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Ann Thorac Surg 2005;80:1479-1483
© 2005 The Society of Thoracic Surgeons
a Department of Trauma Surgery, Hannover, Germany
b Department of Thoracic and Cardiovascular Surgery, Hannover, Germany
c Department of Anesthesiology, Hannover, Germany
d Department of Gynecology, Medical School Hannover, Hannover, Germany
e University of Queensland, Brisbane, Qld, Australia
Accepted for publication December 21, 2004.
* Address reprint requests to Dr Knobloch, Department of Trauma Surgery, Hannover Medical School, Carl-Neuberg-Str 1, Hannover, 30625 Germany (Email: kknobi{at}yahoo.com).
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Abstract |
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DESCRIPTION: Thirty-six patients (67.2 ± 10 years, New York Heart Association functional class 3.1 ± 0.3) undergoing coronary revascularization were prospectively examined postoperatively. One hundred eighty paired CO and stroke volume measurements were compared from the noninvasive USCOM device (Sydney, Australia) and the invasive Swan-Ganz catheter at varying COs. Eighteen measurements were performed intraoperatively by direct insonation of the right ventricular outflow tract.
EVALUATION: Mean noninvasive and invasive CO values were 5.15 ± 1.98 L/min and 4.92 ± 2.0 L/min, respectively (r = 0.870; p < 0.01). The mean difference between methods was –0.23 ± 1.01 L/min greater than a range of CO values from 2.5 to 9.9 L/min. Mean central venous saturation percentage was 72 ± 9%, correlating with both noninvasive and invasive CO (r = 0.474 and 0.606, respectively, p < 0.01). Intraoperatively, both direct and invasive CO were identical.
CONCLUSIONS: Using the ultrasonic cardiac output monitoring (USCOM) device it is possible to determine noninvasive beat-to-beat CO in postcardiac surgery patients without the possible complications associated with invasive right heart catheterization. The USCOM CO and stroke volume showed a very good agreement with invasive Swan-Ganz measures and correlated with central venous saturation percentage.
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Introduction |
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Therefore, noninvasive measurement of CO appears to be a very attractive option. Using the transcutaneous continuous-wave Doppler method it is possible to determine noninvasive transpulmonary and transaortic CO [3]. The ultrasonic cardiac output monitoring (USCOM) device (USCOM, Sydney, Australia) is a novel noninvasive Doppler device designed to measure and trend map both left and right CO. Proprietary algorithms allow determination of flow volumes from raw Doppler data, independent of two-dimensional echo measurement of flow diameters.
To test the reliability of this method we compared noninvasively determined CO measurements using the USCOM with measurements from a continuous cardiac output PAC in patients during cardiac surgery in the operating room as well as postsurgically in the intensive care unit.
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Technology |
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The USCOM system is based on a 300 MHz National Semiconductor GEODE GX1 CPU with 64 MB SDRAM based on Microsoft CE.Net version 4.1 (Microsoft). The system weighs 4.8 kg, its dimensions are 31 cm high x 35 cm wide x 18 cm deep, and it consists of a 12.1"‘ ELO AccuTouch touchscreen, and uses 2.2 MHz and 3.3 MHz acoustic transducers (Figs 1, 2).
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Technique |
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Data was presented as mean ± standard deviation for continuous variables or number and percentages for dichotomous variables. Correlation indices were calculated according to Spearman-Rho, and a p value of less than 0.05 was considered to indicate statistical significance. The Critchley modification of the Bland-Altman method was used to compare the CO measurement methods [4]. The SPSS statistical software package 11.5 for Windows (SPSS Inc, Chicago, IL) was used for statistical analysis.
