Abstract
Background
Accurate assessment of cardiac output (CO) is essential for the hemodynamic assessment of valvular heart disease. Estimation of oxygen consumption (VO 2 ) and thermodilution (TD) are employed in many cardiac catheterization laboratories (CCL) given the historically cumbersome nature of direct continuous VO 2 measurement, the “gold standard” for this technique. A portable facemask device simplifies the direct continuous measurement of VO 2 , allowing for relatively rapid and continuous assessment of CO.
Methods and materials
Thirty consecutive patients undergoing right heart catheterization had simultaneous determination of CO by both direct continuous and assumed VO 2 and TD. Assessments were only made when a plateau of VO 2 had occurred. All measurements of direct continuous and assumed VO 2 , as well as, TD CO were obtained in triplicate.
Results
Direct continuous VO 2 CO and assumed VO 2 CO correlated poorly ( R = 0.57; ICC = 0.59). Direct continuous VO 2 CO and TD CO also correlated poorly ( R = 0.51; ICC = 0.60). Repeated direct continuous VO 2 CO measurements were extremely correlated and reproducible [( R = 0.93; ICC = 0.96) suggesting that this was the most reliable measurement of CO.
Conclusions
CO calculated from direct continuous VO 2 measurement varies substantially from both assumed VO 2 and TD based CO, which are widely used in most CCL. These differences may significantly impact the CO measurements. Furthermore, continuous, rather than average, measurement of VO 2 appears to give highly reproducible results.
1
Introduction
Cardiac output (CO) is an important parameter of cardiac performance, an accurate assessment of which is an important function of cardiac catheterization laboratories (CCL) . This is especially true in valvular disorders, e.g. in patients with equivocal echo findings where aortic valve area must be calculated for suspected aortic stenosis (AS) . A precise measurement of CO and consequently the degree of AS severity is crucial to ensure that only appropriate patients are referred for valve replacement and to prevent both premature or inappropriately delayed aortic valve replacement .
A widely used CO method is the Fick equation, in which the total uptake or release of a substance, such as oxygen, by an organ is the product of the blood flow through the organ and the arteriovenous concentration difference of the substance . When oxygen consumption is directly measured, this is called direct oxygen consumption (direct, VO 2 ) and is widely considered the gold standard for CO measurement. Alternatively CO can be calculated using estimated VO 2 .
Historically VO 2 was directly measured using a Douglas bag over many minutes, a cumbersome affair. Therefore an estimation of VO 2 is used instead in many CCL, with VO 2 values typically estimated from tables or published predictive equations . Reliability and use of predictive equations for CO measurement have been questioned in the CCL setting because of large discrepancies between measured and estimated values , which of course have an influence on subsequent hemodynamic and valve area calculations .
Portable devices are available that measure breath-by-breath oxygen and carbon dioxide levels using a facemask and provide direct continuous VO 2 measurements ( Fig. 1 ). These devices are well validated to produce very reliable VO 2 assessment similar to those assessed by Douglas bag while being less cumbersome to patients and staff . Importantly, VO 2 can vary during the course of a catheterization. We hypothesized that measuring CO at a time when the VO 2 was not only stable but precisely known (i.e., not an average over time) might give the most reliable calculations of aortic valvular dimensions. We investigated the correlation of the CO based on direct continuous VO 2 measurement by this device to the more commonly used techniques of estimated VO 2 and thermodilution CO .
2
Methods and materials
This study was approved by the institutional review board (IRB) of Christiana Care Health System. Thirty patients scheduled for right and left heart catheterization were included in the study. Catheterization was performed in the usual fashion including measurement of right and left sided pressures. Intravenous midazolam and fentanyl were given to patients as needed. Patients were not placed on supplemental oxygen.
In order to measure VO 2 directly and continuously, patients wore a fitted facemask with an airtight seal over the nose and mouth, which was connected to Ultima CardiO2 breathing analyzer (Medgraphics, St. Paul, MN) The device allows comparison of inspired and expired air to determine VO 2 . To minimize the time of mask use, it was placed only after all catheters were in place for pressure measurements. After a baseline of at least 3 minutes to assure patient comfort and a steady-state VO 2 , immediate measurements of femoral artery and pulmonary artery saturations were obtained in triplicate, immediately followed by three thermodilution CO measurements. We could complete these measurements in approximately 90–120 s. This process was repeated again after 3 minutes after a new steady-state VO 2 was confirmed in the same manner to get a second set of values. We waited until the continuous VO 2 was relatively stable ( Fig. 1 ) before performing the CO determinations in triplicate. If the VO 2 became variable subsequently, we waited for a return of a relatively stable VO 2 before performing the repeat CO determinations ( Fig. 1 ). Oxygen consumption was measured continuously for 2 minutes with the mean VO 2 over that time period used for analysis.
Cardiac output can be assessed using the Fick methods utilizing this equation:
Cardiac output = Oxygen consumption A ‐ V O 2 difference
A-VO 2 difference is the difference in the arterial and venous oxygen blood content. Oxygen blood content can be assessed using this equation:
Oxygen blood content = Oxygen saturation × 1.36 × Hemoglobin g / dl
While direct VO 2 was assessed as described above, estimated Fick calculation oxygen consumption was assessed using this equation:
All patients save one had suitable VO 2 tracings for these measurements. This one patient had unreliable VO 2 tracings due to Cheyne–Stokes respiration. The three values from each cardiac output method were averaged, and those values were compared. Another separate set of direct continuous VO 2 , assumed VO 2 and TD CO was then repeated, after steady-state VO 2 was confirmed, to be used as a validation of reproducibility of CO and AVA. The Phillips (XIMS, Philips Xper Connect, Melbourne, FL) system is in use at our CCL.
2.1
Statistics
Correlation of direct continuous VO 2 , assumed VO 2 and TD based CO measurements was assessed by linear regression analysis and by variance component analysis. The Pearson correlation ( r ) derived from linear regression, assesses the rank ordering of variable values whereas the intra-class correlation (ICC), derived from variance component analysis assesses the degree of agreement between variable values. The ICC is the appropriate statistic for assessing agreement between tests purporting to measure the same thing on the same scale of measurement. The ICC ranges between 0, indicating no agreement, and 1, indicating perfect agreement. Scatter plots of the paired Fick and TD based CO variables included the regression line as well as a 45 degree line of equality along which the paired variables would lie if there was perfect agreement between echo-cath measurements (as would the regression line). Bland–Altman plots were also constructed as complementary assessments of different CO measurements.
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