Technology Assessment

Criteria abstracted from The Users' Guides to the Medical Literature series in JAMA

Intensive Care Med 2008 34(6):1060-1064.[abstract]

Review posted December 24, 2008

I. What is being studied?

1. The study objective:

    To compare cardiac output assessments using the non-invasive Ultrasound Cardiac Output Monitor (USCOM) against assessments made using the gold standard technique of thermodilution using a pulmonary artery catheter (PAC) during cardiac catheterization in patients with congenital heart disease.

2. The study design:

   It is a prospective observational study, in which three paired CO determinations were made simultaneously via USCOM and PAC thermodilution technique at the end of cardiac catheterization after achieving hemodynamic stability (e.g., after device closure of an atrial septal defect or ventricular septal defect ). USCOM measurements were obtained by an experienced single cardiologist, who was blinded to CO results obtained by PAC thermodilution technique.

   The Bland and Altman [1] plot method was used to compare the two measurements techniques used in this study as well as other papers quoted here. In this method the differences between the CO obtained via two techniques are plotted against the average CO measured by two techniques. Line of equality (mean difference in cardiac output =0) and lines of 2SD (95% confidence interval) from line of equality are drawn to show the magnitude of systematic bias. Mean percentage error is calculated (2SD/mean CO X100), if it is less than 30% than technology is considered clinically reliable.

3. The patients investigated:

   This study prospectively enrolled 24 anesthetized pediatric patients (0.1-16.7 yr) with congenital heart disease who were undergoing interventional and/or diagnostic cardiac catheterization. Patients with residual intracardiac or extracardiac shunt, significant pulmonary valve or tricuspid valve insufficiency, body weight less than 3 kg and age older than 18 years were excluded from this study. Nine [37%] of the patients were undergoing device closure of an atrial septal defect, 6 [25%]patients were undergoing device closure of ventricular septal defect, 2 [8.3%] had balloon dilatations of the pulmonary valve,  3 [12.5%] had balloon dilatation of branch pulmonary arteries, 1 [4.1%] had stenting of an aortic coarctation, and 3 [12.5%] underwent catheterization for other diagnostic reasons.

II. Does the technology work as it should?

1. Does the Technology Perform to Specifications in a Laboratory Setting?

   Yes, USCOM does perform to specifications in laboratory settings. Critchley et al [2] studied the USCOM technique in dogs. In 6 anesthetized dogs, USCOM was compared with high precision transit time ultrasonic flow probe placed on ascending aorta to measure the CO. The mean bias between the 2 sets of readings was –0.0l L/min, with limits of agreement (95% CI) of (-0.34) to (+0.31) L/min, which represented ±13% error. Results were promising in this study, and then USCOM was studied in adult population across different clinical situations.   

2. Does the Technology Provide Important Diagnostic Information in a Number of Clinical Situations?

   Yes. Tan et al [3] studied the use of USCOM in 22 adult patients admitted to the ICU after cardiac surgery. Comparison of USCOM and PAC thermodilution techniques showed bias of 0.18L/min and 95% limits of agreement (-1.43) to (+1.78) liters/min. These investigators concluded that USCOM is not reliable in very low or very high cardiac output conditions, due to the failure of USCOM to pick up very low or peak velocity signals.  Knirsch, et al, also reviewed previously published literature by Chand et al [4], Knobloch et al [5] and Chan et al [6] who did similar a comparison between USCOM and PAC thermodilution technique in postoperative adult cardiac patients to measure cardiac output. All these studies found a mean percentage error exceeding 30% suggesting poor accuracy of this machine in detecting exact CO by noninvasive method in adult post operative cardiac patients.

3. Does the Technology Provide Information That Allows a More Accurate Assessment of the Presence or Severity of Disease in Patients?

   No. USCOM is not very reliable to measure accurate readings of cardiac output as this Doppler machine is not able to detect very low or very high peak velocities in descending aorta [3] In the current study (Knirsch et al.), mean CO measured by PAC thermodilution technique ranged from 1.30 to 5.30 liters/min (median 3.57) and mean CO values by USCOM were 0.86 to 5.93 l/min (mean 4.04). The mean difference between the two methods was -0.13 liters/min and precision (± 2SD of the differences was 1.34 liters/min). The mean percentage error was 36.4% for USCOM compared to PAC thermodilution technique.

