In Reply:—

We thank Orr et al.  for their interest in our article and are pleased to take this opportunity to reply to their comments.

The objective of our article was the analysis of errors that are intrinsic to the standard partial rebreathing technique as reported by Gedeon et al.  1and others. We did not set out to analyze the performance of any specific instrument. We mentioned the NICO2in the introduction of our article 2because it is one of the more successful instruments commercially available for monitoring pulmonary blood flow using the partial rebreathing method, and because recent studies have questioned its accuracy at both low and high cardiac outputs. 3–5 

We accept that the NICO2instrument may use algorithms that are more sophisticated than the standard partial rebreathing equations and that those algorithms do address some of the sources of error we described. 2Some of the corrections described by Haryadi et al.  6and Jaffe, 7however, are intended to compensate for right-to-left shunt and parallel dead-space. In our analysis, 2our computer model was set up to simulate a healthy lung with negligible shunt and parallel dead-space ventilation, so these corrections were not relevant to our study. We used the same blood gas dissociation curves for calculating carbon dioxide content in our model and partial rebreathing equations, so this potential source of error was also removed.

In their letter, Orr et al.  refer to an algorithm that is used to avoid excessive rebreathing times, but there is not enough information in their letter or in the references cited to enable an independent evaluation of the algorithm. They refer to Heigenhauser and Jones, 8who describe a number of approaches for correcting estimates of quasi-equilibrium end-tidal pressure of carbon dioxide when rebreathing times are inadequate, but do not indicate which method is used in the NICO2. Haraydi et al.  6report an algorithm for correcting carbon dioxide output estimates for differences between airway and pulmonary capillary carbon dioxide flux during rebreathing. This algorithm requires knowledge of alveolar volume, which appears to be calculated as a function of the Fowler dead-space, 6but the relevant equations are not reported or referenced.

We understand that commercial interests might motivate against the publication of some of the intellectual property developed for commercial instruments. However, it is unfortunate that some of the more critical algorithms used in the NICO2to improve the partial rebreathing method have not been published in enough detail to allow the independent evaluation suggested by Orr et al. 

Gedeon A, Forslund L, Hedenstierna G, Romano E: A new method for noninvasive bedside determination of pulmonary blood flow. Med Biol Eng Comput 1980; 18: 411–8
Yem JS, Tang Y, Turner MJ, Baker AB: Sources of error invasive pulmonary blood flow measurements by partial rebreathing, A nesthesiology 2003; 98: 881–7
Tachibana K, Imanaka H, Miyano H, Takeuchi M, Kumon K, Nishimura M: Effect of ventilatory settings on accuracy of cardiac output measurement using partial CO2rebreathing. A nesthesiology 2002; 96: 96–102
Nilsson LB, Eldrup N, Berthelsen PG: Lack of agreement between thermodilution and carbon dioxide-rebreathing cardiac output. Acta Anaes Scand 2001; 45: 680–5
van Heerden PV, Baker S, Lim SI, Weidman C, Bulsara M: Clinical evaluation of the non-invasive cardiac output (NICO2) monitor in the intensive care unit. Anaesth Intensive Care 2000; 28: 427–30
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
Jaffe MB: Partial CO2rebreathing cardiac output: Operating principles of the NICO2system. J Clin Monit Comput 1999; 15: 387–401
Heigenhauser GJ, Jones NL: Measurement of cardiac output by carbon dioxide rebreathing methods. Clin Chest Med 1989; 10: 255–64