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Clinical Experience |
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Invasive and Noninvasive Cardiac Output Data
Invasive CO measured by the PAC was 4.92 ± 2 L/min (95% confidence intervals, 4.63 to 5.22 L/min). The noninvasive Doppler device determined CO by the USCOM system was 5.15 ± 1.98 L/min (95% confidence intervals, 4.86 to 5.44 L/min) with a cardiac index of 2.6 ± 0.2 L/m2. Mean SV was 52 ± 20 mL, and mean CO values ranged from 2.5 to 9.9 L/min.
Comparison of Methods
Correlation of invasive and noninvasively determined CO (n = 180) were significantly associated with a correlation index of 0.794 (p < 0.01, Spearman-Rho). Mean CVS percentage was 72 ± 9%, correlating with both noninvasive and invasive CO (0.474 and 0.606 respectively, p < 0.01). Furthermore, SV was significantly correlated with noninvasive CO, invasive CO, and CVS percentage (0.946; 0.803; 0.474; all p < 0.01).
Bland-Altman analysis of the mean difference between measures was –0.23 ± 1.01 L/min, with a mean value of the means of 5.04 ± 1.92 L/min, and a mean error of –4.67 ± 19.90% (Fig 3). Linear regression of the PAC = 0.897 USCOM + 0.396 L/min (Figs 4–6).
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Comment |
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Applications for this novel device may include the intensive care unit as well as intermediate or regular care units to immediately assess the hemodynamic status of a critically ill, unstable patient. Furthermore, the USCOM system may be of special value in the operation room preoperatively, perioperatively, and postoperatively in the hands of anesthesiologists in which hemodynamic management may be critical. Additional use may be applicable in the transplantation ambulance to assess transthoracic CO noninvasively, allowing early hemodynamic optimization. This device can be used to confirm normal circulation in the at-risk patient to detect and grade abnormal circulation in the unstable patient, and to monitor hemodynamic changes associated with therapy or changed clinical status.
Invasive CO Determination
Thermodilution, Fick, and indicator dilution are the current clinical invasive CO methods, and all three methods have well-described limitations. The PAC thermodilution, although still common in clinical practice, requires placement of a PAC in the pulmonary artery and involves crossing the right atrium and ventricle, and is reportedly associated with a significant morbidity and mortality [1].
Thermodilution requires a meticulous methodology for accurate results and may not be practiced with universally good outcomes [1, 2]. In addition, the method is diminished in the accuracy of cases of tricuspid regurgitation, a hemodynamic abnormality commonly associated with depressed cardiac function, and it is specifically reported to over estimate CO in patients with low output, the clinical group of greatest therapeutic significance [5]. Figure 3 demonstrates that USCOM measurement of low output is generally lower than that for the PAC, suggesting that USCOM may measure lower CO values more accurately than the PAC.
Less invasive or modified transpulmonary thermodilution methods, although not requiring intracardiac catheterization, require a venous and arterial line that remains a potential site of infection and a tedious recalibration for accuracy.
Most invasive techniques depend on multiple cycle signal averaging and may display CO values calculated from signals measured minutes prior to display, and may not reflect the current patient status. Averaging of these beat-to-beat variations may also result in the loss of vital hemodynamic variability information. A beat-to-beat method, such as USCOM, is able to measure hemodynamic variability as an independent parameter that may be useful for determination of optimal fluid loading [6, 7]. Left ventricular angiography can measure CO, but it involves intracardiac catheterization and can be associated with arrhythmias and puncture site complications.
Noninvasive CO Determination
Echocardiography provides a two- or three-dimensional image of the heart and can be used to determine CO, but it is expensive and depends substantially on the expertise of the investigator and the quality of the image. Reported 95% confidence intervals for two-dimensional CO determination by experts are in the order of 10% to 26%, whereas with the Doppler, the confidence intervals are approximately 5%, suggesting that the Doppler is a more sensitive method for assessment of CO [8].