   There are no studies done to compare USCOM and transesophageal doppler technique to measure CO. However, Murdoch et al [7] compared the measurements of CO via transesophageal doppler with PAC thermodilution technique. Mean percentage difference in CO between PAC thermodilution technique and transesophageal doppler was -0.5% (95% CI for the bias -4% to 3%; limits of agreement -10.7 to +9.7%). This suggests that quality of doppler signal remains an important factor in measuring more accurate CO via non-invasive technique.

III. What Is The Impact Of The Diagnostic Technology?

1. Does the Technology Increase Healthcare Worker Confidence?

   No. So far published literature suggests that USCOM has a mean percentage error of more than 30% in both the adult and pediatric population. There are many factors that could explain this finding, such as the doppler insonation angle, intrathoracic air and inter- and intra-rater reliability of measurements. Moreover, the USCOM doppler software uses normal healthy children's aortic valve diameter rather than the patient's own aortic valve diameter  for calculating cardiac output, which may introduce measurement error in growing patients and those with aortic abnormalities.

   Despite these caveats, researchers of this study do suggest that USCOM may be fast and readily usable, and could be a valuable diagnostic tool to measure cardiac output and to do serial monitoring of CO, in pediatric cardiac ICU patients to evaluate the effect of therapeutic interventions such as fluid boluses or increasing the dose of inotropes.  

2. Are Therapeutic Decisions Altered as a Result of the Technology?

   This is not studied.

3. Does Application of the Technology Result in a Benefit to the Patients?

   The authors do not address this issue. Further studies are required to assess the utility of USCOM for evaluating therapeutic interventions and to document the subsequent changes in cardiac output. This machine is portable, non-invasive, fast and a relatively inexpensive tool for monitoring cardiac output at bedside. However it cannot replace the need for cardiac catheterization, which is superior in terms of getting more information such as intracardiac and intravascular pressures and cardiovascular anatomy (especially in congenital heart surgery patients).

1. Can I Expect a Similar Benefit in My Setting?

   Further studies are needed. Meanwhile, USCOM can be used on an experimental basis to monitor trends in cardiac output in the Pediatric Cardiac Intensive Care Unit as an alternative to echocardiography to assess daily ventricular function in post operative cardiac patients, especially those without residual intracardiac or extracardiac shunts and aortic valve disease etc.  

2. Are the Expected Benefits Worth the Associated Costs?

   No studies done to date have included a cost-benefit analysis. Apart from initial machine acquisition costs, the daily operation of USCOM is relatively inexpensive. The learning curve to use the machine is very short, however inter and intra-rater reliability of CO measurements remain an issue as this study was performed by single USCOM operator. If it can be adapted for routine use in the ICU, this device would meet an ongoing need for noninvasive ways to evaluate the effects of interventions on cardiac output in CICU patients. 

References:

1. Bland JM, Altman DG (1986) Statistical methods for assessing agreement between two methods of clinical measurements. Lancet I: 307–310
2. Critchley LA, Peng ZY, Fok BS, Lee A, Phillips RA (2005) Testing the reliability of a new ultrasonic cardiac output monitor, by using aortic flow probes in anesthetized dogs. Anesth Analg 100:748–753
3. Tan HL, Pinder M, Parsons R, Roberts B, van Heerden PV (2005) Clinical evaluation of USCOM ultrasonic cardiac output monitor in cardiac surgical patients in intensive care unit. Br J Anaesth 94:287–291
4. Chand R, Mehta Y, Trehan N (2006) Cardiac output estimation with a new Doppler device after off-pump coronary artery bypass surgery. J Cardiothorac Vasc Anesth 20:315–319
5. Knobloch K, Lichtenberg A, Winterhalter M, Rossner D, Pichlmaier M, Phillips R (2005) Non-invasive cardiac output determination by two-dimensional independent Doppler during and after cardiac surgery. Ann Thorac Surg 80:1479–1484 1064
6. Chan JS, Segara D, Nair P (2006) Measurement of cardiac output with a noninvasive continuous wave Doppler device versus the pulmonary artery catheter: a comparative study. Crit Care Resusc 8:309–314
7. Murdoch IA, Marsh MJ, Tibby SM, McLuckie A (1995) Continuous haemodynamic monitoring in children: use of transoesophageal Doppler. Acta Paediatr 84:761–764

Last Updated: December 24, 2008

Fill out our Feedback Form | Editorial Notes | About Us