conventional AQ, which were 15.6% and 8.6%, respectively, were much improved by harmonic AQ (8.9% and 4.5%, respectively). These results indicate the feasibility of real-time measurement of LV ejection fraction by harmonic imaging, although absolute LV volume can be underestimated even by this technique.

Measurement of cardiac output by transoesophageal echocardiography: a comparison of two Doppler methods with thermodilution

This study assessed the agreement between three methods of cardiac output (CO) measurement, thermodilution, the current clinical standard, and two transoesophageal echocardiographic techniques. Measurements were performed in 37 patients using thermodilution, continuous wave Doppler across the aortic valve and pulsed wave Doppler positioned in the left ventricular outflow tract. The aortic valve area was measured by direct planimetry, and the left ventricular outflow tract area was calculated from its diameter. Weighted least products regression analysis was employed to detect bias, and standard deviation of the difference (SDdiff) was calculated. There was no fixed bias but there was proportional bias between continuous wave Doppler and thermodilution methods (SDdiff 0.92 l/min). There was fixed bias but not proportional bias between pulsed wave and thermodilution methods (SDdiff 1.1 l/min). There was neither fixed nor proportional bias between pulsed wave and continuous wave Doppler methods (SDdiff 1.1 l/min). The transoesophageal Doppler methods described can be clinical alternatives to thermodilution cardiac output measurement.

Online Quantification of Left Ventricular Function: Correlation with Various Imaging Modalities

The need for more objectively quantifiable evaluation of the left ventricular function, obtainable on line with echocardiography, was fulfilled through modifications of integrated backscatter imaging to permit real-time differentiation of the endocardium and the blood pool area in every image frame. Operator-guided study of individual cardiac chambers permitted instantaneous measurement of chamber areas in either the systole or diastole, with resulting physiologically meaningful recordings that relate to cardiac function. Validation studies by various approaches suggest that the methodology is clinically relevant, and further improvements in design will sharpen its applicability in the future.

Acoustic Quantification Today and Its Future Horizons

We briefly review previously published work based on the uses of acoustic quantification (AQ) or validation of this technology. We also discuss the limitations of AQ in a critical review of the literature, including operator dependency, signal noise, and low temporal resolution. We describe some enhancements made to AQ software to address these limitations and improve the accuracy of this technique, including digital beam processing, harmonic imaging, and signal averaging. Several anticipated applications are also briefly described for those interested in the future development of this technology. These future applications include noninvasive long-term monitoring of ventricular function and objective assessment of regional ventricular wall motion in two and three dimensions.

Quantification of mitral regurgitation by automated cardiac output measurement: experimental and clinical validation

OBJECTIVES

To develop and validate an automated noninvasive method to quantify mitral regurgitation.

BACKGROUND

Automated cardiac output measurement (ACM), which integrates digital color Doppler velocities in space and in time, has been validated for the left ventricular (LV) outflow tract but has not been tested for the LV inflow tract or to assess mitral regurgitation (MR).

METHODS

First, to validate ACM against a gold standard (ultrasonic flow meter), 8 dogs were studied at 40 different stages of cardiac output (CO). Second, to compare ACM to the LV outflow (ACMa) and inflow (ACMm) tracts, 50 normal volunteers without MR or aortic regurgitation (44+/-5 years, 31 male) were studied. Third, to compare ACM with the standard pulsed Doppler-two-dimensional echocardiographic (PD-2D) method for quantification of MR, 51 patients (61+/-14 years, 30 male) with MR were studied.

RESULTS

In the canine studies, CO by ACM (1.32+/-0.3 liter/min, y) and flow meter (1.35+/-0.3 liter/min, x) showed good correlation (r=0.95, y=0.89x+0.11) and agreement (deltaCO(y-x)=0.03+/-0.08 [mean+/-SD] liter/min). In the normal subjects, CO measured by ACMm agreed with CO by ACMa (r=0.90, p < 0.0001, deltaCO=-0.09+/-0.42 liter/min), PD (r=0.87, p < 0.0001, deltaCO=0.12+/-0.49 liter/min) and 2D (r=0.84, p < 0.0001, deltaCO=-0.16+/-0.48 liter/min). In the patients, mitral regurgitant volume (MRV) by ACMm-ACMa agreed with PD-2D (r= 0.88, y=0.88x+6.6, p < 0.0001, deltaMRV=2.68+/-9.7 ml).

