Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

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The questions for diagnostic technology articles are based upon: Keenan SP, Guyatt GH, Sibbald WJ, Cook DJ, Heyland DK, Jaeschke RZ. How to use articles about diagnostic technology: gastric tonometry. Crit Care Med. 1999;27(9):1726-31 [abstract]

Article Reviewed:

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An Evaluation of a Noninvasive Cardiac Output Measurement Using Partial Carbon Dioxide Rebreathing in Children.

Levy RL, Chiavacci RM, Nicolson SC et al.

Anesth Analg 2004;99 1642-1647 [abstract]

Reviewed by Mark D. Weber RN, CPNP, West Virginia University Hospital, Morgantown, WV

Review posted March 19, 2005

I. What is being studied?

Study objective:

To assess the accuracy of noninvasive cardiac output (NICO) measurement by using partial rebreathing of CO2 compared with measurement done by thermodilution in children.

Study design:

This was a prospective, unblinded comparison study. Thirty-seven anesthetized, mechanically ventilated patients underwent cardiac catheterization. The simultaneous CO measurements were done by a noninvasive diagnostic technology (NICO monitor) and a reference standard using an invasive pulmonary artery catheterization (thermodilution method). The agreement between two methods was assessed using Bland-Altman analysis.

The patients investigated:

The patients included: Thirty seven patients underwent cardiac catheterization for assessment of hemodynamic function. The ages ranged from 16 mos to 12 years with weights of 9.4 to 42 kg.

The patients excluded: Patients weighing < 8 kg, had intracardiac mixing lesions, or had pulmonary hypertension.

II. Does the technology work as it should?

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

Yes- The principle of NICO is to use partial rebreathing of CO2 and estimates pulmonary capillary blood flow, or CO from Fick equation for CO2.

Q = VCO2 / (CvCO2 - CaCO2)

The assumptions are that alveolar dead-space and CvCO2 are constant during partial rebreathing. The end-tidal CO2 estimates CaCO2. By using a differential form of the Fick equation the need to estimate CvCO2 is eliminated, provided appropriate end-tidal CO2 points are selected where CvCO2 is exactly equal.

There have been a number of studies performed comparing NICO the thermodilution in animals. One study in dogs reported a strong correlation (r² = 0.85) (1). The NICO system has actually already been approved by Food and Drug Administration for use with tidal volume ≥ 200 ml. As the authors state, there have been no clinical studies of the NICO system in children. For an overview of the scientific and technologic data on the NICO system refer to Jaffe MB, J of Clin Monitoring and Computing (2).

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

Unknown. The study group was a highly controlled population of anesthetized children in the cardiac catheterization laboratory. Although NICO could be conceptually used as a diagnostic tool in wide range of clinical settings, there have been several concerns on its accuracy when used in mechanically ventilated critically ill patients.

The NICO cannot be used in several groups of patients. The current technology does not allow the use of NICO in small children since the NICO system is approved by the FDA in situations where the set tidal volume is ≥ 200 ml. In this study the authors expanded on this information stating that the accuracy of the NICO system fell when used in Vt < 300ml and in patients with a BSA < 0.6 m². Smaller children would have gained the most benefit (if the technology allows) since assessment of CO using pulmonary artery catheter has particular technical difficulty and risks in small children.

This principle cannot be applicable to patients with intracardiac mixing lesions. The potential increase of serum CO2 during partial rebreathing excludes the use in patients with pulmonary hypertension and those with increased intracranial pressure due to apparent risks associated with hypercapnia. In further review of the literature the authors have not mentioned the further need to exclude patients with intrapulmonary R to L shunting > 35%, or in patients that are spontaneously breathing (3-5).

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

Yes. Since clinical determination about adequacy of CO is not reliable, and the use of pulmonary artery catheter has its own risks, the NICO could help to monitor and guide therapy in critically ill patients.

