To the Editor:—‘

Yem et al.  used a computer model to describe potential sources of systematic error in the partial rebreathing method of measuring cardiac output. 1They use a complex model to test the basic form of the partial Fick equation first described by Gedeon et al.  2Although the mathematical model of the cardiovascular system described by the authors seems to be elegant and realistic, the partial rebreathing technique that was analyzed using the model bares little resemblance to what is implemented in the actual commercially available system.

It is incorrect to assume that the algorithm used to calculate pulmonary blood flow in a commercially available system is as simple as what is described in the sales literature. Most modern monitoring devices are based on well-known, yet idealized, equations and derivations of these equations. These equations are only used in actual clinical devices after a series of compensations and corrections have been applied to the raw data. Examples of such devices include thermal dilution cardiac output computers and pulse oximeters. Similarly, currently available partial rebreathing cardiac output computers apply various corrections that compensate for the systematic errors described by Yem et al. 

Specifically, Yem et al.  describe excessive rebreathing time as a source for underestimation at high cardiac outputs and insufficient rebreathing times as a source of error when cardiac output is low. Yem et al.  assume that because the patient rebreathes for 50 s, the partial Fick equation must be applied using the data from the last breath observed during rebreathing. Because data are collected throughout the rebreathing period, the algorithm inside the monitor may select any of the breaths that occur during rebreathing, thereby eliminating the need for variable length rebreathing periods.

The other systematic error source cited by Yem et al.  occurs when the rebreathing time is insufficient for equilibrium of the end-tidal carbon dioxide signal to occur. This problem is well documented in the literature related to cardiac output measurement using the total rebreathing method. Various correction techniques dealing with this problem have been described and compared in the literature, 3and one of these techniques is applied in the NICO2system.

These and other corrections based on a mathematical model of the lung similar to what Yem et al.  have described operate within the NICO2partial rebreathing cardiac output system (Novametrix-Respironics, Wallingford, CT). Jaffe 4and Haryadi et al.  5describe many of these corrections in two articles; the former is referenced by Yem et al.  Adequate compensation for the error sources described by Yem et al.  is evidenced in the results published by Odenstedt et al.  6Although Yem et al.  should be complimented on what appears to be an elegant mathematical model, it would be more interesting if this model were used to analyze a realistic partial rebreathing algorithm as implemented in an actual, commercially available device rather than a simple idealized equation.

Yem JS, Tang Y, Turner MJ, Baker AB: Sources of error in noninvasive pulmonary blood flow measurements by partial rebreathing. A nesthesiology 2003; 98: 881–7
Gedeon A, Forslund L, Hedenstierna G, Romano E: A new methods for noninvasive bedside determination of pulmonary blood flow. Med Biol Eng Comput 1980; 18: 411–8
Heigenhauser GJF, Jones NL: Measurement of cardiac output by carbon dioxide rebreathing methods. Clin Chest Med 1989; 10: 255–64
Jaffe MB: Partial CO2rebreathing cardiac output: Operating principles of the NICO™ system. J Clin Monit Comput 1999; 15: 387–401
Haryadi DG, Orr JA, Kuck K, McJames S, Westenskow DR: Partial CO2rebreathing indirect Fick technique for non-invasive measurement of cardiac output. J Clin Monit Comput 2000; 16: 361–74
Odenstedt H, Stenqvist O, Lundin S: Clinical evaluation of a partial CO2rebreathing technique for cardiac output monitoring in critically ill patients. Acta Anaesthesiol Scand 2002; 46: 152–9