CONCLUSIONS

We determined that ACM is a feasible new method for quantifying LV outflow and inflow volume to measure MRV and that ACM automatically performs calculations that are equivalent to more time-consuming Doppler and 2D measurements. Additionally, ACM should improve MR quantification in routine clinical practice.

Acoustic quantification indexes of left ventricular size and function: effects of signal averaging

BACKGROUND

The aim of this study was to evaluate the clinical utility of using signal-averaged acoustic quantification (SAAQ) waveforms for improved assessment of left ventricular (LV) size and function.

METHODS AND RESULTS

Four separate protocols were performed in 47 subjects. SAAQ waveforms were used to assess alterations in LV function induced by dobutamine (15 microg/kg per minute) and esmolol (200 microg/kg per minute) in eight normal subjects. Subsequently, we analyzed SAAQ waveforms obtained in 12 patients with LV dysfunction secondary to dilated cardiomyopathy and 12 age-matched normal subjects. Finally, we developed computer software for monitoring of LV function on the basis of continuous acquisition and repeated analysis of SAAQ waveforms. We compared the interbeat variability in indexes of LV function obtained from raw AQ and SAAQ during 10 minutes of steady-state monitoring in eight patients undergoing transesophageal echocardiography. The feasibility of long-term monitoring in the intensive care setting was then studied in seven patients undergoing abdominal surgery. Our analysis tracked variations in LV function induced by dobutamine and esmolol. Significant differences in all measured indexes were found between normal subjects and patients with dilated cadiomyopathy. Signal averaging during steady-state monitoring significantly reduced the interbeat variability of all indexes (21% to 42%). In the operating room, the SAAQ monitoring system tracked hemodynamic changes in close agreement with invasive measurements.

CONCLUSIONS

SAAQ allows fast and easy quantification of LV function and can track hemodynamic trends in the operating room setting.

The Use of Echocardiography in the Intensive Care Unit

Echocardiography is a powerful diagnostic tool that has become an indispensable part of intensive care medi cine. There is a broad clinical application for the noninva sive real-time structural and functional assessment of the critically ill patient. The echocardiograph provides on-line visual information and software for data manipu lation at the intensive care bedside without significant discomfort or risk. Assessment of ventricular function, hemodynamics, pericardial pathology, valvular status, and the outcomes of cardiac surgical interventions are naturally suited to this modality. Transesophageal echo cardiography is an important adjunct to the standard transthoracic examination, particularly in those pa tients with inadequate precordial images. Anatomic, physiologic, and hemodynamic findings can be corre lated in a variety of clinical conditions to make and confirm diagnoses and to direct management in a manner complementary to routine intensive care. Indi cations for echocardiography in the intensive care unit at this institution included assessment of ventricular function, valvular function, endocarditis, complications of surgery, abnormal hemodynamics, evaluation of intra cardiac source of embolus, and echocardiographic- guided endomyocardial biopsy. In this review, the tech niques, indications, and clinical applications of transthoracic and transesophageal echocardiography in the intensive care setting are explored, with a focus on experience in the cardiac surgical patient.

Automated Left Ventricular Endocardial Border Detection Using Acoustic Quantification in Children

OBJECTIVES: The purpose of this study was to determine the reliability and accuracy of automated border detection using acoustic quantification in children. BACKGROUND: Acoustic quantification has shown promise in adult patients as a method for on-line estimation of left ventricular size and function. However, in children, the smaller ventricular size might magnify the importance of measurement error. METHODS: We compared the cross-sectional area and fractional area change of the left ventricle as measured on line by acoustic quantification with the area and fractional area change derived by hand-digitizing the endocardial border of the left ventricle off line, both with and without the papillary muscles included in the left ventricular cavity. RESULTS: The areas and area change fractions from the two methods were highly correlated, both with inclusion and exclusion of the papillary muscles for off-line analysis. However, the regression slope was closer to unity when the papillary muscles were excluded from the left ventricular cavity during off-line digitization of the endocardial border. Analysis of agreement between the two methods showed good agreement for area measurements and fair agreement for function measurements. The magnitude of the difference between the two methods for area measurement was directly proportional to the size of the ventricle. That is, the larger the ventricle the larger the difference between the area measurements by the two methods. DISCUSSION: Automatic border detection using acoustic quantification appears to be an acceptable method for estimating the cross-sectional area and fractional area change of the left ventricle in children.