The NICO system demonstrated a good correlation with reference the standard for CO measurement (r² = 0.69, p < 0.03). This means 69% of the variance in CO measurement can be accounted for by knowing the variance in CO measurement using NICO system. The Bland Altman analysis revealed an acceptable bias of -0.27 L/min. However, the precision or the second standard error of the differences of ± 1.49 L/min was quite wide resulting the limits of agreement of -1.76 to 1.22. Thus, approximately 95% of NICO measurements may be 1.22 L/min below or 1.76 L/min above the reference standard measurements, which could be unacceptable for clinical purposes, particularly in smaller patients. The NICO system was also found to underestimate CO when the CO was < 3 L/min while it overestimated CO when CO was > 3.0 L/min.

The study failed to prove any significant correlation with reference standard in term of Cardiac index measurement. (r²= 0.45, p = 0.15). This was due to a mathematical reason. The differences between two measurements were magnified when the data were indexed with BSA < 1 m². The Bland Altman analysis revealed a bias of -0.18 L/min/m² and a wide precision of ± 2.13 L/min/m².

The repeatability of CO measurement for both methods in each patient was quite good, i.e., the standard deviation of the mean measurements was only 4% error ± 6. This property makes NICO a reliable method to trend COs continuously, rather than the absolute values. It can provide improved warning of states of reduced CO compared to intermittent clinical assessment or thermodilution technique.

III. What Is The Impact Of The Diagnostic Technology?

1. Does the Technology Increase Healthcare Worker Confidence?

Potentially yes. Although treatment decisions were not assessed in this study, the information could enhance clinicians confidence and comfort. The NICO system could aid in the management of some patients in the PICU. For instance, the CO information added by the NICO system could be useful in the titration of fluid boluses, and vasoactive agents in the patients with shock. Whether the NICO monitor has any impact on patient outcomes is yet to be determined in a randomized controlled trial.

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

Not assessed. It was not the purpose of this study.

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

Not assessed; this was simply a validation trial.

IV. Can I Apply The Diagnostic Technology In My Practice?

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

Yes. There is a potential for this technology to be used in my setting and in other PICU's. The application of the NICO system, however, would be quite limited to only the patients with Vt's > 300 ml.

2. Are the Expected Benefits Worth the Associated Costs?

Unknown. The authors did not state the cost of the NICO system or the associated software. Upon contacting Respironics, the price of the NICO system was quoted at aprox $12,000. Respironics also provides a respiratory monitor that has comparable functions to the NICO except that it does not calculate CO. This CO2SMO monitor has a list price of $8,000. The cost of a standard end-tidal CO2 monitor ranges from $2,000 to $5,000. With the high cost of the NICO monitor, and the limited patient application in relevance to smaller patients, the benefits to a PICU would not seem significant at the present time. The cost could further be justified if the technology could be reengineered for use on smaller pediatric patients.

References

1. Capek JM, Roy RJ. Noninvasive measurement of cardiac output using partial CO2 rebreathing. Transactions on Biomedical Engineering 1988; 35: 653-661. [citation]
2. Jaffe MB. Parital CO2 rebreathing cardiac output - operating principles of the NICO system. J of Clin Monitoring and Computing 1999; 15: 387-401. [abstract]
3. de Abeu MG, Quintel M, Ragaller M, Albrecht DM. Partial carbon dioxide rebreathing: a reliable technique for noninvasive measurement of nonshunted capillary blood flow. Crit Care Med 1997: 25: 675-83. [abstract]
4. Rocco M, Spadetta G, Morelli A, Dell'Utri D, Porzi P, Conti G, Pietropaoli P. A comparative evaluation of thermodilution and partial CO2 rebreathing techniques for cardiac output assessment in critically ill patients during assisted ventilation. Intensive Care Med 2004; 30: 82-7. [abstract]
5. Tachibana K, Imanaka H, Takeuchi M, Takauchi Y, Miyano H, Nishimura M. Noninvasive cardiac output measurement using partial carbon dioxide rebreathing is less accurate at settings of reduced minute ventilation and when spontaneous breathing is present. Anesthesiology 2003; 98: 830-7. [abstract]

 

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