Validation of the Use of Foreign Gas Rebreathing Method for Non-invasive Determination of Cardiac output
|Keywords||rebreathing indirect Fick cardiac output determination thermodilution left ventricle radiography velocity-time integral echocardiography|
Heart diseases are the most important health problem of our population. And congestive heart failure（CHF） is one of the most common ones. In clinical, the determination of cardiac output is always a problem. In the arsenal of cardiac functional parameters, cardiac output （CO）, cardiac index （CI）, and stroke volume （SV） are potentially important determinants of hemodynamics, the measurement of which relies on complicated and sometimes invasive techniques. In the case of congestive heart failure, non-invasive methods to provide accurate measures of these parameters during exercise stress testing would add significant objectiveness to the test result. This could be of considerable importance for precise risk stratification and for adjustment of therapy. In general, non-invasive measurements （such as: NYHA calss, treadmill, 6 minutes walk, etc） are inaccurate and the invasive measurements （thermodilution, direct Fick） can not be accepted easily by the doctors and patients.The foreign gas rebreathing method has been known for more than 50 years and validated against invasive methods （dye dilution, thermodilution and direct Fick）. The principle of the foreign gas rebreathing method is to let the patient breath a gas mixture containing two inactive compounds, one being blood soluble and the other being blood insoluble in a closed rebreathing assembly. When the blood soluble gas comes in contact with the blood in the lung capillaries it is dissolved and is thus washed out by the blood perfusing the lungs. The pulmonary blood flow （cardiac output） is therefore proportional to the rate of washout of the blood soluble compound,（cardiac output） is therefore proportional to the rate of washout of the blood soluble compound, which is measured continuously by a gas analyzer. The blood insoluble compound is used to determine the lung volume, which is also required in the equation used to calculate cardiac output for the measured washout curve of the blood soluble compound. Previous validations of the foreign gas rebreathing method showed that the method gives very accurate measurements of cardiac output in both rest and exercise. However, until recently the method depended on the use of a medical mass spectrometer, which is expensive and quite complicated to operate and maintain.Recently, a new product, Innocor, was introduced using foreign gas rebreathing to measure cardiac output. This product is based on a newly developed insert rebreathing gas analyzer, which is significantly less expensive than a mass spectrometer and much less complex to apply in a clinical environment.Our study is a validation of the rebreathing method by comparison with the standard methods used in clinical practice: thermodilution, echocardiography and left ventricle radiography. And then, we applied this method in clinical following up. In a previous study, we reported there was no correlation between Simpson’s method and the rebreathing method with respect to the measurement of cardiac output parameters. We believe this phenomenon may be due to mitral regurgitation in heart failure patients. We designed and conducted another study in order to confirm this hypothesis.The first part: Validation of the use of foreign gas rebreathing method for non-invasive determination of cardiac output:Objective: To compare a new device （Innocor） for non-invasive measurement of cardiac output （CO） by foreign gas rebreathing method with conventional techniques used in the measurements of cardiac function. Methods: Cardiac outputs measured by Innocor (CORB) were compared with CO obtained by echocardiography (COEC), Swan-Ganz thermodilution (COTD), and left ventricle radiography (COLVR) in 34 patients subjected to cardiac catheterization. Values obtained from the four methods were analyzed by linear regression and paired values were compared by the method of Bland and Altman in SPSS. Results: There was strong positive correlation （r=0.94） between Innocor cardiac output values and the corresponding values obtained by thermodilution and between COEC and COLVR values. Thermodilution appears to overestimate cardiac output when compared to the values obtained with Innocor by （0.66±0.22） L/min （P<0.0001）. There was no correlation between data obtained by Innocor and the corresponding COEC and COLVR values. Conclusion: Innocor CORB is an easy, safe and well established method for non-invasivemeasurement of cardiac output with good prospects for clinical application in heart disease patients.The second part: Comparison of the Correlation of Rebreathing Method and Echocardiography in Heart Failure Patients With Moderate to Severe Mitral RegurgitationObjective: To follow-up and estimate cardiac function in 11 heart failure patients with moderate to severe mitral regurgitation who underwent cardiac resynchronization therapy （CRT） and to compare echocardiography to the rebreathing method （indirect Fick method） which were used for estimation. Design: Prospective, observational, clinical study. Setting: University teaching hospital. Methods: Eleven cases （8 males and 3 females） were selected and followed-up during presurgery, postsurgery, and 1, 3, and 6 months after pacemaker implantation. Stroke volume was measured by echocardiography （Simpson’s method and velocity-time integral method） and rebreathing each time. Results: Correlations were found between stroke volume with the rebreathing method （RSV） and stroke volume with the velocity-time integral （VSV）, R = 0.89, although ANOVA, the q test, and paired t test showed no statistical differences between them. Stroke volume with Simpson’s rule （SSV） was poorly correlated with stroke volume using the Indirect Fick method （RSV） and with stroke volume using the velocity-time integral method （VSV） （R = 0.58 and 0.54, respectively）. Conclusion: The rebreathing method （indirect Fick method） and velocity-time integral method are noninvasive methods with which to measure cardiac function and exhibited good correlation during the follow-up study.Conclusions:1. The rebreathing method （indirect Fick） is an easy, safe and well established method for non-invasive measurement of cardiac output with good prospects for clinical application in heart disease patients.2. The rebreathing method （indirect Fick） provided at least as good an estimate of cardiac output as did the thermodilution technique. But there is no correlation among the rebreathing method and left ventricle radiography method and echocardiography （Simpson’s method）, especially to those patients with moderate to severe mitral regurgitation.3. It is concluded the rebreathing method and velocity-time integral method are excellent noninvasive methods with which to measure cardiac function. They exhibit good correlation, even in these CHF patients with moderate to severe mitral regurgitation.4.Furthermore, the rebreathing method and velocity-time integral method are easy, safe, and well established methods for the noninvasive measurement of cardiac output with good prospects for clinical application. Simpson’s method seems to be poorly correlated with these 2 methods. The difference among them maybe accredit to the mitral regurgitation.