Comparison of continuous left ventricular volumes by transthoracic two-dimensional digital echo quantification with simultaneous conductance catheter measurements in patients with cardiac diseases

Automated border detection enables real-time tracking of left ventricular (LV) volume by 2-dimensional transthoracic echocardiography. This technique has not been previously compared with simultaneously measured continuous LV volumes at rest or during transients in humans. We performed 18 studies in 16 patients (age 50 +/- 15 years, range 22 to 70; ejection fraction 63 +/- 20%, range 15% to 85%) in which continuous LV volumes acquired by digital echo quantification (DEQ) were compared with simultaneous conductance catheter volume obtained by cardiac catheterization. Both volume signals were calibrated by thermodilution-derived cardiac output and ventriculogram-derived ejection fraction. Volume traces acquired at rest were averaged to generate a comparison cycle. The averaged volume waveforms acquired by DEQ and by conductance catheter were similar during all phases of the cardiac cycle and significantly correlated (conductance catheter = slope. DEQ + intercept, slope = 0.94 +/- 0.09, intercept = 5 +/- 8 ml, r2 = 0.86 +/- 0.12, all p <0.0001). Steady-state hemodynamic parameters calculated using either averaged volume signal were significantly correlated. Transient obstruction of the inferior vena cava yielded a 45 +/- 13% decrease in end-diastolic volume. Successful recordings of DEQ volume during preload reduction were obtained in only 50% of studies. End-diastolic volumes from the 2 methods were significantly correlated (mean slope 0.88 +/- 0.31, mean intercept 14 +/- 37 ml, average r2 = 0.89 +/- 0.11, all p <0.01), as were end-systolic volumes: mean slope 0.80 +/- 0.43, intercept = -20 +/- 26 ml, r2 = 0.67 +/- 0.18, all p <0.05). We conclude that automated border detection technique by DEQ is reliable for noninvasive, transthoracic, continuous tracking of LV volumes at steady state, but has limitations in use during preload reduction maneuvers in humans.

Automated cardiac output measurement by spatiotemporal integration of color Doppler data. In vitro and clinical validation

BACKGROUND

A new Doppler echocardiographic technique has been developed for automated cardiac output measurement (ACOM) that assumes neither a flat flow profile nor collinearity with the scan line, but clinical validation of this method is lacking.

METHODS AND RESULTS

In 165 subjects (50 intensive care patients, 10 dobutamine echocardiography patients, and 105 normal volunteers; age, 49.4 +/- 19.3 years; 92 men), ACOM was performed in the left ventricular outflow tract (LVOT), with the color baseline shifted to avoid aliasing. ACOM was also tested in a pulsatile in vitro model. Stroke volume was calculated by double integration of Doppler signals in space (across the LVOT) and in time (through the systolic period), assuming hemiaxial symmetry: integral of integral of pi r v(r,t) dr dt, where v(r,t) is the velocity at a distance r from the center of the LVOT at time t during systole. Stroke volume from ACOM was compared with thermodilution (TD), aortic valve pulsed-wave Doppler (PWAO), and left ventricular echocardiographic (two-dimensional [2D]) methods. There was good correlation between ACOM and PWAO (r = .93). TD (r = .86), and 2D (r = .74), with close agreement seen. ACOM had higher correlation and agreement with TD than did either PWAO (P < .02) or 2D (P < .01). ACOM was also able to track accurately the changes in cardiac output with dobutamine infusion in comparison with PWAO (r = .94). In vitro assessment demonstrated excellent correlation (r = .98, y = 1.0x + 1.94) with little impact of pulse repetition frequency or misalignment up to 30 degrees. Gain dependency was noted but could be optimized by visual inspection of the color image.

CONCLUSIONS

Automatic integration of numerical data within color Doppler flow fields is a feasible new method for quantifying flow. It is simpler and faster, requires fewer assumptions, and uses only one apical view. ACOM is a promising new approach to echocardiographic quantification that deserves further study and refinement.