Transesophageal echocardiography measures descending thoracic aortic SV and is a semi-invasive ultrasound method for determination of CO. This method involves inserting a probe into the esophagus (adjacent to the descending thoracic aorta) to measure flow. Algorithms allow calculation of CO from this flow on the assumption that a constant proportion of blood volume leaves the heart and continues down the descending thoracic aorta, an assumption often challenged in normal and abnormal circulation. This procedure is not universally well tolerated in the conscious patient and although it may be useful for trend information, it remains problematic for accurate measurement of CO [9].
Noninvasive Doppler determination of CO using the USCOM device is noninvasive and easy to achieve by both medical and nursing staff, and it provides real-time assessment of CO. Results correlate highly with invasive methods of CO determination, as this study demonstrates. In addition, the USCOM system is lightweight and portable, so it can be used in serial hemodynamic determinations in the emergency department, the intensive care unit, and even in the operating room prior to and during procedures. As a noninvasive method that does not require calibration, additional measurements for optimizing hemodynamic interventions can be acquired at no patient risk cost. These repeated measurements also result in significant improvements in measurement sensitivity [10]. The USCOM device can measure hemodynamics from both the right and left heart, and a number of acoustic accesses, allowing for versatility in the surgical environment where access may be restricted by dressings and leads. The device is simple to understand and operate, so reliable results should be attainable in general clinical practice, and these may improve with increased use, such as with other ultrasound modalities.
Limitations
This study used PAC thermodilution as the "gold standard" clinical CO measurement method, however significant concerns with regard to the accuracy of measurements using this method have been described [1, 2, 5].
This study compared a single optimized USCOM measure against a PAC CO measurement. The PAC method depends on averaging signals over determined time intervals and displaying the results after a period of processing delay. As USCOM measures instant beat-to-beat information, there may be some disagreement due to the noncontemporaneous nature of the measurements and the fact that each is measuring a different hemodynamic time sample. The USCOM results may be more comparable with continuous CO if a series of USCOM flow profiles were averaged.
Transcutaneous evaluation of CO using the USCOM system depends on the signal quality of the recorded Doppler profile, and similar to transthoracic echocardiography, results may be compromised by suboptimal ultrasonic conditions. Although only the absolute accuracy of CO measurement is evaluated in this study, an important function of the Doppler is the detection of hemodynamic change by serial CO examination. Serial examinations depends on the reliability of measuring the area under the curve, or the velocity time integral, a measure with confidence intervals in the order of 5% [10].
In addition this study was conducted by one expert operator, and results may not be reproducible in less experienced hands.
Conclusion
Using the USCOM system it is feasible to determine the beat-to-beat CO in critically ill patients on the cardiac intensive care unit noninvasively. These measures demonstrate a high correlation to invasively determined CO, CVS percentage, and SV by a Swan-Ganz PAC, without the possible complications associated with invasive right heart catheterization. The device provides instantaneous results and is portable, with applications in the intensive care unit, emergency department, and operating room preoperatively and perioperatively.
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Disclosures and Freedom of Investigation |
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Footnotes |
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This article has been cited by other articles:
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  K. Knobloch eComment: Double-knotted Swan-Ganz catheter - potential for non-invasive ultrasonic cardiac output monitoring? Interactive CardioVascular and Thoracic Surgery, October 1, 2008; 7(5): 862 - 863. [Full Text] [PDF]
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 L. Siu, A. Tucker, S. K. Manikappa, and J. Monagle Does Patient Position Influence Doppler Signal Quality from the USCOM Ultrasonic Cardiac Output Monitor? Anesth. Analg., June 1, 2008; 106(6): 1798 - 1802. [Abstract] [Full Text] [PDF]
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K. Knobloch, M. Spies, and P. M. Vogt The more the better? Cardiac output monitoring during cardiopulmonary rescuscitation. Interactive CardioVascular and Thoracic Surgery, February 1, 2008; 7(1): 157 - 157. [Full Text] [PDF]
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 R. Savage and L. G. Svensson Invited commentary Ann. Thorac. Surg., October 1, 2005; 80(4): 1483 - 1484. [Full Text] [PDF]
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