Lung heat transfer

Abstract

Serikov and colleagues evaluated the effects of the ventilation pattern and pulmonary blood flow on lung heat transfer (Serikov et al. 2004). The authors conclude that the pattern of ventilation substantially affects the values of the overall heat transfer coefficient of the lungs determined at the tracheal level, and thus limits the usefulness of measuring lung heat transfer for determining pulmonary blood flow. A surrogate measure of overall thermal capacitance was determined with the time constant s of the expiratory temperature decay curve between two steady states (warm humidified inspiratory gas at body temperature followed by dry inspriatory gas at room temperature).The results of the study by Serikov et al. (2004) are in contrast with highly encouraging findings, reported by the same author in previous animal and human studies , where the s was not influenced by physiologic changes of minute ventilation (ventilation-to-perfusion ratio <5). The author also assumed the effective coefficient of lung thermal conductivity to be a constant and recommended a new non-invasive method for estimating cardiac output by simply measuring s. Serikov found an excellent correlation between cardiac output and the inverse s in 48 patients (r=0.89). By relating the ''airway thermal volume'' to body size, he furthermore found a reliably high correlation between the cardiac output determined through the s and the cardiac output measured by thermodilution in 15 patients (r=0.96).Despite these interesting previous findings, the recently presented limitations for the use of lung heat transfer to determine pulmonary blood flow seem to be a drawback in the development of this non-invasive technique to quantify cardiac output.However, we want to point to a new aspect of lung heat transfer in human, based on an observation in postoperative cardiac surgical patients (Schlimp et al. 2000), where a highly significant correlation between the simplified acute physiology score (SAPS II) and s was found (r=0.72, P<0.0001, n=30). This suggests that lung heat transfer could be an interesting parameter in critical care medicine and should be investigated further. ReferencesSchlimp CJ, Neugebauer T, Hiesmayr M, Haider W, Haisjackl M (2000) Noninvasive assessment of cardiac output by airstreamthermometry in post-operative cardiac surgical patients. Br J Anaesth 84 [Suppl 1]:29 Serikov VB, Jerome EH (1997) Non-invasive measurements of cardiac output in sheep: a new improved method. J Biomed Eng 19:618-629 Serikov VB, Jerome EH, Fleming NW, Moore PG, Stawitcke FA, Staub NC (1997) Airway thermal volume in humans and its relation to body size. J Appl Physiol 83:668-676 Serikov VB, Fleming NW, Talalov VA, Stawitcke FA (2004) Effects of the ventilation pattern and pulmonary blood flow on lung heat transfer. Eur J Appl Physiol 91:314-323