Doppler myocardial imaging vs. B-mode grey-scale imaging: a comparative in vitro and in vivo study into their relative efficacy in endocardial boundary detection

Doppler myocardial imaging (DMI) is a new ultrasound imaging modality in which colour Doppler algorithms are adapted to visualise the myocardium. It allows measurement of regional intramyocardial velocities and quantification of intramural left ventricular function. However promising the technique is, to date the accuracy of endocardial boundary detection by DMI has not been validated. As Doppler velocity estimation is based on measurement of phase shift rather than signal strength, the technique is relatively independent of chest wall attenuation. In the current study, a series of in vitro and in vivo studies was performed to compare standard B-mode grey-scale imaging (GSI) and DMI techniques in endocardial boundary detection. In vitro, the minimum and maximum volumes of a single-chamber tissue-mimicking phantom were calculated using both imaging techniques. In vivo, left ventricular end-diastolic (ED) volume and end-systolic (ES) volume indices were measured from GSI and DMI images in a group of 40 volunteers. All images were obtained in the freeze-frame mode with the Doppler display turned on and off so that simultaneous DMI and GSI information was obtained. In vitro, the limits of agreement between the minimum volume of the phantom and the minimum volume measured by GSI and DMI was 4% and 3%, respectively. For maximum volumes, limits of agreement were 3% for GSI and 2% for DMI. In vivo, the limits of agreement between the two imaging techniques in volume measurements were 6 mL (9%) for ED and 4 mL (11%) for ES. The comparison of the endocardial boundary detection by GSI vs. DMI showed DMI to be significantly superior: ED (72 +/- 16% vs. 85 +/- 8%, respectively; p < 0.05) and ES (71 +/- 13% vs. 88 +/- 7%, respectively; p < 0.05). The results of the study show that: (1) in vitro, based on two-dimensional algorithms, DMI provides as accurate volume measurements as GSI; and (2) in vivo, there is a very good agreement of left ventricular volume measurements between GSI and DMI. However, the endocardial boundary is more reliably displayed and visually easier to detect using DMI than GSI.

Automatic backscatter analysis of regional left ventricular systolic function using color kinesis

Assessment of regional wall motion by 2-dimensional echocardiography can be performed by either semiquantitative wall motion scoring or by quantitative analysis. The former is subjective and requires expertise. Quantitative methods are too time-consuming for routine use in a busy clinical laboratory. Color kinesis is a new algorithm utilizing acoustic backscatter analysis. It provides a color encoded map of endocardial motion in real time. In each frame a new color layer is added; the thickness of the color beam represents endocardial motion during that frame. The end-systolic image has multiple color layers, representing regional and temporal heterogeneity of segmental motion. The purpose of this study was to validate the use of color kinesis for semiquantitative analysis of regional left ventricular systolic function and quantitatively in measurement of endocardial excursion. Semiquantitative wall motion scoring was performed in 18 patients using both 2-dimensional echo and color kinesis. Scoring was identical in 74% of segments; there was 84% agreement in definition of normal vs. abnormal. There was less interobserver variability in wall motion scoring using color kinesis. Endocardial excursion was quantified in 21 patients. 70% of the imaged segments were suitable for analysis. Correlation between 2-dimensional echocardiographic measurements and color kinesis was excellent, r = 0.87. The mean difference in excursion as measured by the 2 methods was -0.05 +/- 2.0 mm. In conclusion, color kinesis is a useful method for assessing regional contraction by displaying a color map of systolic endocardial excursion. This algorithm may improve the confidence and accuracy of assessment of segmental ventricular function by echocardiographic methods.

Automatic border detection and three-dimensional reconstruction with echocardiography

This article reviews two important innovations in echocardiography resulting from the recent advances in the capabilities of microprocessors. The first, automatic endocardial border detection, has been implemented on computers contained entirely within echocardiograph machines and is gaining wide clinical use. The second, three-dimensional imaging, is currently under intense investigation and shows great promise for clinical application. It requires, however, further development of the specialized transducer apparatus necessary for image acquisition and the sophisticated computer-processing capability necessary for image reconstruction and display.