Lung heat transfer: author?s reply

Abstract

We appreciate the interest of Drs. Schlimp and Wiedermann and welcome the opportunity to comment on the potential clinical utility of our observations. Heat exchange and the transfer of energy during respiration are governed by complex interactions of fluid mechanics, biophysics and heat transfer phenomena. Despite the simplicity of measuring the temperature, humidity and flow rate of exhaled gas, these values are the end result of complex interactions among multiple variables, at least a dozen of which are important and only a few of which can be estimated with reasonable accuracy. Critical variables include the architecture of the bronchial tree and the bronchial walls throughout the lungs, the blood supply to the bronchi and the parenchyma, the amount of tissue in the parenchyma, the presence of a free liquid layer on the airway surface, the pattern of ventilation and the instantaneous gas flow rates. In spite of the complexity of the system structure and behavior, heat exchange processes follow the laws of mechanics and thermodynamics. Multiple attempts have been made to provide robust models which would describe some aspects of this system. These models were based on experimental data and clinical observations and focused on the description of specific phenomena while neglecting other aspects of the process. Our initial observations were in animal studies in which the variables of ventilation and circulation were strictly controlled. This allowed observations regarding correlations of the half-time of exhaled gas temperature decay during transitory phases of heat exchange and the blood supply to the lungs (Serikov and Jerome 1997). These observations allowed us to use a simple lumped-sum heat capacity model and think of the lung as a simple heatexchange unit. We extended these studies to a selected group of humans with well-matched ventilation and perfusion, no known lung pathology and reasonable stability of the other unknown, but possibly predictable, parameters (Serikov et al. 1997). Despite the promising correlations, further extension of these studies into patients with mismatched ventilation and perfusion, low tidal volumes, and possible lung, bronchial or vascular pathology demonstrated that more complex relationships exist between simple indexes of lung heat exchange and pulmonary blood flow. To further clarify these relationships, we undertook this present animal study, combined with complex mathematical modeling, to determine whether there are limits to the use of a simple lumped-sum model of heat exchange (Serikov et al. 2004). These results clearly indicate that the limitations of our previous simple model are substantial. The pattern of ventilation and the instantaneous gas flow rate are critical variables which considerably influence the temperature of the expired gas and other characteristics of lung heat exchange. We do not consider these findings to be a drawback in the development of a non-invasive technique to measure pulmonary blood flow based on lung heat exchange, but rather a substantial and important step forward in understanding the mechanics of the process and its major determinants. We now know that the variables of ventilation must be accounted for and have developed a complex, but robust numerical model to do so. Previously, the application of such a model for common use would be unrealistic, but with the rapid developments in computing power and speed it is, or V. B. Serikov Children’s Hospital, Oakland Research Institute, Oakland, CA 94549, USA