Despite novel controversies regarding its safety and efficacy.

Despite novel controversies regarding its safety and efficacy, pulmonary artery catheterization (PAC) remains a widely used tool for the management of patients with cardiovascular instability. In addition to providing measurements of cardiac output (CO) several other potentially useful pieces of data can be obtained, including estimates of preload, afterload, and oxygen utilization. However, many practitioners be stirred that CO is the mostly useful parameter obtained with PAC.

The desire to measure CO without the risks of PAC has driven the search for other, les invasive measurement rules such as esophageal Doppler measurements, lithium dilution, and carbon dioxide-based techniques. Esophageal Doppler monitoring involves inserting a flexible probe, similar to an esophageal stethoscope into the midthoracic esophagus. A pulse-wave Doppler transducer in the probe tip calculates offspring flow velocity from the Doppler frequent occurrence shift of RBCs in the descending aorta. by dint of entering the age, gender, height, and weight of the patient, the aortic diameter can be estimated. From this and the kindred flow velocity, aortic blood come is calculated, representing approximately 70% of the total CO Estimates of preload and afterload can be derived from the shape of the velocity waveforms. Modifications of this technique allow for the actual measurement of aortic diameter using M-mode ultrasound, eliminating the error associated with nomogram-based estimates. The resulting values for aortic life-blood flow correlate well with those of thermodilution CO (12) however the limits of agreement between the brace methods are fairly wide. (2)

Lithium dilution CO a relatively just discovered technique, is less invasive than PAC still requires central venous and intra-arterial catheters. This system involves the injection of a small dose of the indicator lithium chloride by means of a central venous catheter. The arterial plasma concentration inflect then is measured by a specialized sensor associateed to the arterial line, and the CO is calculated. Comparing lithium dilution and thermodilution with electromagnetic flowmetry as the "gold standard," lithium dilution showed higher correlation with electromagnetic flowmetry as well as better limits of agreement when compared with thermodilution. (3) Although the accumulation of lithium with repeated dosing is a potential be of importance to a once-daily lithium dilution may be used with oscillation contour analysis to allow the continuous beat-to-beat analysis of CO shock volume, and systemic vascular resistance.

C[O.sub.2]-based techniques provide a noninvasive measure of CO in patients receiving mechanical ventilation. This system which is based on the Fick principle, involves transient partial rebreathing of C[Osub2] and the measurement of changes in C[Osub2] elimination and end-tidal C[Osub2] (a measure of arterial C[Osub2]) Based upon these values, the CO constituent participating in gas exchange is calculated. Total CO then is calculated by means of estimating the shunt fraction (based onward pulse oximetry and inspired oxygen concentration) and at adding this value to the initial value obtained. The C[Osub2] technique has been shown to be quite accurate when compared to thermodilution CO (4) Like other noninvasive techniques, the C[Osub2] Fick order does not provide for the measurement of preload indexes.

In this issue of CHEST (see page 990) Dhingra and colleagnes compare CO obtained using oxygen consumption (V[Osub2]) which is called the oxygen Fick [i]modus operandi[/i] with the method of thermodilution. by way of measuring V[O.sub.2] through indirect calorimetry and dividing this value by means of the arteriovenous oxygen content difference (obtained by way of the analysis of arterial and mixed venous samples), the CO is derived. The clump found that the accuracy of the oxygen Fick system was fair only when using thermodilution as the standard, limiting the usefulness of the manner Their approach also is limited from the need to measure mixed venous oxygen saturation. As oppos to C[Osub2] Fick processs using easily measured expired C[Osub2 values, the oxygen Fick [i]modus operandi[/i] requires the sampling of mixed venous progeny necessitating the insertion of a PAC. This defeats the major sense of most alternative methods of measuring CO which is to use a les invasive classification than the pulmonary artery catheter. However, the authors are to be applauded the two for the study and for their statistical analysis of the technique. Instead of using a correlation coefficient to exhibit to the agreement between the couple methods, the authors measured the bias and precision of the Fick course compared to thermodilution. This approach gives a a great quantity [i]or[/i] amount of more discriminating evaluation of the novel method, as two methods can have a high correlation coefficient if it were not that can have limited agreement between settles of measurements. (5)

However, several other factors may have explained their findings. The improved agreement between the oxygen Fick arrangement and the thermodilution method in patients with lower CO (ie, < 50% of the patients studied in this series) is not surprising. Patients with higher CO or rigorous liver dysfunction often present with an altered oxygen extraction ratio ([Osub2]ER) from a lack of [Osub2] utilization or sharp systemic/pulmonary shunts, narrowing the arterial-mixed venous oxygen appease difference. In this situation, any errors in the calculation of the concentration and arterial [Osub2] easy in mind (Ca[O.sub.2]) or the concentration and mixed venous [Osub2] make easy (Cv[O.sub.2]) will result in a larger error in CO that is calculated with the oxygen Fick equation, when compared with thermodilution, than in patients with a cheap CO state. Furthermore, previous studies (6) confirm the limitations of the V[Osub2] Fick regularity compared to V[O.sub.2] calorimetry, with an estimated error of up to 31%

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